The motivation for the study is to rationalise the evaluation of existing bridges with a view to avoiding unnecessary strengthening, repair or replacement of structures that are in practice adequately safe. This is the danger when design codes, or design levels of target safety, are applied to the evaluation of existing bridges.
The goal is to develop a methodology for rationally determining an adequate level of safety for a specific concrete bridge.
Many bridge engineers propose interventions on the basis of simple analysis where strengthening may not, in fact, be necessary. A more detailed analysis could help to decrease the intervention cost significantly. Compared to the design of new structures, existing structures have to be treated differently. There are fewer hazards and less uncertainty once a structure has successfully entered service and performed satisfactorily.
The structural safety of a bridge is a function of its resistance to applied loads. If the resistance and action effects are assumed to be random variables, the structural safety can in theory be modelled by a failure probability. Deterministic codes define loads and partial load factors in an attempt to respect a minimum probability of failure. But these codes have to be applicable to all the bridges without taking into account individual statical systems, load conditions, cross-sections and other bridge-specific information, thus need to be conservative in nature. As a result many bridges may well have far higher load capacities than predicted. Although many papers on structural reliability and probabilistic-based assessment have been published there is still no widely agreed methodology for determining what the acceptable level of safety is.
At the end of the research a report will be produced covering the most important knowledge in this field and which could serve as a reference guide for practising engineers. Information will be given about limit states, basic variables and how to model their uncertainty, load modelling and reliability methods. Plastic methods, e.g. yield-line theory, will be considered to take into account all the bridge’s inherent safety. Further investigations into the deterioration of resistance and including inspection data will also be carried out.
As important as the probability of failure are the consequences in the case of failure. An acceptance criterion including both aspects will be developed. The proposed procedure will be tested on typical road bridges and the results critically analysed.
Daniel Imhof, PhD Student
Dr Campbell R Middleton, Supervisor
Professor Andrew C Palmer, Advisor