Devdas Menon

The strut-and-tie method (STM) is commonly used to design reinforced concrete (RC) pier caps, where the traditional methods of shear and flexural design cannot be used due to nonlinear strain distribution. This paper presents experimental studies on scaleddown pier cap specimens subject to four concentrated loads, to study the influence of various parameters and the applicability of STM. The STM prescribed by ACI 318 and AASHTO are found to give reasonably conservative results only when the beneficial effect of triaxial confinement near the bearing node is invoked; otherwise, the predictions are overly conservative. Variations in bearing plate size, distributed reinforcement, and loading eccentricity did not have much influence on the load capacity, but distributed reinforcement was seen to be beneficial in limiting crack widths. The formation of the assumed STM is validated using strain measurements in concrete and steel, and it is observed that this develops only after significant cracking.

This study was motivated by the observation of unexpected cracking in an actual reinforced concrete (RC) pier cap, consisting of a pair of two secondary corbels, supported on a primary corbel with the pier at its center. The pier cap was analyzed using the strut-and-tie method (STM) by considering a three-dimensional (3-D) model. Tests were carried out on two scaled-down pier cap specimens to assess safety and serviceability performance. The load-carrying capacity of the pier cap was under-predicted by approximately 75% by STM using ACI 318 and AASHTO codes. The test results presented in the paper include evolution of cracking and strains in the steel reinforcing bars, along with load-deflection plots. It was observed that concentrating reinforcing bars near the bearings resulted in an increase in strength by approximately 8.5% without adversely affecting serviceability. In summary, STM is found to provide an effective basis for the design and detailing of such complex pier caps.

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.

This paper presents an investigation on structural cracking observed in a reinforced concrete (RC) pier cap supporting a prestressed concrete box girder of 13m span, with a carriage way width of 20m. The pier cap is designed to carry the heavy concentrated loads transmitted to the pier through elastomeric bearings. 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 1mm at some locations, where the cover provided was found to be 100 mm, which is more than the proposed cover of 50 mm. A detailed analysis using nonlinear finite element analysis (NLFEA) was performed to understand the causes of these cracks using two models: One with full contact of the bearings and the other with half contact. The complete crack pattern was obtained from NLFEA which showed cracking at a similar location as that at the site. The cracking occurred at a lower load in the model with half contact at bearings. This clearly established the reason for cracking as the reduced contact area at the bearings. The wide cracks were perhaps due to the unexpected high cover at the corner location. The safety of the structure at ultimate loads was also checked using NLFEA and strut-and-tie method, and is seen that the structure is safe at ultimate loads.