www.sgh.com
Project 17 Development of Next-Generation Seismic Design Value Maps
Geo Structures 2016 16 February 2016 R.O. Hamburger, SE, SECB
Some History
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• Prior to 1988 - maps provided broad seismic zones • Earthquakes frequently happen
– Bad things happen • Moderate shaking occurs from time to time
– Damage can occur • Can feel an earthquake
– Take minimal precautions • Don’t worry – be happy
Some History
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• Maps were not directly tied to particular ground motion • Maps were based mostly on historic record of where
earthquakes had been felt • Maps changed very little from code edition to code edition
Some History
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• Life (and design) was simple • We didn’t really know what we were doing • It seemed to work reasonably well, most of the time • Sometimes (when we had an earthquake) it didn’t work so
well
ATC 3-06
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• Introduced separate mapping of “short period” and “long period” motion
• Identified “Design Motion” as 475-year return period (10% - 50 year exceedance)
• Identified worst Zone 4 motions as – 0.4g pga – 1.0g short period response – 0.4g Sa(1 second)
• Aa and Av maps were essentially identical – UBC stayed with single zone map from 1988-1994
Aa Av
ATC 3-06
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• Life remained simple • Things worked better (most of the time) • Strong motion instruments proliferated in California
– Some records were really strong – These seemed to occur near the zones of fault rupture – The worst damage seemed to be generally near these records
1997 UBC
• Life was not so simple • Code-specified ground
motion became very site specific
• Special criteria for “near fault” design
• Significant gradation in design values with small change in location
• Stability?
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• Separate Ss and S1 maps • Maps at MCE level
– 2475 years (2%- 50 year exceedance)
• Design Level taken as 2/3 of Site-adjusted MCE values
• Detailing criteria (Seismic Design Categories) based on site-adjusted values
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Ss
S1
2000 NEHRP, ASCE 7-02, 7-05
Maximum Considered Earthquake Shaking ASCE 7.05
• MCE motion is 2%/50 year unless – 2%/50 year is > 150% 1997 UBC Zone 4
• Use greater of 150% of deterministic motion for maximum magnitude event on controlling fault, but not less than 150% 1997 UBC Zone 4 – 150% deterministic approximately represents
mean + 1σ
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Maj
or
Faul
t Distance
Probabilistic Motion @2%-50 years Deterministic Motion
from Characteristic Event
150% of 1997 UBC Zone 4 no near field
Seismic Hazard Process
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Magnitude Recurrence
USGS
CDMG
MWD
Attenuation
Sadigh
Campbell & Bozorgnia
Idriss
Abrahamson & Silva
50% 25% 25%
15% 35% 35% 15%
Final hazard is determined as sum over all faults, all magnitude recurrence relationships, all attenuation relationships (each with “proper” weighting)
• New maps impossible to read – USGS developed web-based
“applet”
• Significant variation in ground motion intensity with location
• Mapped values started to “fluctuate” from edition to edition
• Detailing requirements within a region highly variable
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Ss
S1
2000 NEHRP, ASCE 7-02, 7-05
ASCE 7-10 • Use of PEER NGA Ground Motion Prediction Models to
develop national seismic hazard maps • Use of “maximum direction” component, rather than
geomean as definition of seismic hazard • Conversion from “uniform hazard” basis to “uniform risk”
basis – Rather than 2%-50 year motion, 1%-50 year collapse risk – Return period for MCE shaking is somewhat different at every site – Generally, the return period is less than 2%-50 years
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PEER NGA1 – Major Findings
• Ground motions produced by very large magnitude earthquakes on large active faults, e.g. San Andreas, Hayward produce much less severe ground motions than previously thought – Past activity has “worn out” the roughness on these faults – Soils go nonlinear and are unable to transmit very large motions
• Hazard is generally not as high as previously thought in places like San Francisco & Oakland
• Uncertainty is larger than previously thought – One Standard deviation above mean – 1.5
1.8
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Comparison of NGA & Older Relationships
• Ground motions predicted by PEER NGA relationships in much of the Western U.S. were 70% or less than those underlying the ASCE 7-05 maps
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𝑆𝑆1−2008𝑆𝑆1−2002
2%-50 years
Ground Motion Directionality
• Typical ground motion recording includes – X component – Y component
oriented at 90o
• Ground Motion Prediction Models use “geomean”
• For this motion: X=0.28g, Y=0.5g, GM=0.37g
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𝑆𝑆𝑎𝑎−𝑔𝑔𝑔𝑔 = 𝑆𝑆𝑎𝑎−𝑋𝑋 𝑆𝑆𝑎𝑎−𝑌𝑌
Structural engineers on the committee felt GM had no particular relevance and felt more comfortable designing for the maximum component
Max Direction v. Geomean
• Max/geomean ratios based on: – Huang, Whittaker & Luco, 2008
• Large magnitude events M>6.5
• ASCE 7-10 uses: – Ss – max = 1.1 geomean – S1 – max = 1.3 geomean
• Note that ratio of 84th percentile to median is >1.5 – Deterministic event now taken
as 84th percentile, rather than 150% of median
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Ratio of Maximum to Geomean
2.0/1.1=1.8>1.5 2.3/1.3=1.8>1.5
Uniform Hazard v. Uniform Risk
• Under ASCE 7.05, design values in Memphis, TN San Francisco, CA and Los Angeles, CA are similar – Yet in past 200 years
• S.F. has experienced at least 5 significant earthquakes (1836, 1868, 1906, 1957, 1989)
• LA has experienced at least 8 significant earthquakes (1857, 1933, 1952, 1971, 1979, 1987, 1993, 1994)
• Memphis has experienced only one series of events (all in 1811-1812)
– Engineers in the Memphis region complained that it did not make sense given this experience that the design requirements were the same
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Seismic Risk = Risk of Collapse
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𝑃𝑃 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = # of collapses per yr = 𝑃𝑃 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑆𝑆𝑐𝑐 𝑇𝑇 )𝑃𝑃(𝑆𝑆𝑎𝑎(𝑇𝑇)𝑑𝑑𝜆𝜆𝑆𝑆𝑎𝑎 𝑇𝑇 =∞
𝑆𝑆𝑎𝑎 𝑇𝑇 =0
Hazard Fragility
0.001 annual probability
1.5g 1.5g
30% probability of collapse
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝑐𝑐𝑐𝑐 𝐶𝐶𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑐𝑐𝑎𝑎 1.5𝑔𝑔 =.001𝑦𝑦𝑐𝑐𝑐𝑐𝑦𝑦 ∗ 0.3𝑐𝑐𝑦𝑦𝑐𝑐𝑝𝑝 𝑔𝑔𝑔𝑔𝑔𝑔𝑐𝑐𝐴𝐴 1.5𝑔𝑔
= 0.0003/𝑦𝑦𝑐𝑐𝑐𝑐𝑦𝑦
Uniform Hazard v. Uniform Risk
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• Memphis hazard has much shallower slope (big earthquakes occur less often)
• therefore, although 2%-50 year motion is similar, risk is much less
Hazard Curve Comparison
Collapse Fragility Determination • Procedure outlined in
FEMA P695 – Developed to permit
establishment of building seismic performance factors:
• R • Cd
• Ωo
for new structural systems
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“Standard Structural Fragility”
• USGS integrates standard structural fragility with hazard curve to obtain Risk Coefficients
• Risk Coefficient x 2,500 year motion is new MCE motion
𝑃𝑃 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑆𝑆𝑎𝑎 𝑇𝑇
MCE Sa-MCE(T) Uncertainty (β) taken as value of 0.6
Risk Coefficient Maps
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CRS ranges from 0.7 in eastern U.S . to 1.1 in the western U.S.
CR1 ranges from 0.8 in eastern U.S. to 1.1 in the western U.S.
Resulting Maps – MCER
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New maps look like old maps but at a given site, the motion may either be larger or smaller, typically within a range of about 0.7 t o1.1
NEHRP 2014 (ASCE 7-16)
• USGS produced updated set of maps – Updated fault catalog
• New fault segmentation • New magnitude recurrence defintions
– Updated GMPEs (attenuation relationships) • NGA2
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Project 17
• Joint BSSC – USGS project to develop consensus between earth science and geotechnical communities as to basis for maps in ASCE 7-22
• Initiated in February 2013 • Will complete mid-year 2018 with publication of
preliminary maps for ASCE 7-22
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Major Issues
• Precision vs. Uncertainty • Multi-period spectra • Acceptable Collapse Risk • Use and definition of deterministic caps
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Precision v Uncertainty - 1994
• UBC Zone values were clearly imprecise • Engineers understood these as “design” values • Site specific study would provide different values • Engineers didn’t worry about the imprecision • Advancements in earth science did not effect values
enough to change design values from year to year 30
Z=0.4 Z=0.3 Z=0.2
Precision vs. Uncertainty - 2015
• Small changes in earth science mean radical change in contours • Maps change significantly every cycle • Changes are not statistically significant
Precision v Uncertainty – Possible resolution
• 3-tier procedure for ground motion determination • Tier 1
– Zonation (or contours with coarse gradation) – Applicable to ELF and RSA for modest period structures – Used for determination of detailing
• Tier 2 – Default site specific values – Determined using USGS applet – Applicable for any structure
• Tier 3 – Geotech-performed site specific – Applicable for any structure – Limited to % of Tier 1 or Tier 2 values
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Multi-Period Spectra
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Acceleration Domain
Velocity Domain
Displacement Domain
SDS = 2/3 x SMS = 2/3 x Fa x Ss
TS = SD1/SDS
SD1 = 2/3 x SM1 = 2/3 x Fv x S1
Cs = SDS/(R/Ie)T ≤ Ts
Cs = SD1/T(R/Ie)Ts < T ≤ TL
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0.1 1.0 10.0
Spec
tral A
ccel
erat
ion
(g)
Period (seconds)
84th percentile response spectra of an M8.0, strike-slip, earthquake at R = 5 km for Site Class A (1,520 mps), B (760 mps - Ss = 1.84g, S1 = 0.77g), C (530 mps), D (260 mps) and E (130 mps)
site conditions (2008 NGA relations)
A - Vs,30 = 1,520 mps
B - Vs,30 = 760 mps
C - Vs,30 = 530 mps
D - Vs,30 = 260 mps
E - Vs,30 = 130 mps
Multi-period Spectra
• ASCE 7-16 will require site specific study for all structures with T>1.5 sec on soft soil sites (Site Class, D, E, F)
• Proposal for ASCE 7-22 – USGS Tier 2 “maps” will provide Sa(T) at multi-periods – 0.2, 0.3, 0.5, 0.7, 1.0, 1.5, 2.0, 2.5, 3. 3.5, 4, 4.5 5 seconds – Computation will include site class effects
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Acceptable Collapse Risk
• FEMA P-695 over-estimates the actual collapse risk for real buildings, possibly by a large amount
• Due to deterministic caps, collapse risk achieved by current code is not uniform
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• Return to uniform hazard (rather than uniform collapse risk)
• Accept higher risk (eliminate deterministic cap zones) • More carefully explain in commentary what we believe
real collapse risk is
Summary
• Seismic design value maps in the building codes have become more precise and more complex over the years
• While the scientific basis for the maps has improved, the average engineer’s understanding of them has decreased
• While the design values have become more precise, they are not necessarily more accurate
• Claims of “uniform risk” underlying the maps are false • Continuing advancement of the “science” threatens to
worsen the situation
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Summary
• USGS and BSSC are aware of the issues and working to develop consensus on an appropriate approach: – Take advantage of improved scientific understanding and
knowledge – Convey design values in a manner appropriate to their computed
accuracy – Provide a more stable definition of design values
• This will all come too late for ASCE 7-16 • Look for improvements in ASCE 7-22
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