Keywords: knee braced frame, seismic parameters, energy dissipation.

The seismic design of steel structures must satisfy two main criteria. These structures must have adequate strength and stiffness to control interstorey drift to prevent damage to the structural or non-structural elements during moderate but frequent excitations. Under extreme seismic excitations, the structures must have sufficient strength and ductility to prevent collapse. Since stiffness and ductility are generally two opposing properties, neither of moment-resisting nor concentrically braced frames, alone can economically fulfil these two criteria.

A Moment Resisting Frame (MRF) is an excellent energy dissipating system, but it has to be designed with large beam sections in order to develop sufficient stiffness to prevent excessive drift. The Concentrically Braced Frame (CBF) is much stiffer than MRF with similar sections, but it has poor energy dissipating capability due to the buckling of the bracing elements. In order to overcome these deficiencies a structural system was proposed so-called the Eccentrically Braced Frame (EBF) which has both the sufficient stiffness and ductility. By a suitable choice of eccentricity, a sufficient amount of stiffness from the brace is retained while ductility is achieved through shear yielding of a short segment of the beam created by the eccentrically placed brace member.

In order to dissipate input earthquake energy in all the above mentioned systems, inelastic deformation in main structural members requires high expense to repair or replace the damaged structural parts. The new proposed knee braced frame in which the diagonal brace provide most of the lateral stiffness and the knee anchor that is a secondary member, provides ductility through flexural yielding. In this case, the structural damage caused by an earthquake will be concentrated on these members, which can be easily replaced by reasonable cost.

In this paper eight Knee and X-braced frames (KBF and XBF respectively) are selected using an optimal shape. Gravity and equivalent static lateral seismic loads imposed to the frames. Then using the step by step non-linear static analysis (Pushover), the force-displacement capacity diagram of the each frame is created. In this way, the seismic performance of the frames is investigated. In other words, seismic parameters such as: ductility, over strength factor, ductility effect in reducing strength factor, the factor of behaviour and also mechanism of plastic hinges formation of both frame types are calculated.

By evaluating the non-linear analysis results of KBF and XBF, it can be found that in the KBF system the diagonal brace provides most of elastic lateral stiffness where the beams and columns are hinge-connected. The knee elements prevent collapse of the structure under extreme seismic excitations by dissipating energy through flexural yielding and the area under the force-displacement diagram of the KBF system shows that the energy dissipating capacity is much more than that of the XBF system.

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