Abstract:
The eccentrically braced frame (EBF) is a unique, practical, seismic-resisting structural steel bracing system. The active link is its key feature. Under large lateral displacement it yields via inelastic rotation and dissipates energy. It acts as a ductile structural ‘fuse’ by concentrating inelastic demand away from the other frame components, allowing them to remain elastic. Following a severe earthquake, contingent on the level of cyclic plastic shear strain (Yp ) and its cumulative demand (CPD), it can either be left alone or replaced. Evidence from the 2010-11 Canterbury, New Zealand (NZ) earthquake series and prior research has led to the development of tentative CPD limits beyond which link replacement is recommended. But no account was taken of the effects of strain ageing on link material properties and performance. The current experimentation aimed to quantify and qualify such effects. A robust, empirical relationship between hardness and cyclic Yp was confirmed by this research. Nine pairs of EBF active links were subjected to nine different loading protocols of constant and variable amplitude cyclic Yp. The results generated help expand the scope of prior research by including the effects of strain ageing. The results demonstrate that strain ageing somewhat affects the hardness of the link web and thereby the stresses in the steel. Prior to ageing, as the CPD increases by cyclically loading the link in its inelastic range, the hardness is shown to increase until it reaches a maximum value and thereafter remains constant. The hardness marginally increases after strain ageing. On subsequent loading it returns to its pre-strain aged level. Useful, empirical, exponential relationships were developed to correlate the change in hardness caused by the maximum level of Yp. Both the un- and strain aged conditions were included. These relationships can be used to assess the state of the active link, in that the approximate maximum level of cyclic Yp can be known.
