WIM data are used to assess traffic loads on bridges, for various purposes:
- Development and calibration of bridge loading codes
- Design of non standard bridges, e.g. with span longer than the maximum specified in the bridge code (> 200 m for the Eurocode)
- Calculation or assessment of fatigue damage and lifetime
- (Re-)assessment of existing structures
Bridge Code Calibration
Originally the design loads taken into account in bridge codes were rather artificial and far above the real truck loads, mainly because bridges were designed by the self weight (dead loads), and not accurately calculated. Since the fourth quarter of the 20th century, lighter structures are designed, with advanced computer programmes and the truck weights increased. Therefore, the bridge design loads were more considered. With the semi-probabilistic approach (load and resistance factors, safety factors), the new bridge loading codes are designed and calibrated using real gross weights and axle load distribution (e.g. Ontario bridge code, 1978, Eurocode 1991-2, etc.).
WIM data collected on different routes and during long time periods allow deriving reliable probabilistic distributions. E.g. calculation of extreme loads with a given return period (e.g. 1000-year for traffic loads in the Eurocode), are derived from the WIM data by extrapolation to calibrate the design loads.
Design of non standard structures
Design codes, as Eurocode 1991-2 on traffic loads on bridges, apply to standard structures. But bridges with spans greater than 200 metres, or other exceptional structures, require an accurate and detailed design for both economical and safety reasons. Using real and adapted WIM data makes it possible to make an optimal design or in-service verification. Long span suspended or cable stayed bridges were among the case studies: Storebaelt bridge in Denmark, Severn bridge in UK, Millau viaduct and Normandy bridge in France, Akashi and Tatara bridges in Japan, etc.
Fatigue design and assessment
Fatigue damage and lifetime is a critical (ultimate) limit state above all for steel bridges and parts of composite bridges. Traffic loads induce repeated stress cycles which may initiate and then propagate cracks, until a partial failure or a bridge collapse. “Rain-flow” histograms of stress cycles are calculated using WIM data and influence lines or surfaces, and then combined with fatigue (S-N) curves in a Miner model to assess the whole lifetime of a given detail or the probability of failure for a given lifetime (probabilistic approach).
Re-assessment of existing bridges
Safety of existing structures requires periodically assessing their bearing capacity and traffic loads, above all in case of structural deterioration or structural ageing. Applying the bridge design code loads mostly leads to erroneous conclusion that the structure is not safe, because these loads contain safety margins, and hence may be much greater than the real traffic loads. Moreover, old bridges have been designed with former codes and lower design loads. However, their residual lifetimes are much lower than those of new bridges and they are more frequently monitored and inspected. Reversely, in some case the traffic loads evolved dramatically and reinforcement may be needed. In all cases, an accurate knowledge of the current traffic loads by WIM is needed, and for fatigue re-assessment, the whole traffic load history is required.
Assessment of effect of weights and dimensions of trucks on bridges
Traffic data recorded by WIM systems give a true picture of traffic, at a given location and a given time. The impact of any change in the heavy commercial vehicle weights and dimensions regulation may be assessed by analyzing WIM data before and after the rule change.
Before changing the rules, if increasing some weight limits, it may be useful to check by simulation the impact on the stock of bridges of the new regulation. Assumptions can be made on the evolution of traffic after the load limit change, recorded traffic data files can be modified accordingly, and the effects on bridges can be assessed. This was done before the increase from 40 to 44 t of 5-axle articulated truck in several EU member states, and the introduction of longer and heavier (EMS) truck up to 60 t and 25.25 m in some EU member states.