Report - PEER - University of California, Berkeley

Conceptual design is greatly facilitated by focusing on discrete performancetargets associated with discrete hazard levels — similar to the way it is beingpracticed in most of the performance-based guidelines presently in use. In theconceptual design phase, engineers are used (and likely will be so for many years tocome) to select and rough-proportion structural systems for strength, stiffness (driftlimitations), ductility, and perhaps energy dissipation and floor accelerations. The artof engineering, which should be practiced in this phase, is to use global informationon important performance targets in order to come up with a structural system thatfulfills specified performance objectives in the most effective manner. This impliesexploration of alternatives, which may be utilizing different structural materials andsystems or advanced technologies such as base isolation or internal energy dissipationdevices.The challenge is to provide the designer with a small set of most relevant criteriaon important EDPs on which good conceptual design can be based. In concept, thismeans reversing the information flow discussed before for performance assessment,and working towards quantification of relevant EDPs, given that desired performancecan be expressed in terms of targeted DV values at discrete performance levels. Thisreversal of information flow is indicated in Figure 1 with vertical arrow lines thatflow into two horizontal arrow lines and merge at the EDP level, which then containslimit values of relevant EDPs (strength, stiffness, ductility, floor acceleration, etc.)that drive design decisions.Given EDP limits and associated IM hazards for various performance levels,such as the two illustrated in Figure 1, conceptual design for multiple performanceobjectives can be performed. In general, performance should be concerned withstructural and nonstructural systems as well as building contents. There is no singledesign parameter that will control all performance goals at all performance levels.For instance, nonstructural damage is controlled often by interstory drift limitations,which demand large stiffness. Content damage, on the other hand, is mostlyproportional to floor accelerations, which can be limited by reducing the stiffnessand/or strength of the structure. At the other extreme, life safety and collapseprevention are controlled by the inelastic deformation and energy dissipationcapacities of ductile elements and the strength capacity of brittle ones.This discussion indicates that different performance objectives may imposeconflicting demands on strength and stiffness, and that seismic design is likely tobecome an iterative process in which different performance criteria may lead to tradeoffsbetween strength and stiffness requirements, but in which no compromise can bemade on issues of life safety and collapse prevention. This iterative process can beaccomplished in two phases; a conceptual design phase in which one or moreeffective structural systems are explored and rough-sized, and a performanceassessment phase in which performance of the structural, nonstructural, and contentsystems is evaluated and final design decisions and modifications are made.This paper is concerned with the conceptual design phase. Two challenges needto be addressed in the context of performance-based conceptual design. One is to507