Behavior of Reinforced Concrete Pier Caps Under Concentrated Bearing Loads

At congested highway interchanges, the Texas Department of Transportation (TxDOT) uses narrow concrete piers and shallow depth steel cap girders. Research Project 0-1302 is concerned with the connection detail between these two elements. This report deals with the shear strength and reinforcement details at the top of the concrete pier in the vicinity of the bearings. Since no formal design procedure currently exists for determining the required amount and distribution of reinforcing steel in a pier cap, this research also had the pupose of providing design guidelines for the pier cap. To investigate the behavior of the pier caps, six test specimens were constructed at a 30% scale. Five different reinforcing steel patterns were used in the six specimens to examine the contributions of different reinforcing types to the pier cap strength.

Eleven static load tests were conducted to failure on the six pier caps. For all specimens, load on the pier cap was carried primarily by the action of a tied arch which transferred load from the base plates into the column. Overall, specimens that had a greater quantity of horizontal reinforcing steel and adequate development of horizontal reinforcing had a greater capacity.

Three design methods were used to analyze the strength of the pier caps tested: (1) AASHTO (1992) Corbel Provisions; (2) ACI 318-89 Deep Beam Provisions; and (3) Strut-and-Tie Method. The corbel and deep beam provisions were very conservative in predicting the capacity of the pier cap because they consider only concrete capacity in shear. On average, these two methods underestimated the pier strength by a factor of 3 to 4. Testing showed that the pier cap resisted loads through a tied arch, which is a much stronger load-carrying mechanism than concrete in shear. The strut-and-tie models used were much more accurate than conventional design methods in predicting the capacity of the pier caps because they model the compression arch action ovserved during testing. The strut-and-tie method is suggested for design because strut-and-tie analyses gave the best correlation with test results, modeled true behavior, and were still conservative.

To detail the use of the strut-and-tie method, a design example using a proposed strut-and-tie model is presented. Also, recommendations are given for evaluating existing pier caps through field inspection.