Keywords: brace, connection, seismic, steel frame, earthquake, nonlinear analysis, dynamic analysis.

Steel concentric braced frames are commonly used to resist lateral loads especially in structures that are constructed in high seismic zones. In modelling of X-braced frames it is assumed that diagonals at mid-span cross without interrupting each other and the end connections of the braces are pinned. But in practice one of braces is discontinuous and the end connections of braces could have partial rigidity.

Changing the relative stiffness of the end connections, affects the effective length factor of X-bracing members with semi-rigid ends [1]. Also the effective length factor of X-bracing members is sensitive to rotational stiffness of mid-connection [2].

In this paper the seismic behaviour of simple steel frames with X-bracing members were studied assuming different conditions for brace connections. For this purpose a particular class of braced steel structures, those composed of three, five and ten storeys were modelled as two-dimensional frames and were designed according to the specifications of AISC-ASD 1989 [3]. End and mid-connections of braces were modelled in two pinned and rigid extreme states with different relevant effective length factors. These frames were evaluated by performing nonlinear time-history analysis under different earthquakes. Plastic hinges were assigned to frame elements to allow for nonlinear behaviour.

According to the analytical results, when the elements remain linear, differences in the response of the models with different conditions for brace connections are negligible. Since the brace buckling load capacity depends on the connection conditions and effective length factor, it was observed that under the some earthquakes that several plastic hinges were generated in the members, the differences in maximum base shear and roof displacement of the models are considerable and in some cases reach to 85% and 60% of the common model respectively. This mainly arises from the differences in end and mid-connections of the braces. In identical loading conditions, the process of plastic hinge generation can be different and the first plastic hinge can be formed in columns or in braces when the brace connections are modelled in different ways. It has therefore been demonstrated that it is necessary to select proper connection conditions and effective length factor for the braces when modelling these structures.

1

F. Segal, R. Levy, A. Rutenberg, "Design of Imperfect Cross-Bracings", Journal of Engineering Mechanics, 120(5), 1057-1075, 1994. doi:10.1061/(ASCE)0733-9399(1994)120:5(1057)

2

A. Davaran, "Effective Length Factor for Discontinuous X-Bracing Systems", Journal of Engineering Mechanics, 127(2), 106-112, 2001. doi:10.1061/(ASCE)0733-9399(2001)127:2(106)

3

AISC, "Specification for structural steel buildings", American Institute of Steel Construction, Chicago (IL), 2005.

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