A Re-definition of the Stiffness of Reinforced Concrete Elements and its Implications in Seismic Design

It is postulated that for the purposes of seismic design the ductile behaviour of lateral force-resisting components, elements and indeed the entire building system, can be satisfactorily simulated by simple bi-linear force-displacement relationships. This enables the displacement relationships between the system and its lateral force-resisting elements at a particular limit state to be readily evaluated. To this end some widely used fallacies, relevant to the transition from elastic to inelastic behaviour, are exposed. A re-definition of yield displacements and consequently stiffness, allows much more realistic predictions of the most important feature of seismic response, element displacements, to be made. The concepts introduced are rational yet very simple. Their applications are closely interwoven with the designer's intensions. The strategy provides the designers with unexpected freedom in the assignment of strengths to lateral force resisting elements, such as frames or structural walls. Contrary to current design practice, whereby a specific global displacement ductility capacity is prescribed for a particular class, the designer can determine the acceptable displacement demand to be imposed on the system. This should protect critical elements against excessive displacement demands.