Limit States Testing of a Buried Deep-Corrugated Large-Span Box Culvert
Limit States Testing , Box Culvert
Results are reported from full-scale testing of a buried, deep-corrugated, large-span box culvert with a 2.4 m rise and 10.0 m span under controlled laboratory conditions. A total of twenty-one experiments were conducted on the structure, measuring its response without backfill, during backfilling, under a loaded tandem axle dump truck, and under simulated vehicle loading with force applied by an actuator. Surface strain measurements were used to calculate bending moments and thrusts, while deflections were monitored using an electronic theodolite. Tests conducted to a maximum force of truck loading specified by the Canadian Highway Bridge Design Code multiplied by a dynamic load allowance factor were performed at three cover depths. The maximum moment increased from 6 to 41 kNm/m as the cover was reduced from 1.5 to 0.45 m. This was attributed to less load distribution and decreased soil-structure system stiffness at shallower cover. The maximum bending moments were consistently observed directly beneath the applied force. Tests were also conducted at forces larger than the design values to identify the ultimate limit state(s) of the structure. An ultimate limit state was encountered at approximately 800 kN where the geotechnical resistance beneath the loading pads was exceeded. A subsequent test with the loading pad force spread over larger areas permitted larger forces to be applied. An ultimate limit state of the structure involving the formation of three plastic hinges occurred at 1100 kN. The plastic hinge initially formed at the crown, followed by hinges located at each shoulder. Post-test observations showed evidence of local buckling of the conduit wall, gaps between the plates at the seams, the tilting of bolts along the longitudinal seams, and surface cracks in the soil. Applying the material resistance factor of 0.9 to the ultimate load limit of 1100 kN measured for the structure yields a reserve capacity of 1.7 when compared to the fully factored load including dynamic load allowance and live load factors.