RELIABILITY DURING TEMPORARY ERECTION CONDITIONS

Reliability criteria during bridge erection should be established in accordance with appropriate consideration of the risk. In some cases, the required reliability, or the required safety factors, should be larger than those used during design of the permanent structure. This would be logical where the cost of temporary works is very small relative to the consequences of failure, or where costs are primarily related to access and setup, and are not particularly sensitive to the strength of the work. In other situations, the cost may be very sensitive to the required strength. An example would be where existing equipment would require expensive modification to achieve a higher strength, or where a different scheme could accomplish the task at much reduced cost, but at a lower safety factor. In either case, it is frequently necessary for the contractor to consider the criteria explicitly, since design codes do not necessarily apply to temporary works or temporary conditions of the uncompleted structure. The choice of target reliability and corresponding loads, load factors, and performance or resistance factors requires consideration of the specific situation from the viewpoints of worker safety and of economic optimization for the contractor. When the safety of the structure does not affect worker safety, such as in design against collapse of temporary falsework due to extreme wind, the contractor should be free to set the risk to optimize total costs including net present expected value of failure costs. Safety of personnel requires a standard similar to that for other workplaces, hence a bridge erection project should provide to the workers a net exposure, over the time of employment, no more severe than for alternative industrial worksites. However, it may be possible to consider an unusually high risk for very short periods of time. This paper addresses the choice of reliability criteria for individual projects, based on appropriate decision models that account for costs and consequences from the point of view of the private constructor and that of the personnel exposed on the structure. Economic value achieved by establishment of rational reliability criteria is demonstrated in the context of an example project.