Study On Basic Mechanical Properties Of Steel Frame With Slender Braces

A new lateral force resisting system for steel frame, mainly composed of veryslender braces, is presented and some basic mechanical properties of the system areinvestigated. The new system changes the traditional single brace into a series of slenderbraces. This makes the whole structural system ductile even if the buckling of a singlebrace is brittle. In the slender braces system, flat steel cross-sectional shape, which isunacceptable for traditional braces, can be used. Meanwhile, the new system takes moreadvantages including small residual drifts after earthquake, easy and low-costmaintenance, convenient design, and accurate numerical simulation results.Through theoretical analysis and FEM analysis by Abaqus, it is concluded that thebraces must be very slender. This is because that the more slender the brace is, the moreductility will the brace have no matter the brace is elastic or plastic. Meanwhile, thelarge slenderness ratio makes the axial buckling force of the brace very small, so thatthe tension-only truss element can be used to simulate the braces with enough accuracy.After examining the behavior of the individual brace, the Sap2000program is usedto build a unit model and a whole structural model. The behavior of the slender bracesunder lateral load is analyzed. The results show that the slender braces can decrease theresidual drifts of the structure obviously. The reason of this phenomenon is that whenthe braces in tension are unloaded till tension is reversed to compression, they have nocompression load-carrying capacity due to buckling; meanwhile, the braces in anotherdirection do not yet restore to the original length so that they can carry no tension untilre-straightened. This makes that at this stage the lateral stiffness of the braces becomesso small that only very small restoring force can make the structure back to its originalposition.The slender braces can be arranged as ordinary braces in two orthogonal directions,diagrid shear walls, or unidirectional braces respectively in two bays. Each of thesetypes is essentially equivalent to one another and can be designed in the same way.According to the nonlinear time-history analyses by Sap2000, the proper angle of theslender braces is from35o to50o. In the design procedure, the angle of the braces shouldbe chosen within the range of the proper angle. The specific type of the braces should bechosen according to the given situation. In any cases, the braces in the structure should yield before the frame to protect the structure and dissipate the earthquake energy.The slender braces will lose its good self-centering ability if there is serious plasticdeformation in the frames. In order to enhance the self-centering capability of thestructure, there are two viable measures, i.e., offering additional restoring force andavoiding large plastic deformation in the frames. The post-tension self-centeringbeam-column joints and other joints with self-centering ability are employed to achievethese two targets. Through the analysis of the structure with slender braces as well asthe self-centering joints via Sap2000program, it is proved that offering additionalrestoring force and reducing the plastic deformation in the frames do improve theself-centering performance of the structure.