Assessment dynamic ratio for traffic loading on highway bridges

The determination of characteristic bridge load effect is a complex problem. Usually,
statistical extrapolation of simulated static load effects is used to derive a lifetime
characteristic static load effect. However, when a vehicle crosses a bridge, dynamic
interaction occurs which often causes a greater total load effect. This total load effect is
related to the static load effect through a dynamic amplification factor (DAF).
Specifications often recommend a conservative level for DAF, based on bridge length,
number of lanes, and type of load effect only. Therefore significant improvements in the
accuracy of this calculation are possible if a DAF, specific to the considered bridge, is
applied. In this paper, the authors develop a novel method that considers site-specific
bridge and traffic load conditions and allows for the reduced probability of both high
static loading and high dynamic interaction occurring simultaneously. This approach
utilises multivariate extreme value theory, in conjunction with static simulations and
finite element vehicle-bridge dynamic interaction models. It is found that the dynamic
allowance for the sample bridge and traffic considered, is significantly less than
recommended by bridge codes. This finding can have significant implications for the
assessment of existing bridge stock.