Ultimate Shear Behavior of Post-Tensioned Deep Transfer Girders

Abstract

Deep beams are typically used as transfer girders in tall buildings, cap beams in bridges, in foundations and offshore structures. They are characterized by high shear resistance due to their small shear-span-to-effective-depth ratio a/d that does not exceed about 2.5. Due to the large loads they carry, the design of such members is very important in order to avoid partial or complete collapse of the structure. As opposed to slender beams, deep beams cannot be designed based on the simple but powerful hypothesis that plane sections remain plane. Experimental studies on deep beams have shown that the use of prestressing improves the shear resistance.

The aim of this thesis is to study a Two-Parameter-Kinematic-Theory (2PKT) developed by Mihaylov et al. (2013) for reinforced concrete deep beams and to extend the theory to prestressed deep beams. The original theory is able to predict the ultimate shear strength of reinforced concrete deep beams using only two kinematic parameters. In order to cover the case of prestressed deep beams, an extended model is proposed. This extended model captures the effect of prestressing in three ways: 1) increase of the shear force derived from flexural equilibrium; 2) effect of the prestressing on the geometry and strength of the critical loading zone (CLZ); and 3) dowel action of the prestressing reinforcement. The extended model is validated against a collected database of tests conducted on rectangular deep beams without openings and with straight prestressing tendons. The extended theory is also compared to the original 2PKT approach. It is shown that the ultimate shear strengths predicted by the extended model agree very well with the experimental results. Compared to the results from the original model, the predictions are significantly improved.

In order to further validate the extended theory, non-linear finite element modelling is also performed. It is shown that the 2PKT method that uses only two degrees of freedom produces very similar (or even better) results that the complex numerical models with thousands of degrees of freedom.

Future investigations on this topic can study the effect of curved tendons on the shear resistance of deep beams. Moreover, I-girders can also be studied as they are very common in practice.