Date: Thursday, Feb 10th
Presenter: Christopher Higgins, Professor, Slayden Construction Faculty Fellow, School of Civil and Construction Engineering, Oregon State University
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.
A research program was undertaken at Oregon State University to assess the remaining capacity and life of vintage RCDG bridge bent caps. The research program included field and laboratory testing 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, as shown in Fig. 1. 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). Five of the specimens had a cross-section of 16 x 72 in. (406 x 1829 mm), and one specimen had a cross-section of 16 x 48 in. (406 x 1219 mm). All cap beams had a clear span of 264 in. (6706 mm), and they were supported indirectly by 96 in. (2438 mm) high columns with a square cross-section of 24 x 24 in. (610 x 610 mm), as illustrated in Fig. 2. 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 72 in. (1829 mm) deep specimens prior to failure testing.
The experimental results contribute a new set of data for behavior and strength of very large deep beams and the corresponding analyses provide methods for capacity prediction that incorporate the salient features of vintage RCDG bent cap designs.
Christopher Higgins is a Professor and the Slayden Construction Faculty Fellow in the School of Civil and Construction Engineering at Oregon State University. He is also Associate Director of the Oregon Transportation Research and Education Consortium.
He holds a PhD from Lehigh University, MS from The University of Texas at Austin, and BSCE from Marquette University. He is a registered Professional Engineer.
He teaches graduate and undergraduate courses in Structural and Bridge Engineering and his research is focused on evaluation and rehabilitation of aging and deteriorated infrastructure.
He and his research teams have conducted hundreds of field and laboratory tests of full-scale bridge components.
His research on aging reinforced concrete bridges is reported to have saved State of Oregon taxpayers approximately $500 million (half a billion dollars).