Evaluation of Serviceability Requirements for Load Rating Prestressed Concrete Bridges

Within Texas, the procedures in the AASHTO Manual for Condition Evaluation of Bridges (MCEB) are used to determine the load rating of existing structures. A large number of prestressed concrete bridges that were constructed in the 1950s and 1960s have load ratings that fall below the minimum design vehicle specified in the MCEB. The load ratings for this group of are typically controlled by the serviceability limit state criterion related to the tensile stress in the concrete. A low load rating implies that these bridges have experienced damage under service loads. However, observations made by TxDOT personnel during routine inspections indicate that the condition of these bridges is very good, and that there are generally no signs of deterioration.

Based on the results of the diagnostic load tests and laboratory fatigue tests, it was concluded that the tensile stress criterion in the MCEB should not be used to evaluate existing prestressed concrete bridges. The calculated tensile stress in the concrete is not a reliable indicator of the stresses induced in the strand due to live load. Conservative guidelines for considering the fatigue limit state explicitly in the load rating process were developed.