New PEER Report 2020/21: "Comparison of the Response of Small- and Large-Component Cripple Wall Specimens Tested under Simulated Seismic Loading"

December 16, 2020

PEER has just published Report No. 2020/21: "Comparison of the Response of Small- and Large-Component Cripple Wall Specimens Tested under Simulated Seismic Loading," 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 Brandon Schiller and Tara Hutchinson, Universiity of California, San Diego; and Kelly Cobeen, Wiss, Janney, Elstner Associates, Inc.

Visit the PEER publications page to download a free color pdf of the document.


This  report  is  one  of  a  series  of  reports  documenting  the  methods  and  findings  of  a  multi-year,  multi-disciplinary  project  coordinated  by  the  Pacific  Earthquake  Engineering  Research  Center  (PEER  and  funded  by  the  California  Earthquake  Authority  (CEA).  The  overall  project  is  titled  “Quantifying the Performance of Retrofit of Cripple Walls and Sill Anchorage in Single-Family Wood-Frame Buildings,” henceforth referred to as the “PEER–CEA Project.”

The  overall  objective  of  the  PEER–CEA  Project  is  to  provide  scientifically  based  information  (e.g.,  testing,  analysis,  and  resulting  loss  models)  that  measure  and  assess  the  effectiveness of seismic retrofit to reduce the risk of damage and associated losses (repair costs) of  wood-frame  houses  with  cripple  wall  and  sill  anchorage  deficiencies  as  well  as  retrofitted  conditions that address those deficiencies. Tasks that support and inform the loss-modeling effort are: (1) collecting and summarizing existing information and results of previous research on the performance   of   wood-frame   houses;   (2)   identifying   construction   features   to   characterize   alternative  variants  of  wood-frame  houses;  (3)  characterizing  earthquake  hazard  and  ground  motions  at  representative  sites  in  California;  (4)  developing  cyclic  loading  protocols  and  conducting  laboratory  tests  of  cripple  wall  panels,  wood-frame  wall  subassemblies,  and  sill  anchorages to measure and document their response (strength and stiffness) under cyclic loading; and (5) the computer modeling, simulations, and the development of loss models as informed by a workshop with claims adjustors.

This  report  is  a  product  of  Working  Group  4:  Testing,  whose  central  focus  was  to  experimentally investigate the seismic performance of retrofitted and existing cripple walls. Two testing programs were conducted; the University of California, Berkeley (UC Berkeley) focused on large-component tests; and the University of California San Diego (UC San Diego) focused on small-component  tests.  The  primary  objectives  of  the  tests  were  to  develop  descriptions  of  the  load-deflection  behavior  of  components  and  connections  for  use  by  Working  Group  5  in  developing numerical models and collect descriptions of damage at varying levels of drift for use by Working Group 6 in developing fragility functions. This report considers two large-component cripple wall tests performed at UC Berkeley and several small-component tests performed at UC San Diego that resembled the testing details of the large-component tests.

Experiments  involved  imposition  of  combined  vertical  loading  and  quasi-static  reversed  cyclic  lateral  load  on  cripple  wall  assemblies.  The  details  of  the  tests  are  representative  of  era-specific  construction,  specifically  the  most  vulnerable  pre-1945  construction.  All  cripple  walls  tested were 2 ft high and finished with stucco over horizontal lumber sheathing. Specimens were tested  in  both  the  retrofitted  and  unretrofitted  condition.  The  large-component  tests  were  constructed  as  three-dimensional  components  (with  a  20-ft    4-ft  floor  plan)  and  included  the  cripple wall and a single-story superstructure above. The small-component tests were constructed as  12-ft-long  two-dimensional  components  and  included  only  the  cripple  wall.  The  pairing  of  small-  and  large-component  tests  was  considered  to  make  a  direct  comparison  to  determine  the  following: (1) how closely small-component specimen response could emulate the response of the large-component specimens; and (2) what boundary conditions in the small-component specimens led to the best match the response of the large-component specimens.

The answers to these questions are intended to help identify best practices for the future design of cripple walls in residential housing, with particular interest in: (1) supporting the realistic design of small-component specimens that may capture the response large-component specimen response; and (2) to qualitatively determine where the small-component tests fall in the range of lower-  to  upper-bound  estimation  of  strength  and  deformation  capacity  for  the  purposes  of  numerical  modelling.  Through  these  comparisons,  the  experiments  will  ultimately  advance  numerical  modeling  tools,  which  will  in  turn  help  generate  seismic  loss  models  capable  of  quantifying  the  reduction  of  loss  achieved  by  applying  state-of-practice  retrofit  methods  as  identified in FEMA P-1100Vulnerability-Base Seismic Assessment and Retrofit of One- and Two-Family  Dwellings.  To  this  end,  details  of  the  test  specimens,  measured  as  well  as  physical  observations, and comparisons between the two test programs are summarized in this report.