Thermal Gradients and Their Effects on Segmental Concrete Box Girder Bridges

The thermal gradients in segmental concrete box girder bridge structures are not fully known as a result of variations in location, daily temperature changes, radiant load, wind speed, surface reflectiveness, box shape, and other variables. These variables make the prediction of thermal gradients difficult. In a continuous structure thermal gradient stresses caused by longitudinal moments can equal those of live loads. The number of segmental concrete box structures studied for temperature gradients is fairly low.

An on-site instrumentation program to measure the thermal gradients in a post-tensioned segmental box girder was conducted at the U.S. 183 Segmental Bridge in Austin, Texas. Thermocouples were installed in the bridge superstructure to measure the concrete temperature in the box and wings. Strain gauge devices were developed and placed at several locations in the superstructure to document the effects of temperature gradients on the superstructure. On-site data recording equipment continually acquired concrete temperatures and strains over a ten month period beginning in March of 1995.

Data concerning temperature gradients and thermal induced strains through the superstructure depth, across the webs and slabs of the box are presented. These temperatures and strains are analyzed for their affect on the superstructure. Calculations are made to compare predicted strains to measured strains. Comparisons are made from design thermal gradients to those measured and recommendations are made concerning design gradients for hollow prestressed concrete bridge superstructures.