Optimal Performance-Based Seismic Design

Improving Design Practice

In the traditional design of structures, attention was mostly paid to strength of structure and the deformation control was often a secondary check. The designer aimed to design a structure in such a way that it withstands the applied loads with sufficient reserved resistance capacity. The structure was in fact designed for amplified loads and checked for deflection or side-sway. It is not too far that the philosophy of design changed to load and resistance factored design (LRFD) in which reduced ultimate strengths of structural elements are compared to the corresponding amplified internal forces. In this method, different loads are amplified differently based on the reliability of their evaluation; and the resistance is decreased differently for bending moment, shear, etc. for similar reasoning. After California earthquakes, including the 1989 Loma Prieta and 1994 Northridge events, the need for a better control on performance of structure became more serious. Seismic-related optimization was supposed to address this problem. Alternatively, Performance-Based Design (PBD) of structures that aims to design a structure for required ductility and targeted displacement in expected risk levels was proposed to satisfy this need. This latter new design philosophy is so attractive that has the potential of being the next generation of design philosophy.

In this new design philosophy, the structure is expected to be such designed that it behaves nonlinearly under severe loadings while it behaves linearly under service loads, small wind and minor earthquake effects. The design has to have enough ductility to tolerate specified drift for severe wind and earthquakes. Depending on behaviour of material used in the structure, being ductile or brittle, the importance of the structure, the risk level and the severity of loading, different design criteria may apply. In other words, different performances may be expected from a structure with different material and different load intensities including wind, earthquake, etc. In this chapter, we will learn how to find the performance level of a structure. To that end, since some methods of performance-based designs require nonlinear analysis, a brief introduction to nonlinear analysis procedures is also provided to complete the discussion.