Evaluation and Retrofit Provisions for Bay Area Soft Story Wood Frame Buildings

December 30, 2009
Publication: Improving the Seismic Performance of Existing Buildings and Other Structures - Proceedings of the 2009 ATC & SEI Conference on Improving the Seismic Performance of Existing Buildings and Other Structures, p 331-342
Author(s): Stephen Harris David McCormick Stephen Bono David Bonowitz

The most commonly used code provisions for retrofit of soft story wood frame residential buildings are found in Appendix Chapter A4 of the International Existing Building Code. For practical reasons these provisions continue the practice of modifying the code for new construction to make it suitable for evaluation and retrofit. As policy and technical questions have arisen about this approach, and as more jurisdictions begin in earnest to address this vulnerable building class, the Structural Engineers Association of Northern California has convened a committee to study the issues and to advise local engineers and code officials. This paper reports on the committee's initial study regarding the sufficiency of ground story retrofit and the implications of code-based approaches. Using nonlinear response history analysis of a two-dimensional model, the study finds that code-based approaches can fail to properly balance the stiffness and strength of different stories, leading sometimes to under-designin the ground story or to damaging effects in stories above. A proposal to size the retrofit system instead based on mitigation of irregularity and on relative stiffness and strength appears to offer a more productive approach.