ASCE 7-16 Proposed New Chapter 16 (Differences from BSSC Submittal) Ronald O. Hamburger, SE Senior Principal Simpson Gumpertz & Heger Chair, ASCE 7-16 www.sgh.com Background • ASCE 7-10 Chapter 16 governs linear and nonlinear response history analysis – Chapter was originally developed in 1997 NEHRP Provisions / ASCE 7-98 • Based on FEMA 273/274 Procedures for Seismic Rehab – Analysis is performed at Design Earthquake shaking • Minimum of 3 accelerograms – Maximum demands from suite of 3 or more records – Average demands from suite of 7 or more records • Acceptance criteria set a 2/3 “limit” values 2 Background • 2009 NEHRP Development effort attempted major rewrite of Chapter 16 – failed • 2015 NEHRP completed major rewrite of Chapter 16 – Required suite of at least 11 motions – Not more than one unacceptable response for Risk Category II structures – Statistically based acceptance criteria intended to provide: • <10% probability of failure resulting in collapse for Risk Category II structures • <6% probability of failure of elements that could result in collapse for Risk Category III structures • <3% probability of failure of elements that could result in collapse for Risk Category IV structures • ASCE 7-16 is now balloting revised version of 2015 BSSC version 3 Similar to BSSC Provisions • Selection suite of not less than 11 motions – Scale or spectrally match to: • Maximum Direction Uniform Risk (MCER) spectrum • Scenario Spectra selected compatible with MCER spectrum and hazard – At least 2 spectra needed for scenario approach • For Risk Category II Structures not more than 1 motion can produce “unacceptable response”. • Mean response, factored for load/resistance factors to account for uncertainty in demand and capacity used to evaluated acceptable performance 4 Different from BSSC Provisions 5 Vertical Response Analysis • Explicit vertical response analysis is required when: – Vertical elements of the Seismic Force Resisting System are Discontinuous – For non-building structures, when Chapter 15 requires explicit consideration of vertical response • Separate vertical target MCE spectra must be determined 6 Ground Motion Selection & Scaling • Near-fault site defined: – Sites within: • 15km of the surface projection of a fault capable of producing M7 or larger events • 10km of the surface projection of a fault capable of producing M6.5 or larger events – Exceptions: • Faults with estimated slip rate less than 1mm per year • Surface projection need not include portions of fault deeper than 10km from the surface 7 Scaling/Matching Range • Horizontal – 0.2T1 – 2.0T1 • Upper range can be 1.5 T1 if dynamic analysis indicates that period elongation does not exceed 1.5 T1 • Lower bound period must be selected to achieve at least 90% mass participation • Vertical – Similar, but lower bound period need not be taken less than 0.1 second, or lowest period at which significant vertical response occurs 8 Amplitude Scaling 1. Construct “max direction” spectrum for each pair of horizontal motions 2. Select scale factors for each “pair” such that average of “max direction” spectra does not fall below 90% of target spectrum over scaling range – For vertical response, the average of the vertical spectra shall envelope the vertical target spectrum 9 Spectral Matching • Each horizontal component spectrally matched and scaled such that the average of the spectra for all horizontal components shall not be less than the target over the matching range • Each vertical component matched to the vertical target 10 Application to Model • Near Fault – Rotate components to Fault Parallel and Fault Normal directions – Apply to model in the Fault Parallel and Fault Normal directions, respectively • Not Near Fault – Apply motions to model in orthogonal orientations such that the average of the spectra applied in each direction is within +/-10% of the spectrum for all components within the scaling range 11 Progress • Proposals have worked through the Seismic Task Committee • Now in ballot to the ASCE 7 Main Committee • Will likely take 2 ballots to resolve issues 12