Seismic behaviour of knee braced frames

This paper describes the development of the concept of knee bracing for improving the seismic performance of steel frames. In a knee braced frame the main diagonal braces are connected to short knee elements which span diagonally across the beam-column joints. The knee elements act sacrificially, reducing the demands on the main structure during a small or medium earthquake, and can be replaced after the event. Results from a series of parametric analyses are presented, which demonstrate the potential benefits of the approach. However, careful design of the knee elements is required to ensure that they are able to absorb energy through repeated large plastic deformations without suffering collapse or instability. To optimise the design, an extensive programme of finite element modelling and experimental testing has been undertaken. It is shown that excellent performance can be achieved using standard hot-rolled sections, modified by the addition of web stiffeners to prevent localised buckling failure. Weakening of the knee elements so that they yield very early in an earthquake has potential benefits, but is shown to be unsafe as it promotes premature failure of the element.