Limit State Design of CIP Post-Tensioned Concrete Box Girder Bridges

Abstract

The cast-in-place post-tensioned box girder bridge has been a popular bridge type in the western United States, particularly in the state of California. This type of construction allows for an efficient and cost-effective alternative, while being a very aesthetically pleasing structure. While post-tensioning provides the required service strength, the monolithic pier construction enhances the seismic performance of the bridge. Due to the relatively high torsional rigidity, these bridges have typically been designed as a unit, rather than individual girders. The LRFD specifications have introduced several changes that affect the design of this type of bridge. These include live load application, live load distribution factors, live load reduction due to skew, loss of prestress and post-tension, partial prestressing, and revised load factors and load combination Limit States. As the implementation of the LRFD continues for this type of bridge, the effect of the new specifications needs to be understood. This paper will present some of the key issues affecting the CIP box girder bridges. Analytical results comparing the AASHTO Standard and LRFD specifications will be presented to highlight their differences.