A NEW KNEE-BRACE-FRAME SYSTEM FOR SEISMIC RESISTANT STEEL BUILDINGS

Abstract

A new structural framing system for seismic resistant steel buildings, called the knee-brace-frame (KBF), is presented in this thesis. The KBF consists of a non-buckling brace with one end anchored to a knee element instead of the beam-column joint. The brace provides the necessary stiffness to control the drift under moderate earthquakes. In the event of a severe earthquake, the knee element yields in flexure to dissipate energy. The brace is thereby prevented from buckling, resulting in a full and unpinched hysteretic response of the structure. The damaged knee element can be easily replaced since it is a secondary structural element. A dimensional analysis was first conducted to investigate the parameters that affect the elastic and inelastic behaviour of KBF. Next, large scale models of one and two storey KBFs were constructed for pseudodynamic testing. The experimental results verified that the KBF possesses the necessary stiffness, strength and ductility for seismic resistant design. A maximum displacement ductility of six for the KBF structure can be obtained, prior to the failure of the knee element when the latter is designed against lateral torsional buckling and local buckling. An analytical bilinear moment-rotation relationship for the knee element was derived. based on the assumption of an anti-symmetric bending of each of the two segments of the knee element, viz. between the knee-beam and knee-brace and between the knee-brace and knee-column joints. The model was shown to correlate well with test results. Using this model, the inelastic response of the KBF was determined analytically through the DRAIN-2D computer program. Lastly, the application of KBF in multi-storey buildings was illustrated through an analytical example comprising a full-scale seven storey frame. Two eccentric braced frames (EBF) with selected eccentricities were chosen for comparison. The results showed that the KBF is suitable for multi-storey buildings. Further, the KBF system is able to control the vertical deformation of the floor very effectively, in contrast to the EBF system where large vertical displacement of the floor is a necessary requirement for the shear link to be activated. The KBF system thus offers an attractive alternative system in the design of seismic resistant steel buildings.