- Many conventionally reinforced concrete deck girder (RCDG) bridges were built in the US during the 1950s, throughout the expansion of the Interstate System. Designs followed the AASHO standard of the time, which permitted higher shear stress in concrete and reduced detailing requirements than permitted by current specifications. Many of these bridges exhibit diagonal cracking of the main girders and bent caps that has been attributed to the design as well as increased traffic volume and truck load magnitudes, and temperature and shrinking effects. While these RCDG bridges are nearing the end of their design lives, wholesale replacements or renewals are not possible due to the large numbers of bridges, and many remain in service.
In order to develop a methodology for assessment of vintage RCDG bridge bent caps, a total of six realistic full-scale replicas of in-service bent caps with 1950s vintage details, including the overall geometry, reinforcement configuration, and material properties were constructed for laboratory tests. The test specimens were a subassemblage of the pertinent bridge components at the bent cap region including the integral columns, cap beam, and portions of the monolithic internal girders that frame into the cap. Test variables included shear span-to-depth (a/d) ratio, number of flexural bars anchored in the columns, flexural reinforcement cut-off locations, web reinforcement size and grade, and loading type (static and fatigue loading). The bent caps were loaded indirectly, similar to their in-field counterparts, via portions of the integral girders. The specimens were loaded to failure using an incremental cyclic load protocol. To simulate the effect of 50 years of ambient traffic loading, 1,000,000 cycles of fatigue loading, based on a unique load protocol derived from in-situ measured stress ranges from three in-service bridges, was applied to one of the specimens prior to failure testing.
Various analytical methods were applied to the laboratory specimens for capacity prediction, including ACI 318 shear design methods, the Modified Compression Field Theory (MCFT) sectional analysis approach, strut-and-tie models, mechanical models, and non-linear finite element analysis.
The embedded reinforcement at the anchorage zone, the web reinforcement, and the a/d ratio were all found to be pertinent parameters which affect the strength of bent cap specimens. High cycle fatigue did not cause a significant degradation in the ultimate capacity for a specimen with light web reinforcement, although debonding of the stirrups was observed at the characteristic diagonal crack vicinity. Best results for capacity prediction were achieved with non-linear finite element analysis and with The Modified Zararis Mechanical Model. Further finite element analyses showed that strength of specimens with heavy web reinforcement may be more sensitive to the effects of bond fatigue.