Saving half through girder bridges using non-linear finite element analysis

In the UK there are a large number of half through girder bridges dating from the latter part of the 19th Century that continue to provide a vital part of the transport infrastructure. The main edge girders of these bridges are frequently found to fail strength assessments even though there is no evidence of any impending failure. Their strength, according to codes of practice, relies on U-frame behaviour and a moment connection with the deck. However, in many cases it is difficult to quantify the connection due to a lack of detailed information, difficult inspection conditions and uncertainty regarding deterioration. Consequential strength assessment conservatism leads to a low connection moment capacity and, therefore, little lateral restraint to the top flange and an almost inevitable strength assessment failure. This paper describes a new way of considering girder restraint that is not reliant on U-frame moment connections but instead utilises more definite horizontal lateral restraint provided by the in-plane shear stiffness of the deck. Current assessment rules, that consider effective length and slenderness of parts in compression, cannot be easily used to quantify it. Instead numerical simulation of girders based on non-linear finite element analysis has been used to comprehensively model failure and predict strength. Gifford have now used this technique to model 30 half through girder bridges, significantly raising strength assessment ratings and, therefore, negating costly strengthening work. This paper describes the methodology and presents a typical case study. For the covering abstract see ITRD E126028.