Technical Background Report for Structural Analysis and Performance Assessment (PEER-CEA Project), PEER Report 2020/22

Abstract: 

This report outlines the development of earthquake damage functions and comparative loss metrics for  single-family  wood-frame  buildings  with  and  without  seismic  retrofit  of  vulnerable  cripple  wall and stem wall conditions. The underlying goal of the study is to quantify the benefits of the seismic  retrofit  in  terms  of  reduced  earthquake  damage  and  repair  or  reconstruction  costs.  The  earthquake  damage  and  economic  losses  are  evaluated  based  on  the  FEMA P-58  methodology,  which incorporates detailed building information and analyses to characterize the seismic hazard, structural response, earthquake damage, and repair/reconstruction costs. The analyses are informed by  and  include  information  from  other  working  groups  of  the  Project  to:  (1)  summarize  past  research on performance of wood-frame houses; (2) identify construction features to characterize alternative variants of wood-frame houses; (3) characterize earthquake hazard and ground motions in California; (4) conduct laboratory tests of cripple wall panels, wood-frame wall subassemblies and sill anchorages; and (5) validate the component loss models with data from insurance claims adjustors.  Damage  functions  are  developed  for  a  set  of  wood-frame  building  variants  that  are  distinguished by the number of stories (one- versus two-story), era (age) of construction, interior wall and ceiling materials, exterior cladding material, and height of the cripple walls. The variant houses are evaluated using seismic hazard information and ground motions for several California locations,  which  were  chosen  to  represent  the  range  seismicity  conditions  and  retrofit  design  classifications outlined in the FEMA P-1100 guidelines for seismic retrofit.

The resulting loss models for the Index Building variants are expressed in terms of three outputs: Mean Loss Curves (damage  functions),  relating  expected  loss  (repair  cost)  to  ground-motion shaking intensity, Expected Annual Loss, describing the expected (mean) loss at a specific building  location  due  to  the  risk  of  earthquake  damage,  calculated  on  an  annualized  basis,  and  Expected RC250 Loss, which is the cost of repairing damage due to earthquake ground shaking with  a  return  period  of  250  years  (20%  chance  of  exceedance  in  50  years).  The  loss  curves  demonstrate the effect of seismic retrofit by comparing losses in the existing (unretrofitted) and retrofitted condition across a range of seismic intensities.

The  general  findings  and  observations  demonstrate:  (1)  cripple  walls  in  houses  with  exterior wood siding are more vulnerable than ones with stucco siding to collapse and damage; (2) older pre-1945 houses with plaster on wood lath interior walls are more susceptible to damage and losses than more recent houses with gypsum wallboard interiors; (3) two-story houses are more vulnerable  than  one-story  houses;  (4)  taller  (e.g.,  6-ft-tall)  cripple  walls  are  generally  less  vulnerable  to  damage  and  collapse  than  shorter  (e.g.,  2-ft-tall)  cripple  walls;  (5)  houses  with  deficient stem wall connections are generally observed to be less vulnerable to earthquake damage than equivalent unretrofitted cripple walls with the same superstructure; and (6) the overall risk of losses  and  the  benefits  of  cripple  wall  retrofit  are  larger  for  sites  with  higher  seismicity.  As  summarized in the report, seismic retrofit of unbraced cripple walls can significantly reduce the risk of earthquake damage and repair costs, with reductions in Expected RC250 Loss risk of up to 50%  of  the  house  replacement  value  for  an  older  house  with  wood-frame  siding  at  locations  of  high seismicity. In addition to the reduction in repair cost risk, the seismic retrofit has an important additional benefit to reduce the risk of major damage that can displace residents from their house for many months.

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Author: 
David P. Welch
Gregory G. Deierlein
Publication date: 
November 1, 2020
Publication type: 
Technical Report
Citation: 
Welch, D.P., & Deierlein, G.D. (2020). Technical Background Report for Structural Analysis and Performance Assessment (PEER-CEA Project) (Report No. 2020/22). Pacific Earthquake Engineering Research Center (PEER). https://apps.peer.berkeley.edu/publications/peer_reports/reports_2020/2020_22_Welch_TechnicalBackground.pdf