BRIDGE SUPERSTRUCTURE RESEARCH. TASKS 4-7: DESIGN FOR OVERLOADS IN PRESTRESSED CONCRETE BRIDGES. FINAL REPORT

In prestressed concrete bridge beams, regions that are initially subjected to compression are subsequently subjected to tension when overload conditions are encountered. In the design of such beams, an allowable tensile concrete stress may be specified for the precompressed tensile region of the beam for this overload condition. Since this allowable stress will affect the final design of the beam, it will subsequently have an effect on the behavior of the beam. Because AASHTO gives no clear guidelines on this matter, a questionnaire was distributed to 61 agencies to determine the state of current practice and thereby establish the range of tensile stress to be considered in this study. An exhaustive parameter study was conducted to examine a number of serviceability limit states as a function of the allowable tensile stress. These states include live-load deflection, crack width, and fatigue stress range. The study included simply supported beams as well as continuous structures. In the latter case, beams of unequal depth were considered, and the structures could be treated as simply supported for dead load and continuous for live load. Load histories for Pennsylvania were studied to determine the frequencies and intensities of overloads, and recommendations are made regarding how to account for the increases in loads that were evident.