Evaluating Seismic Capacity of Steel Frames with Knee Bracing

Author Name(s): Azam Raeisi, Hamed Jalavand
Author Email: raeisi.Azam@yahoo.com

Abstract

Knee Braced Frames (KBF) is a new system of energy dissipation in steel frames which provides good combination of ductility and lateral hardness. In KBF, at least one end of brace connects to the brace member which locates between beam and column sidelong instead of connecting to the beam-column junction. Coaxial bracing supply hardness of system while ductility is obtained under influence of lateral loads through knee member flow. When earth quack happens, knee member acts as ductile fuse and prevents coaxial bracing buckling and energy dissipation happens through bruised of knee member. In this study, we investigate three buildings (3, 5 and 7 story ones) of 2 types (3 and 5 bays) with coaxial bracing, chevron, moment frames, and knee braces through non-linear static analysis in uniform lateral load distribution and the first mode of vibration. Also concentric bracing, chevron and moment frames performance in construction frames in comparison with knee bracing frames is discussed. As an energy dissipating system, Knee bracing frame has a combination of excellent ductility and lateral stiffness that has a high performance against lateral loads, especially earthquake.

Keywords

moment frame, knee bracing, level of performance, non-linear static analysis, x bracing

Introduction

In 1986,  designer engineer named Aristizabel-Ochoa had introduced  Steel a system called disposable knee bracing. The end of brace connects to the point of knee member which locates between beam and column or column and support instead of connecting to the beam and column junction[1,2]. bracings are one of the systems that resist lateral forces and show a good seismic performance like reduced inter-story drifts. [1, 2] no sway structures induce high lateral stiffness and considerably reduce structure drift against seismic loads. However, some engineers tend to use divergent braced frames, but in practice, usage of convergent braced frames is highly increased. [3] X bracings induce to design structures for higher base shear by development of high stiffness, but they are still more economical than moment frames. This frames start to lose compressive strength and cause to concentrate damage in weaker stories by entering the non-elastic area. [2] Also one the most important deficiency of this system is buckling of the bracing. On the other hand, a convergent bracing system with knee members is recommended [4], that prevents from bracing buckling and knee element acts as a fuse and dissipates earthquake forces or lateral force by creating plastic hinge. [5] In recent years, usage of this systems has been research topic of many engineering science researchers. [8, 6]

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