Steel Bracing of RC Frames for Seismic Retrofitting

The use of steel bracing systems for retrofitting seismically inadequate reinforced concrete frames is examined. Diagonal bracing provides an excellent approach for strengthening and stiffening existing buildings for lateral forces. A variety of retrofitting objectives, ranging from drift control to collapse prevention, can be achieved. The designer can determine the force path in the retrofitted structure and adjust the strength and stiffness as needed. An analytical study is performed to gain understanding into the behavior of a braced frame under cyclic lateral loading, particularly frames with weak short columns. Inelastic buckling of the braces influences detrimentally the inelastic cyclic behavior of a braced frame. Instability can be prevented by using braces that yield in compression or buckle elastically at low axial loads. The advantages of altering beams of a braced frame with weak short columns are described. The beam strength can be reduced to produce a more favorable (ductile) frame failure mechanism. Combining bracing with beam alterations can significantly improve inelastic behavior of the braced frame.