Assessment loading criteria for bridge decks, piers and parapets

The volume of road traffic in Great Britain has grown rapidly since the 1950s. This has been accompanied by considerable increases in gross vehicle weights. In response to these developments, the traffic loading requirements for bridge design were significantly upgraded in the 1980s. Notwithstanding these enhancements, the network still contains a large number of existing decks, parapets and piers designed to the earlier requirements, a proportion of which may now be at some risk. The authorities are aware that these elements need to be identified and upgraded as soon as possible. Wholesale upgrading of all structures or structural elements to their full design requirements, and maintaining them at that level, is not a practical proposition. Nor is this justifiable as the conditions on some routes represent negligible risk to even the ‘sub-design standard’ structures. Assessment specific criteria for bridge decks which are less onerous than the design requirements have therefore been used formally from the 1960s in order to identify the existing structures in need of upgrading. This paper attempts to derive similar assessment criteria for bridge piers and parapets in respect of accidental vehicle impact. The first part of the paper examines four possible methods that can be used for the purpose deterministic, risk based cost-benefit, target reliability and whole life cost optimisation methods. These methods are somewhat related to each other, in the sense that they all consider the probability of failure in some way. It is concluded that the deterministic criteria, which can be justified as being based on simple engineering judgement, produce in reality similar results to the other approaches. In the second part of the paper, the assessment criteria for existing piers and parapets are therefore derived using a deterministic approach based on the maximum speeds of vehicles on specific types of road.