Cost and safety optimization of structural design specifications

The design of buildings, bridges, offshore platforms and other civil infrastructure systems is controlled by specifications whose purpose is to provide the engineering principles and procedures required for evaluating the safety of structural systems. The calibration of these codes and specifications is a continuous process necessary to maintain a safe national and global infrastructure system while keeping abreast of new developments in engineering principles, and data on new materials, and applied loads. The common approach to specification calibration is to use probabilistic tools to deal with the random behavior of materials and to account for the uncertainties associated with determining environmental and other load effects. This paper presents a procedure to calibrate load factors for a structural design specification based on cost and safety optimization. The procedure is illustrated by determining load factors that may be applicable for incorporation in a bridge design specification. Traditional code calibration procedures require a set of pre-determined safety levels that should be used as target values that each load combination case should satisfy. The procedure in this paper deduces the failure cost implied in present designs, and provides consistent safety levels for all load combination cases. For greater accuracy, load effects showing variance in time have been modeled by separating them into two random variables; time dependent r.v. (wind speed, vehicular loads, etc.) and time independent r.v. (modeling uncertainties). The total expected lifetime cost is used in the optimization to account for both initial construction cost and future equivalent failure costs.