Seismic Design Category

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PCI
th
6
Edition
Building Systems
(Seismic)
Presentation Outline
• Building System Loads
– Seismic
•
•
•
•
Structural Integrity
LFRS – Walls
LFRS – Frames
Diaphragms
Seismic Changes
• Based on new changes to ASCE 7 and
ACI 318
• Based current seismic research and
observations
Seismic Changes
• Some of these changes are:
– Recognition of jointed panel construction
– Recognition of strong and ductile
connections in precast frames
– Recognition and requirements for
connections in precast walls
Seismic Changes
• Additional changes are:
–
–
–
–
–
Modification of drift computation and limiting drift
Deformation compatibility of elements
Additional soil type classifications
Special considerations locations near seismic faults
Consideration of redundancy and reliability in
strength design requirements
Seismic Changes
• Design Forces are Based on Risk
– Previous codes based on 10% chance of
exceedance in 50 years
– IBC 2000, 2003 codes based on 2%
chance of exceedance in 50 years
Seismic Risk
• Soil factors
– Other regions of high seismic risk - not just west coast anymore
Practically every precast, prestressed
concrete structure designed under IBC
2000 will require some consideration of
seismic effects.
Seismic Performance Objectives
• Current design - minor damage for
moderate earthquakes
• Accepts major damage for severe
earthquakes
• Collapse is prevented of severe events
Seismic Performance Objectives
In order to achieve the design objectives,
the current code approach requires
details capable of undergoing large
inelastic deformations for energy
dissipation.
Seismic Design Approach
• Emulation
– No special requirements for low seismic risk
– Chapter 21 requirements for moderate and
high seismic risk
• Non-emulative design
– PRESSS
– Acceptance criteria for frames
Earthquake Loads – Equivalent Lateral
Force Method
• Base Shear, V
V= Cs·W
Where:
Cs - Seismic Response Coefficient
W - Total Weight
Equivalent Lateral Force Method Limitations
• This method may not apply to buildings
with irregularities in Seismic Design
Categories D, E, or F
Earthquake Loads – Total Weight, W
• Dead Load of structure plus:
– 25% of reduced floor live load in storage
areas
– live load in parking structures not included
– Partition load if included in gravity dead
– Total weight of permanent equipment
– 20% of flat roof snow load, pf
where pf > 30 psf
Seismic Response Coefficient, Cs
• Function of
– Spectral response acceleration
– Site soil factors
– Building Period
– Response modification factors
– Importance factor
Seismic Response Coefficient, Cs
•
•
•
•
Step 1 - Determine SS and S1
Step 2 - Determine site Soil Classification
Step 3 - Calculate Response Accelerations
Step 4 - Calculate the 5% Damped Design
Spectral Response Accelerations
• Step 5 - Determine the Seismic Design Category
• Step 6 - Determine the Fundamental Period
• Step 7 - Calculate Seismic Response Coefficient
Step 1 – Determine SS and S1
• From IBC Map
• From local building
codes
• IBC 2003 CD-ROM
– Based on
• Longitude / Latitude
• Zip Code
Step 2 – Determine Site Soil Classification
• If site soils are not known use Site Class D
• Figure 3.10.7 (a) (page 3-111)
• From soil reports
Step 3 – Calculate Response Accelerations
• SMS = Fa·SS
• SM1 = Fv·S1
Where:
– Fa and Fv are site coefficients from Figure 3.10.7
(b) and (c) (page 3-111)
– SS spectral accelerations for short periods
– S1 spectral accelerations for 1-second period
– All values based on IBC 2003
Step 4 – Calculate the 5%-Damped Design
Spectral Response Accelerations
• SDS = (2/3)SMS
• SD1 = (2/3)SM1
Step 5 – Determine the Seismic Design Category
• Table 3.2.4.1.
• Sometimes this restricts
the type of Seismic Force
Resisting System (SFRS)
used (see Figure 3.10.8)
(page 3-112)
Step 6 – (Approximate Period) Determine
the Buildings Fundamental Period
Ta  C thnx
Where:
Ct = 0.016 for moment resisting frame systems of
reinforced concrete
0.020 for other concrete structural systems
x = 0.9 for concrete moment resisting frames
0.75 for other concrete structural systems
hn = distance from base to highest level (in feet)
Step 6 – (Exact Period) Determine the
Buildings Fundamental Period
Rayleigh’s formula
n
T  2
2
w

 ii
i1
n
g Fii
i1
Where:
wi = dead load weight at Floor i
δi = elastic displacement at Floor i
Fi = lateral force at Floor i
g = acceleration of gravity
n = total number of floors
Step 7 – Determine Seismic Response
Coefficient, Cs
Lesser of
Cs 
SDS
R
I
or C s 
SD1
TR
I
Where:
R = Response Modification
Factor
Figure 3.10.8 (page 3-112)
Ι = Seismic Importance Factor
Step 7 – Determine Cs
Minimum Value of Cs
Cs = 0.044·SDS·Ι
Special Cases In Seismic Design Categories E and F
Cs 
0.5  S1
R
I
Vertical Distribution of Lateral Force
Fx  C vx  V
C vx 
w x  hkx
n
k
w

h
 i i
i1
Where:
Fx = Force per floor
Cvx = Vertical distribution factor
V = Base shear
k = 1 - buildings with a period ≤ 0.5 sec
= 2 - buildings with a period > 2.5 sec
hi and hx = height from base to Level i or x
wi and wx = Level i or x portion of total gravity load
Location of Force in Plane
• Accidental Torsion
– calculated by assuming that the center of mass is
located a distance of 5% of the plan dimension
perpendicular to the applied load on either side of
the actual center of mass
• Total torsion = sum of the actual torsion plus
the accidental torsion
Seismic Drift Requirements
• Elastic Displacement Amplification
Factor, x
• Stability Coefficient Limits, q
• P-D Effects
Drift Limits
• Figure 3.10.9 (page 3-113)
Drift Amplification Factor, x
x 
C d   xe
I
Where:
δx = Amplified deflection of Level x
δxe = Deflection of Level x determined from elastic
analysis, includes consideration of cracking
Cd = Deflection amplification factor
(Figure 3.10.8)
Ι
= Seismic Importance Factor
Stability Coefficient, θ
q
Px  D
Vx  hsx  C d
Where:
Px = Total vertical unfactored load including and above
Level x
∆ = Difference of deflections between levels x and x-1
Vx = Seismic shear force acting between levels x and
x-1
hsx = Story height below Level x
Cd = Deflection amplification factor
Stability Coefficient, θ
The stability coefficient is limited to:
qmax 
0.5
 0.25
  Cd
Where:
β = ratio of shear demand to shear capacity between
Levels x and x-1
P-D Effects
• To account for P-∆ effects, the design story
drift is increased by
(1− θ)-1
• If θ < 0.10, P-∆ effects may be neglected
Reliability Factor, ri
• Required in High Seismic Design Categories
D, E, and F
• The Earthquake Force is increase by a
Reliability Factor, ri
• 1.5 Maximum Required Value
ri = 1.0 for structures in Seismic Design
Categories A, B and C
Reliability Factor, ri For Moment Frames
ri  2 -
20
rmax i  A i
Where, for each level:
Ai = floor area
rmaxi = For moment frames, the maximum of the sum of
the shears in any two adjacent columns divided by the
story shear. For columns common to two bays with
moment-resisting connections on opposite sides, 70%
of the shear in that column may be used in the column
shear summary.
Reliability Factor, ri For Shear Walls
ri  2 -
20
rmax i  A i
Where, for each level:
Ai = floor area
rmaxi = For shear walls, the maximum value of the
product of the shear in the wall and 10/lw divided by
the story shear.
Load Combinations
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•
•
•
•
•
•
U = 1.4(D+F)
U = 1.2(D+F+T) + 1.6(L+H)
U = 1.2D +1.6(Lr or S or R) + (1.0L or 0.8W)
U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R)
U = 1.2D + 1.0E + f1L + 0.2S
U = 0.9D + 1.6W + 1.6H
U = 0.9D + 1.0E + 1.6H
f1 = 1.0 Parking garages
= 1.0 Live load ≥ 100 psf on public assembly floors
= 0.5 All others
Modification for Vertical Acceleration
• E = ρ·QE ± 0.2·SDS·D
Seismic Load Combinations Become
• U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S
• U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H
Notice Building weight increase as Ground move Up
Where
QE = Horizontal Seismic Force
Modification for Vertical Acceleration
• E = ρ·QE ± 0.2·SDS·D
Seismic Load Combinations Become
• U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S
• U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H
Notice Building weight decreases as Ground move Down
Overstrength Factor, Wo
• Components within the Diaphragm
– Chord ties
– Shear Steel
– Connectors
• Ωo = 2.0 - Seismic Design Categories
C, D, E and F
• Ωo = 1.0 - Seismic Design Categories
A and B
Special Load Combinations
• U = 1.2D + fi·L + Em
• U = 0.9D + E
Where:
Em = Wo·QE + 0.2·SDS·D
and
Wo = Overstrength Factor
Overstrength Factor, Wo
• Connections from Diaphragms to Seismic
Force Resisting System (SFRS)
– Ωo = Seismic Design Categories C and higher
Figure 3.10.8 (page 3-112)
Structural Integrity Requirements
• All members must be connected to the Lateral Force
Resisting System (LFRS)
• Tension ties must be provided in all directions
• The LFRS is continuous to the foundation
• A diaphragm must be provided with
– Connections between diaphragm elements
– Tension ties around its perimeter
• Perimeter ties provided
– Nominal strength of at least 16 kips
– Within 4 ft of the edge
• Column splices and column base connections must have
a nominal tensile strength not less than 200Ag in pounds
Structural Integrity Requirements
• Precast vertical panels connected by a minimum of
two connections
• Each connection is to have a nominal strength of 10
kips
• Precast diaphragm connections to members being
laterally supported must have a nominal tensile
strength not less than 300 lbs per linear ft
• Connection details allow volume change strains
• Connection details that rely solely on friction caused
by gravity loads are not to be used
Lateral Force Resisting Systems (LFRS)
• Rigid frames and
shear walls exhibit
different responses
to lateral loads
Influential Factors
• The supporting soil and footings
• The stiffness of the diaphragm
• The stiffness LFRS elements and
connections
• Lateral load eccentricity with respect to center
of rigidity of the shear walls or frames
Shear Wall Systems
• Most common lateral force resisting
systems
• Design typically follows principles used
for cast-in-place structures
International Building Code
(IBC) Requirements
• Two categories of shear walls
– Ordinary
– Special
ACI 318-02 Requirements
• Created an additional intermediate
category, but has assigned no distinct
R, Ωo and Cd
ACI 318-02 Wall Definitions
• Defines all shear walls as “structural
walls”
• Three levels of definition
– Ordinary structural (shear) wall
– Intermediate precast structural (shear) wall
– Special precast structural (shear) wall
Ordinary Structural (Shear) Wall
• Wall complying with the requirements of
Chapters 1 through 18
• No special seismic detailing
Intermediate Precast Structural
(Shear) Wall
• Wall complying with all applicable
requirements of Chapters 1 through 18
• Added requirements of Section 21.13
– Ductile connections with steel yielding
– 1.5 factor for non-yielding elements
• IBC imposes restriction that yielding be
in the reinforcing
Special Precast Structural (Shear) Wall
• Precast wall complying with the requirements
of 21.8.
• Meeting the requirements for ordinary
structural walls and the requirements of 21.2
– Requires precast walls to be designed and
detailed like cast-in-place walls, “emulative”
design
– Meet the connection requirements of Section
21.13.
Design Guidelines for Shear Wall Structures
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•
•
Evaluation of building function and
applicable precast frame
Preliminary development of shear wall
system
Determination of vertical and lateral loads
Design Guidelines for Shear Wall Structures
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•
•
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Preliminary load analysis
Selection of shear walls
Final load analysis
Final shear wall design
Diaphragm design
Moment Frame Classifications
• Three Classifications
– Ordinary Moment Frame
– Intermediate Moment Frames
– Special Moment Frames
• Based on Detailing
• Seismic Design Categories
Ordinary Moment Frames
• Seismic Performance Categories A & B
• ACI 318 Chapters 1 to 18
• Response modification factor, R = 3
Intermediate Moment Frames
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•
•
Seismic Performance Category C
ACI 318 only defines intermediate as
cast-in-place
Response modification factor, R = 5
Special Moment Frames
• Seismic Performance Categories D, E,
and F
• Yielding will be concentrated in the beam,
Strong column -weak beam behavior
• Special Moment frames
– ACI 318 Sections 21.2 through 21.6
• Response modification factor, R = 8
Diaphragms
• A diaphragm is classified as rigid if it can
distribute the horizontal forces to the vertical
lateral load resisting elements in proportion to
their relative stiffness
• Long-span applications suggest that many
precast diaphragms may in fact be flexible
Diaphragm Design
• The distinction between rigid and
flexible diaphragms is important not
just for diaphragm design, but also
for the design of the entire lateral
force resisting system.
Diaphragm Classification
• Flexible diaphragm
– Lateral deflection twice average story drift
• Rigid diaphragm
– Not flexible
– Implies capability to distribute load based
on relative stiffness of LFRS elements
Steps in the Design Method
Step 1 - Calculate and compare distribution and
diaphragm forces
 Based on rigid diaphragm action
 Based on flexible diaphragm action
Step 2 - Check of diaphragm deformation with
respect to drift limits
Step 3 - Check attached element drift limits
Step 4 - Adjustments in vertical element
stiffness and placement to limit drift
Diaphragm Design Forces
• Based on Wind and Seismic Events
• Wind
– Combined windward and leeward wind pressures
– Act as uniform load on building perimeter
– Distributed to the LFRS based on diaphragm
behavior
Seismic Diaphragm Design Forces
• Separate calculations from the design of the LFRS
• Diaphragm Design force, FP
• Seismic Design Categories B or C
Fp = 0.2·IE·SDS·Wp + Vpx
Where
Vpx – represents forces from above levels that
must be transferred through the diaphragm due to
vertical system offsets or changes in stiffness.
Seismic Diaphragm Design Forces
• Seismic Design Category D
n
Fpx 
F
ix
n
i
w
i x
wpx
i
0.2·IE·SDS·wpx< Fp < 0.4·IE·SDS·wpx
Diaphragm Detailing
• Wind and Low Seismic Hazards
• Moderate Seismic Hazards
• Seismic Design Category D Topped Systems
• High Seismic Hazards - Untopped
Systems
Wind and Low Seismic Hazard
• Seismic Design Category A
– Strength requirements imposed by the applied
forces, No Amplification
• Seismic Design Category B
– Requires the design of collector elements
– Does not require forces to be increased by over
strength factor, Ωo (Revised from IBC 2000)
Moderate Seismic Hazard
• Topped and Pretopped Systems
• Seismic Design Category C
• Concrete wall systems have special
requirements IBC 2003
• Diaphragm must include
– special continuous struts or ties between
diaphragm chords for wall anchorage.
– use of Sub-Diaphragms, the aspect ratio of is
limited to 2½ to 1
Moderate Seismic Hazard
• Walls classified as Intermediate Precast Walls
– Collector elements, their connections based on
special load combinations
– Need to include overstrength factor
– Ductile connections with wall interface
– The body of the connection must have sufficient
strength to permit development of 1.5fy in the
reinforcing steel
Seismic Design Category (SDC) D
• Topped Systems
• Untopped Systems
– Not implicitly recognized in ACI 318 - 02
– Section 21.2.1.5
• permits a system to be used if it is shown by
experimental evidence and analysis to be equivalent in
strength and toughness to comparable monolithic cast-inplace systems
SDC D – Topped Systems
• High strain demand across the joints
• Reinforcing steel needs to be compatible with
this demand
• Use of larger wire spacing or bars may be
needed
• Mesh in the topping must take the entire
shear across the joint.
• Correct lapping to maintain diaphragm
integrity
SDC D – Topped Systems
• Specific provisions in ACI 318-02
• Chord steel determined from flexural analysis
• Shear strength based entirely on
reinforcement crossing the joint:
Vn = Acv·rn·fy
Where
Acv = thickness of the topping slab
ρn = steel ratio of the reinforcement
SDC D – Topped Systems
• ACI 318-02
– minimum spacing requirement of 10 in
– Diaphragm f -factor ≤ vertical element fshear -factor
– May result in f = 0.6, based on ACI 318-02
Section 9.3.4
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