PEER has just published Report No. 2020/22: "Technical Background Report for Structural Analysis and Performance Assessment," a report for the "Quantifying the Performance of Retrofit of Cripple Walls and Sill Anchorage in Single-Family Wood-Frame Buildings" Project. It was authored by David P. Welch and Gregory G. Deierlein, John A. Blume Earthquake Engineering Center, Department of Civil and Environmental Engineering, Stanford University.
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.