Strengthening of Existing Light-Framed Buildings with Gypsum Shear Walls Using a Newly Developed Fiber Reinforced Polymer (FRP) Assembly

Recent earthquakes have demonstrated the weakness of light-frame construction buildings that rely on gypsum drywall sheathing as shear walls. Seismic design codes now limit the use of gypsum sheathing to resist seismic loads and therefore have created a large inventory of seismically suspect buildings — including many two- three- and four-story residential apartment buildings constructed between 1950 and 1980. Potential earthquake damage to these buildings may be heavy enough to require vacating these buildings for repair. The required displacement would impact the residents and the surrounding communities significantly. The conventional method to seismically upgrade these buildings has been to remove existing gypsum sheathing, install new structural plywood, upgrade sill bolting and hold-downs, and then re-apply gypsum and finish. This approach is expensive and intrusive, resulting in very few buildings being retrofitted. A test program at the University of California Irvine (UCI) was developed to investigate the performance of gypsum sheathed shear walls reinforced with glass fiber-wrap material epoxied to the gypsum, and using glass fiber anchors to connect the system to the foundation. The test specimens had 1/2-inch and 5/8-inch gypsum wallboard sheathing, and framing and nailing that simulate typical light-framed construction from 1950 to 1980. The test program included 9 half-scale (4 ft x 4 ft) and 8 full-scale (8 ft by 8 ft) wall specimens loaded cyclically to obtain the force/displacement relationships and strength parameters for use in retrofit design. All testing was conducted using computer-controlled, cyclical test protocols [ASTM E 2126-07]. Half-scale tests to validate the capacity of the glass-fiber anchors followed a force-controlled protocol, while full-scale tests for overall panel strength used a displacement-controlled protocol. This paper presents experimental results including: (a) composite anchor strength results and failure modes; (b) control and retrofitted panel strength results, force-displacement relationships and failure modes; and (c) preliminary concepts and recommendations for application in seismic retrofit design.