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Thermal Gradients in Southwestern United States and the Effect on Bridge Bearing Loads
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Thermal gradients became a component of bridge design after soffit cracking in prestressed concrete bridges was attributed to nonlinear temperature distribution through the depth of the bridge. While the effect of thermal gradient on stress distributions has been previously investigated in concrete bridges, less research has been done investigating the effect on bearing loads. The climate condition of the southwestern portion of the United States may cause larger thermal gradients than recommended by AASHTO LRFD Bridge Design Specifications. The main objective of this study was to evaluate the effect of thermal gradients in the southwestern region of the United States on bearing design. This study consisted of two parts, heat flow analysis using long-term meteorological data and two case study bridges in Nevada analyzed for bearing loadings including several variations of thermal gradient loading. One bridge was a two-span concrete posttensioned box girder bridge in Las Vegas, the second bridge was a two-span composite steel girder bridge in Reno. Heat flow analysis was conducted using meteorological data from weather stations in Northern and Southern Nevada to evaluate the AASHTO LRFD thermal gradient recommended for Nevada. Results showed that AASHTO LRFD Zone 1 thermal gradient is an unconservative estimate of conditions in the southwestern states for both concrete and composite superstructures. Analysis in CSiBridge using area models of the concrete bridge in Las Vegas indicated that the largest predicted thermal gradient obtained through heat flow analysis increased total exterior bearing loads 12% relative to total load including the AASHTO thermal gradient. Analysis using area models of the composite steel girder bridge in Reno indicated that the unaltered temperature profile obtained through heat flow increased the total exterior bearing 27% relative to total load including the AASHTO thermal gradient at Abutment 1. Variation of constant temperature through the steel girder influenced both longitudinal and transverse loading. Reducing the temperature through the girder maximized bending moment and support reactions, while unaltered temperature through the girder maximized individual bearing loads. Thus, it is uncertain whether constant temperature through girder should be included.
Report No. CCEER-17-01