Truck Load Effects for Michigan Bridges

Truck Load Effects for Michigan Bridges

  The paper provides a statistical background for bridge live load model, including static and dynamic components, and girder distribution factors. The static portion of live load includes truck weights (gross vehicle weight), axle loads, load effects (moments, shear forces, strain and stress). The data is based on field measurements carried out in Michigan. A considerable percentage of illegally overloaded trucks was observed, in particular on bridges located far from truck weigh stations. The cumulative distribution functions are shown for various locations in Michigan, and they are strongly site-specific. Dynamic load was measured on girder bridges. It turned out that there is almost no correlation between static and dynamic portions of live load. This means that dynamic load factor decreases for increasing static live load, or dynamic load factor decreases for heavier trucks. For a single heavy truck, the dynamic load factor is less than 0.20, and for two trucks side-by-side, dynamic load factor is less than 0.10. Girder distribution factors were also based on measurements, and compared to analytical values calculated using the code-specific procedures. It was observed that the code specified values are conservative in most cases. The statistical data can be used to update a basis for reliability analysis of girder bridges.