Load rating of cast-in-place reinforced concrete box culverts based on load testing

Many in-service Reinforced concrete box culverts (RCBCs), designed according to older design codes using lighter design truck weights, may require restrictive load posting when rated for current design and permit loads although no structural distress is observed. Most load rating engineers typically utilize 2 dimensional (2D) structural frame software, which leads to fast but highly conservative results. The ratings are negatively affected by the conservative live-load attenuation through soil fill, along with the inability of the 2D method to account for any lateral load distribution. The present study aimed to load rate RCBCs based on strains obtained from field testing. The testing evaluated strains induced by live-load distribution through soil fill when load rating eighteen (18) cast-in-place RCBCs in Kentucky. RCBCs with one-, two-, three-, and four-cells were load tested and the effects of truck live loads analyzed. Fill heights varied from 0 m to 3.6 m, cell spans from 3.0 m to 6.7 m, and culvert spans from 6.7 m to 18.6 m. Strain gauges were attached to the interior surfaces of culverts and the data recorded corresponding to known truck loads and positions. Irrespective of the number of cells in the RCBC, strain variations in the top slab were similar with change in fill height. Maximum strains in the top slabs measured less than 10 microstrain, a value at which live-load effects could be considered negligible, at fill heights greater than 3.05 m. The load rating based on field tests improved both Inventory and Operating rating factors for all tested RCBCs.