The Effects of Beam Characteristics on Seismic Behavior of Chevron Braced Frames with Different Configurations

One of the most common types of steel frames is Concentrically Braced Frame (CBF). A certain type of CBF called Chevron braced frame has always been interesting to designers due to architectural advantages. When lateral earthquake-induced displacements are applied to these frames, their nonlinear behavior starts with buckling of compression brace. The buckled brace experiences a considerable reduction in resistance. On the other hand, the tension brace tends to increase its internal force. The unsymmetrical behavior of braces in tension and compression leads to application of a force known as unbalanced force to the beam. The unbalanced force can yield the chevron beam and result in failure of this kind of CBF configuration.
In this paper the seismic behavior of ordinary chevron braced frames, super-X braced frames, zipper braced frames and suspended zipper braced frames are investigated. In order to achieve this goal, six types of chevron braced structures are considered. These structures are symmetrical and are braced in one bay of the perimeter frames. The braces of the structures are selected so that that they comply with the seismic provisions of Chapter 10 of Iranian National Building Code. By changing the section properties of chevron beams, six types of chevron braced frames are obtained. Then, five types of these frames are converted to super-X frames, six of them to zipper-braced frames and one of them to suspended zipper frame. The performance of these structures is assessed by non-linear static analysis. For each structure, the pushover curves are plotted and the capacity demand ratios (DCRs) of different elements for Life Safety performance level are determined. The performance of different elements is investigated by interpreting the outputs. It is observed that in ordinary chevron frames slight strengthening of the beam is not helpful unless it is fully strengthened so that it can resist the unbalanced force linearly. Otherwise, strengthening the beam may exacerbate the global performance of the frame. It is also observed that the conversion of inverted-V configuration to super-X configuration improves the behaviors of these frames. The non-linear analyses show that adding the zipper column to the frame can improve the seismic behavior of the frame only when the structure has the characteristics of suspended-zipper frames.