Reliability analysis of a Super-T prestressed concrete girder at serviceability limit state to AS 5100:2017

Abstract

The level of safety or reliability index achieved by a design code is an important measure of its performance. Presently, such a measure is not readily known for the prestressed concrete (PSC) girders common in Australia – the Super-T girder – which is designed to the Australian bridge design code (AS 5100). To ascertain the performance of the latest edition, this paper determines the annual reliability indices of a Super-T PSC girder at serviceability limit state (SLS) designed using AS 5100 (2017) and compares the results to an equivalent design using the Eurocode. A single-span bridge deck consisting of five prestressed concrete Super-T girders spanning 33.5 m is used as the basis for comparison. Several design scenarios are conducted, considering both dead and traffic loads, for SLS conditions. The reliability analysis is conducted using first-order reliability method for the SLS limit state functions, using load and resistance random variables based on previous literature. Sensitivity analyses are conducted to consider both the relative contribution of these variables and the effect of their distributions on the annual serviceability reliability indices. It is found that a girder designed to AS 5100 has a larger design moment and requires more prestress to satisfy stress limits compared to a Eurocode design. Moreover, the annual serviceability reliability index of the girder designed to the Eurocode is slightly higher, particularly for compression stress at transfer and tension stresses at service. Service limit state functions are found to be particularly sensitive to traffic load, permissible stresses and prestress losses as well as their respective distributions. Hence, future revisions of the Australian Standard should focus on these aspects to achieve consistent levels of reliability performance.