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Seismic behavior and design of friction concentrically braced frames for steel buildings Tremblay, Robert


This study explores the possibility of improving the seismic response of concentrically braced frames (CBFs) by including friction connections at the end of the bracing members. These connections will slip at a predetermined load level in order to absorb and dissipate by friction most of the energy input by earthquake ground motions, and then avoid yielding and buckling of the bracing members. The characteristics of CBFs and other alternative systems are reviewed and a FCBF system is proposed which includes bolted brace connections with slotted holes in the gusset plates. 42 connection samples including various faying surface materials and bolting configurations were subjected to dynamically applied cyclic loading. These tests revealed that a stable response can be achieved by using proper sliding material together with an appropriate bolt clamping force level. The behavior of a full-scale braced frame assembly undergoing severe interstorey drifts was also investigated. The results of the quasi-static tests performed showed that the system behaves in a very predictable and satisfactory manner. An analytical study including nonlinear dynamic analyses of typical single- and multi-storey FCBFs was then performed in order to develop design guidelines for their stability under seismic loading. For single-storey buildings with a rigid roof diaphragm, design spectra were developed for predicting the ductility demand and the threshold of instability. For single-storey structures with a flexible roof diaphragm, a case study including 48 buildings revealed that this type of structure experiences more significant nonlinear response than the equivalent SDOF system. Analyses of eight braced frames varying from 2 to 12 storeys in height showed that instability in multi-storey FCBFs occurs in a collapse mechanism involving only a few stories. Such phenomena can be inhibited by providing the columns in the building with sufficient bending strength and stiffness. Values were proposed for the stiffness which appeared to yield a stable response of the frames. Columns designed solely for gravity loads were found to have sufficient strength for sustaining the moments likely to develop during ground motions. Design criteria for the brace induced column loads and for the loads acting in the collecting elements were also proposed.

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