Cost and safety optimization of structural design specifications

The design of buildings, bridges, offshore platforms and other civil infras tructure systems is controlled by specifications whose purpose is to provid e the engineering principles and procedures required for evaluating the saf ety of structural systems. The calibration of these codes and specification s is a continuous process necessary to maintain a safe national and global infrastructure system while keeping abreast of new developments in engineer ing principles, and data on new materials, and applied loads. The common ap proach to specification calibration is to use probabilistic tools to deal w ith the random. behavior of materials and to account for the uncertainties associated with determining environmental and other load effects. This pape r presents a procedure to calibrate load factors for a structural design sp ecification based on cost and safety optimization. The procedure is illustr ated by determining load factors that may be applicable for incorporation i n a bridge design specification. Traditional code calibration procedures re quire a set of pre-determined safety levels that should be used as target v alues that each load combination case should satisfy. The procedure in this paper deduces the failure cost implied in present designs, and provides co nsistent safety levels for all load combination cases. For greater accuracy , load effects showing variance in time have been modeled by separating the m into two random variables; time dependent r.v. (wind speed, vehicular loa ds, etc.) and time independent r.v. (modeling uncertainties). The total exp ected lifetime cost is used in the optimization to account for both initial construction cost and future equivalent failure costs. (C) 2001 Elsevier S cience Ltd. All rights reserved.