Soft?Story Woodframe Buildings: What are They and How Do We Protect Them from Earthquakes?

In a new paper in the Proceedings for the American Society of Civil Engineers (ASCE) 2014 Structures Congress, FPL’s Douglas Rammer discusses wood work pertinent to earthquake-prone areas of the world. In the recently published paper, Overview of the NEES?Soft Experimental Program for Seismic Risk Reduction of Soft?Story Woodframe Buildings, Rammer states that the existence of thousands of soft-story woodframe buildings in California is considered a disaster preparedness problem, which has resulted in mitigation efforts throughout the state.

A soft-story building is a building that has one or more stories with significantly less stiffness (and strength) than the stories above or below. This condition usually occurs at the bottom story of a multi-story building and is often the result of large openings that are used for main building entrances, store fronts, or parking garages. These buildings were generally built before 1970 and many as early as the 1920s, which means that the contractors used construction practices not considered acceptable by today’s codified standards. The wall lengths available to resist lateral loads, in general, are too short at the bottom story, thereby resulting in a soft-story.

The considerable presence of these large multi-family buildings in San Francisco prompted the city to mandate their retrofitting over the next seven years. The NEES-Soft project is a five-university multi-industry three-year project that has many facets including improved nonlinear numerical modeling, outreach, retrofit methodology development, and full-scale system-level experimental validation of soft-story retrofit techniques.

In 2013, two full-scale buildings were tested within NEES-Soft. A hybrid test of a three-story building consisting of a one story numerical substructure and a two-story physical structure above at the University at Buffalo, and a shake table test of a four-story building at the University of California-San Diego. A series of retrofits, based on methodologies ranging from FEMA P-807 to performance-based seismic retrofits developed as part of the project, were tested at both sites. Collapse testing for both building specimens was also conducted at the end of each test program. This paper presents a summary of selected test results for these full-scale building tests within the NEES-Soft project.