INTERNAL STEEL BRACING OF RC FRAMES

Steel bracing systems can be used effectively for seismic retrofitting of existing RC buildings as well as for seismic design of new buildings. Although adaptation of bracing to upgrade the lateral load capacity of existing RC frames has been the subject of a number of successful studies, guidelines for its use in newly constructed RC frames need to be further developed. This paper reports on some recent experimental and numerical work conducted by the author and his colleagues on internal bracing of RC frames using direct connections between the bracing system and the frame. The effects of X-bracing and knee bracing on enhancing the seismic capacity of the frames are investigated experimentally through pushover tests as well as cyclic tests. A compression release device has also been introduced and tested to enhance the seismic performance of the bracing system by avoiding the buckling of the compression member. An important consideration in the design of steel-braced RC frames is the level of interaction between the strength capacities of the RC frame and the bracing system. In this paper, results of experimental and numerical investigations aimed at evaluating the level of capacity interaction between the two systems are also discussed. It is found that the capacity interaction is due primarily to connections verstrength. Based on the numerical results the connection overstrength has been quantified and guidelines for the seismic design of the internally braced RC frames with direct connections are provided.