PEER Tall Buildings InitiativeTask 2

1
PEER Tall Buildings InitiativeSeismic
Performance Objectives for Tall Buildings(Task 2)

• William T. Holmes
• Charles Kircher
• Laurence Kornfield
• William Petak
• Nabih Youssef

2
Original Description of Task 2

• Official Charge Using an appropriate
  methodology, develop a consensus on performance
  objectives for tall buildings.
• Current design of many tall concrete condos under
  Alternate Means provisions of the codeforcing
  performance-based designsgenerates the question
  of target performance and several sub-issues
• What is the performance expected of code
  buildings (that is to be matched in Alternate
  Means designs)?
• Should these buildings be better than normal
  code?
• Better in what performance category and how much
  better?

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Background

• The recent surge in high rise condos, many of
  which do not incorporate the required back-up
  moment frame, has focused attention on the
  alternate means of compliance section in the
  building codes.
• Normal interpretation is that alternate designs
  must have equivalent performance as a code
  compliant building.
• What is the technical definition of equivalent
  performance?

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Equivalent Performance

• Three level SEAOC description is not clearly
  translatable into technical language.
• Recent NEHRP Provisions (and the IBC) focus
  primarily-but not exclusivelyon preventing
  collapse in the MCE.
• ATC 63, intended to define a method for
  establishing an R-factor, focused on reliability
  against collapse in the MCE.
• Using sophisticated non-linear analysis methods,
  found that several code systems demonstrated
  about a 10 probability of collapse in the MCE.
• LA Tall Buildings Council defines performance in
  PBD nomenclature
• Serviceable for 50 in 30 years
• Life Safe for 10 in 50 years
• Collapse Prevention for MCE
• SF AB-083 defines
• Code level design
• MCE level Acceptably low probability of
  collapse under severe earthquake ground motions
• Serviceability Evaluation under certain conditions

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Methodology

• Interview stakeholders
• Ownerscondo units
• Owners---long term building owners
• Developers
• City Planners
• Building Officials
• Lenders
• Insurers
• Urban Economists
• Hold workshop to discuss and focus results
• Task 2 Committee to make recommendations for the
  balance of the Tall Buildings Initiative.

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More Realistic Output from Task 2

• A Consensus (National? State? SF? and LA?) is
  difficult, if not impossible, to identify within
  the confines of this project.
• Based on interviews and the workshop, Task Group
  2 will recommend
• Improved parameters to communicate earthquake
  risk to stakeholders
• Clarify, if necessary, interpretation of
  normal code performance if this is apparently
  acceptable
• Summarize the level of feelings among
  stakeholders that tall building should be
  designed to perform better than normal buildings
  and for what reasons
• It is further understood that a recommendation to
  design tall buildings for enhanced performance
• Nationally is a code change issue and would be
  required to go through normal code change
  processes
• Locally, as an alternate, is a public policy
  issue
• If recommended, will be an important
  consideration for the balance of the Tall
  Buildings Initiative

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Interview Material

• Prep materials developed
• Two page primer on building codes
• Target Life Safety
• Buildings judged important designated by
  occupancy and function
• Tall buildings generally not so designated (only
  if gt5000 occupants)
• Tall Building and Loss Scenario

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Interviews and Workshop

• Interview outline and response form
• Explain background of project
• Discuss background material
• Specifically Performance Level A, B, and C
• Specifically that code can not provide a
  no-risk condition
• Seismic performance of tall buildings
• Should tall buildings be designed to have seismic
  performance any different that normal
  buildings?
• If better, in what way?
• Does opinion vary by occupancy (condos vs.
  office/hotel, etc.)
• Committee performed 15 interviews.
• A total of 21 engineers, interviewees, and other
  interested parties attended an all day workshop
  to discuss responses given in interviews.

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Preliminary ResultsUnderstanding Risk and
Disclosure

• Stakeholders were surprised by, and not
  necessarily accepting of, current code
  performance expectations for normal buildings,
  which could include long term closure or
  economically infeasible repair costs, and even a
  very small, but real, chance of collapse in the
  MCE.
• The public needs education to develop reasonable
  expectations of building performance building
  condo owners might reasonably use a method
  similar to the PML analysis used for lenders to
  evaluate their own risks.
• The great need is to be able to clearly
  articulate performance levels and risks
• Stakeholders strongly advocate disclosure of
  risks and anticipated performance, including
  serviceability and fire risks. This disclosure
  is necessary as a part of the risk management
  of development, where risk is transferred from
  the developers to the owners/occupants/insurers/Ci
  ty. It must be made clear what risk is being
  transferred.
• Most stakeholders would pay up to a 10 premium
  for better performance

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Preliminary ResultsTall Buildings should have
better (or at least more reliable) performance
than normal buildings

• Essentially all interviewees thought tall
  buildings should be better than normal buildings.
• Two of three breakout groups at the workshop
  concluded that a higher standard should be
  required for tall buildings.
• This is considered a special class of buildings.
  Noted that the approval of tall buildings
  requires resolution of many issues having greater
  impacts on occupants, neighbors and the City than
  other/low-rise buildings
• that tall buildings have a great impact on the
  City and City services that there are high
  occupant loads on small land area in high-rises
• because of few exits and the other special
  conditions of high-rises, there is a need to
  increase resistance to the potential impacts of
  building fire, structural damage or failure.
• Many stakeholders felt that the loss consequences
  of collapse, long term closure, or even serious
  damage would be devastating for commercial
  property owners, condo owners and the community.

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Is Performance Based Design Good Enough?

• Current Procedures
• Nonlinear analysis for MCE response histories
• Peer Review
• Normal building target performance

• Or Higher Target?

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Recommendations from Task Group 2

• Disclosure of risk is important.
• Various methods of describing risk should be
  further investigated
• Rating system was mentioned several times
• Significant interest in providing better
  performance
• Highest reliability of no-collapse
• Improved probabilities (consider cost benefits)
• Reasonable repair costs
• Avoiding loss of use
• The Tall Buildings Initiative should include
  methods to provide better than code performance
  in the research program
• Implementation is currently unclear
• Financial incentive for developer
• Local public policy
• Building code change in Occupancy Category