Edinburgh Napier University

  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; 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 the conclusions presented herein to be
drawn with respect to the effects of support degradation, and the consequent effect 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 have been investigated through altering the support
offered to the plate by a series of springs, each with a different stiffness, to simulate bearings
with different stiffnesses. Vibrations of varying frequency have also been forced upon the plate,
and its response recorded. Displacement values provided the clearest indication of the effect of
bearing stiffness, with the least stiff spring resulting in the largest displacement. Alteration of
support stiffness in the model can have a marked effect on the resonant frequency of the plate
(approx. 23 % frequency change between spring 1 (1.22 Nmm 1) and 5 (15.62 Nmm 1)).
Full-scale testing on a highway bridge at Berwick-upon-Tweed on the Al, 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 describe the dynamic properties of the structure,
the force generated by each type of vehicle traversing the structure was determined using
instrumentation already in place on the bridge. Statistics drawn from the data are presented,
which indicate that the bearings are functioning as expected, but are subjected to forces of a
much larger magnitude due to overloaded HGVs than in current design specifications. Larger
HGVs made up a small percentage of overall vehicles recorded, but contribute a much higher
proportion of damage caused to the bridge. Displacement and frequency were both valid
measures of bearing performance and therefore state of degradation.
The method is offered as a condition monitoring test for bridges and their bearings.