Load Distribution in Reinforced Concrete Slab Span Bridges

Changes in vehicular loading over time have caused some concrete slab span bridges to rate poorly when considering modern truck weights, which leads to load posting and restrictions that in turn lengthen trucking routes. A potential way to allow heavier vehicles to cross these bridges would be to increase the load rating by utilizing a more accurate live load distribution factor generated from field testing and computational modeling. The focus of this study was to determine the live load distribution factor for two slab span bridges using results from live load testing and plate model analysis. Field testing utilized a suite of instrumentation, and simple isotropic or orthotropic plate models of the slab span bridges were validated with field data to further investigate live load distribution. Field-test estimates of equivalent width computed using displacement data were more precise than those using strain data, but modeling indicated that strain data may better capture the distribution of stresses in the slab. Despite significant visible damage, the field bridge behaved more like an isotropic plate. AASHTO equivalent widths were conservative when compared with those computed from the field results and isotropic model for single-lane truck loading but were similar to each other for multilane loading. However, model results also indicated that the equivalent width depends on the load configuration, such that single-axle loading will be concentrated into a narrower strip compared with tandem loading. In addition, an orthotropic slab will further narrow the equivalent width compared with an isotropic slab, such that AASHTO equivalent widths may no longer be conservative for single-axle loading of a slab so degraded that it may be treated as effectively orthotropic.