Experimental and analytical investigations of the thermal behavior of prestressed concrete bridge girders including imperfections.

摘要

Bridge engineers have increased the span of prestressed concrete bridge girders by using high-strength concrete and optimized cross-sections. However, the lengthening of the girders has also increased the possibility of a stability failure in the girders, especially during construction. In particular, unexpected imperfections in the girder and the supports during fabrication and construction could adversely affect the stability of the girders especially when the girders are exposed to thermal effects from the environment.An experimental and analytical study was conducted on a BT-63 prestressed concrete girder segment to investigate the thermal effects on the girder. A 2D finite element heat transfer analysis model was then developed which accounted for heat conduction in the concrete, heat convection between the surroundings and the concrete surface, heat irradiation from the sun, and heat radiation to the surroundings. The solar radiation was predicted using the location and geometry of the girder, variations in the solar position, and the shadow from the top flange on other girder surfaces. The girder temperatures obtained from the 2D heat transfer analysis matched well with the measurements. Using the temperatures from the 2D heat transfer analysis, a 3D solid finite element analysis was performed assuming the temperatures constant along the length of the girder. The maximum vertical displacement due to measured environmental conditions was found to be 0.29 inches and the maximum lateral displacement was found to be 0.57 inches.Using the proposed numerical approach, extremes in thermal effects including seasonal variations and bridge orientations were investigated around the United States to propose vertical and transverse thermal gradients which could then be used in the design of I-shaped prestressed concrete bridge girders. A simple beam model was developed to calculate the vertical and lateral thermal deformations which were shown to be within 6% of the 3D finite element analyses results. Finally, equations were developed to predict the maximum thermal vertical and lateral movements in terms of the span length of the girders for four AASHTO-PCI standard girders.To analyze the combined effects of thermal response, initial sweep, and bearing support slope on a 100-foot long BT-63 prestressed concrete girder, a 3D finite element sequential analysis procedure was developed which accounted for the changes in the geometry and stress state of the girder in each construction stage. The final construction stage then exposed the girder to thermal effects and performed a geometric nonlinear analysis which also considered the nonlinear behavior of the elastomeric bearing pads. This solution detected an instability under the following conditions: support slope of 5° and initial sweep of 4.5 inches.This research also performed a sensitivity study to evaluate the effects of changes in the thermal properties of concrete, as well as the solar absorptivity and emissivity of concrete surface on temperature distributions in the prestressed concrete girder. The solar absorptivity was determined to have the largest effect on the girder temperatures. In general, for the prestressed concrete bridge girder subjected to environmental thermal effects, the influences of the thermal properties of concrete would be minimal when thermal properties are within reasonable ranges. The thermal behavior of the girder was then evaluated using the 3D thermal stress analysis with variations in the coefficient of thermal expansion (CTE) of concrete. With increases in the CTE, the vertical and transverse movements proportionally increased.