Low temperature performance of elastomeric bearings in a full size field experimental bridge
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Elastomeric bearings have been extensively used as bridge and seismic isolation bearings. Seismic bearings support gravity and earthquake-induced axial loads and can accommodate large lateral displacements. Past experimental studies demonstrated that elastomers stiffen at low temperatures. Stiffening at low temperatures takes two forms: immediate (or instantaneous) and crystallization. Most tests to characterize behavior have been performed on small size rubber specimens at controlled temperatures for a short period of time, which is not representative of full-scale bearings installed in the field. This study investigates the long-term behavior of elastomeric seismic isolation bearings subjected to a wide range of temperatures. A full size bridge was constructed and seismically isolated by others using low damping rubber (LDR) bearings. A series of experiments was performed to obtain the mechanical properties (e.g., effective stiffness, effective damping) of the bridge and the bearings. Most of the tests were performed in winter to enable an understanding of how the in-service mechanical properties of bearings change with cold temperatures: such as those experienced in the northeast of the United States. The experimental results show that effective stiffness of LDR bearings increase at low temperatures, especially below 0°C (32°F). A finite element (FE) model was developed to investigate how rapidly the internal temperature of a bearing would change with time. Weather data from the site of the bridge was used as input to the FE model for performing thermal analysis. The numerical results show that the bearings were likely in thermal equilibrium prior to the winter tests at low temperatures, indicating that the measured change in effective stiffness, from subsequent days of testing in winter, is largely due to crystallization stiffening or softening.