Stiffening Requirements for Unbonded Braces Encased in Concrete Panels

Abstract

This paper addresses the design and behavior of a composite brace system comprised of unbonded steel flat-bar brace embedded in a precast reinforced concrete panel. Debonding material is painted onto the steel flat-bar brace to remove the bond stress between the steel flat-bar brace and the precast concrete panel, so that axial load is applied only on the steel flat-bar brace. The precast concrete panel of a stiffening member is designed to prevent the steel flat-bar brace from buckling in order to effectively mobilize the compressive brace together with the tensile brace during seismic loading. The stiffening requirements to restrain the buckling of the steel flat-bar brace are derived from equilibrium considerations, and are verified by tests. Two types of tests were performed: (1) fundamental buckling tests; and (2) tests of frame with the stiffened brace subjected to the earthquake type of loading. By using the stiffened brace as a hysteretic damper, a designer does not have to consider the complicated postbuckling behavior of steel braces, and can easily control the horizontal stiffness, strength, and energy absorption capacity of steel buildings.