From Code Compliance to Holistic Approaches in Structural Design of Bridges

Abstract

In the process of designing structures such as bridges, engineers have to make design choices in light of uncertainty with the expectation that the structures they design, once built, perform as intended and do so with a sufficient degree of safety against failure and nonperformance. Consequences as a result of structural failures can be very high indeed, and it is vital that events such as these rarely occur. To achieve this end, the risks of failure should be addressed, understood, and controlled in such a way that they are deemed within acceptable limits for all stakeholders involved. According to Elms (1992), two primary strategies are used for controlling risks in engineering design: the first “is to be more conservative in design to allow for uncertainties : : : [and the second] : : : is to put more effort into careful risk assessment, to maintain safety levels and reduce risk while refining designs and reducing costs.” The prior strategy is a form of engineering heuristics (Koen 2003), which by definition cannot be absolutely proven but is, nevertheless, considered legitimate because it has been successfully implemented by generations of engineers in the past without any disproportionate rates of failure; examples include factors of safety applied in allowable stress design (ASD). The second approach to control risk, although arguably also a form of heuristic (e.g., Koen 2003), has its basis in the rational treatment of uncertainty as a calculable entity— something that can theoretically be reduced and altered such that risk-optimal design solutions can be obtained that are both safe as well as being cost-effective. The scientific rationalization and formalization of risk assessments in engineering design has led to theories, such as structural reliability theory (Freudenthal 1956), which are necessarily confined in their application compared with previous tried-and-tested methods of risk control based on design conservatism and engineering judgment. It is the virtue of a competent engineer to understand the limitations of such theories and give equal thought to the importance of heuristics in engineering design and the role of engineering judgment in yielding good (quality) designs (Addis 1990; Vick 2002; Davis 2012; Bulleit et al. 2014). The aforementioned approaches focus on what engineers themselves can do to help control risks in the process of designing structures. Viewed from a wider societal standpoint, the management of risks in engineering design requires addressing the broader issue of providing assurances that engineering professionals achieve this goal satisfactorily. To help address these issues (and to help legitimize the engineer as a professional), design codes and other formal standards of practice have been established (Shapiro 1997). Some of the primary purposes of these documents, hereafter simply referred to as design codes, include codifying current best engineering knowledge and practices as well as helping to ensure (and assure) that design risks are adequately treated by engineers, i.e., the design codes act as instruments for controlling risk. This development is important because it indicates a transfer of responsibility for controlling risk away from the individual engineers carrying out the design to the code-writing committees.