Effect of live loads on top slab of cast-in-place reinforced concrete box culverts

Reinforced concrete box culverts (RCBCs) designed according to old codes, using lighter truck weights (e.g., H15), may require posting when rated for current design and permit vehicles. When load ratings are calculated based on current AASHTO Specifications, they can be negatively affected by the conservative live-load attenuation through soil fill. This study evaluates the positive bending strains induced by live-load distribution through soil fill when load rating cast-in-place (CIP) RCBCs. Eighteen (18) single and multicell RCBCs in Kentucky were load tested and the effects of truck live loads analyzed. Fill heights, HF, varied from 0 m (0 ft) to 3.57 m (11.7 ft), clear cell spans, S, from 3.05 m (10.0 ft) to 6.71 m (22.0 ft), and culvert spans from 6.71 m (22.0 ft) to 18.59 m (61.0 ft). Field data were collected using strain gauges attached to the interior surfaces of culverts. Irrespective of how many cells the RCBCs had, strain variations with fill height in the top slabs were similar. At fill heights greater than 3.05 m (10.0 ft), maximum strains in the top slabs measured less than 10 microstrain, a value at which live-load effects are considered negligible. Combining live-load strains measured on the Kentucky culverts with ones from field tests in Missouri and Louisiana, a live-load tensile strain envelope is proposed to load rate the top slabs of RCBCs for positive bending when cell spans are less than 4.6 m (15.0 ft).