Effects of Dead Load and Multiple Earthquake Loadings on Seismic Performance of Wood-Frame Shear Walls

Goals of this preliminary study are to better understand (1) earthquake performance of wood-frame shear walls carrying gravity loads, compared with walls without gravity load, and (2) performance of walls subjected to a sequence of earthquake motions, compared with walls subjected to a single earthquake. Tests with simulated earthquake ground motions were conducted on 2,440 by 2,440-mm (8 by 8-ft) walls with 38 by 89mm (nominal 2 by 4) Douglas-fir studs at 610 mm (24 in.) on center. Two oriented strand board (OSB) panels were installed and fastened vertically to the frame, and two gypsum wallboard panels were installed opposite the OSB. Partially anchored (PA) walls had two anchor bolts on the sill plate. In addition to the anchor bolts, fully anchored (FA) walls included holddowns installed at the end studs. Ground motions were scaled to the 10 percent in 50 years probability of exceedance design level for Seattle, Washington, the traditional level associated with life safety performance. For PA walls with dead load, failure modes were consistent with tests without dead load; however, additional fastener damage, common to FA walls, resulted from the additional resistance to overturning. PA walls realized a greater improvement in performance from dead load application compared with FA walls; performance appears to approach that of FA walls when dead load is applied. FA and PA walls subjected to a sequence of earthquake motions showed wall performance about the same as that of walls subjected to a single scaled earthquake motion. Earthquakes are relatively common in the Pacific Northwest. According to the Pacific Northwest Seismograph Network (2005), each year several thousand earthquakes are recorded, although only a few dozen are large enough to be felt. Lateral loads imposed upon buildings from these earthquakes, and also as a result of wind, are random and cyclic, and these loads are resisted by the building’s lateral force resisting system (LFRS). The Portland Cement Association (1997) reported that more than 90 percent of US residences have shear walls as their primary LFRS. Usually, shear walls resist lateral loads and also provide support to the weight (or gravity load) of the structure above; however, shear wall design capacities are most often based on tests that do not account for the vertical load. A research program was therefore developed to better understand shear wall behavior under realistic loading conditions. The objectives of this study were the following: 1. To evaluate earthquake performance of residential shear walls, following provisions of the International Residential Code (IRC) (International Code Council [ICC] 2006) carrying gravity loads, and to compare the results with those of walls carrying no gravity loads. These walls have only anchor bolts and no hold-downs. 2. To evaluate residential shear wall performance when subjected to a sequence of earthquakes, and to compare the results with those of walls subjected to a single earthquake ground motion. The authors are, respectively, former Graduate Research Assistant, Dept. of Wood Sci. and Engineering ([email protected]), Associate Professor, School of Civil and Construction Engineering ([email protected]), and Professor, Dept. of Wood Sci. and Engineering ([email protected]), Oregon State Univ., Corvallis. This paper was received for publication in July 2009. Article no. 10663. Forest Products Society 2010. Forest Prod. J. 60(2):150–156.