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CAN
BRIDGE REPAIR AND REPLACEMENT COSTS BE REDUCED BY DEVELOPING
"SMART" BRIDGES?
There
is a lot of European research going on in bridge loading at the
moment. The Eurocode for bridge loading, EC1, Part 3, is the first
bridge loading code in the world to be based on statistical
principles. With the techniques used to develop the EC1 notional
traffic loading – equivalent of BS5400 “HA” – it is now
possible to derive a site-specific traffic loading for a bridge that
is being assessed. In many cases, particularly if the bridge is
subject to a low traffic flow, it is possible to prove that bridges
are safe even if they can only carry a small portion of what the
assessment code prescribes.
The
approaches used at the moment vary considerably and it is questionable
if this approach is yet sufficiently well established to be used for
general bridge assessment. However, there is some degree of
convergence in European thinking as can be seen from the COST 345
action, “Procedures for the Assessment of Highway Structures”, the
final report of which is due out this year.
Research to improve our understanding of traffic loading is ongoing at
UCD. For example, the Eurocode allowance for dynamic impact is quite
conservative – it is based on tests carried out in Switzerland of
single trucks crossing bridges. It is known that dynamic excitation
occurs when the frequency of the truck crossing speed coincides with
the natural frequency of the bridge. However, the vast majority of
bridges have two or more lanes and the critical loading event consists
of two or more trucks meeting (or passing) on the bridge.
The
dynamics of a two-truck loading event is not easy to predict – it is
affected by the relative speeds and the point at which the trucks
meet. However, you can reasonably expect interference in the dynamic
excitation effect of each truck which would give a much reduced
dynamic amplification overall. This is being modelled at UCD at the
moment and tested in the field through a collaboration with the
Slovenian National Building and Civil Engineering Institute. The
results may form the basis for a future revision to the Eurocode
traffic load model.
So,
what will happen in the future? One idea that is being investigated
through a European research project is a bridge that communicates with
the truck so as to guarantee that dynamic excitation does not occur when two trucks
meet on it. The bridge will detect the approaching trucks and, if
their speeds and calculated meeting point are such as to cause a
problem, the bridge will instruct one or both of them to slow down. It
may sound far fetched but this is the kind of thinking being used to
try to get the maximum out of the infrastructure that we already have
and to minimise the need to strengthen or replace bridges using
non-renewable materials.
Eugene OBrien
Civil
Engineering Department, UCD
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