Should Design Spectra in Building Codes Be Specified from the Maximum Component or the Average Horizontal?

Size: px

Start display at page:

Download "Should Design Spectra in Building Codes Be Specified from the Maximum Component or the Average Horizontal?"

Hannah Stokes
7 years ago
Views:

1 Should Design Spectra in Building Codes Be Specified from the Maximum Component or the Average Horizontal? Jonathan P. Stewart University of California, Los Angeles LATBSDC Meeting May 7, 2010

2 Outline Problem definition Background on the code change Directionality in structural response Logic behind bias concern Technical summary Where to go from here? References

3 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) Basis for most pre-nga GMPEs H 2 H 1 GM 1

4 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 2

5 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 3

6 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 4

7 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 5

8 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GMRotD50=Median(GM i ) Find single rotation angle with GM closest to GMRotD50 Basis for NGA GMPEs Values similar to GM GM 6 Boore et al., 2006

9 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 Max Direction (MD) Used in 2009 code Median converted from GMRotI50 Sigma (σ) larger than for GM or GMRotI50 Huang et al., 2008, 2009

10 Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 Max Direction (MD) Arbitrary Component Larger σthan for GM or GMRotI50 Huang et al., 2008, 2009

11 Problem Definition This presentation: Was the change to MD appropriate? Could it bias the hazard levels intended for application in design?

12 Background on the Code Change Change to MD is part of broad revisions Update to NGA GMPEs Adoption of risk targeted ground motions Change to MD from GM Full disclosure I opposed the change because of MD issue Co-authored EERI letter justifying institutional No vote to NEHRP Provisions Update Comm.

13 Directionality in Structural Response Azimuth-independent response Flagpole

14 Directionality in Structural Response Azimuth-independent Flagpole Buildings with similar str/stiffness in both directions --? Christovasilis et al., 2009

15 Directionality in Structural Response Azimuth-independent Flagpole Buildings with similar str/stiffness in both directions --? Christovasilis et al., 2009

16 Directionality in Structural Response Azimuth-independent Flagpole Buildings with similar str/stiffness in both directions --? Can conceptualize MD controlling collapse, but Research has not demonstrated this

17 Directionality in Structural Response Azimuth-independent Azimuth-dependent response

18 Directionality in Structural Response Azimuth-independent Azimuth-dependent

19 Directionality in Structural Response Azimuth-independent Azimuth-dependent Both directions contribute, but Out of phase Modal combination (Sec ASCE7) nisee.berkeley.edu Transv: T % = 0.74sec Long.: T % = 1.25sec Stewart et al., 1999

20 Directionality in Structural Response Azimuth-independent Azimuth-dependent Azimuth-specific response requires directional ground motions

21 Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km Watson-Lamprey et al., 2007

22 Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km 2D analysis: best to use arb. comp or GM (same µ, different σ) Baker and Cornell, 2006 Watson-Lamprey et al., 2007

23 Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km 2D analysis: best to use arb. comp or GM (same µ, different σ) USGS hazard maps use GMRotI50 ( GM)

24 Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km 2D analysis: best to use arb. comp or GM (same µ, different σ) USGS hazard maps use GMRotI50 ( GM) APE 2% APE in 50 yr Acceleration USGS

25 Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km 2D analysis: best to use arb. comp or GM (same µ, different σ) USGS hazard maps use GMRotI50 ( GM) If MD, using lower APE (return period ) APE Lower APE Acceleration NEHRP (Design maps)

26 Change of Median Beyer and Bommer (2006): MD/GM Beyer and Bommer, 2006

27 Change of Median Beyer and Bommer (2006): MD/GM Huang et al. (2008): Huang et al., 2008

28 Effect of NGA on USGS Maps

29 NEHRP Revision Process Procedure Technical Subcommittee drafts proposal PUC feedback Revision in Subcommittee PUC approval Member balloting Final edits and approval

30 NEHRP Revision Process Procedure Conservative process

31 NEHRP Revision Process Procedure Conservative process Peculiarities in this case: No technical basis for revisions PUC input ignored Member ballot objections ignored

32 NEHRP Revision Process Procedure Conservative process Peculiarities in this case The unwritten last bullet

33 Technical Summary MD motions can be rationalized for structures with equal stiff./str. in both directions Research needed to justify Rarely the case in real buildings

34 Technical Summary MD motions can be rationalized for structures with equal stiff./str. in both directions MD motions should bias the APE used in design for structures with azimuth-dependent props. If motivation was to avoid decreased motions from NGA, better to phase in adoption of NGA Bias concern is widely held and was presented to NEHRP PUC

35 Technical Summary MD motions can be rationalized for structures with equal stiff./str. in both directions MD motions should bias the APE used in design for structures with azimuth-dependent props. USGS maps do not account for additional uncertainty of MD Will lead to additional future increases of design spectra

36 Where to go from here? NEHRP Provisions and Commentary approved ASCE-7 updated approved IBC re-write in process IBC does not become code until adopted Adoption can occur with revision Inform leaders in design profession; seek input Document concerns Bring concerns to regulatory agencies

37 References Baker, JW and CA Cornell (2006). Which spectral acceleration are you using? Earthquake Spectra, 22(2), Beyer, K and JJ Bommer (2006). Relationships between median values and between aleatory variabilities for different definitions of the horizontal component of motion, Bull Seism Soc Am, 96(4a), Boore, DM, J Watson-Lamprey, and NA Abrahamson (2006). Orientation-independent measures of ground motion, Bull Seism Soc Am, 96(4a), Christovasilis, IP, A Filiatrault, MC Constantinou, and A Wanitkorkul (2009). Incremental dynamic analysis of woodframe buildings, Eq Engrg & Str Dyn, 38, Huang, YN, AS Whittaker, and N Luco (2008). Maximum spectral demands in the nearfault region, Earthquake Spectra, 24(1), Huang, YN, AS Whittaker, and N Luco (2009). Orientation of maximum spectral demand in the near-fault region, Earthquake Spectra, 25(3), Stewart, JP, RB Seed, and GL Fenves (1999). Seismic soil-structure interaction in buildings. II: Empirical findings, J. Geotech. & Geoenv. Engrg., 125 (1), Watson-Lamprey, J and DM Boore (2007). Beyond Sa GMRotI : Conversion to Sa Arb, Sa SN, and Sa MaxRot, Bull Seism Soc Am, 97 (5),

Design Example 1 Design Spectral Response Acceleration Parameters 11.4

Design Example 1 Design Spectral Response Acceleration Parameters 11.4 OVERVIEW For a given building site, the risk-targeted maximum considered earthquake spectral response accelerations S S, at short