The Earthquake Engineering Online Archive

Study of X-braced steel frame structures under earthquake simulation

Ghanaat, Yusof

UCB/EERC-80/08, Earthquake Engineering Research Center, University of California, Berkeley, 1980-04, 241 pages (530/G48/1980)

Diagonal steel bracing systems are intended to limit lateral displacements of buildings when subjected to wind or other lateral loadings. Many existing buildings with such bracing were designed on the basis of nominal building code requirements for wind, with no consideration of the function of ductility in seismic response. Although the seismic behavior of such structures has been studied analytically, no experiments had been performed previously for verification of the analytical results. This report presents experimental shaking table test results on the seismic performance of a model three-story building frame, both unbraced and with three different wind bracing systems; and correlates these results with analytical predictions. Considerable compression buckling and tension yielding of the diagonal bracing members were observed in the tests, but the bracing provided significant reductions in the lateral displacements when compared with the unbraced frame response. Analytical techniques employing three different hysteresis models to represent the three types of bracing systems are shown to predict the response of braced frames with excellent accuracy. Analytical response predictions for the unbraced frame, employing concentrated bilinear plastic hinges for all members including joint connections, also are shown to be very accurate for the levels of nonlinearity encountered. The results of this study indicate that diagonal bracing systems such as pipe and double angle braces are very effective in reducing lateral displacements of buildings for moderate earthquakes and that their energy dissipation will be significant if their compressive capacity is not less than 50 percent of their tension capacity. Consequently, damage to both the primary structural members as well as nonstructural components can be reduced by the use of appropriate lightweight diagonal bracing systems.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-80-08.pdf (20 MB)