Seismic Ratcheting Considerations

Abstract

This paper describes recent research conducted to understand the effects of structural characteristics on the seismic response of single storey structures with a ratcheting tendency. The ratcheting tendency was characterised by a moment at the top of the structure. This represented a cantilever structure subjected to gravity forces. The structures had flag shaped hysteresis loops with different amounts of hysteretic energy dissipation. The range of behaviour from these loops was from non-linear elastic to bilinear. The strength was varied to show the effect of the lateral force reduction factor. Time-history analysis was conducted with a suite of earthquake records. It is shown that as the difference in strength in the forward and reverse directions increased, the tendency for increased seismic displacements in one direction increased. Ratcheting of the model with fatter hysteresis was significantly greater than that of the models with more pinched loops in terms of both their peak and permanent displacements. The relationships obtained were consisted with that found previously for both steel and reinforced concrete structures as proposed for the New Zealand (NZ) loadings standard, but significantly more information was obtained because of the range in hysteresis loop able to be investigated. Increased post-elastic stiffness also decreased the ratchetting. From the results obtained, simple recommendations are provided for use in structural design.