Experimental Behavior of Skewed Cast-in-Place Concrete Box Culverts

Abstract

Cast-in-place reinforced concrete box culverts are widely used for roadway drainage and are the most common type of bridge in many states. The current culvert design methodology is performed in accordance with the AASHTO LRFD Bridge Design Specifications. However, there is no specific guidance for skewed culverts, where the culvert and roadway centerlines are not perpendicular. Due to the lack of explicit provisions for skewed culverts, state departments of transportation (DOTs) adopt conservative design practices and vary extensively, highlighting the need for consistent design approaches. This study investigates the structural effects of skew angle on cast-in-place concrete box culverts through full-scale laboratory tests. Three culvert specimens with 10 foot clear spans, identical reinforcement layouts, and skew angles of 0 degrees (control), 30 degrees, and 60 degrees were constructed and tested under simulated vehicular loading. Experimental results revealed that the skew angle had minimal impact on the overall stiffness, service-level behavior, and ultimate strength of the culvert. All specimens displayed nearly rigid slab-to-wall joint behavior, validating rigid assumptions commonly used in analysis. Cracking was not observed at service design load levels, and ultimate load capacities significantly exceeded AASHTO LRFD requirements. The skewed culverts exhibited beneficial two-way bending behavior, engaging both longitudinal slab reinforcement and integral parapet beams, enhancing load distribution. Differences in failure modes emerged at high skew angles, transitioning from beam shear failures at 0° and 30° to slab punching shear at 60°, highlighting the importance of detailing in highly skewed configurations. This research demonstrates that current conservative practices, such as doubling reinforcement at skewed regions, are unnecessary. A standard, uniform reinforcement design adequately meets structural demands for skewed culverts, offering DOTs opportunities to optimize designs and achieve significant material and cost savings without compromising structural performance.