Project 17 Development of Next-Generation Seismic Design Value Maps

Transcription

1 Project 17 Development of Next-Generation Seismic Design Value Maps Geo Structures February 2016 R.O. Hamburger, SE, SECB

2 Some History Prior to 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 2

3 Some History 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 3

4 Some History 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 4

5 ATC 3-06 A a A v 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

6 ATC 3-06 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 6

7 1997 UBC 7

8 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? 8

9 2000 NEHRP, ASCE 7-02, 7-05 Separate S s and S 1 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 S 1 S s 9

10 Maximum Considered Earthquake Shaking ASCE 7.05 Major Fault 150% of 1997 UBC Zone 4 no near field Deterministic Motion from Characteristic Event Probabilistic years 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 Distance 10

11 Seismic Hazard Assessment Process S g M 6.0 S 11

12 Seismic Hazard Process Magnitude Recurrence Attenuation 50% USGS Sadigh 15% 25% CDMG Campbell & Bozorgnia 35% 25% MWD Idriss 35% Abrahamson & Silva 15% Final hazard is determined as sum over all faults, all magnitude recurrence relationships, all attenuation relationships (each with proper weighting) 12

13 2000 NEHRP, ASCE 7-02, 7-05 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 S 1 S s 13

14 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 14

15 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

16 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 2%-50 years SS SS

17 Ground Motion Directionality Typical ground motion recording includes X component Y component oriented at 90 o Ground Motion Prediction Models use geomean SS aa gggg = SS aa XX SS aa YY For this motion: X=0.28g, Y=0.5g, GM=0.37g Structural engineers on the committee felt GM had no particular relevance and felt more comfortable designing for the maximum component 17

18 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 84 th percentile, rather than 150% of median Ratio of Maximum to Geomean 2.0/1.1=1.8> /1.3=1.8>1.5 18

19 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 ) Engineers in the Memphis region complained that it did not make sense given this experience that the design requirements were the same 19

20 Seismic Risk = Risk of Collapse PP cccccccccccccccc = # of collapses per yr = SSSS TT = SSaa TT =0 PP cccccccccccccccc SSSS TT )PP(SS aa (TT)ddλλ Fragility Hazard annual probability 30% probability of collapse 1.5g 1.5g AAAAAAAAAAAA CCCCCCCCCCCCCCCCCC aaaa 1.5gg =.001 yyyyyyyy = /yyyyyyyy 0.3pppppppp gggggggggg 1.5gg 20

21 Uniform Hazard v. Uniform Risk Hazard Curve Comparison Memphis hazard has much shallower slope (big earthquakes occur less often) therefore, although 2%-50 year motion is similar, risk is much less 21

22 Collapse Fragility Determination Procedure outlined in FEMA P695 Developed to permit establishment of building seismic performance factors: R C d Ω o for new structural systems 22

23 Standard Structural Fragility PP cccccccccccccccc SS aa TT MCE S a-mce (T) Uncertainty (β) taken as value of 0.6 USGS integrates standard structural fragility with hazard curve to obtain Risk Coefficients Risk Coefficient x 2,500 year motion is new MCE motion

24 Risk Coefficient Maps C RS ranges from 0.7 in eastern U.S. to 1.1 in the western U.S. C R1 ranges from 0.8 in eastern U.S. to 1.1 in the western U.S. 24

25 Resulting Maps MCE R 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 25

26 Web-based Applet 26

27 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 27

28 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

29 Major Issues Precision vs. Uncertainty Multi-period spectra Acceptable Collapse Risk Use and definition of deterministic caps 29

30 Precision v Uncertainty Z=0.4 Z=0.3 Z=0.2 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

31 Precision vs. Uncertainty Small changes in earth science mean radical change in contours Maps change significantly every cycle Changes are not statistically significant

32 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 32

33 Multi-Period Spectra S DS = 2/3 x S MS = 2/3 x F a x S s S D1 = 2/3 x S M1 = 2/3 x F v x S 1 C s = S DS /(R/I e ) C s = S D1/T(R/I e ) T T s T s < T T L Acceleration Domain T S = S D1 /S DS Velocity Domain Displacement Domain Spectral Acceleration (g) th 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 Period (seconds) 33

34 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, , 4, seconds Computation will include site class effects 34

35 Acceptable Collapse Risk Acceptable collapse risk currently set at 1% chance 50 years 35

36 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 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 36

37 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 37

38 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

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