Use of steel braces to resist seismic forces in precast concrete structures.

摘要

Steel bracing systems, when properly designed and detailed, provide steel structures with good earthquake resistance. This report investigates the feasibility of extending the use of steel bracing to precast concrete structures to resist seismic forces. In the proposed system, the frames consisted of precast concrete beams and columns designed primarily for gravity loads. The connections between beams and columns were simple non-moment connections, except in the braced bays. The lateral loads were resisted mainly by the steel braces. The beams and columns in the braced bays were composite members consisting of built-up steel trusses embedded inside precast concrete members.; A design procedure, which took into account different load combinations, was developed and used to design a six story structure. Nonlinear static analyses showed that the design procedure achieved its objectives. The structure's lateral load capacities and the force distribution were close to those desired.; Nonlinear dynamic analyses showed that dual systems provided better seismic resistance than systems without moment resisting frames. Using R{dollar}sb{lcub}rm w{rcub}{dollar} = 8 in the equivalent static load design provided a safe level of lateral load design. Although the unbraced bays did not increase the lateral load capacity of the systems, they made the story drifts more uniform and reduced the ductility demands on columns, braces and beams. The ductility demand on columns in the first story was reduced significantly when the bases of the unbraced bays were fixed.; An experimental program studied the behavior of composite beams under cyclic loading and compared it with that of its components, namely the built-up steel truss and the reinforced concrete beam. The tests showed that, due to the interaction between the steel truss and the reinforced concrete beam, the composite beam had about 30% higher capacity than the sum of the capacities of its components. Primary reasons for the strength increase were the participation of concrete in flexural compression and the concrete confinement which prevented buckling of members of the steel truss.