United States Department of Transportation
i
Behavior of reinforced concrete pier caps under concentrated bearing loads.
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1995-02-01
[PDF-3.79 MB]
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Abstract: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.
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