Implication of increased live loads on the design of precast concrete bridge girders

Abstract

American Association of State Highway and Transportation Officials (AASHTO) design live loads are sometimes increased as directed by relevant authorities to reflect actual traffic conditions. A 50% increase is adopted in some regions of the Middle East based on comparisons of AASHTO LRFD specifications HL93 live loads with the British Standards HA + HB live loads. Even in some U.S. jurisdictions, the AASHTO live loads may not represent modern truck configurations. This paper examines the effects of increasing AASHTO LRFD specifications HL93 live loads on the design of precast, prestressed concrete girders. A parametric study was first conducted for this purpose. Design aids in the form of charts that relate the precast concrete girder size and spacing to the span length as a function of the 28-day concrete compressive strength and environmental classifications were developed for HL93 and 1.5 × HL93 live loads. It was shown that the increase in live loads can be economically accommodated by increasing the 28-day concrete strength. The charts not only set up the basis of comparisons but also provided a practical solution that can be simply used by precast concrete designers for optimizing the precast concrete girder size and spacing.