Study of Inelastic Behavior of Zipper Braced Frames

Naeimi Hefzabad, Soheila and Eimani Kalehsar, Houshyar and Mohammadi, Yaghoub and Ghorbani, Morteza Ali (2009) Study of Inelastic Behavior of Zipper Braced Frames. Other thesis, University of Mohaghegh Ardabili.

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Abstract

Chevron frames are one of the ordinary Concentrically Braced Frames. The behavior of such a system is controlled by the buckling of the first story braces in compression, resulting in a localization of the failure and loss of lateral resistance. By the increase of lateral loads the compressive members buckle under the pressure and plastic hings will merge, and because plastic hings share not distribution, buckling is seen in the lower stories, and only these members dissipate the seismic forces, and the higher floor braces practically stay in elastic state, the shear capacity of the frame decrease if the structural members of lower stories buckle.In the lower floors due to buckled compressive members an unbalanced vertical force will be imposed to the intersection of braces and the above beam, which will make an oversized displacement at the beam mid-span point and so it results in strong beam design which not proportionate to other members. In order to ensure the system behaves in a stable manner, diffirent concentrically braced configuration has been studied. One such frame is the Zipper Braced Frame. The frame has the same geometry as the Chevron Braced Frame, except a vertical structural element, the zipper column, is added at the beam mid-span points from the second to the top story of the frame. In order to evaluate this new system using Opensees software, an in-plane frame with zipper braced system was modeled along with Chevron frame systems. These models were analyzed under pushover conditions and their ductility, drift and internal forces of the members were compared with each other using Matlab and Excel softwares. The development of plastic hings to higher stories and the participation of all braces in dissipating the seismic forces and also reduction of displacement in the beam mid-span point were the results of this analysis. In addition the improvement of ductility and response modification factor of this new system is noticeable in comparison with other concentric braces.

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