Transcript PUC Briefing on IT-8 Liquefaction/Site Problems Subgroup

PUC Briefing on
IT-8 Liquefaction/Site Problems Subgroup
Requirements for Foundations and Structures
on Liquefiable Sites
Bob Bachman
Chair, IT-8 Subgroup
May 8, 2012
PUC Meeting
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Issue
• In 2009 NEHRP (and ASCE 7-10 and 2012 IBC) we now require that
liquefaction be evaluated for MCEG and that geotechnical reports
that options for mitigation measures be provided for foundations.
• Before the 2012 IBC, liquefaction was evaluated for the DE instead
of the MCEG
• Folks generally understood that the DE liquefaction demands were
to be combined with other DE seismic demands but they do not
understand what to do with MCEG demands
• And frankly, how structures and foundations were actually designed
at liquefiable sites was all over the map since there are currently no
specific requirements for foundations or structures in ASCE 7-10 or
the 2012 IBC especially lighter structures on shallow foundations
where liquefaction effects are generally ignored.
May 8, 2012
PUC Meeting
2
Scope/Focus of IT-8 Subgroup
• Develop enforceable code requirements for structures,
foundations and ground improvements consistent with
the intent of Section 11.8.3 of ASCE 7-10 and Section
1803.5.12 of the 2012 IBC for MCEG ground motions for
liquefiable sites including lateral spreading and seismic
settlement effects. Initial focus is structures on shallow
foundations
• Intended that the requirements will be reasonable,
practical and apply to the full range of structures
governed by ASCE 7
• Requirements are currently not intended to be
probabilistically based (i.e. risk targeted collapse) except
for ground motion hazard
May 8, 2012
PUC Meeting
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IT 8 – Subgroup Members
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Bob Bachman, REBachman Consulting SE
C.B. Crouse, URS
Dom Campi, Rutherford & Chekene
Gyimah Kasali, Rutherford & Chekene
Geoff Martin, USC
Omar Jaradat, Moffet & Nichol
Jon Siu, City of Seattle
Bob Hanson, FEMA Technical Monitor
May 8, 2012
PUC Meeting
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IT 8 – Subgroup Schedule
• April XX, 2012 – Kickoff teleconference
• May 2, 2012 – Subgroup meeting in SF
• June 15, 2012 – Initial draft for subgroup review
• July 18, 2012 - Subgroup meeting in SF
• Sept 6, 2012 – Initial Draft for Review to PUC for
Discussion at Oct 9 – 10, 2012 PUC building
• May, 2013 – Ballot to PUC
May 8, 2012
PUC Meeting
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Subgroup Initial Thoughts
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Overall performance goal is collapse prevention at the MCEG
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Secondary performance goal is functionality in essential facilities following the DE
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Need to have prescriptive requirements with exceptions if more detailed procedures
are performed (ala Chapter 16 of ASCE 7)
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Types of prescriptive requirements. For example
- less than say 2 inches of lateral sliding or seismic settlement – you don’t need to do
anything
- one option if greater than 2 inches..improve ground so that settlements are less
than 2 inches
- another option – shallow foundations with detailing (eg use foundation requirements
for expansive soils or greatly increase tie forces between foundations)
- another option – deep foundations with detailing that limits foundation displacement
limits that structure can be assumed to tolerate
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Exception performance option – show that foundation & structure do not collapse
when subjected to MCEG motions and essential structures continue to function
following the DE. Need specific evaluation rules which means we need evaluation
requirements for MCEG which we currently do not have codified.
May 8, 2012
PUC Meeting
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Initial Suggested Outline for New
Section 12.13.7 – Requirements for Foundations
on Liquefiable Sites
12.13.7 Requirements for Foundations on Liquefiable Sites
1. General Requirements – Apply to all foundations
- foundation design required for DE assuming no liquefaction occurs
- refers to section 11.8.3 regarding geotechnical report determined lateral
spreading and seismic settlement displacements
- reduced bearing capacity for MCEG loadings (reference procedures
2. Additional Specific Foundation Requirements
2.1 Shallow Foundations
- ground improvement
- MCEG – bearing check for dead load only
- Prescriptive Limits – Vertical and Horizontal Displacements
Foundation Prescriptive Design Requirements
- Exception to permit site specific evaluations and designs
May 8, 2012
PUC Meeting
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Initial Outline Continued
2.2 Deep Foundations for Buildings
- Ground improvements
- Prescriptive foundations
- Exception to permit site specific evaluations and designs
2.3 Deep Foundations for other Structures
- Ground improvements
- Prescriptive foundations
- Exception to permit site specific evaluations and designs
May 8, 2012
PUC Meeting
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Additional Efforts
• Case History Research Effort to justify this is
problem that needs to be addressed
- will include observations from New Zealand
Japan, Chile and Taiwan
• Will include case history in reason and
commentary associated with the proposal
• We will take full advantage of other existing
documents
May 8, 2012
PUC Meeting
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Additional Issue
• Do we also address sliding stability where its
somehow can be associated with the structure?
May 8, 2012
PUC Meeting
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