This paper presents an investigation on structural cracking observed in a reinforced concrete (RC) pier cap supporting a prestressed concrete box girder of 13 m span. Unexpected vertical cracks were observed at service loads on the sides directly under the bearings. A site visit revealed that the elastomeric bearings were compressed on one side with loss of contact on the other. The crack width measurements showed a crack width as high as 1 mm at some locations, where the cover provided was found to be 100 mm, more than the proposed cover of 50 mm. A detailed analysis using non-linear finite element analysis (NLFEA) established the reason for cracking as the reduced contact area at the bearings. The wide cracks were due to the unexpected high cover. The safety of the structure at ultimate loads was also checked using NLFEA and the strut and tie method, and is seen that the structure is safe at ultimate loads.

Unexpected Cracking in an RC Bridge Pier Cap—A Case Study

At congested highway interchanges, horizontal and vertical clearance requirements may dictate the use of narrow piers and shallow depth cap girders to accommodate the various roadways and overpasses. In situations such as this, the state of Texas uses horizontally curved steel plate girders as the bridge structural system, supported on integral steel cap girders at single column piers. Two bearings are used to connect the steel cap girder to the concrete pier. Owing to the narrow pier, unbalanced loading may produce a transverse overturning moment at the pier; the bearings resist this moment by developing a couple, with one bearing loaded in compression and one bearing loaded in tension. When the unbalanced loading is caused by truck traffic, which is cyclic, the bearing resisting the uplift is subject to fatigue loading. The standard connection used by the state of Texas is an in-house design that comprises a line rocker bearing, which accommodates the horizontal rotation, and embedded anchor bolts that are used to both resist potential uplift and to provide a positive connection from the cap girder to the pier. The behavior of this connection, however, is not well understood and the detailing is complex. The objectives of this research were to examine and categorize the behavior of the existing connection and to develop a new detail that is simpler and cost effective, and to develop design guidelines for steel reinforcement in the concrete pier cap. The research showed that the standard Texas Department of Transportation connection performs adequately with respect to horizontal rotation but that it cannot resist uplift because of the poor fatigue characteristics of threaded anchor bolts. While the fatigue problem may be mitigated by post-tensioning the uplift bearing, this option in not available for the standard connection. Two new connection details were developed, one to be used for situations in which uplift does not occur and one that is capable of resisting uplift. The new details replace the line rocker with a rolled wide-flange section, and the threaded anchor bolts are replaced with high-strength threadbar, which is specifically designed for post-tensioning. The new connections proved to be more cost effective than the standard connection. At the top of the concrete pier, a strut-and-tie model is recommended for designing the steel reinforcement to support the bearing loads.

The connection between a steel cap girder and a concrete pier. final report

The connection between a steel cap girder and a concrete pier

At congested highway interchanges, the Texas Department of Transportation (TxDOT) uses narrow concrete piers and shallow depth steel cap girders. Research Project ()"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 purpose of providing design guidelines for the pier cap. To in'vestigate 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-TIeMethod. 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 observed 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.

https://rosap.ntl.bts.gov/view/dot/31676/dot_31676_DS1.pdf

R. J. Denio

Papers

Behavior of Reinforced Concrete Pier Caps Under Concentrated Bearing Loads