United States Department of Transportation
i
Evaluation of Reserve Shear Capacity of Bridge Pier Caps Using the Deep Beam Theory
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2018-12-01
[PDF-10.04 MB]
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Edition:Final Report
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Abstract:Many bridge pier caps are deep due to short shear spans. When analyzed using the slender beam theory (i.e., the sectional method), a large number of pier caps are found to be shear-overloaded even though they don't exhibit any noticeable cracking or signs of distress. AASHTO LRFD 2017 recommends the use of either a strut-and-tie or nonlinear finite element model for the analysis and design of deep members. Both methods are more sophisticated and require more effort than the sectional method. The objective of this study was to simplify the strut-and-tie method for pier caps to obtain larger and less conservative shear capacity predictions. For this purpose, a solution algorithm (computer program) was developed based on Section 5.8.2 Strut-and-Tie Method of AASHTO LRFD 2017. The program, named STM-CAP (Strut-and-Tie Method for pier CAPs), is implemented in Microsoft Excel using Visual Basic macro codes. An adaptive graphical solution procedure was employed to minimize the input errors and give the analyst options for optimizing the automatically generated model. STM-CAP calculates the utilization ratio for every element, which reflects the condition (overload or reserve capacity percentage) of the pier cap. If overloaded, STM-CAP indicates the calculated failure mode and its location. Suitable rehabilitation methods and load limits can then be determined accordingly. STM-CAP is verified using a general purpose strut-and-tie software, CAST (Computer Aided Strut and Tie) and VecTor2 (a nonlinear finite element analysis software) for eight existing pier caps located in Ohio. In addition, the sectional method calculations were performed to demonstrate the extra shear capacity predictions obtained from the strut-and- tie method. The strut-and-tie method predicted 2 to 3 times higher shear capacities for beams with shear span-to-depth ratio (a/d) of 0.50. The predictions by STM-CAP and the sectional method converge as the a/d ratios approach 3.0. The research results have a potential to result in significant cost savings by rehabilitating fewer pier caps and reducing the associated construction work and traffic disruption. The developed program, STM-CAP, can also be used when load rating concrete pier caps.
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