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The most important characteristic of brace frames is their significant and appropriate stiffness as well as their compression strength against earthquake forces. Built-up special concentrically braced frames (SCBFs), which contain double angle braces, are among the common steel structural systems resisting lateral loads. Along the built-up brace length, the stitch and connector distances make significant role in cyclic and ductility behavior of braced frames due to possibility of out of plane buckling.The results of experimental studies of built-up double angle braces illustrate that setting the stitches closer to each other can improve the post buckling behavior of systems, resulting in increasing the final compression strength, close to box-shaped brace strength. In addition, an individual member buckling is possible by increasing the stitch distances along built-up braces. According to AISC seismic provisions regarding built-up SCBFs, the slenderness ratio of individual elements between the connectors should not exceed 0.4 times the governing slenderness ratio of the built-up member. Also, connecting built-up members by the use of welding is not permitted within the middle one-fourth of the clear brace length. In fact, AISC seismic provision has prohibited the use of stitches and connectors in the protected zones of built-up specially concentrically braced frames such as the center one-fourth of the clear brace length and a zone adjacent to each connection equal to the brace depth in the plane of buckling.
In this research, seismic provisions related to built-up diagonal and X-braced SCBFs are numerically investigated under cyclic loading using a single-bay single-story frame. The numerical study is performed on models, which contains parameters such as back-to-back and face-to-face connection types of built-up members. Seismic behavior of these braces are investigated from the view points of cyclic and failure behavior. This investigation is performed on both types of diagonal and X-braced steel frames. The cyclic behavior of systems is studied based on post buckling capacity, structure initial stiffness, and final compression strength. Failure behavior of systems is investigated with regard to failure cycle and ductility capacity. In order to evaluate of seismic behavior and ultimate ductility of the numerical models, regarding to proximity of initiation and propagation of steel cracks, the concept of plastic equivalent strain is used to predict system failure.
The results of this study show that increasing the number of stitches or decreasing their distances along the length of the built-up members may not necessarily improve behavior of braced systems.That means inelastic deformation consent will probably occur in individual elements between stitches resulting in earlier failure of braces. Therefore, current seismic provisions such as not exceeding the slenderness ratio of individual elements between stitches from 0.4 times of the governing slenderness ratio of the built-up member for compression sections, are conservative in SCBFs and can be changed according to the type of braces. In addition, Failure of double angle back-to-back diagonal braces occurs sooner in comparison to face-to-face braces. Also, in X-braced frames, cyclic and failure behavior of built-up face-to-face braces are more desirable than the similar back-to-back braces in general.

Keywords: Build-up concentrically braced frames, double angle, plastic equivalent strain, back-to-back, face-to-face

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