Short-Period Building Performance Paradox
Resolution of the Short-Period
Building Seismic Performance
Paradox and Recommendations for
Improving Seismic Codes and Design
Collection of FEMA P-695 Collapse Results
(NIST GCR 12-917-20)
Why do analytical models of
code-compliant designs predict
high probabilities of collapse for
short-period buildings contrary
to damage observed in actual
earthquakes and the judgment
of earthquake engineers?
Charlie Kircher (Kircher & Associates)
Kelly Cobeen (Wiss, Janney, Elstner Associates)
2
Why does this matter?
ATC-116 Project
 Seven-year project (2013 – 2020)
 Funded by FEMA
 Conducted by the Applied
Technology Council (ATC)
 Four FEMA P-2139 reports
 Short-period buildings (T ≤ 0.5
seconds) constitute the vast majority
of buildings
 Do short-period buildings meet the
collapse safety objective of seismic
codes?
 Are improvements in seismic building
codes, design, and assessment
methodologies warranted?
 Overarching Findings, Conclusions, and
Recommendations (Vol. I)
 Light-Frame Wood Buildings (Vol. II)
 Reinforced Masonry Buildings (Vol. III)
 Steel Special Concentrically Braced
Frame Buildings (Vol. IV)
 Reports available at:
https://femap2139.atcouncil.org/
Credit: KlausVonVilver, WikimediaCommons
3
4
1
Project Objectives
 Identify causes of the short-period building paradox and
develop solution concepts
 Improve and validate numerical modeling methods for
short-period buildings
 Develop recommendations for improving seismic codes
and engineering practice and for future research initiatives
5
Observed Response and Collapse Performance
 Post-Earthquake Surveys
1971 San Fernando
1978 Miyagi ken-oki (JP)
1987 Whittier
1994 Northridge
1995 Kobe (JP)
2011 Christchurch (NZ)
Northridge Meadows Apartment Collapse
Reinforced-Masonry Test Structure – UC San Diego
 Full-Scale Shake Table Testing
E-Defense (Miki, Japan)
University of Buffalo and UC San Diego
7
6
Selection of Systems
 Analysis of low-rise occupancies
from HAZUS inventory of about 2
million buildings in Northern
California
 Wood light-frame: W1, W1A, W2
(77%)
 Reinforced masonry: RM1, RM2
(7%)
 Steel braced frame: S2 (1.3%)
 More than 85% of square footage
of low-rise occupancies
S3 S4
C1
C2
PC
S1
S2
RM
W2
W1A
W1
8
2
Approach of Analytical Studies (each SFRS)
 Identify Representative Archetypes
 Develop Designs of Archetypes
 Develop Numerical Models of
Archetype Designs
 Analyze Numerical Models (P-695)
Archetypes
Light-Frame Wood
2.33 g
Archetype (1.2 x 1.94 g)
SMT = 1.5 g
System
3.1 g
Wood light-frame walls
with wood structural panel
sheathing (wood)
Single-Family Dwelling (SFD)
ACMR = 3.1 g/1.5 g = 2.07
Reinforced Masonry
Archetype
 Pushover Analysis (Vmax/W)
 Incremental Dynamic Analysis (IDA)
 Evaluate Analysis Results (P-695)
 Dynamic Response
 Collapse Statistics
 Evaluate Collapse Performance (P-695)
 ACMR = SSF x SCT / SMT
 P[Collapse|SMT] = Lognormal (median, bTOT)
Steel SCBF
ST [T=0.25s] (g)s
Archetype
Figure
Figure4-16
4-17of
ofFEMA
FEMAP-2139-2
P-2139-2(Wood
(WoodReport)
Report)
showing
median
collapseand
showing IDA results and
(collapse
fractions)
acceleration
(SCT =curve,
1.94 g)
and collapse
collapse fragility
median,
SCT = 3.1 g, and
displacement
capacity
(drift ratio
8.17%)
lognormal standard
deviation
of b=TOT
= 0.5.
No. of
Stories
Response
Modification
Coefficient (R )
Design Seismicity
1, 2
6.5
High, Very High
Multi-Family Dwelling (MFD),
Commercial (COM)
1, 2, 4
6.5
High, Very High
Special reinforced
masonry shear walls
(masonry)
Commercial (COM)
1, 2, 4
5
High, Very High
Steel special
concentrically braced
frame (SCBF) (steel)
Commercial (COM)
1, 2, 4
6
High, Very High
Where, High: SMS = 1.5g; Very High: SMS = 2.25g
9
Archetype Designs
 Engineered designs prepared
using ASCE 7-10, assuming
Risk Category II
 Designs represent typical
modern practice in areas of
significant seismicity
 Configurations selected to be
realistic and representative of
actual buildings in size and
proportion
Occupancies
10
Wood Archetypes
Two-story
Masonry
Foundation Plan
 Single-Family Dwelling (SFD)
 32 ft x 48 ft plan
 One and two stories
 OSB shear walls and diaphragms
Two-story
Masonry Floor
Framing Plan
11
12
3
Wood Archetypes
Wood Archetypes
 Multi-Family Dwelling (MFD)
 Commercial (COM)
 48 ft x 96 ft plan
 One, two and four stories
 Two-story: four-unit row
houses/townhouses
 One- and four-story: apartments with
central corridor
 OSB shear walls and diaphragms





48 ft x 96 ft plan
One, two and four stories
One-story: retail or repair shop
Two- and four-story: office
OSB shear walls and diaphragms
13
Masonry Archetypes
14
Steel Archetypes
 Commercial (COM)
 Commercial (COM)
48 ft x 96 ft plan
One, two and four stories
Office building
Rigid floor diaphragms, flexible roof
diaphragms
 Load bearing, fully grouted,
reinforced hollow-unit concrete
masonry with cantilever shear walls
90 ft x 150 ft plan
One, two and four stories
Office building
Rigid floor diaphragms, flexible
roof diaphragms
 Steel SCBF with square HSS
braces








15
16
4
3D Nonlinear Numerical Models
Parametric Studies
 Baseline Configuration
Steel SCBF Model
 Best-estimate of each modeling parameter,
used to compare against benchmark collapse
rates
Variations in Backbone Curve
Shape for Wood Models
 Variant Configurations & Modeling
Parameters
Light-Frame Wood
Model
 Collapse displacement capacity (wood,
masonry)
 Soil structure interaction and foundation
flexibility (wood, masonry, steel)
 Nonstructural wall finishes (wood)
 Backbone curve shape (wood)
 No redundancy (steel)
 No reserve moment frame (steel)
Reinforced
Masonry Model
17
18
U.S. Earthquake Losses – 1964 – 20141,2
Earthquake
Observed Response and
Performance Benchmarks
Name
Fatalities
Building Loss (billions)
Date
Magnitude
Total
Building
EQ Date
1964 Anchorage
3/28/1964
M9.2
128
10
$0.3
Est. 2014
$5.0
1971 San Fernando
2/9/1971
M6.6
65
53
$1.5
$20.0
1989 Loma Prieta
10/17/1989
M6.9
63
12
$6.5
$17.5
1994 Northridge
1/17/1994
M6.7
57
20
$25.0
$55.0
Other Earthquakes (23)
33
14
$3.7
$7.4
Total All Earthquakes
346
109
$37.0
$104.9
1. Date and magnitude of selected large-magnitude (> M5.5) U.S. earthquakes, 1964 - 2014,
and associated fatalities and building-related economic losses.
2. Approximately 2 building-related fatalities per year (109 fatalities/50 years).
19
3. Estimated as many as 3,400 (day time) fatalities for a repeat of the 1906 San Francisco
earthquake (Kircher et al., “When the Big One Strikes Again,” Earthquake Spectra, 2006)
20
5
Summary and Comparison of Earthquake Losses
Sources of Earthquake Data Used by this Project
Sources of Damage Data on SFRSs
Considered by the Project
Earthquake
Date
Name
Magnitude Wood Light- Reinforced
Frame
-Masonry
1971 San Fernando
02/09/1971
M6.6
1978 Miyagi-ken-okI (JP)
06/12/1978
M7.4
Steel
SCBF
6
7
3
1987 Whittier
10/01/1987
M5.9
1994 Northridge
01/17/1994
M6.7
1
1995 Kobe (JP)
01/17/1995
M6.8
2
2011 Christchurch (NZ)
02/22/2011
M6.2
4
8
9
5
1. OES, 1995; HUD, 1994, ATC, 2000; Schierle, 2003;
2. AIJ, 1995a; Yamaguchi and Yamazaki, 2000; Yamazaki and Murao, 2000;
3. Hart et al., 1988; 4. OES, 1995; TMS, 1994; 5. Dizhur, 2011.
6. Simpson, 2017; NOAA, 1973; 7. Simpson, 2017; Tanaka et al., 1980;
8. OES, 1995; Krawinkler, 1995, Tremblay et al., 1995; 9. AIJ, 1995a, AIJ, 1995b; Tremblay, 1996.
1994 Northridge Earthquake
1995 Kobe Earthquake
• 60 fatalities (20 due to building
collapse – 4 wood buildings)
• 1,044 hospitalized injuries
• 11,088 displaced households
• 14,500 Yellow/Red tagged buildings:
• Less than 1% of buildings with Red
Tags in strongest (MMI IX) shaking
• $26 - $40 billion of total direct
economic loss (1994 dollars)
• $18.5 - $25 billion of building-related
economic loss (1994 dollars)
6,340 fatalities (most due to collapse
of smaller buildings)
25,000 serious injuries
300,000 homeless
150,000 collapsed/destroyed bldgs:
More than 20% of buildings
collapsed within 5 km of fault rupture
$100 -$200 billion of total direct
economic loss (1995 dollars)
$80 - $150 billion of building-related
economic loss (1995 dollars)
21
1994 M6.7 Northridge Earthquake
Single-Family Dwelling (SFD) Damage – Wood Frame Buildings
22
1994 M6.7 Northridge Earthquake
Multi-Family Dwelling (MFD) Damage – Wood Frame Buildings
Woodland Hills
Apartment Complex
Older 2-story home
with cripple walls
(not in the study)
Post-1960 2-story home
Northridge Meadows
Apartment Complex
23
24
6
1995 M6.8 Kobe Earthquake
Severe (Heavy) Damage and Collapse of Short-Period Buildings
Observations of Collapse from Shake Table Testing
Examples of Large (> 10%) Drift Displacement Capacity
Severe damage (incipient collapse)
of a 2-story mixed-use building
1-story reinforced masonry
coupled T-wall test structure
(UC, San Diego)
Modern Japanese 2-story wood
house (E-Defense, Miki, Japan)
Collapse of a 2-story house with
weak walls and heavy clay-tile roof
25
Observed Performance – Key Findings for Modern
Short-Period Buildings
Collapse Probability Quantified with Red-Tag Data
Post-1960 Wood Buildings - 1994 Northridge Earthquake
Collapse Probability or Red Tag Percentage
3.0%
2.5%
 First-story collapse failure mode
MCER Ground
Motion Intensity
Red Tag % - 186 Post-60 Census Tracts
Red Tag % - 22 Equal Count Groups
Best Fit (MLE) of Red Tag % (1st 21 Grps)
P[Col] - 'Newer' W1 Bldgs. (FEMA P-155)
 Large lateral displacements at incipient collapse.
Example Statistic
Census Tract 115200
Buildings
Count
w/Red Tag
23
Total
905
2.0%
26
 Low probability of collapse for MCER ground motions (where
SMS ≤ 1.5g)
1.5%
 Benchmark collapse probabilities derived from Red-Tag data:
 1-story buildings - P[Collapse|SMS = 1.5g] ≤ 2 percent
1.0%
 ≥ 2-story buildings - P[Collapse|SMS = 1.5g] ≤ 5 percent
0.5%
0.0%
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.3-Second Response Spectral Acceleration (g)
27
28
7
Three Seismic Force Resisting Systems (SFRSs)
1. Wood light-frame (aka Wood)
2. Special reinforced masonry
Archetypes, Parametric Studies
and Findings
2-story wood light-frame
MFD
3. Steel SCBF
Representative short-period (T ≤ 0.5 sec)
building archetypes of each SFRS:
2-story special reinforced
masonry COM
 Modern code-compliant construction.
 Height (one-, two-, and four-story).
 Occupancy (COM, SFD and MFD).
2-story steel SCBF COM
 Design level (High, Very High Seismic)
29
Two Seismic Ground Motion Design Levels
 High Seismic archetypes:
 Very High Seismic archetypes:
 Short-period MCER spectral response
acceleration adjusted for site class effects
SMT = SMS = 2.25g.
 Not required by FEMA P-695.
 Used in to investigate collapse
performance under 1.5 x MCER ground
motions that could occur in areas of very
high seismicity (e.g. sites located close to
fault rupture).
Parametric Studies of Building Archetypes
2.0
Spectral Acceleration (g)
 Short-period MCER spectral response
acceleration adjusted for site class effects
SMT = SMS = 1.5g.
 SDC Dmax per Table 5-1A of FEMA P-695.
1.8
MCE SDC D (maximum)
1.6
MCE SDC D (min) or SDC C (max)
1.4
MCE SDC C (min) or SDC B (max)
1.2
MCE SDC B (minimum)
1.0
0.8
0.6
0.4
0.2
0.0
0
0.5
1
1.5
30
2
2.5
3
3.5
4
Period (seconds)
Figure 5-2 FEMA P-695
 Objective
Investigate the effects of
variations of archetype
configuration and
modeling parameters on
the collapse potential of
each of the three SFRSs.
 Nine Parametric Studies:
 Some common to various
SRFSs.
 Some for specific SFRS.
31
32
8
Key Findings of High Seismic Baseline Studies Generically
Applicable to all Three SFRSs
 Numerical models representing best-estimate response
behavior and collapse performance.
 Failure mechanism of all high seismic baseline
archetypes characterized by P-Delta failure at
large displacement of the first story.
 Important to consider P-Delta effects in numerical
models, particular for taller archetypes.
 Baseline archetypes incorporated all elements of real buildings
 e.g. nonstructural wall finishes for wood building archetypes.
33
40%
 Common Failure Mechanism
30%
 Improved Collapse Performance
 In general, the MCER collapse probabilities of onestory and two-story wood light-frame, special
reinforced masonry and steel SCBF archetypes of
this project comply with benchmark metrics.
 Exception for COM wood light-frame archetypes for
which MCER collapse probabilities are much greater
than benchmark values.
 Directly related to relatively low strengths of
these archetypes.
Wood MFD (1, 2, 4-story)
Wood SFD (1 and 2-story)
Masonry COM (1, 2, 4-story)
Steel COM (1, 2, 4-story)
35%
MCER Collapse Probablity
 Investigated variation in response behavior and collapse
performance of representative archetypes of three short-period
SFRs.
25%
20%
4-story
15%
ASCE/SEI 7-16
10%
2-story
5%
≥ 2-story BM
1-story
1-Story BM
0%
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Period, T (seconds)
40%
Wood High h/b (1 to 5-story)
2-story
35%
Wood Low h/b (1, 2, 3-story)
Masonry HG (1, 2, 4-story)
30%
MCER Collapse Probablity
Baseline Configuration Parametric Study
Steel (2-story and 3-story)
25%
2-story
20%
15%
1-story
4-story
2-story
10%
5-story
ASCE/SEI 7-16
3-story
4-story
5%
3-story
≥ 2-story BM
1-Story BM
0%
0
0.1
0.2
0.3
0.4
0.5
0.6
Period, T (seconds)
0.7
34
Key Findings of Very High Seismic Baseline Studies
Generically Applicable to all Three SFRSs
Key Findings of High Seismic Baseline Studies Generically
Applicable to all Three SFRSs
 Key findings for High Seismic archetypes apply also to archetypes
designed for Very High Seismic (e.g., same failure mechanism)
 Strength (Vmax/W)
 Strength found to be the most
important parameter influencing
collapse performance of the high
seismic baseline archetypes.
 Collapse performance is worse for an archetype designed and
evaluated for Very High Seismic loads than that of the same
archetype designed and evaluated for High Seismic loads.
 Consistent trend in MCER
collapse probability with strength
for all archetypes of this project
and, in general, those of prior
FEMA P-695 studies
 Seismic loads for Very High Seismic regions are 50% higher than those for
High Seismic regions.
 Actual strength for a Very High Seismic archetype is typically less than 50%
greater than that of same archetype designed for High Seismic loads (e.g.,
less over-strength).
 “…the stronger the
archetype the better the
collapse performance…”
 Reasons for differences in over-strength between Very High Seismic and
High Seismic archetypes are different for each SFRS.
35
36
9
Reasons for Differences in Over-Strength of Very High
Seismic and High Seismic Archetypes
Collapse Displacement Capacity Parametric Study
 Wood light-frame baseline archetypes:
 Only structural wood shear designed for 50 percent greater strength.
 Nonstructural walls (interior gypsum and exterior stucco) - similar for
high seismic and very high seismic archetypes, hence combined strength
of structural and nonstructural walls of very high seismic archetypes less
than 50 percent higher than that of corresponding high seismic
archetypes.
 Special reinforced masonry baseline archetypes:
 Wall cross-sectional area – More efficient designs for higher loads
 Investigated effects of collapse displacement capacity on response
behavior and collapse performance.
 Motivated by results of shake table and pull tests of full-scale structures
showing collapse displacements at story drift of 10% or greater without
loss of stability.
 Significantly greater than collapse displacement capacity of nonlinear
models of prior FEMA P-695 studies.
 Variations in the collapse displacement capacity of numerical models of
archetypes were represented by different modeling assumptions of postcapping residual strength.
 Steel SCBF baseline archetypes:
 Brace Section – More efficient designs for higher loads
37
38
Modeling of Collapse Displacement Capacity
Modeling of Collapse Displacement Capacity
 Wood light-frame archetypes:
 Special reinforced masonry
archetypes:
 Displacement capacity expressed in
terms of a residual strength based on
a percentage of model peak strength.
 Displacement capacity
expressed by slope of postpeak base shear – vs. – story
drift curves.
 Affects more negatively collapse
performance of weaker models (due
to P-Delta effect).
 In reality, residual strength ratio of
a weaker model should be larger
than that of a stronger model.
 Lower collapse probabilities with
increased collapse displacement
capacity or increased post-capping
residual strength.
Residual Strength
Plateau
39
 Properties of detailed and
simplified nonlinear models
emulate post-peak behavior
 Strong trend of lower collapse
probabilities with increased
collapse displacement
capacity.
40
10
Soil-Structure Interaction (SSI) and Foundation
Flexibility Parametric Study:
Key Findings of Collapse Displacement Capacity
Parametric Studies
 Investigated SSI inertial and kinematic effects and foundation flexibility for
two soil conditions (stiff and soft sites) on response behavior and collapse
performance.
 SSI inertial effects modeled with a distributed set of discrete nonlinear soil
springs and dashpots below flexible foundation elements.
 Kinematic SSI effects were evaluated by modifying frequency content of
ground motion records (filtered records) used for response history
analysis.
 Response and collapse results compared with those of corresponding
archetype models on a rigid foundation excited by using unfiltered
records.
 Displacement capacity at point of incipient collapse found to be
the second most important parameter influencing collapse
performance of wood light frame and special reinforced
masonry high seismic baseline archetypes.
 Why? – For wood, displacement capacity resists the effects
of P-Delta on side-sway collapse of the archetype model
 Specific value of displacement capacity at point of incipient
collapse to incorporate in numerical models of a given building
archetype is not well understood.
 Root Cause - Lack of test data at very large displacements
41
42
Key Findings of SSI and Foundation Flexibility
Parametric Studies
 In general, modeling of SSI and
foundation flexibility is not required for
accurate calculation of short-period
building response and collapse
performance.
Generic Collapse Performance
 Exception: Rocking of spread footing
below single-bay of steel SCBF
 Results suggest that reduction in
design base shear of short-period
buildings due to SSI and foundation
flexibility included in ASCE/SEI 7 is not
justified except for unusual cases such
as buildings with very large plan
dimensions and/or deep foundations.
43
44
11
Short List of Prior Analytical Studies of Bilinear (SDOF) Models
Equal Energy Criterion of an Elasto-Plastic SDOF Model
Force
 Veletsos and Newmark, 1960, 2WCEE
 ”Effect of Inelastic Behavior on the Response of Simple Systems to
Earthquake Motions”
FE
 Miranda and Bertero, 1994, Earthquake Spectra, EERI
 “Evaluation of Strength Reduction Factors for Earthquake-Resistant
Design”
Equal areas according to
equal energy criterion
 Improvement of Nonlinear Static Seismic Analysis Procedures, FEMA
440, June 2005
FI
 Effects of Strength and Stiffness Degradation on Seismic Response,
FEMA 440A, June 2009
 Tentative Framework for Development of Advanced Seismic Design
Criteria for New Buildings, NIST GCR 12-917-20. NIST, 2012, Section 3.1
“Study of Short-Period Systems”
45
Comparison of Strength Reduction Factors from Various Studies
dY
Areas common to both
elastic and inelastic responses
dE
dI
Displacement
Concepts by Veletsos and Newmark, 1960, 2WCEE
Artwork by Andre Filiatrault, 2020, FEMA P-2139-1)
46
FEMA P-695 collapse results for bilinear SDOF systems with collapse
displacement capacity 10 times yield displacement (NIST GCR 12-917-20)
(Figure 8, “Evaluation of Strength Reduction Factors for Earthquake-Resistant Design”,
Miranda and Bertero, 1994, Earthquake Spectra)
47
48
12
Comparison of Results of the SDOF study of FEMA P-2139-1
with those of Prior SDOF Studies of NIST GCR 12-917-20
Collapse Criteria of the SDOF Study
 Analysis methods and models:
 Same set of earthquake records and IDA methods
 Similar SDOF models
 Collapse displacement criteria:
 Similar ductility-based limits
 Different drift-based limits - 1st-story failure
 Different drift-based limits – multi-story failure
“1-Story” Models
 Peak inelastic response results:
 Same trends and similar values
“4-Story” Models
 Collapse results (e.g., ACMR/ACMR10%):
 Ductility-based – essentially the same trends and values
 Drift-based – very different trends and values
49
Plots of ACMR and the ratio of ACMR/ACMR10% as a
function of the model period of SDOF models (V = 0.4W)
Ductility-Based Collapse ( = 8.0)
Drift-Based Collapse (7.5 Percent)
Elastic
Period
of
SDOF
Model
(sec.)
Yield
Displ.
(in.)
Collapse
Displ.
(in.)
Yield
Displ.
(in.)
Collapse
Displ.
(in.)
Yield
Displ.
(in.)
Collapse
Displ.
(in.)
Model
Height
(ft.)
Collapse
Displ.
(in.)
Vmax = 0.2W
Vmax = 0.4W
Vmax = 0.8W
1st-Story Failure1
Multi-Story Failure2
Model Collapse
Height3
Displ.
(ft.)
(in.)
0.1
0.02
0.16
0.04
0.31
0.08
0.63
10
9.0
10
9.0
0.15
0.04
0.35
0.09
0.71
0.18
1.41
10
9.0
10
9.0
0.20
0.08
0.63
0.16
1.25
0.31
2.5
10
9.0
10
9.0
0.25
0.12
0.98
0.25
1.96
0.49
3.9
10
9.0
12.3
11.1
0.30
0.18
1.41
0.35
2.8
0.71
5.6
10
9.0
15.7
14.2
0.35
0.24
1.92
0.48
3.8
0.96
7.7
10
9.0
19.3
17.4
0.40
0.31
2.5
0.63
5.0
1.25
10.0
10
9.0
23.1
20.8
0.45
0.40
3.2
0.79
6.4
1.59
12.7
10
9.0
27.0
24.3
0.50
0.49
3.9
0.98
7.8
1.96
15.7
10
9.0
31.1
28.0
1. 1st-story failure assumes that all inelastic story drift occurs at the 1st-story of a multi-story building represented by the
SDOF model, where the height of the 1st-story is H = 10 feet.
2. Multi-story failure assumes that inelastic story drift is uniformly distributed equal over the height of a multi-story building.
3. Model height, H, represents the effective height which is assumed to be 2/3 of total height (hn) of a multi-story building,
where the total height (hn) is calculated from the elastic period (T) of the model using the approximate fundamental period
formula of Section 12.8.2.1 of ASCE 7-16, i.e., H = 2/3 exp[ln(T/1.4(0.02))/0.75].
50
Summary of SDOF Model Comparisons
 Provide a theoretical basis for understanding why:
 short-period buildings have been observed to perform well in
past earthquakes and
 improved numerical models of this project have found better
collapse performance of archetypes with shorter periods.
 But .....
 Collapse performance of theoretical bilinear elasto-plastic
SDOF models may not be realistic representations of the
collapse performance of the SFRS of interest.
Ductility-based trend
same as Fig. 3-1 NIST
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52
13
Plots of notional collapse surfaces for SDOF models with
periods, T = 0.15 s and T = 0.45 s, assuming 1st-story failure
Notional Collapse Surfaces
 During the course of the ATC-116 project, the concept of a
“collapse surface” was developed as a valuable tool to describe the
interaction of primary building response properties affecting
collapse performance of structures.
 Data from the generic collapse performance investigation of SDOF
models was used to develop a series of notional collapse surfaces.
 Notional collapse surfaces (based on SDOF models) are
conceptual and collapse surfaces of a given SFRS would need to
be developed from collapse (ACMR) results of FEMA P-695
analyses (IDAs) of the SFRS of interest.
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54
Collapse Surface Utilization
 Collapse surfaces describe the amplitude of a collapse metric (e.g.,
ACMR) as a function of key building response properties, i.e.,
strength, displacement capacity, elastic period and failure mode,
that significantly influence the collapse performance of the system
of interest.
Paradox Solved
 For the system of interest, collapse surfaces could be used to
either:
 Estimate collapse performance given values of building
response properties, or, conversely
 Estimate the value of a building response property (e.g.,
strength) required to achieve a specific collapse objective.
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56
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The Short-Period Building Performance Paradox
Resolution of the Short-Period Paradox
 Observed good performance:
Why do analytical models of code compliant designs
predict high probabilities of collapse for short-period
buildings contrary to damage observed in actual
earthquakes and the judgement of earthquake
engineers?
“Show me the bodies”
 Very few collapses of modern short-period buildings (e.g., evidenced
by only 109 U.S. building-related earthquake fatalities in the last 50
plus years)
 Collapse trends reversed due to:
 Representative (realistic) archetypes
 Improved numerical models
 Theoretical insight (gained from the SDOF study)
Bill Holmes
 MCER collapse probabilities (low)
 Collapse performance of improved nonlinear models of representative
archetypes is generally consistent with observed good performance
57
58
Recommendations for Improving Seismic Design
Recommendations for
Improving Seismic Design
For Improved Seismic
Design Codes
 Intended primarily for
seismic-code-development
committees
59
60
15
Recommendations for Improving Seismic Design
Recommendations for Improving Seismic Design
For advanced
seismic design and
analysis practice
For enhanced modeling,
testing and data
collection
 Intended primarily for
engineering
practitioners
 Intended primarily for
research engineers and
academicians
Figures: FEMA P-2139-3
61
Figures: FEMA P-2139-3
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Seismic Design Codes and Standards
Seismic Design
Codes and Standards
 Develop performance-based design criteria (collapse
surfaces) to inform development of seismic design
63
Figures: FEMA P-2139-1
Update: ATC-154
Project is currently
exploring
64
16
Seismic Design Codes and Standards
Seismic Design Codes and Standards
 Consider actual strength and deformation capacity
 Recognize that future refinements in
design and construction can affect
undesigned overstrength and therefore
seismic performance
Figures: PEER 2020/20
65
Figures: FEMA P-2139-3, FEMA P-2139-4, CUREE W-29
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Seismic Design Codes and Standards
 Review current ASCE 7 checks for deformation compatibility of
Non-SFRS components
 Address increased collapse potential with very high seismic demand
Very
High
29.0
%
Very
High
29.0
%
Update: ATC-154
Project is currently
exploring
Very
High
26.0
%
R
COL I MCE ]
P [COL IP [ MCER]
Seismic Design Codes and Standards
Very
High
High
High
7.3%
Very
High
15.7
%
High
19.0
%
13.4
%
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17.2
%
High
1.0%
Figures: FEMA P-2139-3
High
MODEL
COM1 B
COM2B
COM3B
MODEL
COM4 B
COM5B
COM6B
COMMERCIAL
5.5%
1.8%
MFD1B
MFD2B
MFD3B
MFD4B
MFD5B
MFD6B
MULTI-FAMILY RESIDENTIAL
Figure: Based on FEMA P-2139-2 Table 6-1
Very
High
7.9%
Very
High
8.4%
2.6%
SFD1B
SFD2B
SFD3B
SFD4B
SINGLE-FAMILY RESIDENTIAL
68
17
Seismic Design Codes and Standards
Seismic Design Codes and Standards
 Undertake research to address targeted collapse probability:
 Undertake research to address targeted collapse probability:
1. Should the target for conditional probability of collapse be revised
to be more in line with empirical experience? If so, what changes
would be needed in the FEMA P-695 methodology to
accommodate the revised target collapse probabilities?
2. Should the methodologies for estimating collapse probability be
revised to explicitly consider strength from elements not a formal
part of the seismic-force-resisting system, and, if yes, how would
their contribution be controlled in design?
3. Should the target conditional probability of collapse be different for
very high-seismic (i.e., near fault) sites than for other sites? If no,
are more rigorous design rules needed for very high-seismic sites
in order to achieve the target conditional probability of collapse?
4. Do the observations identified in item 4 and item 5 above hold true
for longer-period structures?
Figures: FEMA P-2139-1
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Figures: FEMA P-2139-1
Seismic Design Codes and Standards
Seismic Design Codes and Standards
 Update FEMA P695 seismic criteria including resolution of the
following discrepancies:
 Revisit current ASCE 7
provisions for soilstructure interaction to
determine whether
current SSI reductions
in design forces for
short-period structures
lead to worse collapse
performance
 The seismic criteria of FEMA P695 are about 90% of those in ASCE 722 for short-period buildings and 60% of those in ASCE 7-22 for the
velocity domain
 The seismic criteria of FEMA P-695 ignore higher levels of ground
shaking typical of site closer to major faults, thereby implicitly permitting
probabilities of collapse in excess of 10%,which is at odds with the
stated intent of NEHRP and ASCE 7 seismic design provisions
Figures: FEMA P-2139-1
71
Figures: FEMA P-2139-3
70
72
18
Seismic Design Codes and Standards
 Revisit current ASCE 7 provisions for soil-structure interaction to
determine whether current SSI reductions in design forces for shortperiod structures lead to worse collapse performance
Figures: FEMA P-2139-3, Figure 5-12
Advanced Seismic
Design and Analysis
73
Advanced Seismic Design and Analysis
74
Advanced Seismic Design and Analysis
 Evaluate collapse safety issues associated with foundation rocking
occurring prior to development of mechanisms in vertical elements
 Recognize overstrength not included in seismic design can create
higher that anticipated element seismic forces, inconsistent with
building code capacity design principals.
A SHEARWALL STRONGER
THAN NECESSARY MAY
LEAD TO NEAR=ELASTIC
FORCES, INCREASE LOADS
TO DIAPHRAGMS AND
THEIR CONNECTIONS
Figures: FEMA P-2139-3
75
76
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Enhanced Modeling, Testing and Data Collection
Enhanced Modeling, Testing,
and Data Collection
 Develop cyclic testing protocol up to and including incipient collapse
or collapse
77
Figures: FEMA P-2139-3
Enhanced Modeling, Testing and Data Collection
Enhanced Modeling, Testing and Data Collection
 Emphasize shake table testing to large displacements including
insipient collapse, as well as bi-directional ground motions
 Emphasize testing of assemblies with realistic boundary conditions
over component testing
Figure:
Ron Gallagher
79
Figures: PEER 2020/20
78
80
20
Enhanced Modeling, Testing and Data Collection
Closing & Thank you
 Put plans in place for rapid and thorough collection of post EQ data
PDH certificates
Provided for participants of live webinar (not the
recording)
Distributed via email within four weeks
Q&A
Distributed via email within four weeks
Figures: FEMA P-2139-1
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