Soft/Weak Story Problems and Solutions for Residential Structures

Wood-framed residential structures with soft, weak, or open-front walls are common in areas of high seismicity like the west coast of the United States. Parking below the residential portions of the building, or "tuck-under" parking, is particularly problematic in many buildings built in the 1960s through the 1980s, when stucco and gypsum board were widely used to resist lateral loads in two-, three-, and even four-story structures. The 1994 Northridge earthquake demonstrated the problematic nature of these types of structures, as a number of these structures experienced significant damage and even collapse. Standards for designing strengthening measures for wood-framed structures with soft/weak stories were adopted into Appendix Chapter 4 of the International Existing Building Code (IEBC). This paper addresses problems with these provisions in the IEBC, including what the authors believe are excessive engineering analysis requirements as well as excessive design force levels, neither of which is conducive to economical designs or inducement to owners. Several case studies are presented to demonstrate problems with the current provisions in the IEBC and to demonstrate how, if engineering judgment is used, reasonable solutions can be implemented in actual buildings in what the authors believe to be an effective and economical manner (i.e. significant reduction of risk for a reasonable cost and a reasonable amount of engineering effort). Issues regarding owner reticence to invest in structural strengthening are also discussed, and possible modifications to the IEBC are suggested.