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

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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 Assessment Process

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“S” 2 3 4 5 6 7

0.004

M 6.0 0.2g

“S”

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

Web-based Applet

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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

• Acceptable collapse risk currently set at 1% chance 50 years

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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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