The Earthquake Engineering Online Archive

Inelastic behavior of eccentrically braced steel frames under cyclic loadings

Roeder, Charles W.; Popov, Egor P.

UCB/EERC-77/18, Earthquake Engineering Research Center, University of California, Berkeley, 1977-08, 335 pages (530/R6/1977)

A unique, practical structural system, the eccentric bracing system, which possesses many advantages in the seismic design of steel structures, is described in this work. This system employs diagonal braces with deliberately large eccentricities with respect to the beam-column joint. The eccentricity is introduced to provide a ductile fuse which will prevent brace buckling at extreme loads, such as those that may occur during a severe shake, and to avoid the poor energy dissipation characteristics which result from this buckling. The system is also stiff structurally, since linear elastic analysis indicates that the lateral stiffness remains essentially constant over a wide range of small-to-moderate eccentricities. Short beams which initially yield in shear are tested in cyclic loading. These beams were designed to simulate the behavior of an eccentric element. It was found that cyclic shear yielding of the eccentric element is the most desirable energy dissipation mechanism because of its greater stability during large cyclic deflections. An analytical model for predicting the behavior of such beams is developed from the test results. The model is based on sandwich beam theory, which includes the effect of cross-sectional warping caused by shear yielding. The inelastic model is used to perform inelastic dynamic analysis of a 20-story eccentrically braced prototype structure under the 1.5 times El Centro and unreduced Pacoima Dam acceleration records. The results of these analyses are compared with the computed response for similar ordinary braced and moment-resisting frames. Two one-third scale model eccentrically braced test frames were designed and tested. The frames were three stories high, and were modeled to represent the lower corner of the 20-story prototype structure. The tests are compared with the inelastic model. Finally, design recommendations are made.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-77-18.pdf (15 MB)