The efffect of elastomeric bearing degradation on bridge dynamic response.

The dynamic response of highway bridges is a topic that has been thoroughly researched over many years. However, understanding of how the dynamic response of bridges is affected by the performance of their bearings over an extended period of time is, at present, not clearly defined. Although health monitoring of bridge structures is relatively advanced, the scope for further research is wide. The study presented in this thesis contains research on plate structures, ranging from a simple Euler–Bernoulli method to determine natural frequencies, modal analysis of a plate structure in the laboratory, finite element analysis (FEA) of the plate structure, modal analysis on a full-scale structure subjected to vehicle loading, and FEA of a simplified model representing the full-scale structure. A combination of these methods has allowed conclusions to be drawn regarding the effects of support degradation and the consequent impact on structural performance. In the laboratory, modal analysis of a small-scale, thin rectangular plate of Perspex has been completed. A series of boundary conditions were investigated by altering the support offered to the plate using springs of varying stiffness, simulating bearings with different stiffnesses. Vibrations of varying frequency were applied to the plate and its response recorded. Displacement values provided the clearest indication of the effect of bearing stiffness, with the least stiff spring producing the largest displacement. Changes in support stiffness had a marked effect on the resonant frequency of the plate, with approximately a 23% frequency change observed between spring 1 (1.22 Nmm⁻¹) and spring 5 (15.62 Nmm⁻¹). Full-scale testing on a highway bridge at Berwick-upon-Tweed on the A1, over the River Tweed, was completed in May 2005. These data form the baseline for future dynamic testing and condition monitoring of the structure. To characterise the dynamic properties of the bridge, the force generated by each type of vehicle crossing it was determined using existing instrumentation. Statistical analysis indicated that the bearings were functioning as expected, but were subjected to forces of a much greater magnitude due to overloaded heavy goods vehicles (HGVs) than anticipated in current design specifications. Although larger HGVs constituted a small percentage of total vehicles, they contributed a disproportionately high level of damage to the bridge. Both displacement and frequency were found to be valid measures of bearing performance and degradation. The method is proposed as a condition monitoring approach for bridges and their bearings.