Steel bracing systems for seismic strengthening of reinforced concrete slab-column structures.

Abstract

This study is concerned with developing a rational design procedure for the use of ductile steel bracing systems for strengthening existing seismically weak RC slab-column building structures. A representative one-third scale, two-bay, two-story frame model was used for analytical and experimental investigation. The dual role played by the horizontal and vertical steel elements of the bracing system as truss and frame elements was considered in the analysis and design. Connection of horizontal steel elements to RC slabs was required in order to transfer the lateral inertia forces from the floors. However, the vertical steel elements were simply wrapped and tied around the RC columns without using shear connectors. A supplementary experimental study was conducted to better understand the behavior of RC members strengthened by jacketing steel without using shear connectors. Two RC beams were jacketed by steel angles, and were subjected to reversed cyclic loads. The behavior of the control RC beam was significantly improved by the strengthening steel. The results were implemented in designing the vertical steel elements for strengthening the columns of the RC frame. Three tests were performed on the strengthened frame where gravity and cyclic lateral loads were applied simultaneously. In the first test, the horizontal and vertical steel elements were added to the exterior bay. The objective was to verify the steel frame action of the steel elements. In the second test, chevron braces were added to the exterior bay. In the third test, a more ductile steel bracing system was designed and attached to the interior bay after removing the braces from the exterior bay. The test results showed dramatic improvement in the behavior of the RC frame. Excellent semi-composite flexural action developed between the vertical steel elements and the core RC columns which were able to force their curvature on the jacketing steel. Analytical models for the strengthened frame were developed and used for nonlinear analyses. The models were subjected to loads similar to those applied experimentally. The experimental and analytical results compared very well. Conclusions are drawn for practical design and construction of the strengthening system.