The development of a rational procedure to determine structural performance factors

Abstract

The New Zealand Loadings Standard, NZS4203:1992 incorporates a Structural Performance Factor, Sp, in the expressions for the lateral force coefficient “C”. The reason given for its introduction was to allow for effects that were not addressed directly by other design spectra factors. The Sp factor was assigned a value of 0.67 at its introduction. This was considered to be “a reasonably average value”, which it effectively maintained a similar average level of seismic demand in the transition from a 150 year design return period of the previous code to the 450 year return period of the new code.

This report examines the reasoning behind the introduction of the Sp factor. It concludes that many of the reasons that were quoted as justifying its introduction are already allowed for in the design process. However, there are two factors that can justify the use of the Sp factor. These are the “duration effect” and the influence of “structural redundancy”. The first of these arises from the different ways the term “ductility” is used in the definition of the “C” factors, and the methods of assessing ductility levels in structural testing. Structural redundancy in statically indeterminate structures increases the robustness and reliability of the structure allowing it to perform better than the single degree of freedom structure, for which the “C” coefficients are developed.

A method of developing rational Sp factors is proposed by assessing damage levels using time history analyses and damage indices. It is concluded that a single factor for use with all limit states and design ductility levels cannot be justified. The value that is used should be specified in the materials code taking into account the ductility level together with the reliability and robustness of the structure.