ASCE 7-16 Proposed New Chapter 16 (Differences from BSSC Submittal)

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
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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
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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
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Different from BSSC Provisions
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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
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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
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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
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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
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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
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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
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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
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