2007 Structures Congress
Selected New Provisions
of ASCE/SEI 7-2005:
Jim Harris
J. R. Harris & Company
Denver, Colorado
John Hooper
Magnusson Klemencic Associates
Seattle, Washington
May 18, 2007
•
•
•
•
•
•
•
•
•
Overview
Reorganization
New maps; Long period map
New systems, revised R factors and limitations
Diaphragm assumptions
Redundancy factor
Dynamic analysis triggers
Near fault spectral shape applicability
Modal response spectrum analysis
Simplified design method
2007 Structures Congress
Seismic Design Under ASCE 7-2005
2
ASCE 7-05 Reorganization
Goals of Seismic Section Reorganization
1.
2.
3.
4.
5.
To improve clarity and use
Reduce depth of section numbering from 6 max
typical to 4 max typical
(i.e. Sec. 9.5.2.5.2.2 is now Sec. 12.5.3)
Simplify table and figure numbering
(i.e. Table 9.5.2.5.1 is now Table 12.6-1)
Create logical sequence of provisions aim at the
structural engineering community
Improve headings and clarify ambiguous provisions
2007 Structures Congress
Seismic Design Under ASCE 7-2005
3
ASCE 7-05 Reorganization
1. Changed major subjects to Chapters rather than
Sections (similar to the IBC)
2. Replaced Section 9 with Chapters 11-23
3. Incorporated the material appendices
4. Put the Chapters into a logical sequence
5. Rewrote ambiguous headings
6. Examined and rewrote sections to eliminate
ambiguity
7. Provided Cross Reference Table C-11-1…02 to 05
2007 Structures Congress
Seismic Design Under ASCE 7-2005
4
Comparison of Contents
ASCE 7-2002 Sections
ASCE 7-2005 Chapters
1. General
2. Combinations of
Loads
3. Dead Loads
4. Live Loads
5. Soil and Hydrostatic
…and Flood Loads
1. General
2. Combinations of
Loads
3. Dead Loads, Soil …
and Hydrostatic
4. Live Loads
5. Flood Loads
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Comparison, continued
ASCE 7-2002 Sections ASCE 7-2005
Chapters
6. Wind Loads
6. Wind Loads
7. Snow Loads
7. Snow Loads
8. Rain Loads
9. Earthquake Loads 8. Rain Loads
9. (not used)
10. Ice Loads
A. Supplemental (QA) 10. Ice Loads
11. - 23. Seismic
B. Serviceability
A & B. QA & Existing
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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Seismic Contents
ASCE 7-2002
ASCE 7-2005
9.1 General Provisions
9.2 Definitions/
Symbols
9.3 (not used)
9.4 Ground Motion
9.5 Structural Design
Criteria, Analysis,
and Procedures
11. Seismic Design
Criteria
12. Seismic Design
Requirements for
Building Structures
13. Seismic Design
Requirements for
Nonstructural Comp.
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Seismic Design Under ASCE 7-2005
7
Seismic Contents, continued
ASCE 7-2002
ASCE 7-2005
9.6 Arch, Mech, Elect
Comp and Sys
9.7 Foundations
9.8 Steel
9.9 Concrete
9.10 Composite Struct.
9.11 Masonry
14. Material Specific
15. Nonbuilding
Structures
16. Response Hist Anal
17. Seismic Isolation
18. Damping Systems
19. Soil-Struct.
Interact.
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Seismic Contents, continued
ASCE 7-2002
ASCE 7-2005
9.12 Wood
9.13 Seismic Isolation
9.14 Nonbuilding
Structures
A9. Quality
Assurance
20. Site Classification
21. Site-Specific
Ground Motions
22. Maps
23. Reference Docs
11A. Quality
Assurance
11B. Existing
Buildings
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11 Seismic Design Criteria
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
General
Definitions
Notation
Seismic Ground Motion Values
Importance Factor
Seismic Design Category
Design Req’ts for Category A
Geologic Hazards & Geotechnical Invest.
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Seismic Design Under ASCE 7-2005
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11.1 Purpose
“…specified earthquake loads are based upon
post-elastic energy dissipation in the
structure, and because of this fact, the
requirements for design, detailing, and
construction shall be satisfied even for
structures and members for which load
combos w/o EQ exceed those with EQ…”
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Seismic Design Under ASCE 7-2005
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11.4 Seismic Ground Motions
1 Determine basic values from maps for
bedrock conditions
2, 3 Classify soil conditions at site and
determine site coefficients
4 Determine site-adjusted values
5 Take two-thirds for use in design
6 Construct design response spectrum
7 Site-specific studies permitted/required
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Seismic Design Under ASCE 7-2005
12
Mapped Acceleration Parameters
• Two updated sets of basic maps for the response
spectrum accelerations
– SS for spectral response acceleration at 0.2 secs
– S1 for spectral response acceleration at 1.0 secs
• New map for long period transition: TL in
seconds
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Ground Motion Parameters & Seismic Hazard
Mapped Contours of SS
SS and S1 are the
mapped 2% in 50 year
spectral accelerations
for firm rock
SDS and SD1 are the
design level spectral
accelerations (modified
for site and “expected
good performance”)
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Seismic Design Under ASCE 7-2005
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General Comparison of Maps
With ASCE 7-02
• Changes everywhere,
but mostly minor
• Deterministic area
around New Madrid
2007 Structures Congress
With UBC 97
• Lots of change
• Lower in most areas
• Higher in high hazard
areas, except near fault
in California
• Three maps, not one
Seismic Design Under ASCE 7-2005
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Long Period Transition Maps (Fig 22.15)
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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Site Specific Studies
• Clarification of two types:
– Basic ground motion hazard at a point in rock
– Site amplification in overburden soil
• First type never required, but permitted;
limits placed upon results
• Second type encouraged; required in some
instances
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Seismic Design Under ASCE 7-2005
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Cumulative Nature of Provisions
by Seismic Design Category
A
2007 Structures Congress
B
C
D
E
F
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18
Seismic Design Category A
• 11.7 is a self-contained section; defines E
• Horizontal force = 1% of dead load
• Load path for horizontal forces
– connections = 5% of weight of smaller part
• Beam, truss connections = 5% D + L
• Anchor concrete and masonry walls
– 280 pounds per foot
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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Geologic Hazards and
Geotechnical Investigations
• SD Category E and F:
– Do not locate on active fault
• SD Category C:
– Evaluate slope instability, liquefaction,
differential settlement, surface displacement
• SD Category D, E, F:
– More detail than C plus lateral pressures on
basement walls and retaining walls
2007 Structures Congress
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20
•
•
•
•
•
•
•
•
•
Overview
Reorganization
New maps; Long period map
New systems, revised R factors and limitations
Diaphragm assumptions
Redundancy factor
Dynamic analysis triggers
Near fault spectral shape applicability
Modal response spectrum analysis
Simplified design method
2007 Structures Congress
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21
12 Seismic Design of Building Structures
1
2
3
4
5
6
7
Design Basis
Structural Systems
Diaphragm Flexibility;
Configuration; &
Redundancy
Load Effects &
Combinations of Loads
Direction of Loading
Selection of Analysis
Modeling Criteria
2007 Structures Congress
8 ELF Method
9 Modal RS Method
10 Diaphragms, Chords
Collectors
11 Structural Walls (outof-plane)
12 Drift and
Deformation
13 Foundation Design
14 Simplified Alternate
Seismic Design Under ASCE 7-2005
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12.1.1 Basic Requirements
•
•
•
•
Strength, Stiffness, Energy Dissipation
Design Motion in Any Horizontal Direction
Construct Mathematical Model
Evaluate Model for Effects
– Limitations on methods of evaluation
– Modifications to internal forces (R, 0)
– Modifications to deformations (Cd)
• Alternate Procedures Must Be Consistent
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Minimum Connection Forces
• Very Similar to 11.7 for SD Category A
• Continuous Load Path: “…from point of
application to final point of resistance…”
– 0.133 SDS WP  0.05 WP
– Does not apply to overall design of SFRS
(Seismic Force Resisting System)
• Beams, Trusses to Support 5% of D + L
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12.2 Structural System Requirements
1.
2.
3.
4.
5.
System identification and limitations
Combos of systems: different direction
Combos of systems: same direction
Combos of systems: detailing
Specific system requirements
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Seismic Design Under ASCE 7-2005
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System Identification
• “Basic lateral and vertical SFRS shall
conform to one (or a permitted combo) of
the systems from Table 12.2-1…”
• “Selected SFRS shall be designed and
detailed per referenced requirements…”
• SFRS not from table permitted only if
analytical and test data establish basis
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Basic system parameters
Obtain from table and use in seismic design:
• R Response Modification Factor
- measure of system inelastic capabilities
• Cd Deflection Amplification Factor
- increase elastic  to total 
• o System Overstrength Factor
- accounts for actual strength greater
than design strength; used to protect
vulnerable items.
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R factor comparisons
System
97
Special RC Shear Wall (bearing)
4.5 5
5
Ordinary RC Shear Wall (bearing)
4.5 4
4
Special RC Shear Wall (bldg frm)
5.5 6
6
Ordinary RC Shear Wall (bldg frm)
5.5 5
5
Intermed Precast Shear Wall (b f)
--
--
5
Ord Precast Shear Wall (b f)
--
--
4
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28
R factor comparisons
System
97
Special RM Shear Wall (bearing)
4.5 5
Intermed RM Shear Wall (bearing)
4.5 3.5 3.5
Ord RM Shear Wall (bearing)
4.5 2
Special RM Shear Wall (bldg frm)
5.5 5.5 5.5
Intermed RM Shear Wall (bldg frm)
5.5 4
Ord RM Shear Wall (bldg frm)
5.5 2.5 2.5
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5
2
4
29
R factor comparisons
System
97
Special Steel Concentric BF
6.4 6
6
Ordinary Steel Concentric BR
5.6 6
3.25
Special RC Shear Wall (bldg frm)
5.5 6
6
Ordinary RC Shear Wall (bldg frm)
5.5 5
5
Eccentrically Braced Frame (with)
7
8
8
Eccentrically Bracked Frame (w/out) 7
7
7
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R factor comparisons
System
97
Light Frame with SWP (bearing)
5.5 6
6.5
Light Frame with other (bearing)
4.5 2
2
Light Frame with SWP (bldg frm)
6.5 6.5 7
Light Frame with other (bldg frm)
5
Light Frame with straps (bearing)
2.8 4
2007 Structures Congress
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05
2.5 2.5
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31
Height Limits
• Most shear wall and braced frame systems
limited to 160 feet high in SD Categories D
and E, and to 100 feet in SD Category F
• These limits can be increase to 240 feet and
160 feet, respectively for some structures
– No line resists more than 60% of base shear
– Torsional force < 20% of total force in the line
• Many exceptions, especially for nonbuilding
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Concrete Shear Wall - Frame
• Limited to SD Category B
• Ordinary detailing for wall and frame
• Analyze for interaction and provide as a
minimum
– Walls strong enough for 0.75 Vx at each story
– Frames strong enough for 0.25 Vx at each story
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12.3 Diaphragms, Configuration,
and Redundancy
1. Diaphragm Flexibility in Analysis
2. Identification of Irregularities in System
Configuration
3. Limitations on and Penalties for
Irregularities
4. Redundancy
–
Significant changes from prior edition
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Diaphragm Flexibility
Assume Flexible if
Assume Rigid if
• Wood or steel deck with
concrete or masonry
walls
• 1 or 2 family residential
if light frame
• Compute
ΔDia > 2 * δvert
• Concrete slab (or filled
deck) with span to depth
< 3 and no horizontal
irregularity
2007 Structures Congress
Otherwise:
• Must analyze system
including actual
stiffness of diaphragm!
Seismic Design Under ASCE 7-2005
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Configuration: Basic Parameters
Size
Proportion
2007 Structures Congress
Shape
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Plan Irregularities
Type
Measure
1a Torsional irregularity
 corner > 1.2  center
1b Extreme torsional irregularity  corner > 1.4  center
Note: torsional irregularity not checked for flexible diaphragms
2 Re-entrant corners
Both projections > 15%
of respective sides
3 Diaphragm discontinuity
50% change in a level or
from level to level
4 Out-of-plane offsets
absolute
5 Nonparallel systems
absolute
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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Vertical Irregularities
Type
Measure
1a Stiffness-Soft Story
Story stiffness < 70% above
1b Stiffness- Extreme Soft Story Story stiffness < 60% above
2
Weight (Mass)
More than 150% adjacent story
Note: 1 and 2 dropped if no story drift exceeds 130% of story above
3
Vertical Geometric
Length of SFRS >130% of that
in adjacent story
4
In-Plane Discontinuity
Offset > length of element or a
reduction in stiffness below
5a Capacity-Weak Story
Lat strength < 80% of above
5b Extreme Weak Story
Lat strength < 65% of above
2007 Structures Congress
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Configuration Limitations
•
•
•
•
Horiz 1b not permitted in SD Cat E+
Vert 1b, 5a not permitted in SD Cat E+
Vert 5b not permitted in SD Cat D+
Vert 5b limited to 2 stories or 30 feet in SD
Cat B or C, unless weak story strength
capable of 0 times design force
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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Configuration Penalties
• Horiz 4 and Vert 4 (column, slab, beam, or
truss elements supporting discontinuous
elements) to resist 0 force (all SD
Categories)
• Horiz 1, 2, 3, 4 and Vert 4 have 25%
increase in force for connection of
diaphragm to vert element and collectors in
SD Cat D+; also req’d for collectors except
those already designed for 0 force
2007 Structures Congress
Seismic Design Under ASCE 7-2005
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•
•
•
•
•
•
•
•
•
Overview
Reorganization
New maps; Long period map
New systems, revised R factors and limitations
Diaphragm assumptions
Redundancy factor
Dynamic analysis triggers
Near fault spectral shape applicability
Modal response spectrum analysis
Simplified design method
2007 Structures Congress
Seismic Design Under ASCE 7-2005
41
Redundancy factor
Seismic Design Category
Reliability Factor
B or C
ρ  1.0
D, E or F
ρ  1.0 or 1.3
 is always 1.0 for drift and P-delta calcs and for design of:
• Nonstructural components
• Nonbuilding structures not similar to buildings
• Members designed for 0 forces
• Diaphragms
• Structures with damping systems
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Redundancy Factor  = 1.3
Unless following loss does not an extreme
torsional irregularity and does not reduce
story strength by more than 33%:
• Braced frame: removal of a single brace
• Moment frame: loss of moment resistance
at both ends of a single beam (or at base of
a single cantilever column)
• Shear walls: removal of any single pier
with h/l > 1.0 (or collector to such a pier)
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12.6 Analysis Method Selection
Methods Defined
• Equivalent (Static) Lateral Force: ELF
• Modal Response Spectrum: MRS
• Seismic Response History (Linear and
Nonlinear): SRH (Defined in section 16)
Alternate classifications:
Static / Dynamic Max / Dynamic History
Linear / Nonlinear
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What type of Analysis?
• The answer depends on:
– what performance level you
are hoping to achieve
– the configuration of the
structure
– how accurate you need to be
• A wide range of choices are
available-
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Seismic Design Under ASCE 7-2005
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Superior Performance Levels
Joe’s
Beer!
Food!
• Behavior will be essentially elastic
– For regular structures with short periods, linear static
procedures are fine
– For regular structures with long periods and all
irregular structures - linear dynamic procedures are
better, response spectra accurate enough
2007 Structures Congress
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Poorer Performance Levels
• Inelastic behavior is significant (elastic analyses
are the wrong approach!)
– For structures dominated by first mode response,
pushover analysis may be adequate
– For structures with significant hire mode response,
nonlinear time history necessary
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Methods Permitted
• SD Cat B and C: any defined method
• SD Cat D+: ELF permitted for
–
–
–
–
–
Occ Cat I/II < 3 stories
Occ Cat I/II of light frame < 4 stories
Reg structures with T < 3.5 TS
Reg structures of light frame any T
Irreg structure with T < 3.5 TS limited to horiz
types 2, 3, 4, or 5 and vert types 4, 5a, or 5b
• Other SD Cat D+ must use MRS or SRH
2007 Structures Congress
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Design Response Spectrum
0.7
0.6
Sa = SD1 / T
Sa = SDS(0.4 + 0.6 T/T0)
Spectral Acceleration, g
0.5
Drawn for
SS = 1.0, Fa = 1.0
S1 = 0.4, Fv = 1.5
TL = 4
Sa = SD1 TL / T2
0.4
0.3
0.4SDS
0.2
0.1
0.0
0
T0 TS
1
2
2007 Structures Congress
3
4
5
6
7
Period, seconds
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Design Response Spectrum
0.7
Drawn for
SS = 1.0, Fa = 1.0
S1 = 0.4, Fv = 1.5
TL = 4
Sa = SD1 / T
0.6
Spectral Acceleration, g
0.5
0.4
0.3
0.4SDS
Sa = SDS(0.4 + 0.6 T/T0)
Sa = 0.5 S1
0.2
Sa = SD1 TL / T2
0.1
0.0
0
T0 TS
1
2
2007 Structures Congress
3
4
5
6
7
Period, seconds
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12.8 ELF Method of Analysis
1.
2.
3.
4.
5.
6.
7.
Seismic Base Shear: V = CSW
Period Determination
Vertical Distribution of Seismic Forces
Horizontal Distribution of Forces
Overturning
Story Drift Determination
P-Delta Effects
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Seismic Coefficient
Basic rules are the design spectrum adjusted
for R and I: C  S DS  S D1  S D1TL
 R
 R
2 R 
  T  T  
I
I
I
Cs  0.01 (new minimum)
s
Also, where S1 > 0.6:
2007 Structures Congress
S1
C s  0. 5
 R
 
I
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Adjustments to Base Shear
• Soil-Structure Interaction per Section 19 is
permitted
• Low rise buildings in high ground motion
areas:
If stories < 6 and T < 0.5 seconds
Can use SS = 1.5 max
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12.8.2 Period of Vibration
• Follow modeling
criteria in 12.7 to
compute T
• Upper bound for forces:
T  CuTa
2007 Structures Congress
SD1
> 0.4
0.3
0.2
0.15
<0.1
Cu
1.4
1.4
1.5
1.6
1.7
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Approximate Period
Ta  C h
x
t n
Structure
100% Moment Frames:
Steel
Concrete
Eccentrically Braced
0.028 0.8
0.016 0.9
0.03 0.75
All others
0.02
2007 Structures Congress
Ct
x
0.75
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What is hn? – Concept of Base
Masonry
wall
RC frame
hn
hn
RC wall
Base
Base
RC wall
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Alternate Estimates for Ta
• 100% moment frames up to 12 stories with
story heights at least 10 feet: , Ta = 0.1N
• Shear walls of concrete or masonry:
0.0019
Ta 
hn
Cw
2007 Structures Congress
2
Ai
100  hn 
 
Cw 

2
AB i 1  hi  
 hi  
1  0.83  

 Di  
x
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Vertical Distribution of Force
Equivalent static force at level x: Fx  CvxV
Cvx 
k
x x
wh
n
wh
i 1
k
i i
n
Story Shear: Vx   Fi
ix
where
wi , wx: Portion of W assigned to level i or x
hi , hx: Height of level i or x above base
k sets the shape of distribution and depends on T
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Shape of Vertical Distribution
1 ≤ k ≤ 2: Varies with T
For T ≤ 0.5, k = 1 (linear distribution)
For T ≥ 2.5, k = 2 (parabolic distribution;
impact of higher modes)
For 0.5 < T < 2.5,
k = 2 or
k = 0.75 + T/2
(interpolation)
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ELF - Story Shears
n
Vx   Fi
Sum the story forces from the top down
i x
Distribution of story forces intended to give proper envelope
of maximum story shears for a regular building. It does not
give envelope of maximum story forces.
F
2007 Structures Congress
V
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ELF - Horizontal Distribution
• Distribute story shear to vertical elements
per relative stiffness of vertical elements and
diaphragm
• Account for computed (inherent) torsion - eccentricity
between mass and resistance
• Add accidental torsion, except for flexible diaphragms
• Amplify torsion if torsionally irregular
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Torsional Irregularity
max
 av g
min
 max 1.2 irregular 


 avg 1.4 extreme 
For S.D. Category C, D, E or F accidental
eccentricity must be multiplied by Ax where Ax
2007 Structures Congress
2
  max 
  3 .0

 1.2 
avg 

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ELF - Overturning Moments
n
M x   Fi hi  hx 
Story force times height
to level under consideration.
ix
• Overestimate where higher
modes are significant
.
• Prior “codes” allowed up to a 20% reduction in tall buildings
• Now require modal analysis for such structures, thus this
provision is now deleted
• Moment can be reduced 25% at foundation, permitting
some rocking
F
2007 Structures Congress
V
M
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Definition of Drift
Cd   xe
Structural displacement,  x 
I
where,
 xe  Elastic deflection calculated
from design forces
Cd  Deflection amplification factor
I  Importance factor
No reduction for ASD,
but, can ignore limit on T
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Stability: P-Δ Effects
Δ
P
Deflection introduces P-Δ moment
which increases deflection, which
increases moment …..
Structure must be designed to
prevent collapse due to P-Δ
effects
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Stability: P-Δ Effects
• Determine stability coefficient, θ, for each story
Px 

Vx hsxCd
• If θ > 0.10 at any level, then all design forces and
moments must be increased by factor 1+ad

where ad 
1
• Check
   max
2007 Structures Congress
0 .5

 0.25
 Cd
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Derivation of Stability Factor θ
δf
P
V
P
V
k
V
h
Undeformed
A
Deformed
B
Equilibrium at B:
Response
C
Vh  P f  k f h  0
δ
V f h
k o  V , then Vh  P f 
0
o
o
o

Rearranging terms:  f 
P o 1  
1
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Define:
P-Delta
• What if your analysis program “includes”
P-Delta and you don’t want to make a
second set of output?
• max must still be checked
• Compute * from displacements that
include P-Delta, then
*

  max
1 *
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12.9 Modal Response Spectrum
Analysis Method
1. Minimum Number of Modes
2. Adjustment of Response Parameters by
R/I (forces) and Cd/I (displacements)
3. Combining Modes for Total Response
4. Scaling of Design Values
5. Horizontal Shear Distribution, Torsion
6. P-Delta
7. Soil Structure Interaction
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M.R.S. Analysis
• Include enough modes to obtain a combined
modal mass participation of at least 90% of
the actual mass in each of the orthogonal
directions of response
• Short period branch of spectrum is usable
• Divide spectrum by (R/I) to obtain force
responses
• Multiply each displacement by (Cd/I)
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Combining Modal Results
• Basic rule is Square Root of Sum of
Squares (SRSS)
• Complete Quadratic Combination (CQC)
always permitted (see ASCE 4)
• CQC required where modal periods are
closely spaced or where translational and
torsional modes are cross correlated
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Design Response Spectrum
0.7
0.6
Sa = SD1 / T
Sa = SDS(0.4 + 0.6 T/T0)
Spectral Acceleration, g
0.5
Drawn for
SS = 1.0, Fa = 1.0
S1 = 0.4, Fv = 1.5
TL = 4
Sa = SD1 TL / T2
0.4
0.3
0.4SDS
0.2
0.1
0.0
0
T0 TS
1
2
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3
4
5
6
7
Period, seconds
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Scaling Modal Results
• Compute limiting base shear, V, by ELF; if T
exceeds CuTa, then use T = CuTa
• Compare 85% of this force with combined
modal base shear, Vt
• If Vt < 0.85V then multiply all combined
response quantities from modal analysis by
0.85V / Vt
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M.R.S. Analysis
• Critical direction of load applies (orthogonal
combinations)
• Inherent torsion automatically included
• Accidental torsion: two choices:
– Offset mass to achieve accidental eccentricity
- Include static torsion as a load case
• P-Delta applies as for ELF
• Soil Structure Interaction analysis permitted
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12.12 Drift and Deformation
1. Story Drift
–
–
–
Satisfy limits per table; occupancy is factor
If irregular, include torsion effect in SD Cat C+
Divide allowable by  for MF in SD Cat D+
2. Diaphragm Deflection
3. Building Separation
4. Compatibility for SD Category D+
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Drift Ratio Limits
Structure
Occupancy Category UBC
I or II III
IV
4 stories, no masonry
0.025 0.020 0.015 0.025*
Masonry cantilever
0.010 0.010 0.010
Other masonry
0.007 0.007 0.007
All other
0.020 0.015 0.010 0.020*
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Deformation Compatibility
Applies to
• SD Category D+
• All structural components not in SFRS
• Check capacity for gravity load combined
with effects induced from design drift;
rational analysis of restraint required
• ACI 318 Chap 21 acceptable alternate
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•
•
•
•
•
•
•
•
•
Overview
Reorganization
New maps; Long period map
New systems, revised R factors and limitations
Diaphragm assumptions
Redundancy factor
Dynamic analysis triggers
Near fault spectral shape applicability
Modal response spectrum analysis
Simplified design method
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12.14 Simplified Alternate
1.
2.
3.
4.
5.
6.
General (Limitations/Eligibility)
Design Basis (& Load Combinations)
SFRS Identification (& Combinations)
Diaphragm Flexibility
Direction of Loading
Design & Detailing: Load path connections,
collectors, wall anchorage
7. ELF Analysis
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Limitations
•
•
•
•
Occupancy Category I or II
Site Class A, B, C, or D
1, 2, or 3 stories
Bearing Wall or Building Frame System
– Braced frames or shear walls
– No unbraced (moment) frames
• “Regular”
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Torsional Regularity
• Flexible diaphragms:
– Overhang (cantilever) < depth / 5
– Controls displacement at edge
– Controls torsion in non-flexible
• Non-flexible diaphragm
– Eccentricity < 15% width of diaphragm
– Minimum torsional stiffness
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Torsion
m
n
i 1
j 1
2
2
k
d

k
d
 1i 1i  2 j 2 j
e1 2 m
 2.5(0.05  )b1  k1i
b1
i 1
•k1i - lateral stiffness, wall “i” parallel to 1
•k2j - lateral stiffness, wall “j” parallel to 2
•d1i,- the distance from the wall “i” to the
center of rigidity, perpendicular to axis 1
•d2j is the distance from the wall “j” to the
center of rigidity, perpendicular to axis 2
•e1 is the distance perpendicular to axis 1
between the center of rigidity and the
center of mass
•b1 is the width of the diaphragm
perpendicular to axis 1
•m is the number of walls in direction 1
•n is the number of walls in direction 2
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Regularity
• Limit skewed alignments to 15 degrees
• Use simplified method for design in both
horizontal directions
• No in-plane or out-of-plane offsets
– Exception: shear walls in 2 story light frame
– Must use Ω0 ( = 2.5 for all structures)
• No weak stories (80% rule)
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Seismic Design Category
• Limited to Occupancy groups I and II
• Only use SDS, therefore
SDS < 0.33 gives Category B
SDS < 0.50 gives Category C
SDS > 0.50 gives Category D
• Can have Category E if S1 is high
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Combined Effects
• Vertical Seismic Load =  0.2 S DS  W
• Combine positive vertical seismic load
where gravity and effect of horizontal
seismic add; combine negative vertical
seismic load where gravity offsets effect of
horizontal seismic
• Orthogonal combinations not required
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R Factor Table
• Includes wood shear walls, all concrete and
masonry walls and all steel bracing systems
– special, ordinary, intermediate, plain,
detailed, etc – and composite steel/concrete
walls
• Includes specific citations to reference
standards for detailing the systems
• No Cd or Ω0 factors here
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Detailed Rules
•
•
•
•
Connections: 0.2SDSwi or 0.05wi
Openings and corners in shear panels
Collectors: Ω0 = 2.5 (except light frame)
Diaphragms:
– use the story force
– provide continuous ties
• Anchor concrete/masonry walls (flexible)
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Detailed Rules
•
•
•
•
No orthogonal combinations
Redundancy factor = 1.0
Bearing walls: out of plane = 0.4SDSwc
Nonstructural components – same as any
other building
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Simplified ESF Analysis
• Base Shear
F  S DS
V
W
R
• Story Force
F  S DS
Fi 
wi
R
2007 Structures Congress
• Story factor F =
1.0 for one story
1.1 for two stories
1.2 for three stories
• Same acceleration at all
levels of building (very
simple!)
• No I factor, No period T
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Simplified Response
Acceleration
• Use only SDS; don’t use S1S
• Equation
S DS
 2
   Fa  SS
 3
• Site response amplifier Fa =
1.0 for rock
1.4 for soil
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Drift and Separation
• Do not have to check drift
• Use 1% drift for purposes of building
separation, nonstructural component
compatibility, etc, unless actually computed
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Overview
• Introduction to Earthquake Engineering
• Basic Criteria in ASCE 7 – 2005
– Ground Motions
– Response Spectrum
– Occupancy and Seismic Design Categories
• Seismic Design for Buildings
–
–
–
–
–
Basic Requirements
System Requirements
Analysis
Diaphragms, Walls, Foundations
Simplified Method
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Errata
• Go to the SEI website
www.seinstitute.org
• Go to the “Publications” in the bar at the top
• Click on the “Errata” tab
• Download pdf files for ASCE 7-05 (and any
other structural standards you may need)
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