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
Outline Problem definition Background on the code change Directionality in structural response Logic behind bias concern Technical summary Where to go from here? References
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
Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 2
Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 3
Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 4
Problem Definition Three-component ground motions ln(gm)=avg(lnh 1, lnh 2 ) GMRotI50 GM 5
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
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
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
Problem Definition This presentation: Was the change to MD appropriate? Could it bias the hazard levels intended for application in design?
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.
Directionality in Structural Response Azimuth-independent response Flagpole www.flagpolesetc.com
Directionality in Structural Response Azimuth-independent Flagpole Buildings with similar str/stiffness in both directions --? Christovasilis et al., 2009
Directionality in Structural Response Azimuth-independent Flagpole Buildings with similar str/stiffness in both directions --? Christovasilis et al., 2009
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
Directionality in Structural Response Azimuth-independent Azimuth-dependent response www.newtonconsultants.com
Directionality in Structural Response Azimuth-independent Azimuth-dependent
Directionality in Structural Response Azimuth-independent Azimuth-dependent Both directions contribute, but Out of phase Modal combination (Sec. 12.9.3 ASCE7) nisee.berkeley.edu Transv: T % = 0.74sec Long.: T % = 1.25sec Stewart et al., 1999
Directionality in Structural Response Azimuth-independent Azimuth-dependent Azimuth-specific response requires directional ground motions
Logic Behind Bias Concern MD arbitrary for R rup > 3-5 km Watson-Lamprey et al., 2007
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
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)
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
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)
Change of Median Beyer and Bommer (2006): MD/GM 1.2-1.3 Beyer and Bommer, 2006
Change of Median Beyer and Bommer (2006): MD/GM 1.2-1.3 Huang et al. (2008): 1.2-1.5 Huang et al., 2008
Effect of NGA on USGS Maps
NEHRP Revision Process Procedure Technical Subcommittee drafts proposal PUC feedback Revision in Subcommittee PUC approval Member balloting Final edits and approval
NEHRP Revision Process Procedure Conservative process
NEHRP Revision Process Procedure Conservative process Peculiarities in this case: No technical basis for revisions PUC input ignored Member ballot objections ignored
NEHRP Revision Process Procedure Conservative process Peculiarities in this case The unwritten last bullet
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
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
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
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
References Baker, JW and CA Cornell (2006). Which spectral acceleration are you using? Earthquake Spectra, 22(2), 293-312. 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), 1512-1522. Boore, DM, J Watson-Lamprey, and NA Abrahamson (2006). Orientation-independent measures of ground motion, Bull Seism Soc Am, 96(4a), 1502-1511. Christovasilis, IP, A Filiatrault, MC Constantinou, and A Wanitkorkul (2009). Incremental dynamic analysis of woodframe buildings, Eq Engrg & Str Dyn, 38, 477-496. Huang, YN, AS Whittaker, and N Luco (2008). Maximum spectral demands in the nearfault region, Earthquake Spectra, 24(1), 319-341. Huang, YN, AS Whittaker, and N Luco (2009). Orientation of maximum spectral demand in the near-fault region, Earthquake Spectra, 25(3), 707-717. Stewart, JP, RB Seed, and GL Fenves (1999). Seismic soil-structure interaction in buildings. II: Empirical findings, J. Geotech. & Geoenv. Engrg., 125 (1), 38-48. 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), 1511-1524.