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

摘要

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 as a shear resisting elements creating 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 could be devastating, heavy enough to require the displacement many residents, greatly impacting the surrounding communities. The conventional method to seismically upgrade these buildings has been to remove existing gypsum sheathing, install new structural plywood, upgrade sill bolts and hold-downs, and then re-apply gypsum and finish. This approach, although effective, is expensive and intrusive, resulting in the retrofit of only a few buildings. 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. The glass fiber wrap material would provide an alternative load path from the wall's top plates to the foundation that would more effectively mitigate the shear forces developed in the structure during a seismic event. The test specimens utilized both 1/2-inch and 5/8-inch gypsum wallboard sheathing, with framing and nailing simulating typical light-frame construction from the 1950s to the 1980s. The test program encompassed 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 to be used in retrofit design. All testing was conducted using computer-controlled, cyclical test protocols [ASTM E 2126-07]. The Half-scale tests validated the capacity of the glass-fiber anchors through the use of a force-controlled protocol, while full-scale tests examined the overall panel strength. 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.