Seismic collapse performance of Los Angeles soft, weak, and open-front wall line woodframe structures retrofitted using different procedures

The Los Angeles Soft-Story Ordinance was enacted with the goal of reducing the collapse risk of woodframe buildings with soft, weak and open-front (SWOF) wall lines. Four alternative retrofit methods are permitted under the Ordinance including a SWOF-wall-line-only retrofit in accordance with the Department of Building and Safety requirements or a full-story retrofit based on Appendix A4 of the 2012 IEBC, ASCE 41-13 or FEMA P807. A comparative assessment of the increase in collapse safety provided by the four alternative retrofit methods is presented. Nonlinear static and dynamic collapse analyses are conducted on a set of archetypical structural models, which have been developed based on an extensive survey of Los Angeles SWOF buildings. The effect of several building characteristics (e.g. number of stories, wall layout in 1st story) on the relative enhancement in collapse safety of the retrofitted buildings is also investigated. The number of stories is shown to have the greatest effect on the relative collapse safety benefits derived from the alternative methods. The number of SWOF wall lines and the ductility of the upper stories also impacted the extent to which the retrofits enhanced collapse safety.