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Contents
1.
Code Requirements for Two-Way Slab Systems ............................................................... 1
1.1. General Requirements for Two-Way Slab Systems .................................................... 1
1.1.1. Concrete Strength .................................................................................................. 1
1.1.2. Load Types and Load Combinations .................................................................. 1
1.1.3. Deflection ................................................................................................................ 1
1.2. Requirements for Mild Steel Reinforced Two-Way Slabs .......................................... 3
1.2.1. Slab Thickness .......................................................................................................... 3
1.2.2. Flexural Reinforcement .......................................................................................... 3
1.2.3. Shear Reinforcement ............................................................................................. 4
1.3. Requirements for Post-Tensioned Two-way Slabs ...................................................... 5
1.3.1. Slab Thickness .......................................................................................................... 5
1.3.2. Flexural Reinforcement .......................................................................................... 5
1.3.3. Shear Reinforcement ............................................................................................. 6
1.4. Local practice and recommendations ...................................................................... 7
1.4.1. Concrete Strength and Modulus of Elasticity (Florida)...................................... 7
1.4.2. Concrete Cover...................................................................................................... 7
1.4.3. Minimum Flexural Reinforcement ......................................................................... 8
2.
Modeling Techniques Using RAM Concept ..................................................................... 10
2.1. General Modeling Techniques .................................................................................. 10
2.1.1. Support Definition ................................................................................................. 10
2.1.2. Reinforcing Callouts ............................................................................................. 11
2.1.3. Live Load Reduction ............................................................................................ 12
2.2. Modeling Techniques for Mild Steel Reinforced Concrete Slabs .......................... 13
2.2.1. Design Strip Definition........................................................................................... 13
2.2.2. Program Execution ............................................................................................... 17
2.2.3. Reinforcement Detailing ..................................................................................... 19
2.2.4. Shear Reinforcement ........................................................................................... 20
2.2.5. Local Practices...................................................................................................... 21
2.3. Modeling Techniques for Post-Tensioned Slabs ....................................................... 22
2.3.1. Design Strip Definition........................................................................................... 22
2.3.2. Tendons .................................................................................................................. 26
2.3.3. Program Execution ............................................................................................... 27
2.3.4. Reinforcement Detailing ..................................................................................... 31
2.3.5. Shear Reinforcement ........................................................................................... 32
2.3.6. Local Practices...................................................................................................... 33
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2.4. RAM Concept Template Model Description ........................................................... 35
2.4.1. Template General Description ........................................................................... 35
3.
Modeling Techniques Using SAFE ..................................................................................... 43
3.1.
Overview ....................................................................................................................... 43
3.2. The DXF File ................................................................................................................... 44
3.2.1. Creating the DXF File............................................................................................ 44
3.2.2. Importing The DXF File Into SAFE ......................................................................... 47
3.3. Define Model Properties ............................................................................................. 48
3.3.1. Defining Concrete Elements with Stiffness, without Mass: .............................. 51
3.3.2. Defining Reinforcing Steel Material Properties: ................................................ 52
3.3.3. Defining Slab Properties: ...................................................................................... 53
3.3.4. Defining Drop Properties: ..................................................................................... 54
3.3.5. Defining Beams Properties:.................................................................................. 55
3.3.6. Defining Column Properties: ............................................................................... 56
3.3.7. Defining Wall Properties: ...................................................................................... 59
3.3.8. Defining Mass Source: .......................................................................................... 60
3.4. Property Assignment .................................................................................................... 61
3.4.1. Assigning/Drawing Slabs and Drops .................................................................. 62
3.4.2. Assigning/Drawing Columns ............................................................................... 64
3.4.3. Assigning/Drawing Walls ...................................................................................... 65
3.4.4. Assigning/Drawing Beams as Drops ................................................................... 66
3.4.5. Assigning/Drawing Beams as Drops ................................................................... 70
3.5. Define Loads ................................................................................................................. 71
3.5.1. Defining Load Patterns ........................................................................................ 71
3.5.2. Load Cases ............................................................................................................ 72
3.5.3. Load Combinations.............................................................................................. 73
3.6. Load Assignment ......................................................................................................... 74
3.6.1. Slab Loads (Area Loads) ..................................................................................... 74
3.6.2. Line Loads .............................................................................................................. 77
3.6.3. Point Loads ............................................................................................................ 79
3.6.4. Displaying Loads ................................................................................................... 80
3.7. Preparing the Analysis ................................................................................................. 81
3.7.1. Design Preferences .............................................................................................. 81
3.7.2. Design Combinations ........................................................................................... 83
3.7.3. Design Strips........................................................................................................... 84
3.7.4. Automatic Slab Mesh Options ............................................................................ 88
3.7.5. Reinforcement Options for Cracking Analysis .................................................. 89
3.7.6. Advanced Modeling Options ............................................................................. 90
3.7.7. Advanced SAP Fire Options ................................................................................ 91
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3.7.8.
Running the Analysis............................................................................................. 92
3.8. Slab Analysis and Design Output............................................................................... 93
3.8.1. Display Reactions ................................................................................................. 93
3.8.2. Display Slab Stresses ............................................................................................. 93
3.8.3. Display Strip Forces ............................................................................................... 93
3.8.4. Display Deformed Shape/ Checking Deflections ............................................ 93
3.8.5. Display Punching Shear Ratios............................................................................ 94
3.8.6. Display Slab Design/Reinforcement .................................................................. 95
3.9.
4.
Design ............................................................................................................................ 96
Appendices.......................................................................................................................... 97
4.1.
Appendix for RAM Concept ...................................................................................... 97
4.2. Appendix for SAFE...................................................................................................... 103
4.2.1 Rebar Detailing ........................................................................................................ 103
4.2.1.1 Column Bars ........................................................................................................... 103
4.2.2 Design: Stress Line Analysis (NYC) .......................................................................... 106
4.2.2.1 Design: Stress Line Analysis (NYC) ....................................................................... 106
4.2.2.2 Pulling Top Reinforcement .................................................................................. 111
4.2.2.3 Pulling Bottom Reinforcement ........................................................................... 116
4.2.3 Design (MIAMI) ......................................................................................................... 119
4.2.3.1 Pulling Top Reinforcement ................................................................................... 119
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1. Code Requirements for Two-Way Slab Systems
The objective of this document is to highlight the important factors to consider when
designing two-way slabs and to outline the main steps to follow in Ram Concept and
Safe. Users new to RAM Concept and SAFE should read the Ram Concept and SAFE
manual for examples and description of software capabilities and limitations.
Note: All ACI 318 references in the document are with respect to ACI 318-14. In
addition, the designer should refer to any other local building codes that apply.
1.1.
General Requirements for Two-Way Slab Systems
1.1.1. Concrete Strength
The default early compressive strength (f’ci) of 3000 psi is in accordance with
DeSimone’s General Notes. Default settings in Ram Concept uses the modulus of
elasticity per code. The designer must be aware that lower values of modulus of
elasticity are usually used in Florida due to the limestone as coarse aggregate. If the
deflection criteria govern the slab design, it is possible to specify the concrete to have
full modulus of elasticity at stressing but there is an extra cost associated which should
be discussed with the project manager.
1.1.2. Load Types and Load Combinations
All the load types pertinent to the design shall be included in the model. RAM Concept
by default contain most of the load types associated with ASCE 7’s load combinations.
Other load types such as wind or cladding, can be added with proper factored load
combinations. An example of adding load type is shown in following modeling
technique chapters.
1.1.3. Deflection
Deflection criteria for slabs are set by ACI 318 table 24.2.2, shown below. Per the table
foot note [2], ACI allows to reduce the amount of deflection calculated before the
attachment of nonstructural elements from the total long-term deflection. In other
words, once the total long-term deflection is calculated, the self-weight deflection
can be subtracted. Both the long-term deflection and long-term (-) self-weight are
available in the RAM Concept template.
For long-term calculations, part of the live load is considered as a sustained load. The
amount of the live load used for time-depended deflections depends on the
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occupancy. An excerpt from ADAPT Technical Note is provided as reference,
however exercise engineering judgment and consult with your PM regarding the
appropriate percentage of live load for long-term deflection. By default, RAM
Concept uses a value of 0.5. The RAM Concept template uses a value of 0.3 as most
of the building fall in this category.
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1.2.
Requirements for Mild Steel Reinforced Two-Way Slabs
1.2.1. Slab Thickness
As a starting point, select slab thickness based on the following rule of thumb i.e.
thickness(in)= Span(in)/Factor per Table. Slab thicknesses per ACI table 8.3.1.1 below,
is a conservative starting point since slab deflection limits are always checked.
Without Drop Panels3
Exterior Panels
fy, (psi)2
Without
Edge
Beams
With
Edge
Beams4
40,000
ln/33
ln/36
60,000
ln/30
75,000
ln/28
Interior
Panels
With Drop Panels3
Exterior Panels
Interior
Panels
Without
Edge
Beams
With
Edge
Beams4
ln/36
ln/36
ln/40
ln/40
ln/33
ln/33
ln/33
ln/36
ln/36
ln/31
ln/31
ln/31
ln/34
ln/34
1) ln is the clear span in the long direction, measured face-to-face of supports (in.).
2) For fy between the values given in the table, minimum thickness shall be calculated.
3) Drop panels as given in 8.2.4.Vibration & Fire Resistance criteria are not considered in the
table above.
4) Slabs with beams between columns along exterior edges. Exterior panels shall be
considered to be without edge beams if αf is less than 0.8. The value of αf for the edge
beam shall be calculated in accordance with 8.10.2.7.
1.2.2. Flexural Reinforcement
Minimum Flexural Reinforcement:
Flexural reinforcement shall be provided based on strength and minimum
requirements. ACI requires every section to have at least a minimum amount of
reinforcement per ACI Table 8.6.1.1 for mild-reinforced slabs. Consult with your PM to
ensure that minimum reinforcing ratios are in accordance with local practice.
Reinforcement Type
fy, psi
As,min in2
Deformed bars
< 60,000
0.0020𝐴𝑔
Deformed bars or
welded wire
reinforcement
3
≥ 60,000
Greater of:
0.0018 × 60,000
𝐴𝑔
𝑓𝑦
0.0014𝐴𝑔
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Top and Bottom Reinforcement:
ACI requires top and bottom reinforcement to be provided at both column and
middle strip per ACI figure 8.7.4.1.3a. Reinforcing minimum extension beyond the
support and hooks shall be per ACI figure 8.7.4.1.3a.
Section 8.7.4.2.2 requires that at least two bottom bars, in each direction, pass within
the region bounded by the longitudinal reinforcement of the column and shall be
anchored at each exterior support.
Maximum Reinforcement Spacing:
Maximum spacing of deformed reinforcement at the critical section (see section
8.7.2.2) shall be the lesser of two times the slab thickness and 18 inches. At other
sections, the maximum spacing shall be the lesser of three times the slab thickness and
18 inches.
1.2.3. Shear Reinforcement
One-Way Shear:
One-way shear rarely controls in a two-way slab design. However, this shall be
checked per section 8.4.3.
Two-Way Shear:
Two-way shear or punching shear often controls the design, and stud rails are provided
for slab shear reinforcement if required. There are multiple requirements laid out in ACI
section 22.6; these requirements are considered in RAM Concept program.
Additionally, read RAM Concept User manual chapter 25 for two-way shear check as
performed by the program.
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1.3.
Requirements for Post-Tensioned Two-way Slabs
1.3.1. Slab Thickness
As a starting point, select slab thickness based on the following rule of thumb i.e.,
thickness(in)= Span(in)/Factor per PTI manual Table.
Continuous Spans
Simple Spans
Floor1
Roof2
Floor1
2-way solid slabs
(supported on columns only)
40-45
45-48
35-38
Beams
One-way solid slabs
Cantilever
30
45
14-15
35
50
26
38-40
Roof2
30
45
1. PT slabs are generally not recommended for heavily loaded areas such as amenity decks,
cooling towers, etc.
2. PT slabs are not recommended for roof slabs if many mechanical units/openings are
expected to be placed after slab has been cast.
3. Vibration & Fire Resistance criteria are not considered in the table above.
4. Where required, drop panels may be needed to control the stress or deflection. Minimum
thickness of drop panels should be 25% of the slab thickness and should be extended not
less than the (span/6) from the column center line in all directions.
1.3.2. Flexural Reinforcement
Minimum Flexural Reinforcement:
Some bonded reinforcement is required by Code to ensure flexural performance at
nominal strength per ACI Table 8.6.2.3. However, at the positive moment locations,
reinforcement is not required unless the tensile computed flexural tensile stress at
service load exceeds 2√𝑓𝑐′ .
Region
Positive moment
Calculated ft after all
losses, psi
As,min, in2
𝑓𝑡 ≤ 2√𝑓𝑐′
Not required
(a)
2√𝑓𝑐′ < 𝑓𝑡 ≤ 6√𝑓𝑐′
𝑁𝑐
0.5𝑓𝑦
(b)[1],[2],[4]
Negative moment at
0.00075𝐴𝑐𝑓
(c) [3], [4]
𝑓𝑡 ≤ 6√𝑓𝑐′
columns
1) The value of fy shall not exceed 60,000psi.
2) Nc is the resultant tensile force acting on the portion of the concrete cross section that is
subjected to tensile stress due to the combined effects of service loads and effective
prestress.
3) Acf is the greater gross cross-sectional area of the slab-beam strip of the two orthogonal
equivalent frames intersecting at a column of a two-way slab.
4) For slabs with bonded tendons, it shall be permitted to reduce A s,min by the area of the
bonded prestressed reinforcement located within the area used to determine N c for
positive moment, or within the width of slab defined in 8.7.5.3(a) for negative moment.
Mild-Reinforcement Spacing:
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If bonded reinforcing is required for tensile stress per ACI table 8.6.2.3, bars shall be per
placed per ACI 8.7.5.3.
1. Placed between the lines that are 1.5h outside opposite faces of the column.
2. Provide at least four deformed bars in each direction.
3. The maximum spacing shall not exceed 12 inches.
For slabs with unbonded tendons, the minimum area of longitudinal reinforcement to
be provided is
where Act is the area of that part of the cross section between the flexural tension face and
the centroid of the cross section.
Post-tension cable spacing:
•
•
•
Maximum spacing of post-tensioned cable in distributed direction shall be 8h
or 5 ft per ACI 8.7.2.3.
Minimum average prestressing force in the slab shall be 125 psi per ACI 8.6.2.1.
Minimum of two tendons over the column in both the directions per ACI
8.7.5.6.1.
1.3.3. Shear Reinforcement
One-Way Shear:
Similar to RC slabs, one-way shear must be checked per ACI section 8.4.3.
Two-Way Shear:
For post-tensioned slabs, average compressive stress in the concrete and the vertical
component of all effective prestressing forces has to be taken into account in
additional to typical RC two-way shear design. Vertical component of prestressing
forces is not included in RAM Concept two-way shear design by default and can be
included by going to Criteria> Cal Options as presented in chapter 2 of RAM Concept
modeling techniques.
Other ACI two-way shear requirements for prestressed slabs are taken into account in
RAM Concept. Read RAM Concept User manual chapter 25 for two-way shear check
as performed by the program.
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1.4.
Local practice and recommendations
1.4.1. Concrete Strength and Modulus of Elasticity (Florida)
Example – South Florida
For projects with reduced modulus of elasticity, the engineer must check that the PT
provider uses the adequate modulus of elasticity at the stressing phase when
calculating the PT losses due to elastic shortening.
The formula and table below shows reduced Modulus of Elasticity used in Florida.
1.4.2. Concrete Cover
Cover in RC Slabs:
Defining the cover depends on the application, such as exterior/interior areas, rebar
outer/inner layer, among others.
Interior Areas
(Not Exposed to
Weather)
#11 or smaller
0.75”
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Parking Garage
(South Florida)
#11 or smaller
1”
Parking Garage
(NY)
(Zone III)
#11 or smaller
2” top (1)
1” bot (1)
Exposed to weather (3)
#5 or smaller
1 ½”
#6 or larger
2”
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Consult
PM
1. ACI 362.1 (Guide Design of Durable Parking Structures)- See Appendix Figure 07.
2. When waterproofing or roofing is used, the exterior area might be treated as non-exposed
to weather, but it is a good practice to use at least 1” of top cover for mild reinforcement
and shear studs as extra protection.
Cover in PT Slabs:
Interior Areas
(Not Exposed to
Weather)
#11 or
PT
smaller tendons
0.75”
0.75”
1” to
C.G.S
Parking Garage
(South Florida)
#11 or
smaller
1”
PT
tendons
1”
1.25” to
C.G.S
Parking Garage
(NY)
(Zone III)
#11 or
PT
smaller
tendons
2” top(1)
1” bot(1)
Consult
PM
Exposed to weather (3)
#5 or
smaller
1 ½”
Consult
PM
#6 or
larger
2”
PT
tendons
1”
1.25” to
C.G.S
1. ACI 362.1 (Guide Design of Durable Parking Structures)- See Appendix Figure 07.
2. When waterproofing or roofing is used, the exterior area might be treated as non-exposed
to weather, but it is a good practice to use at least 1” of top cover for mild reinforcement
and shear studs as extra protection.
1.4.3. Minimum Flexural Reinforcement
Minimum Reinforcement in RC Slabs:
Standard DeSimone Practice in certain parts of the United States is to provide bottom
mat reinforcing to avoid/limit restraint/shrinkage cracks.
•
•
For condominium projects, provide #4@12” each way.
For all other projects, provide #5 or #6@12” each way.
NOTE: Project Engineers should consult with the Project Manager, Associate or
Principal prior to specifying a bottom mat. There are regions of the country where
specifying a bottom mat is no longer a standard practice. If this steel exceeds the
calculated reinforcing, no additional reinforcing is needed. If not, added bars must be
provided based upon the RAM CONCEPT output.
Minimum Reinforcement in PT Slabs:
Standard DeSimone Practice in certain parts of the United States is to provide bottom
mat reinforcing to avoid/limit restraint/shrinkage cracks.
•
•
For condo projects, provide #4@24” each way.
For all other projects, provide #4@36” each way.
NOTE: Project Engineers should consult with the Project Manager, Associate or
Principal prior to specifying a bottom mat. There are regions of the country where
specifying a bottom mat is no longer standard practice. If this steel exceeds the
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calculated reinforcing, no additional reinforcing is needed. If not, added bars must be
provided based upon the RAM CONCEPT output.
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2. Modeling Techniques Using RAM Concept
2.1.
General Modeling Techniques
2.1.1. Support Definition
It is a realistic assumption to model the columns and walls with fixity to the slab. It is also
recommended to model the columns and walls above the slab, since they will be
present at the time service and strength loads are applied.
At the early phase of the project, the designer might choose to model all supports as
pinned as a conservative approach for deflection and bottom stresses. After this first
pass, the designer should model the supports as recommended below. Assuming fixity
in the concrete supports built integrally with the slab is a more realistic assumption. The
fixity in the supports will reduce the slab deflection and bottom stresses while the
punching shear ratios and top stresses will increase.
Columns:
Column below built integrally with slab
RC Corbel
Walls:
Shear Walls
RC walls
CMU walls
Designer should consider how the moment reactions at the supports affect the design
of the columns and walls. Under normal conditions, the moment reactions obtained
at interior columns or thick shear walls do not govern the design. However, moments
may control design of columns in some conditions such as edge/corner columns,
slender columns, or columns supporting transfer beams/girders. ACI section 8.4.2.3.2
provides a formula which calculates the factored slab moment resisted by the
column, which is assumed to be transferred by flexure.
In high rise buildings with large column dimensions, the designer may choose to model
the large columns as:
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•
•
Walls:
It should be noted that RAM checks spans center to center of columns whereas
it accounts for the true dimension of wall widths to decrease the clear span
distance. Therefore, modelling large columns as wall elements will reduce the
slab deflection. However, the designer must be aware that RAM does not
check punching shear at wall elements.
A thick and stiff slab object:
Bentley recommends modelling this slab object with a higher priority than the
main slab that overlays the column volume like a mini drop cap. Since the
columns are not solid objects, they are modeled as a single node located at
the column centroid. Therefore, deflections are observed at column faces but
not wall faces. It is recommended to model this thicker slab with an elevated
top of concrete elevation such that the main slab centroid aligns with the middepth of the added slab object to avoid eccentricity at this joint.
2.1.2. Reinforcing Callouts
In RAM Concept, the reinforcement design layout can be modified to be shown
according to office preference and can be exported to DWG for quick
implementation into CAD or BIM. See below for an example.
Default
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Modified
DWG
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2.1.3. Live Load Reduction
Reduction of live loads is permitted under ASCE 7. Go to Criteria> Cal Options to switch
the pull-down menu from “None” to the appropriate ASCE 7 code used for the project.
See extract below.
Also, ensure reducible live loads are assigned to the correct loading. Go to Layers>
Loadings> Live (Reducible) Loading as shown below.
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2.2.
Modeling Techniques for Mild Steel Reinforced Concrete Slabs
2.2.1. Design Strip Definition
2.2.1.1.
General Tab
Environment:
Select Class U, which is recommended for two-way slab systems.
Net Axial Force:
Ram Concept generally recommends selecting this option. With this option, the
program will design the cross section based on both bending moment and the axial
force, similar to design of columns. This option will ensure that sufficient tensile
reinforcement is provided. The use of this option has a more signification influence in
systems with deep beams – larger force couples.
Don’t reduce integrated M and V due to sign change:
When this option is selected, conservative designs are obtained in the regions with
moment/shear with opposite signs.
2.2.1.2.
Strip Generation Tab
Select “Automatic” for the span width calc, as RAM will automatically detect supports
for the design strip. Selecting “Manual” for column strip width calc will limit the column
strip width as set by the user. Draw “Column Strip Boundary Polylines” to set the design
width of the column strip base on section 8.4.1.5. Per Code, it is possible to have
different column strip widths along the gridline, however the designer can adjust the
column widths for similar spans as required to have a consistent design strip width.
Refer to Ram Concept manual for further information. Once all the strips are properly
generated, it might be useful to use “lock generated strip” option to decrease the
amount of the time required to run the model.
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The previous information (General Information Input and Strip information input) is
summarized in the following extract.
2.2.1.3.
Column Strip and Middle Strip Tab
CS Cross Section Trimming:
For reinforced concrete slabs use “Max Rectangle.” This option is well suited for typical
two-way elevated slabs with and without steps or depression. Other trimming options
such as “None” could remove parts of the slab and exclude them from shear
calculations. For other conditions, change the cross section at those specific locations
for the proper cross section trimming option (refer to Ram Concept manual for further
information). It is always possible to provide two different strips at each side of the
step/slab thickness transition rather than one continuous strip.
CS Shear Effective Depth Calc:
Select “All tension reinforcement,” which uses the location of the tension
reinforcement to calculate the effective depth.
CS Torsion Design:
Select “None” for slabs.
CS Design System:
Select the right system (two-way, one-way or beam). It is important to distinguish
between one-way and two-way slabs because the tensile stress limits are different as
indicated previously. Also, in a one-way system, the slab properties must be modified
so that the moments in the slabs are transferred in one direction only.
CS Span detailer:
Select “Code.”
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CS Code Min. Reinforcement Location:
Use “Elevated Slab” option for slabs. This option places minimum reinforcement at top
near supports and bottom near mid span. Use “Tension Face” for beams, as this option
places the minimum reinforcement according to the location of the tension face of
the beam.
Middle Strip:
Check the box with “Middle Strip uses Column Strip Properties” for slabs. Selecting this
option will utilize the options selected from the “Column Strip,” regardless of what is
grayed out.
Please note that the shown cover values are for the case of outer layer in a slab not
exposed to weather condition with #7 bars.
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2.2.1.4.
Punching Shear
In Layers> Design Strip> Punching Checks Plan, add the punching shear check at all
columns. In cases where large columns are modeled as walls, review areas where
punching shear might occur. Default value of 8ft for “Maximum Search Radius” is
appropriate. Refer to RAM Concept for further information.
Enter the “Cover to CGS,” which is the distance from the top of the slab to the bottom
of the top bar. The extract below shows a ¾” clear cover plus a #6 top bar.
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2.2.2. Program Execution
In some instances, it may be required to model only a portion of the total slab where
shear walls are not modeled. In these cases, RAM might flag the model as unstable.
In these situations, check the box with “Auto-stabilize structure in X and Y directions.”
See RAM Concept excerpt below.
In other instances, it may be needed to verify the slab capacity with existing or user
reinforcement in the slab. This way, RAM Concept will not design, but only check if the
user reinforcement is structurally adequate to carry the loads. Similar to typical design,
RAM Concept will not determine if deflections are acceptable per ACI tables. The user
must confirm that deflections are acceptable and code compliant. See RAM
Concept excerpt below.
Run the model and verify:
a) All design strip cross-sections passed the design check. To display the service
stresses: Layers> Design Status> Status Plan
i.
If design sections do not pass the stress check, verify the loads applied and if
any LL reductions are applicable.
ii.
Because column and middle strips are designed and detailed independently in
RAM Concept, it is possible that the column strip requires one-way shear
reinforcement near the column.
b) Punching shear stress ratios below 1.0. To display the stress ratios: Layers> Design
Status> Punching Shear Status Plan. Typically, USR ration higher than 0.9 should be
checked and shear stud should be provided.
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c) Long term deflections are below the limits previously described. To display the total
long-term deflection: Layers> Load History Deflections> 11. Long-Term Total
Deflection> Std Deflection Plan
When stresses and deflections are verified, output the following:
i.
ii.
iii.
iv.
v.
Top reinforcement, this includes column and middle strips.
Bottom reinforcement, this includes column and middle strips.
Service load deflections (5. Live Load – see section 2.4 ).
Total long-term service load deflections (11. Long-Term Total Deflection –
see section 2.4).
Punching shear stress ratios.
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2.2.3. Reinforcement Detailing
Top Reinforcement:
To view RAM Concept’s design reinforcement length and extension beyond the
support, after running the program go to Layer> Reinforcement> Standard Plan or Top
Bars Plan.
Always provide an equal or higher number of reinforcement bars than what is output
by RAM Concept. It is advised to provide at least two additional top bars at all
supports.
Bottom Reinforcement:
Bottom reinforcement shall be provided per strength or minimum requirements. To
view RAM Concept’s design reinforcement length and extension beyond the support,
after running the program go to Layer> Reinforcement> Standard Plan or Bottom Bars
Plan.
Ram Concept allows the designer to place a reinforcement mat in Layers>
Reinforcement. When drawing the mat, make sure that the circles are not located in
openings, beams, or slabs with different elevations, otherwise the mat will not be
considered in calculations.
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2.2.4. Shear Reinforcement
One-Way Shear:
Often times, RAM Concept will require to provide one-way reinforcement at critical
locations, such as adjacent to column. This is because RAM Concept checks the
column strip and both middle strips independently for one-way shear. Sometimes the
shear demand is larger than the shear capacity of the concrete section within the
column or middle strip. A simple hand calculation, using the total tributary width
(column strip and both middle strips) of the span can easily justify the design. Another
way is to change the “Column Strip Width Calc” from “Manual” to “Full Width” (see
example below). This makes RAM Concept utilize the full tributary width in one-way
shear calculations. Reinforcement at this span can remain as grouped by the column
and middle strip.
Two-Way Shear:
RAM Concept provides the design of stud rails at locations where the punching shear
ratio is 1 or above. Check with your PM if additional stud-rails should be provided at
locations where the ratio is slightly below 1 (0.950-0.999). To display the punching shear
ratio: Layers> Design Status >Punching Shear Status Plan. To display the stud rail design:
Layers> Design Status> SSR Plan.
It is also important to confirm that the stud rail design provided by RAM Concept
matches that of the stud rail details in the drawings. As shown below, the stud spacing,
and other rail parameters can be changed to match the drawings.
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2.2.5. Local Practices
• Slab bottom mat
• Shoring and reshoring timeline for deflection check
• Cold weather concreting
• High strength steel
• Local jurisdiction standard of design
• Seismic requirements for punching shear and diaphragm forces
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2.3.
Modeling Techniques for Post-Tensioned Slabs
2.3.1. Design Strip Definition
2.3.1.1.
General Tab
Environment:
Per ACI 318-14, two-way prestressed slab systems must be designed as Class U
(uncracked members) (𝑓𝑡 ≤ 6√𝑓𝑐′ ). For other elements, such as beams or one-way
slabs, the class category definition is the designer’s choice. For one-way slab, it is
recommended to use Class U. Consult with your PM prior to assuming Class T in oneway slabs. For PT beams, Class T (transition between cracked and uncracked) may be
used in interior spaces and non-corrosive environment. Using the load combination
method for deflection calculation will not accurately capture the long-term deflection
of elements modeled as Class T transition zones since the load combination method is
only for elastic deflections of uncracked sections. Thus, deflection must be checked
as a cracked section for Class T.
Net Axial Force:
As explained in the General Tab for the RC section, Ram Concept generally
recommends selecting this option, which is particularly significant due to the inherent
compression force.
Don’t reduce integrated M and V due to sign change:
Similar to RC design, select this option.
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2.3.1.2.
Strip Generation Tab
Select “Automatic” for “Span Width Calc.” This will detect the type of support and
offset the first design section accordingly. Select “Full Width“ for “Column Strip Width
Calc.” Two-way PT slabs are designed using the equivalent frame method, therefore
there is no column and middle strips. Once the model has been run, verify the
generated strip extents. Use span boundary polylines or manual supports. Refer to Ram
Concept manual for further information. Once all the strips are properly generated, it
might be useful to use “lock generated strip” option to decrease the amount of the
time required to run the model.
The previous information (General Information Input and Strip information input) is
summarized in the following extract:
2.3.1.3.
Column Strip Tab
CS Cross Section Trimming:
Similar to RC design, select “Max Rectangle.” This option is also recommended well for
typical PT slabs with and without steps or depression.
CS Shear Effective Depth Calc:
Similar to RC, select “All tension reinforcement.”
CS Torsion Design:
“None” for slabs
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CS Design System:
Select the right system (two-way, one-way or beam). It is important to distinguish
between one-way and two-way slabs because the tensile stress limits are different as
indicated previously. Also, in a one-way system, the slab properties must be modified
so that the moments in the slabs are transferred in one direction only.
CS Span detailer:
Select “Code.”
CS Code Min. Reinforcement Location:
For slab, use ‘Elevated Slab’ option. This option places minimum reinforcement at top
near supports and bottom near mid span.
Please note that the shown cover values are for the case of outer layer in a slab not
exposed to weather condition with #5 bars.
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2.3.1.4.
Punching Shear
Go to Layers> Design Strip> Punching Checks Plan and follow the punching shear
properties of RC. However, in Criteria> Cal Options check the box to “Include tendon
component in punching check reaction.”
Including the vertical component increases vc, as per section 22.6.5.5. However, if this
vertical component is included, then the PT band center line must be placed at the
location where it will be in the field. Likely, the band center line will not coincide with
the column center line and will create an off-centered vertical reaction that will
increase demand. In this situation some capacity will be gained, by including the PT
vertical component, but demand may increase more or less than the gained
capacity. Consult with your PM if this component should be included.
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2.3.2. Tendons
Tendon Placement:
Place tendons based on two rules of thumb. At times, these rules may be mutually
exclusive. Engineering judgment must be used to determine which rule takes
precedence:
i.
ii.
Place distributed tendons in the direction of cantilevered slabs (i.e. balconies).
Place banded tendons in the long direction.
If rules i and ii do not result in a clear-cut choice of direction, place banded tendons
in the direction of the long spans, as this will produce less cables and uniform stress
distribution.
The profile of the cables should take precedence over the placement of the
reinforcing steel. If the cables have been detailed with minimum clear cover at the
slab bottom, a typical detail should call for the reinforcing steel to be raised above
the cables at the low point of the cables. Also, per section 8.7.5.6, a minimum of two
strands must be provided in each direction at columns.
Tendon Profile:
Initial high and low tendon profile points should be based on a minimum clear cover
plus the tendon diameter (unbonded system) or half the duct diameter (bonded
system). ½” unbonded single-strand (mono-strand) tendon composed of seven wires
is typically used in PT slabs. However, follow IBC table 721.1(1) for locations where a 1
½” cover is required for two-hour fire rating. This provision is not strictly enforced in all
the jurisdiction, ask PM for direction on tendon cover at restrains.
Banded Tributary Width:
Find tributary width of banded tendons (column strip width) and select initial number
of banded tendons based on a minimum P/A of 150psi. Some designers prefer to start
with 175psi.
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Uniform Tributary Width:
Place the distributed cable groups so that the pre-compression in the slab is a
minimum of 125 psi per ACI 318 requirement (8.6.2.1). However, some references
recommend higher pre-compression values depending on the structure type. See
Table 1 in the Appendix for further information. The spacing between the groups of
tendons should not exceed the smaller of 8 times the slab thickness and 5 ft. For 8”-9”
PT slabs, it is common to provide the distributed tendons in groups spaced every 4 ft.
For example, in an 8” slab, place 2 strands every 4ft-6in and in a 9” slab, place 2 strands
every 4 ft.
2 𝑠𝑡𝑟𝑎𝑛𝑑𝑠 ∗ 27 𝑘𝑖𝑝𝑠/𝑠𝑡𝑟𝑎𝑛𝑑 ∗ 1000 𝑙𝑏/𝑘𝑖𝑝
= 125 𝑝𝑠𝑖
8 thick slab*54 " 𝑡𝑟𝑖𝑏.
2 𝑠𝑡𝑟𝑎𝑛𝑑𝑠 ∗ 27 𝑘𝑖𝑝𝑠/𝑠𝑡𝑟𝑎𝑛𝑑 ∗ 1000 𝑙𝑏/𝑘𝑖𝑝
= 125 𝑝𝑠𝑖
9 thick slab*48 " 𝑡𝑟𝑖𝑏.
2.3.3. Program Execution
Similar to RC slabs, it may be required to model only a portion of the total slab where
no shear walls are modeled. In these cases, without shear walls, the structure may be
unstable. In these situations, check the box with “Auto-stabilize structure in X and Y
directions.” See RAM Concept excerpt below.
In other instances, it may be needed to verify the slab capacity with existing or user
reinforcement in the slab. This way, RAM Concept will not design, but only check if the
user reinforcement is structurally adequate to carry the loads. Similar to typical design,
RAM Concept will not determine if deflections are acceptable per ACI tables. The user
must confirm that deflections are acceptable and code compliant. See RAM
Concept excerpt below.
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Run the model and verify:
a) Tensile stresses at service loads are below the limits specified on section 24.5.2.1 i.e.
for Class 𝑈 ≤ 6 √𝑓𝑐′ . To display the service stresses: Layers> Rule Set Design> Service
Design> Top Stress Plan or Bottom Stress Plan
i.
If the tensile stresses are exceeding ACI section 24.5.2.1 limits or the long-term
deflection does not meet the deflection criteria, then adjust mid-span profile
points in PT design, the designer must be mindful of how a concentrated load
at the center of only one span can affect the bending diagram of the
continuous spans. For instance, increasing the mid span profile of the backspan of a large cantilever will help to decrease the deflection in the cantilever
portion. Once the tendon profiles have been adjusted, repeat step i. If the
model does not meet the stress limit and/or the deflection criteria yet, increase
the number of cables in the direction where required. Limit the pre-compression
to 300 psi, as recommended by ACI 423. If more cables are needed that cause
the pre-compression to be above this value, reconsider the tendon layout. If
pre-compression exceeds 300 psi, cracking in the slab is more likely to occur,
especially in areas around restraints like shear walls and basement walls.
ii.
Repeat step ii. If the tensile stresses still exceed the ACI 318-14 (24.5.2.1) limits
and/or deflection does not meet the deflection criteria, then:
Option 1: Provide 2 high points rather than 1 high point at the supports. The
tendons are usually placed with a horizontal profile at the supports as illustrated
in the extract below. The high points can be placed 2-3 inches away from the
support faces. Modeling 2 high points in RAM Concept will help to reduce the
tendon span and to reduce tensile stresses at the same time.
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Option 2: Consider increasing the slab thickness or adding drop panels to lower
the top stresses and the deflection.
Option 3: Consider increasing the concrete strength up to 8 ksi. Higher concrete
strength will reduce the deflection due to the higher modulus of elasticity. In
addition to this, the stress limits will increase since they are a function of √𝑓𝑐′ . In
the same way, the slab shear capacity will also increase.
iii.
29
Also, adjust initial banded and distributed tendon profile points based on
balancing approximately 75% of the total dead load (rule of thumb generally
is 75% of only self-weight but RAM will include the superimposed load as well if
this load case was defined as “Dead” in loading type). To display the balanced
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Load percentages: open the design strip window and check balanced load
percentages in the visible objects menu.
For slabs with short cantilevers, beware of upward deflections on the cantilever.
Such upward deflection can potentially cause water intrusion issues.
For slabs with heavy superimposed values such as transfer slabs or slabs with
planter loads, the design may involve providing PT to balance > 100% DL. In
such cases, the user must check initial stresses and provide mild reinforcement
where necessary to counter these effects. In other words, bottom reinforcement
might be required at supports and top reinforcement might be required at mid
spans. The two easiest ways to see the mild reinforcement required due to initial
stresses are:
·
·
In Layers>Design Status>Reinforcement plan, check “controlling criteria” in
visible objects.
Open the window for reinforcement plan in Layers> Rule Set Design> Initial
Service.
b) Punching shear stress ratios are below 1.0. To display the stress ratios: Layers> Design
Status> Punching Shear Status Plan.
c) Long term deflections are below the limits previously described. To display the longterm deflections: Layers> Load History Deflections> 11. Long-Term Total Deflection>
Std Deflection Plan.
When stresses and deflections appear to be okay, output the following:
vi.
vii.
viii.
ix.
x.
xi.
xii.
Initial service stresses (for checking ACI 24.5.3.1).
Final service stresses (for checking ACI 24.5.2.1).
Top reinforcement (Usually located over columns and walls).
Bottom reinforcement (Usually located at mid-spans).
Service load deflections (5. Live Load).
Total long-term service load deflections (11. Long-Term Total Deflection).
Punching shear stress ratios.
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2.3.4. Reinforcement Detailing
Top reinforcement:
Always prove an equal or larger number of reinforcement bars than what is output by
RAM Concept. It is recommended to provide at least two additional top bars at all
supports.
Bottom reinforcement:
Per section 24.4.1, the minimum reinforcement to resist shrinkage and temperature
stresses is not required in two-way slabs.
On section 8.7.5.6, a minimum of two strands must be provided in each direction at
columns. However, in some slabs, tendon layout constraints make it difficult to provide
these integrity tendons. In such situations, Bottom deformed reinforcement in each
direction shall be the greater of (a) and (b) below, where bw is the width of the column
face through which the reinforcement passes.
(a)
𝐴𝑠 =
(b)
𝐴𝑠 =
4.5√𝑓𝑐′𝑏𝑤 𝑑
𝑓𝑦
300𝑏𝑤 𝑑
𝑓𝑦
Similar to modeling RC slabs, ensure that the circles are not located in openings,
beams, or slabs with different elevations, otherwise the mat will not be considered in
calculations.
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Shear Reinforcement
One-Way Shear:
At some instances, RAM Concept will require to provide one-way reinforcement at
critical locations, such as locations adjacent to columns. Although RAM Concept
already uses the span’s tributary width to determine vc of the slab, verify that the
appropriate “CS Cross Section Trimming” is being used. Some section trimmings
remove portions of the slab that carry shear, which reduces the slab’s shear capacity.
See below for an example. Review chapter 23.9 of the manual for more information.
Sections
considered in
shear and
flexure design.
Section Trimming:
“Max Rectangle”
Sections
considered in
flexure design
only.
Section Trimming:
“None”
Two-Way Shear:
RAM Concept provides the design of stud rails at those locations where the punching
shear ratio is 1 or above. Check with your PM if additional stud-rails should be provided
at locations where the ratio is slightly below 1 (0.950-0.999). To display the punching
shear ratio: Layers> Design Status >Punching Shear Status Plan. To display the stud rail
design: Layers> Design Status> SSR Plan.
Similar to RC slabs, confirm that RAM Concept’s design output matches that of the
drawings.
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2.3.5. Local Practices
See local standard of practice per RC section 2.2.5.
The Post-Tensioned Manual suggest to add shrinkage reinforcement near stiff
elements, such as shear walls. The figure below (Figure 6.16 from the Post-Tensioned
Manual) shows the size and location of such reinforcement.
Added reinforcement at bearing and shear walls (Figure 6.16 of the Post-Tensioned Manual 6th)
Providing appropriate amounts of bottom reinforcement reduces the need to satisfy
other shrinkage reinforcement requirements such the one from section 6.4.2.4 of the
Post-Tensioning Manual, which reduces slab cracking at stiff element.
Another benefit of providing appropriate bottom reinforcement is at the locations
where prestressing is less than 100 psi, following ACI section 24.4.4.1. The figure below,
shows a sample location.
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Arrangement of temperature and shrinkage reinforcement (Figure 2.17.2-1 of the Design
Fundamentals of Post-Tensioned Concrete Floors - PTI)
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2.4.
RAM Concept Template Model Description
2.4.1. Template General Description
2.4.1.1.
Ram Concept Model Settings
Cal Options Tab - General:
Go to Criteria> Cal Options and select “Element Size” to be two feet. The finite element
mesh should be refined to produced 12 or more elements per bay, as recommended
by RAM. Create more elements as needed by reducing the Element Size when
generating the mesh.
As mentioned in section 2.3.1.4 this guide, review with your PM if the tendon vertical
component is to be included in the calculations of punching shear.
Cal Options Tab – Load History/ECR:
As referenced by Bentley, external restraint to shrinkage is a simple way to account for
cracking due to the gradual buildup of tensile stress that can occur when shear walls
are modeled. In general, as the input percentage increases, the tensioning stiffening
effect will be reduced, cracking will occur at earlier times and lower loading levels,
and load history deflections will increase. This effect should be distinguished from
internal restraint to shrinkage due to reinforcement. See excerpt below.
Application of this restrain is more noticeable in PT slabs, where external restrain
reduced prestressing.
Creep accounts for the increase in concrete strain with time due to sustained loads.
Typical concrete creep strains range between 2-4 times the elastic strain and are
important for accurate prediction of long-term deflections as a result. RAM Concept
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calculates a differential creep strain over the duration of each loading stage using
ACI 209R.
As a default, RAM Concept uses a 20% of shrinkage restrain.
recommendations are the following:
·
·
·
Bentley’s
0% - Unrestrained or very lightly restrained slabs (flexible columns only, single stiff
element)
10% - Normally restrained slabs (more than one stiff element, some flexibility)
20% - Completely restrained slabs (basement walls around entire perimeter, etc.
causing a high degree of restraint)
As a rule of thumb, it is recommended to start with 10% and to increase or decrease
this number using engineering judgment.
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2.4.1.2.
Load Definition
Create a new load and name it Cladding by going to Criteria> Loadings. Select Dead
as the loading type. Also ensure that cladding loads are assigned under this loading.
See below.
This new loading “Cladding,” as a superimposed dead load, has already been in
included in all other load combinations with dead load in the template. See below for
example of added “Cladding” loading to the “All Dead LC” load combination.
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2.4.1.3.
Load Combinations
The newly created Cladding load, being a dead load too, is already include into other
load combinations.
RAM Concept already provides service and strength load combinations. However, for
easier breakdown of long-term deflections, it is recommended to create new
combinations as shown below.
Note that RAM Concept requires the self-weight to always be present in the load
combinations. As shown in the following section, when certain deflections are being
checked (such as cladding, or live load), the self-weight is removed.
Loading Type
New Load Combination
RAM Extract
All Live LC
Cladding LC
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SD Load LC
SW + Cladding LC
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2.4.1.4.
Load History
To account for the effects of cracking, creep, shrinkage, tension stiffening, and load
history, RAM Concept uses an iterative approach to find deflected equilibrium of a
slab under each of the loads in a sequential Load History. The duration of each step
contributes to the age of the concrete under consideration. This age is used to
calculate time-dependent properties, such as elastic modulus. See Chapter 65 of the
RAM Concept Manual for information on Load History calculations. Follow the extract
below for naming the Load History Steps and for selecting the appropriate load
combination.
The following procedure is executed at each Load History Step and results such as
cracking factors carry over to the next Load History Step:
1. Solves cross section forces.
2. For each cross section, calculates curvatures including long term effects and
the effects from the previous load history step:
· Gross cross section curvature (using gross section properties)
· Uncracked cross section curvature (using uncracked transformed
· section properties)
· Cracked cross section curvature (using cracked transformed section
properties)
· Creep cross section curvature (considers cracking history of the cross
section)
3. Using the calculated curvatures and the tension stiffening model, calculates an
“average” curvature for each cross section.
4. For each element in the structure, uses the average calculated curvatures for
the tributary cross sections to set stiffness factors for the element.
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5. Re-analyzes the structure with the adjusted element stiffnesses and checks for
convergence. Convergence is measured by the deflection difference
between two iterations as measured at a key node.
6. Repeats steps 1-5 for each load history step until convergence.
2.4.1.5.
Deflection Checks
The following describes the load history steps mentioned above.
1. Post-Shoring (Initial Service LC)
This represents the deflection of the floor under its full self-weight after forms are
stripped and before shoring load is applied.
2. Pre-Sustained Cracking (Service LC: D + L)
This represents the largest realistic load the slab will see during its life. Per the
Release Notes for RAM Concept, cracking that occurs during the Sustained load
step can result in an under-prediction of the deflection. Therefore, the slab should
be "pre-cracked" before entering the Sustained load step.
3. Cladding Load (Cladding LC)
This represents the short-term deflection due to cladding only.
Diff. Layer used: “6. Self-Weight Load”
4. SD Load (SD Load LC)
This represents the short-term deflection due superimposed dead load only.
Diff. Layer used: “6. Self-Weight Load”
5. Live Load (All Live Load LC)
This represents the short-term deflection due to live load only. This layer is used to
check the deflection criteria against L/360 (Table 24.2.2).
Diff. Layer used: “6. Self-Weight Load”
6. Self-Weight Load (Initial Service LC)
This represents the short-term deflection due to slab self-weight only.
No Diff. Layer is used.
7. SW+CLADD Load (SW + Cladding LC)
This represents the short-term deflection due to slab self-weight plus cladding.
8. SW+CLADD+SD Load (All Dead LC)
This represents the short-term deflection due slab self-weight, cladding and
superimposed dead load.
9. Sustained Load (Sustained Service LC (0.3LL))
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This is a long-term service step in which all the dead load and a portion of the live
load is applied for 5000 days to cause creep of the concrete. The percentage of
live load used for long-term service depends on the space occupancy as
mentioned before.
10. Post Creep (Initial Service LC)
This represents the deflection of the floor, under self-weigh only, after creep has
occurred.
An option under this layer is the “10. Post Creep: Minus 1. Post Shoring Plan.” This
layer shows the differential deflection between the time the forms were removed
(post-shoring) and the long-term deflection caused by creep. This layer can
provide an insight to how much deflection is accumulated on the slab over time
under its own weight. This optional layer can be used to compare or confirm
deflections on slabs that remained unloaded for long time.
11. Long-Term Total Deflection (Service LC: D + L)
This represents the total long-term deflection, which also includes self-weight and
all nonstructural components. This deflection layer is used to check the total longterm deflection criteria against L/240.
An option under this layer and following note [2] on table 24.2.2 of ACI-318, is “11.
Long-Term Total Deflection: Minus 6. Self-Weight Load Plan.” This layer removes the
deflection that occurs before the attachment of nonstructural elements, selfweight. This optional layer can be further refined so that the self-weight and a
portion of the superimposed dead load is removed. This deflection layer is used to
check the total long-term deflection criteria against L/240.
2.4.1.6.
Local Practices
Other than the deflection requirements per ACI mentioned above, the following
criteria is followed as per local practice:
Interior Spans:
•
Full service: Lesser of L/240 or ¾”
Exterior Spans:
•
•
Full service: Lesser of L/480 or ½”
Full service masonry façade: L/600 or 3/8”
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3. Modeling Techniques Using SAFE
3.1.
Overview
The DeSimone SAFE User Guide was created to inform engineers how to properly use
the CSI SAFE software for concrete slab design. The goal of this user guide is to provide
a clear and consistent step by step procedure for conventionally reinforced slab
design using the CSI SAFE software that can be used by engineers across DeSimone
offices. This user guide will show the user how to set up the model, run the analysis, and
interpret the output for an efficient and effective concrete slab design. This user guide
can be used in conjunction with other DeSimone documents/spreadsheets for
concrete slab design. The related documents include:
•
•
DeSimone Punching Shear Application v2.0.0
DeSimone CLTD-Column Load Take Down Spreadsheet v19.10
The CSI SAFE software is recommended by DeSimone for most reinforced concrete
slab design. The CSI SAFE 2016 V16.0.2 software can be downloaded from ZENworks.
New Users are encouraged to visit the CSI user guide found via the following link:
(https://wiki.csiamerica.com/display/safe)
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3.2.
The DXF File
The objective of this section is to show the user how to create a .dxf file and import it
into the CSI SAFE software. The .dxf file will accurately capture the slab geometry as
well as facilitate in the modeling of certain elements.
3.2.1. Creating the DXF File
Step 1: Export a floor plan from Revit into CAD format
Step 2: Select the entire drawing and use the OVERKILL command. This will delete any
double lines that can cause SAFE to wrongly interpret the information. Use with
discretion, as perimeter columns that are flush with the slab edge will cause the slab
edge to be split and lose connectivity.
Note: The .dxf file should be as simple and minimal as possible to ensure proper import
into SAFE. This means that any text on plan or any unnecessary elements should be
deleted when first importing the .dxf file into CAD.
•
For expediency, in CAD right-click on the element, or text, that you wish to
delete and left click on the “select similar” option. This will select anything on
plan that is similar, and then click on the “delete” button.
o This process allows the user to delete everything that’s similar all at once.
Step 3: Select “A-flor” layer, which should be the slab edge layer in CAD, then right
click > select similar. Once all similar objects are selected, right click > Isolate Objects.
•
•
•
Create a new layer called “slab”
Trace the layout of the slab with the “PL” aka Polyline command
Go to Tools > Isolate > End Object Isolation
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Step 4: For walls and beams, the user should draw the centerlines for these elements
in CAD. This can be seen in the image provided below:
•
45
The center-line element model is utilized for SAFE to carry out an accurate
finite element analysis.
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Step 6: Save the drawing as .dxf format and close the file. Make sure that the file “Files
of Type:” is “AutoCAD 2013/LT2013 DCF (*.dxf)” since this file type is most compatible
with most programs.
•
SAFE will not be able to open/import the .dxf if you have the file open in CAD.
It makes things easier to name the .dxf file the same name as the SAFE
filename, as the .dxf import process renames the SAFE files to the .dxf filename.
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3.2.2. Importing The DXF File Into SAFE
Step 1: When the .dxf file is complete, save the dxf and close the program. The .dxf file
MUST be closed prior to import it into SAFE.
Step 2: Open the DeSimone CSI SAFE template file and go to File>Import> DXF/DWG
Architectural Plan > Select your DXF file
The template file is located here: (Insert tentative location)
Step 3: Save As your SAFE model with correct file name.
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3.3.
Define Model Properties
The objective of this section is to specify how to define model properties. Model
properties include material properties (such as concrete and reinforcing steel) and
structural properties (such as slabs, beams, columns, walls, etc.).
The “Define” tab will be used to define the following model properties:
Defining Material Properties:
The material properties for a slab design include the type/strength of concrete and
the grade of steel for reinforcement. Certain concrete elements will be modeled with
properties that have stiffness with mass and other concrete members will be modeled
with properties that have stiffness without mass. This will be explained in more detail
later.
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Defining Concrete Elements with Stiffness and Mass:
Note: Concrete elements with stiffness and mass typically include the concrete
properties for slabs, slab drops and beams (beams are also defined as “drop”
elements in SAFE)
Step 1: Go to Define>Materials. Select “Add New Material”
Step 2: Fill out Material property data:
•
•
•
•
•
•
•
•
Name: Name your property
Material Type: Concrete
Weight per unit Volume: Use default value (150 pcf or 0.0868 pci)
Note: Create separate materials with 0 pcf weight, for columns and walls.
Modulus of Elasticity: 57000*sqrt(f’c)
Note: For projects in Florida, reduced modulus of elasticity to be used,
based on discussion with Project Manager.
Poisson’s Ratio: Use default value (0.2)
Coefficient of thermal Expansion: Use default value
Concrete Strength: Concrete compressive strength. If lightweight concrete
is used, values for ‘weight per .
For “Modulus of rupture for Cracked Deflections” select user specified and
insert the value for 7.5sqr(f’c) of the concrete.
See example for 5,000psi concrete below:
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3.3.1. Defining Concrete Elements with Stiffness, without Mass:
Note: Concrete elements with stiffness without mass typically includes the concrete
properties for walls and columns.
Step 1: Go to Define>Materials. Select “Add New Material”
Step 2: Fill out Material property data:
•
•
•
•
•
•
•
•
Name: Name your property but be sure to indicate “NM” for No mass
Material Type: Concrete
Weight per unit Volume: Use 0 (stiffness without mass)
Modulus of Elasticity: 57000*sqrt(f’c).
If concrete is light weight, the value shall be adjusted based on ACI 19.2.2.
Poisson’s Ratio: Use default value (0.2)
Coefficient of thermal Expansion: Use default value
Concrete Strength: Concrete compressive strength. If concrete is light
weight, the value shall be adjusted based on ACI 19.2.2.
Modulus of rupture for Cracked Deflections: Select user specified and insert
the value for 7.5sqr(f’c) of the concrete. If concrete is light weight, the
value shall be adjusted based on ACI 19.2.2. Discuss with Project Manager
about other modifications based on ACE 435R-95.
See example for 5,000psi concrete with stiffness without Mass below:
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3.3.2. Defining Reinforcing Steel Material Properties:
Step 1: Go to Define>Materials. Select “Add New Material”
Step 2: Fill out Material property data:
•
•
•
•
•
•
Name: Input reinforcing steel (e.g., A615Gr60 or A615Gr75)
Material Type: Rebar
Weight per unit Volume: 490 pcf
Modulus of Elasticity: 29000 ksi
Minimum yield Stress, Fy: Input yield stress for rebar property (e.g., Fy=60 ksi)
Minimum Tensile Stress, Fu: Input tensile stress for rebar property (e.g., Fu=90
ksi)
See example for A615Gr60 rebar below:
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3.3.3. Defining Slab Properties:
Once the material properties have been established within the model, the slab
properties can be created/defined.
Step 1: Go to Define>Slab Properties. Select “Add New Property”
Step 2: Fill out property data
•
•
•
•
•
•
Property Name: Slabs are to be named as follows (X)-SLAB-(Y)KSI, where “X”
represents the thickness of the slab and “Y” represents the concrete strength.
(For example: 8SLAB7KSI represents an 8-inch slab with an f’c=7ksi)
Slab Material: Use the drop down to select one of the defined materials
Type: Select Slab form the drop down
Thickness: Indicate the slab thickness
Discuss with Project Manager, conditions for checking or unchecking ‘Thick
Slab’ option.
Uncheck Orthotropic
See Example for 8SLAB7ksi below:
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3.3.4. Defining Drop Properties:
Note: As stated above, beams are recommended to be modeled as “drops” rather
than frame elements (qualify why?). Please note that the “drop” property supersedes
“slab” properties. Therefore, if a drop is drawn over a slab, the drop properties will
replace the slab properties.
Step 1: Go to Define>Slab Properties. Select “Add New Property”
Step 2: Fill out property data
•
•
•
•
•
•
Property Name: Drops are to be named as follows (X)-DROP-(Y)KSI, where “X”
represents the depth of the beam and “Y” represents the concrete strength.
(e.g. 24DROP7KSI represents an 24-inch deep beam with an f’c=7ksi)
Slab Material: Use the drop down to select one of the defined materials
Type: Select DROP form the drop down
Thickness: Indicate the beam thickness (i.e. beam depth)
Discuss with Project Manager, conditions for checking or unchecking ‘Thick
Slab’ option.
Uncheck Orthotropic
See Example for 24DROP7KSI below:
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3.3.5. Defining Beams Properties:
Note: Alternatively, beams can be defined as line elements and later be converted to
drops (section xxx will show the procedure for converting line elements to drops). The
beams in this user guide are qualified as link beams because it’s a component of the
lateral system for the structure (i.e., it connects the shear walls).
Step 1: Go to Define>Beam Properties. Select “Add New Property”
Step 2: Fill out property data
•
•
•
55
Property Name: Link beams are named as follows (X) x (Y)-LB-(Z)KSI, where “X”
represents the dimension in the x-direction, “Y” represents the dimension in the Ydirection and “Z” represents the concrete strength in KSI (e.g., 14x24LB7KSI
represents a 14-inch by 24-inch link beam with an f’c=7ksi)
Material: Define the proper material using the drop-down window.
Beam Type and Dimension: Use the drop down to select the desired beam
shape. Note, a preview of the beam shape will appear in the adjacent window.
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3.3.6. Defining Column Properties:
As previously mentioned, column properties are defined using the materials with
stiffness without mass.
Step 1: Go to Define>Column Properties. Select “Add New Property”
Step 2: Fill out property data
•
•
•
•
•
•
Property Name:
o Rectangular columns are to be named as follows (X) x (Y)-COL-(Z)KSI,
where “X” represents the dimension in the x-direction, “Y” represents the
dimension in the Y-direction and “Z” represents the concrete strength in
KSI (e.g. 36x16COL7KSI represents an 36-inch by 16-inch column with an
f’c=7ksi)
o Circular Columns are to be named as follows: (X)-COL-(Z)KSI, where “X”
represents the diameter of the column and “Y” represents the concrete
strength in KSI (e.g. 24COL7KSI represents a 24-inch diameter column with
an f’c=7ksi)
o For all other irregular column shapes (such as L-shape, T-Shape, etc),
naming convention is to be established by project team.
Material: Define the proper material using the drop-down window (Stiffness
without mass!)
Column Type and Dimensions: Use the drop down to select the desired column
shape. Note, a preview of the column shape will appear in the adjacent
window
Check “Include Automatic Rigid Zone Over Column”
Automatic Drop Panel Dimensions: If a drop panel is required, indicate
dimensions and associated property as prompted
Automatic Column Capital (Drop Cap) Dimensions: If a drop cap is required,
indicate dimensions and associated depth as prompted.
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See Example for 36x16COL7KSI below:
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See Example for 24COL7KSI below:
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3.3.7. Defining Wall Properties:
Like columns, wall properties are defined using the materials with stiffness without mass.
Step 1: Go to Define>Wall Properties. Select “Add New Property”
Step 2: Fill out property data
•
•
•
•
•
Property Name: Walls are to be named as follows: (X)-WALL-(Z)KSI, where “X”
represents the wall thickness in inches and “Y” represents the concrete strength
in KSI (e.g. 14WALL7KSI represents a 14-inch wall with an f’c=7ksi)
Wall Material: Define the proper material using the drop-down window (Stiffness
without mass!)
Thickness: indicate the wall thickness
Check “Include Automatic Rigid Zone Over Wall”
Check “Wall Takes Out-of-plane Moment”
See Example of 14WALL7KS
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3.3.8. Defining Mass Source:
The Mass Source is defined in SAFE for the dynamic properties of the slab
Step 1: Go to Define>Mass Source.
Step 2: Add DEAD, SDL and CLAD to the Load Pattern list. Apply a factor of 1.0 for
each. If the DeSimone template file is used, the SDL and CLAD load pattern should
already be defined within the model. If the template file is not being used, the CLAD
and SDL patterns must be defined (Refer Section 3.5 Load Definitions)
Step 3: Check “Ignore Lateral Mass”
See example of Mass Source definition below:
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3.4.
Property Assignment
The objective of this section is to specify how to assign the defined properties to the
model.
Note: When drawing element in SAFE, the user can isolate the layers imported from the
.dxf file by going to Options> Architectural Plan Options.
•
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This option makes it easier to view specific elements when working with a larger
floor plate.
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3.4.1. Assigning/Drawing Slabs and Drops
Slabs and drops can be assigned to an area element that was imported using the .dxf
file or the slab can be manually drawn:
•
If the area of the slab was imported with the .dxf file, the slab property can be
easily assigned by clicking on the area and going to the Assign>Slab data>
Properties. Select the desired property to be assigned to the area. Check that
the correct property was assigned by right clicking on the element and
checking the “Assignments” tab.
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•
If the area was not imported using the .dxf file, then the slab can be drawn using
the draw tool by going to the “Draw” tab and selecting one of three slab draw
tools indicated below.
Be sure to specify the desired slab or drop property in the “Draw Slabs/Areas” window
prior to drawing:
Note: if a “drop” property is drawn over a slab, the drop properties supersede the slab
properties (i.e. the drop will replace the slab).
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3.4.2. Assigning/Drawing Columns
Step 1: go to Draw> Draw Columns.
Step 2: Indicate the proper column property for the column above and below.
Indicate the appropriate column height above and below as well. The column can
be rotated as desired by adjusting the Angle. Offsets to the insertion point can also be
specified as desired in the “Plan Offset.”
See preview of window below:
Step 3: Place your column by selecting the imported points from the dxf file.
Step 4: Check column properties by viewing model in 3D and displaying column
section properties
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3.4.3. Assigning/Drawing Walls
Step 1: go to Draw> Draw Walls.
Step 2: Indicate the proper wall property for the wall above and below. Indicate the
appropriate wall height above and below as well.
Step 3: Draw walls by tracing line elements that were imported with the dxf file.
Note: Walls should be segmented at each intersecting point (i.e. walls should not run
continuous over an intersecting wall)
Step 4: Check Wall properties by viewing model in 3D and displaying section column
section properties
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3.4.4. Assigning/Drawing Beams as Drops
As previously mentioned in section 3.3.5, it’s recommended for beams to be modeled
as “drops” and in Section 3.4.1 these drops were drawn as area elements. An alternate
procedure is provided below for drawing these beams as drop.
Step 1: Go to Draw>Draw Beams/Lines
Step 2: Indicate the proper beam property for the beam.
Step 3: Draw beams by tracing line elements that were imported with the dxf file.
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Step 4: Select the two beams that were drawn in Step 3, then go to Edit>Edit Lines>
Convert Beams to Slabs.
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Step 5: Right-Click on the drop that was just created and a window will appear titled
“Slab-Type Area Object Information”.
•
If the “Slab Property” is not defined by the respective drop, then click on it and
select the correct slab property, which was defined in section 3.3.5.
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Step 6: Delete the line element drawn in step 3 since they are now defined as null
lines.
•
69
If the user wishes the elements extruded, as seen above, they must go to View>
Set Display Options>Extrude View
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3.4.5. Assigning/Drawing Beams as Drops
Step 1: Go to Draw>Draw Rectangular Slabs/Areas
Step 2: Indicate the ”Type of Object” for the opening
Step 3: Draw opening by tracing line elements that were imported with the dxf file.
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3.5.
Define Loads
The objective of this section is to specify how to define loads within the model.
Note: The DeSimone SAFE template file should have all load patterns, cases and
combinations already defined; however, it is good practice to always check the
model. The load patterns, cases and combinations have been provided for the user
below.
3.5.1. Defining Load Patterns
Go to “Define”> “Load Patterns”
The load patterns should include:
•
•
•
•
•
DEAD
LIVE
TRIB
SDL
CLAD
Note: Only DEAD has a self-weight multiplier. Assign 1000 psf TRIB load to all slabs for
CLTD tributary area calculation and consequent column load takedown.
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3.5.2. Load Cases
Once the Load patterns are defined, the load cases can be defined.
Go to “Define”>”Load Cases”
The load cases should include:
•
•
•
Load Patterns (mentioned above)
Deflection/Service Cases:
o Case 1: D + S+ C + L (Cracked)
o Case 2: D + S + C + 0.25L Cracked)
o Case 3: D + S + C + 0.25L (Long Term Cracked)
o Case 4: D (Cracked)
o Case 5: D + C (Cracked)
o Case 6: D + S + C (Cracked)
o Case 7: D + S + C (Long Term Cracked)
Dynamic
o Stiffness to use: Zero Initial Conditions
o Load Case Type: Modal
o Types of Modes: Eigen Vectors
o Number of Modes: 12
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3.5.3. Load Combinations
Once the Load patterns are defined, the load combinations can be defined.
Go to “Define”>”Load Combinations”
The Service load combinations should include:
•
SD - Short term Dead load deflection
• Combination type: Linear Add
• Case 4
• SL - Short term Live load deflection
• Combination type: Linear Add
• Case 1 – Case 6
• SC - Short term Cladding deflection
• Combination type: Linear Add
• Case 5 – Case 4
• SDC - Short term Dead load plus Cladding deflection
• Combination type: Linear Add
• Case 5
• SDSC - Short term Dead load plus SDL plus Cladding deflection
• Combination type: Linear Add
• Case 6
• LDSCL - Long term deflection: Total
• Combination type: Linear Add
• Case 1 + Case 3 – Case 2
• LSCL - Long term deflection: Total – Selfweight
• Combination type: Linear Add
• Case 1 + Case 3 – Case 2 – Case 4
The strength Combinations should Include:
•
•
•
StD: Strength 1.4D
• Combination type: Linear Add
• 1.4(Case 7)
• Design Selection: Strength Ultimate
StDL: Strength Dead plus Live load - 1.2D + 1.6L
• Combination type: Linear Add
• 1.2(Case 7) + 1.6(Case 1) – 1.6(Case 6)
• Design Selection: Strength Ultimate
StE: Strength – Envelope
• Combination type: Envelope
• StDL, StD
• Design Selection: Strength Ultimate
Note: For more detailed discussion on how to measure long-term deflections in SAFE, see
link https://wiki.csiamerica.com/display/safe/Cracked-section+analysis.
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3.6.
Load Assignment
The objective of this section is to specify how to properly assign Area loads, line loads
and point loads to the model.
3.6.1. Slab Loads (Area Loads)
3.6.1.1.
Types of Slab Loads
Slab Loads are usually uniformly distributed area loads (units typically psf). Slab loads
can be applied under any of the load patterns described above in section 5.1.
Examples of some typical slab loads are shown below:
•
Superimposed floor dead loads (assigned under the SDL pattern) include but are
not limited to the following:
o Floor finishes (tile, stone, etc.)
o Equipment pads
o Topping Slabs
o Mechanical units and allowance for hung mechanical
o Terrace Pavers
• Floor Live loads (assigned under the LL pattern) include but are not limited to the
following:
o Residential Live loads
o Office
o Parking
o Storage
o Mechanical
o Truck
o Balconies
o Terraces
o Stairs/corridors
o Amenity
Note: Refer to project design criteria, ASCE 7, the appropriate building code, and the
Project Manager for appropriate floor loading.
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3.6.1.2.
Assigning Area Loads Directly to the Slab
To assign uniformly distributed area loads to a slab or area element:
Step 1: Select the slab and go to Assign>Load data> Surface loads
Step 2: Indicate the load pattern. Select gravity as the direction. Indicate the
magnitude
Step 3: Select “Replace Existing Loads” or “Add to Existing Loads” as it applies
Note: The program default setting is “Add to Existing Loads”
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3.6.1.3.
Applying Area Loads Using a Null Area Element
If there are portions of the slab that require different loading criteria, a null area element can
be drawn over the slab to apply additional loading to that portion of the slab.
Step 1: Draw>Draw Slabs/Areas. Ensure the “NONE” property is selected in the drop down.
Step 2: Draw the null area in the desired location
Step 3: Select the null area and go to Assign>Load data> Surface loads
Step 4: Indicate the load pattern. Select gravity as the direction. Indicate the magnitude
Step 5: Select “Replace Existing Loads” or “Add to Existing Loads” as it applies
Note: if a null area is drawn over a slab, the loads applied to the null area are additive to the
loads applied to the slab area.
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3.6.2. Line Loads
3.6.2.1.
Types of Line Loads
Line Loads are usually uniformly distributed linear loads (units typically PLF or KLF). Line
loads can be applied under any of the load patterns described above in section 5.1.
Examples of some typical line loads are shown below:
•
•
•
•
3.6.2.2.
Façade loads (Typically CLAD Load pattern)
o Curtain wall
o Window wall
o CMU
o Brick
o Stone
Non-loadbearing wall loads (typically the SDL load pattern)
CMU Wall loads supporting stair landings etc. (typically the SDL & LL load
pattern)
Stair loads (typically the SDL & LL load pattern)
Applying Line Loads using a Null Line
Step 1: Got to Draw>Draw Beams/Lines. Ensure the “NONE” property is selected in the drop
down.
Step 2: Draw the null frame in the desired location
Step 3: Select the null frame and go to Assign>Load data> Distributed Loads on Lines
Step 4: Indicate the load pattern. Select gravity as the direction. Indicate the magnitude (Note:
if the line load is distributed, indicate it as a uniform load).
Step 5: Select “Replace Existing Loads” or “Add to Existing Loads” as it applies
See example below of uniformly distributed superimposed dead load of 1.2 KLF:
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3.6.3. Point Loads
3.6.3.1.
Types of Point Loads
Point Loads are usually in the units typically lbs or Kips. Point loads can be applied
under any of the load patterns described above in section 5.1.
Examples of some typical point loads are shown below:
•
•
3.6.3.2.
Transfer loads
o Cumulative DL input as SDL
o Cumulative LL input as LL
Miscellaneous point loads
o Davits
o Screen wall posts
o Window washing
o Dunnage
Applying Point Loads
To apply point loads to a slab, a point element must be drawn where the point load is to be
applied.
Step 1: Got to Draw>Draw Points. An offset from the insertion point can be applied as desired
to facilitate with modeling.
Step 2: Draw the point in the desired location
Step 3: Select the point and go to Assign>Load data> Point Loads.
Step 4: Indicate the load pattern. Select gravity as the direction. Indicate the magnitude. If the
point load represents a post/column up, the dimensions of the column can be inserted so that
the punching shear can be evaluated. Select replace existing or add to existing as it applies.
See example below of superimposed dead load due to a 12”x24” post up:
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3.6.4. Displaying Loads
Once all the loads have been inserted into the model, the user should check that
everything has been assigned correctly by displaying the applied loads. To do this, go
to Display>Show Loads. Area loads, line loads and point loads for each load pattern
can be displayed as desired. Check the “Show Loading Values” to display the load
magnitudes. Note, viewing the model in 3-D may help display the loads.
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3.7.
Preparing the Analysis
The objective of this section is to specify how to properly assign Area loads, line loads
and point loads to the model.
3.7.1. Design Preferences
It is important that the user check that the design preferences are set up properly.
Step 1: Go to “Design”>”Design Preferences”:
Step 2: Check the “Code Tab”. Ensure the proper ACI code and associated resistance
factors are correct. See example below:
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Step 3: Check the “Min Cover Slabs” Tab. Ensure the top and bottom cover are
specified correctly. Take note of the inner slab rebar layer as this will need to be
consistent with the design strips (see section 7.3) and will also need to be properly
called out on plan when publishing the design. The inner layer can also be adjusted
as required to benefit the design BUT ensure the correct direction is indicated in the
drawings.
Note: Beams are recommended to be modeled as drops and should be designed
outside of SAFE, therefore checking the “Min. Cover Beams” tab is not required.
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3.7.2. Design Combinations
To check the design combinations are specified correctly.
Step 1: Go to “Design”>”Design Combos…”
Step 2: Check that all of the strength combinations per section 5.3 are specified for
the Ultimate Load combination type. See below:
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3.7.3. Design Strips
This next section will indicate how to properly define and draw design strips.
Step 1: Go to “Draw”> “Draw Design Strips”
Step 2: Fill out the “Draw Design Strips” window
•
•
•
•
•
•
•
Type of Object: Strip
Strip Layer: Select A or B from the drop down. Ensure this is consistent with the
“Inner Slab Rebar Layer” indicated in section 7.1
Strip Design Type: Column Strip
Start width left: indicate 1 ft (for a 2 ft strip)
Start width right: indicate 1 ft (for a 2 ft strip)
End width left: indicate 1 ft (for a 2 ft strip)
End width right: indicate 1 ft (for a 2 ft strip)
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Step 3: Draw a single strip in the desired direction (vertical or horizontal). Ensure the
strip length covers the entire slab length. Right click on the strip to ensure the correct
properties (including reinforcing steel) has been assigned.
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Step 4: Select the single strip and replicate (via the “Edit” tab) so that is covers the
entire slab. Note that the strip should be replicated in increments that match the strip
width. Therefore, a 2ft strip should be replicated at every 2ft on center. See below:
See below for the result of replicating Strip A (drawn in the vertical direction)
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Step 5: Repeat steps 1 through 4 but indicate Strip layer B. Strip Layer be is to be drawn
orthogonal to strip layer A.
See below for the result of replicating Strip B (drawn in the horizontal direction)
There should be a cross hatch of design strips across the entire slab.
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3.7.4. Automatic Slab Mesh Options
To specify the automatic slab mesh options.
Go to “Run”>”Automatic Slab Mesh Options…”
•
•
Select Use rectangular mesh
• Check “Use Localized Meshing”
• Check “Merge Points Where Possible”
Mesh Size = 2 feet
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3.7.5. Reinforcement Options for Cracking Analysis
To specify the reinforcement options for cracking analysis:
Go to “Run”>”Cracking Analysis Options…”
•
•
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Select “From Finite Element Based Design”
Minimum reinforcing ratios for cracking analysis
o Tension reinforcing = 0.0018
o Compression Reinforcing = 0*
*Note: Compression reinforcing ratio may be modified to aid
in deflection, consult project manager!
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3.7.6. Advanced Modeling Options
To specify the advanced modeling options:
Go to “Run”>”Advanced Modeling Options…”
•
•
•
Check 2D plate-UZ,RX,RY only.
Note: This option gets unchecked when saving it to a different drive or
when the computer has restarted. Check this option every time before
running analysis.
Uncheck “Rigid Diaphragm constraint at Top of Columns and Walls
Above”
Check “Ignore Vertical Offsets in Non P/T Models”
• Vertical offsets to be modelled as line releases
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3.7.7. Advanced SAP Fire Options
To specify the advanced SAP Fire Options:
Instructions:
Go to “Run”>”Advanced SapFire Options…”
•
•
•
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Under “Solver Options” – Select “Advanced Solver”
Under “Analysis Process Options” – Select “Auto”
Under “Other Options”-Uncheck “Always run Analysis as 32-bit, even on
64-bit computers.
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3.7.8. Running the Analysis
Once all the analysis options are implemented, it is time for the user to Run the Model.
Go to “Run”> “Run Analysis & Design”
The program will proceed to run the analysis. The analysis may take a few minutes to
run on larger slab models.
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3.8.
Slab Analysis and Design Output
The objective of this section is to specify how to display analysis output such as reaction
forces, deformed shape, punching shear ratios and slab reinforcement.
3.8.1. Display Reactions
To display the reaction forces of columns and walls:
Go to Display>Show Reaction forces. Here the user can choose to display column
point reactions or Integrated wall reactions for a designated load case or
combination.
3.8.2. Display Slab Stresses
To display the slab stresses:
Go to Display>Show Slab Forces/Stresses. Here the user can choose to display forces
or stresses for a designated load case or combination.
This option comes in handy for checking one-way and two-way shear stresses for
foundation design, refer to DeSimone Foundation Design Guide for det ailed
explanation.
3.8.3. Display Strip Forces
To display the design strip forces:
Go to Display>Show Strip Forces. Here the user can choose to display the design strip
forces for a designated load case or combination. The user can choose to display
Moment, Axial, Shear or Torsion forces for each strip layer.
3.8.4. Display Deformed Shape/ Checking Deflections
To display the deformed shape or deflections:
Go to “Display”>”Show Deformed Shape”
Select the desired case or combination to display the associated deflections. Refer to
the project design criteria, the Project Manager, and the appropriate design codes
for deflection criteria.
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3.8.5. Display Punching Shear Ratios
To display the punching shear design ratios:
Go to Display>Show Punching Shear Design. Once selected, a ratio will appear next
to each column or point load (if the size was specified). Ratios exceeding 1.0 require
stud rails.
SAFE results have been observed to be quite accurate for interior columns without
openings. For corner and edge conditions, and/or columns with openings, it is
recommended to use DeSimone Punching Shear Application or Decon Studrails to
check for punching shear and for stud rail design.
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3.8.6. Display Slab Design/Reinforcement
To display the program determined reinforcement:
Go to “Display”>”Show Slab Design”
The user can choose to display top and/or bottom reinforcement for both strip
directions.
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3.9.
Design
Depending on the office/ region of the project RC slabs may be designed
according to the local industry standards. For example, New York City slabs
are generally designed using stress line analysis. In Miami, RC slabs are
generally designed utilizing a top and bottom mat with additional rebar as
required. The engineer should consult with the project manager to see
which method of design is most appropriate for the project. The engineer
should refer to the typical details for each type of slab to become familiar
with the detailing. Refer to the appendix for more information regarding
each method.
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4. Appendices
4.1.
Appendix for RAM Concept
Figure 01. Estimate of creep and shrinkage shortening for typical post-tensioned
slabs.
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Figure 02. Typical closure strip detail.
Figure 03. Typical closure strip detail.
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Figure 04. Closure strip: Cantilever design.
Figure 05. Closure strip: Shoring Required
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Figure 06. Additional reinforcement at RC walls to mitigate cracking.
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Figure 07. Exposure Zones Map
(ACI 362.1R-97 Reapproved 2002: Guide for the Design of Durable Parking Structures)
Exposure Zones Definition:
Zone I represents the mildest conditions where freezing is rare and salt is not used. This area is
generally defined as all areas south of Zone II and south and west of Zone III except those areas
above an elevation of 3000 feet where freezing occurs.
Zone II represents areas where freezing occurs, and deicing salts are not or rarely used. This
area is generally defined as the area south of Zone III and within 100 miles south of interstate
highway 40 from the Atlantic Ocean west of the Continental Divide, plus all areas in Zone I
above an elevation of 3000 feet and below an elevation of 5000 feet, plus areas in the State
of Oregon and Washington west of the Cascade Range except for those areas above an
elevation of 5000 feet.
Zone III represents the areas where freezing and deicing salts are common. This area is generally
considered to be areas north of and within 100 miles south of Interstate Highway 70 from the
Atlantic Ocean west to Interstate Highway 15, then north to Interstate Highway 84, then
northwest to Portland, Oregon then west to the Pacific Ocean plus areas with Zones I and II
above an elevation of 5000 feet when deicing salts are used.
Coastal Chloride Zone I (Zone CC-I) represents areas with Zone I and within 5 miles of the
Atlantic Ocean, Gulf of Mexico, Pacific Ocean, and the Great Salt Lake.
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Coastal Chloride Zone II (Zone CC-II) is areas within Zones I and II within one-half mile of the salt
water bodies described in Zone C-I.
Structure Type
Parking garage slab (Zone II and III)
Parking garage slab (Zone I)
Parking garage beam (Zone II and III)
Parking garage beam (Zone I)
Building slab
Building beam
Building transfer girder
See Figure 07 for Exposure Zones Map.
Min. P/A, psi
Max. P/A, psi
175
125
200
125
125
125
200
300
300
500
500
300
500
700
Table 1: Pragmatic guidelines for average prestress as function of structure type
(Tips for Post-Tensioning-Part II- Concrete International, October 2016)
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4.2.
Appendix for SAFE
4.2.1 Rebar Detailing
4.2.1.1 Column Bars
•
For typical slabs, use 2 x 0.30Ln* + Column Width when Ln1 and Ln2 are within a
few feet of each other
Otherwise
Use 0.30Ln1 + Column Width +0.30Ln2 and use the R or L notation to indicate
Placement of rebar.
•
For non-typical slabs (I.e. transfer slabs), verify if 2 x 0.3Ln + Column Width or 2 x
(0.2Ln+Ld) + Column Width is controlling. Use the controlling value to determine
the rebar
Bar lengths, for top bars, do not have to be specified on plan since the contractor
follows DeSimone's typical details for top bar lengths, as seen in the image provided
below. Engineer to confirm with project manager if top bar lengths should be specified
on plan. For more information on bar lengths, refer to ACI 318-14 figure 8.7.4.1.3a.
Engineer should review and become familiar with project typical details.
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4.2.2 Design: Stress Line Analysis (NYC)
In New York, slabs are designed to contain a bottom mat of reinforcement, but not a
top mat of reinforcement. The placement of top bars is based on stress lines. Stress lines
are determined based on the trajectory of internal forces.
4.2.2.1 Design: Stress Line Analysis (NYC)
A step by step process for stress line analysis goes as follows:
Step 1: Go to “Display”>”Show Slab Design”
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For the definition of each of the highlighted options see section 3.8.6.
The user can toggle between “Layer A”, design strip layer A going in the
vertical direction, and “Layer B”, design strip layer B going in the horizontal
direction, under “Choose Strip Direction” to show the trajectory of the internal
forces.
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•
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The following will be displayed for the user in SAFE, when the process displayed
above is complete:
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Step 2: Stress lines can be drawn based on the image displayed in step 1, for layer A,
and can be seen displayed by a blue line in the image provided below:
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Step 3: Repeat steps 1 and 2 for Layer B now (layer B is in horizontal direction):
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•
The stress lines based on layers A and B can be seen below superimposed:
These stress lines are to be indicated on project plans.
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4.2.2.2 Pulling Top Reinforcement
As mentioned in the previous section, in New York, slabs are designed to contain a
bottom mat of reinforcement, but not a top mat of reinforcement. For top bars, bars are
placed along the stress lines and over the column. In this section the method for pulling
top reinforcement will be discussed, starting with top layer A:
Step 1: Determine what is the minimum bar size required for a slab. For this example, we
will be looking at an 8-inch slab:
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Based on the calculation provided above, the smallest bar size than can be used
for the bottom mat is a #4 bar, which has a nominal area of 0.20 in^2.
Engineer to confirm with project manager bottom and mid strip bars.
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Step 2: Go to “Display”>”Show Slab Design”
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•
As mentioned in the previous section, this will display the “total rebar area for strip”
as seen in the image below:
•
In section 3.7.3 the design strips were replicated at an increment of 2 ft, and in
section 4.2.1.1, for the typical details, it was mentioned that the bars are placed
at 12-inch spacing. For example, this means that if there are two #4 bars between
each design strip then this equals a total rebar area of 0.40 in^2. Therefore, if a
value greater than 0.40 in^2 appears when displaying the total rebar area in SAFE,
then the user must add rebar over column.
Note: It is good practice that bars over a column and additional bars should be 2 sizes
larger than the minimum bar size used for the mat. This means, for this example, a #6
bar will be used over the columns and for additional reinforcement.
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Step 3: Draw a boxed area around locations where reinforcement is required over the
top of the column:
Step 4: Count the number of strips in the boxed area, which is 5 per the example above.
Step 5: Then multiply the number in step 4 by two since there are two bars per strip. This
now equals a total number of 10 bars. Keep in mind that it is good practice to indicate
an odd number of bars in order to assure that the contractor places 1 bar directly over
the column. This results in a total of 11 bars.
Step 6: Calculate the area of steel required, which is the total area of steel boxed in step
3. This number is 4.97 in^2.
Step 7: Multiply the total number of bars from step 5, 11 bars, by the nominal area of the
bar that will be used over the column, which is a #6 bar.
•
Therefore, the notation for the bar placement is 11T612.
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Step 7: Subtract area of steel for 11 #6 bars from the area of steel required:
Step 8: If additional reinforcement is required, which is the case for this example, then
place add the additional reinforcement. For this example, 2 additional bars will be added
and the proper notation for this would be 2-1AT612.
Note: An image is provided below for how to track the rebar pull when recording calcs
or compiling a calc package.
Step 9: Repeat steps 2 through 8 but for layer B (rebar pull for bars in the horizontal
direction).
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4.2.2.3 Pulling Bottom Reinforcement
Pulling bottom reinforcement is similar to pulling top reinforcement, though the bottom
rebar placement spans between columns. As previously mentioned, slabs in NY are
designed to have a bottom mat, therefore only additional bars will be placed for bottom
bars. In this section the method for bottom bar placement will be discussed, starting with
top layer A:
Step 1: Go to “Display”>”Show Slab Design”
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•
Since a bottom mat is being implemented, then make sure to unclick “Impose
Minimum Reinforcing” and to click on “Typical Uniform Reinforcing Specified
Below” under “Show Rebar Above Specified Value”. This will make it so SAFE only
displays the additional rebar that is required.
This is similar to step 2 in section 4.2.2.2, with the exception that the use selects the
“Typical Uniform Reinforcing Specified Below” option instead of selecting “none”
under “Show Rebar Above Specified Values”.
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o
The reason for this is that a bottom mat will be implemented. Therefore, only
additional bottom bars need to be displayed, as seen below.
Step 2: Draw a boxed area around locations where reinforcement is required over the
top of the column:
Step 3: Count how much rebar area is located within the boxed region in step 2. For this
example, there is a total of 1.18 in^2 of rebar that’s required.
Step 4: Based on step 2 of section 4.2.2.2, #6 bars are being used as additonals. The
calculation of additional bottom bars goes as follows:
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Step 5: Repeat steps 1 through 4 but for layer B (rebar pull for bars in the horizontal
direction).
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4.2.3 Design (MIAMI)
In Miami, or anywhere else for that matter, the slab design process does not require stress
line analysis since this is only a requirement in NYC. Another way the design process differs
by is that in Miami a bottom and top mat of reinforcement is implemented, unlike in NYC
where it’s only a top mat.
4.2.3.1 Pulling Top Reinforcement
Since Miami requires a bottom and top mat of reinforcement and no stress line analysis
is needed due to this, then all bottom and top reinforcement is considered additional
reinforcement. The following steps will demonstrate a step-by-step process for pulling
additional top reinforcement, starting with top layer A.
Step 1: Go to “Display” > ” Show Slab Design”
•
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Since a top mat is being implemented, then make sure to unclick “Impose
Minimum Reinforcing” and to click on “Typical Uniform Reinforcing Specified
Below” under “Show Rebar Above Specified Value”. This will make it so SAFE only
displays the additional rebar that is required.
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Step 2: Draw a boxed area around locations where reinforcement is required over the
top of the column:
Step 3: Count how much rebar area is located within the boxed region in step 2. For this
example, there is a total of 1.18 in^2 of rebar that’s required.
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