Tiered Approach to PerformanceBased Seismic Design of Wood Frame Buildings

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Tiered Approach to Performance-Based Seismic
Design of Wood Frame Buildings

• Weichiang Pang1, Shiling Pei2, Hongyan Liu2, John
  van de Lindt2, and David Rosowsky1

Texas AM University1 Colorado State
University2
Structures Congress 2008, Vancouver BC.
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Overview

• PBSD Background
• Tiered Approaches
• Design Examples
• Displacement-based design (DDD)
• Nonlinear time history analysis (NLTHA)
• Loss-based design (LBD)
• Conclusions

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Background

• Seismic design for woodframe structures
• Current practice and Challenges
• PBSD
• Concept
• Tools and approaches
• Displacement-based approaches, Fragility based
  study, Nonlinear dynamic analysis based, IDA,
  IMA, Loss analysis, etc.

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Three tiered approaches
"While no single approach to building codes can
be simple, reliable, and economical, we can have
any two of these - and maybe that's just what we
need."
-- Hamburger, R.O. (2007) Simple, But No
Simpler, Modern Steel Construction

• Tier 1 Direct displacement design (DDD)
• Objective Control excessive drifts
• Methods Modal analysis, equivalent stiffness,
  database/table
• Tier 2 Nonlinear time history analysis (NLTHA)
• Objective Control overall structural performance
• Methods NLTH simulation, explicit reliability
  analysis
• Tier 3 Loss based design (LBD)
• Objective Control economic loss
• Methods NLTH simulation loss simulation,
  statistical analysis

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Numerical Example

• The structure
• A two-story single family home, approximately 220
  m2 (2400 ft2).
• Shearwall nail spacing to be determined
• The seismic hazard
• San Francisco, California
• Structure assumed to be founded on stiff soil
  (Site Class D)

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TIER 1 Displacement-based Design (DDD)

• Three performance levels (NEESWood)
• IO(0.5) LS(2) CP(5)
• Easy-to-implement steps guide through design
  process
• Shearwall database or design value tables
  developed based on CASHEW

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DDD method (cont.)

• Examine possible wall configurations to match the
  design target points
• Equivalent stiffness curve for final design
  satisfies all requirements
• Currently, torsional effects not included.

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TIER 2 Nonlinear time history analysis (NLTHA)

• A research tool made available to design through
  easy-to-use software package (SAPWood)
• Essentially can be used to design for any
  structural responses (performances)
• Simulation with multiple earthquake records
• Start with tier 1 result, focus on drift.
  Over-strength identified.

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NLTHA (Cont.)

• Two options with NLTHA
• Perform IMA to explore the hidden load carrying
  capacity
• Optimize the initial design while keeping the
  desired performance
• More complex
• Extra information needed (building model,
  earthquake records)
• Torsion automatically included
• Seismic uncertainty included
• Need a software package.

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Tier 3 Loss based design (LBD)

• Loss simulation framework
• NLTH simulation with ground motion database
• Assembly based vulnerability
• Loss fragility
• Long term simulation
• Event loss (Structure, Intensity) Long term
  loss (Structure, Location, Time)
• Integrated in SAPWood program

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LBD (Cont.)

• A structuralized trial-and-error process
• Binary search which narrows the design quickly
• Incremental optimization refines in the target
  region
• Very computationally intensive, must be performed
  by automated tools.

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LBD (Cont.)

• Event loss performance target
• Loss should not exceed 75 of the total house
  value under an intensity level of Sa1.5 g (0.2
  sec, 5 damping) with only 5 probability of
  exceedance
• First story altered in order to satisfy the
  performance target. Require more material than
  DDD and NLTHA design.

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Conclusions and recommendations

• Tiered approach represents three different levels
  of possible PBSD implementations for wood frame
  structures. Validity verified through numerical
  examples.
• DDD simple to apply and reliable, may not
  provide the most optimized solution, only
  controls drifts.
• NLTHA provides more freedom in specifying
  performance targets, design can be economical and
  reliable, more involvement in modeling and
  simulation, software package needed.
• LBD focuses on financial losses. Can be used to
  design for customized financial performance
  targets.
• The selection of tiers in the proposed framework
  should be based on
• The resources available, the accuracy needed, the
  performance target type, etc.

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Acknowledgement

• This material reported in this paper is based
  upon work supported by the National Research
  Initiative of the USDA Cooperative State
  Research, Education and Extension Service, grant
  number 2005-35103-15250 with additional support
  from the National Science Foundation under Grant
  No. CMMI-0529903 (NEES Research) and CMMI-0402490
  (NEES Operations). Any opinions, findings, and
  conclusions or recommendations expressed in this
  paper are those of the authors and do not
  necessarily reflect the views of the sponsors.

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Thank you