Serviceability criteria in prestressed concrete bridge girders

In the design codes, acceptability criteria are formulated in terms of limit state functions. Safety reserve is ensured by specifying conservative values of load and resistance parameters. Recently, load and resistance factors have been derived for the ultimate limit states design of highway bridges. Therefore, in this study a probability-based procedure is developed to calculate load and resistance parameters for the serviceability limit states.;The work is focused on prestressed concrete bridge girders. A practical range of spans for AASHTO type girders is 30 to 120 ft and spacing between 4 and 12 ft. Current AASHTO requires that girders should be designed to satisfy initial and final allowable stresses. Based on this requirement, working stress design controls the selection of prestressing strands.;The serviceability limit states considered in prestressed concrete bridge girders include allowable tensile, allowable compression and flexural cracking. Limit state functions are formulated. The statistical models of resistance parameters are determined based on available test results and special simulations. Results of simulation showed that resistance distribution for permissible stresses can be approximated by a normal distribution while resistance of flexural cracking can be approximated by a lognormal distribution.;For each considered limit state, and for girders designed according to the current AASHTO, the reliability indices, ;Based on the evaluation of performance of structures designed using the current AASHTO, allowable tension has been increased to the limit of cracking stress and a target reliability index equal to 1.0 has been selected for this serviceability limit state. Whereas, target reliability index equal to 3.0 has been selected for the serviceability limit state of permissible compression. The proposed design criteria are formulated using the new LRFD AASHTO code.