Steel Bracing of RC Frames for Seismic Retrofitting

TLDR

Steel bracing, especially diagonal systems, is a widely used retrofit technique for strengthening seismically inadequate reinforced concrete frames, enabling designers to tailor force paths and achieve objectives from drift control to collapse prevention. The study aims to analyze the cyclic behavior of braced frames with weak short columns and to evaluate the benefits of beam alterations in such systems. The authors conduct an analytical investigation of cyclic lateral loading on braced frames with weak short columns, examining how beam modifications influence performance. The analysis shows that brace inelastic buckling degrades cyclic performance, but using braces that yield in compression or buckle elastically at low axial loads, and reducing beam strength to promote ductile failure, can markedly improve the inelastic behavior of the braced frame.

Abstract

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.