Devdas Menon

Reinforced concrete (RC) members with circular cross sections are sometimes preferred over rectangular cross sections in members such as columns, piers, and piles, because of their identical strength properties in all directions. In practice, the shear strength of a circular section is generally based on an equivalent rectangular section, using the formulation provided in ACI 318-14. This paper establishes the need to introduce an additional correction factor of 2/π to the shear strength estimation of circular stirrups, using the formulation applicable for rectangular stirrups. Use of this modified formula is validated in the experimental results of 30 tests on RC circular beams, reported in this paper, as well as 27 test results reported by Ang et al. (1989) and Jensen (2010). The theoretical predictions of shear strength have been based on ACI 318-14 and IS 456-2000.

The application of yield line analysis to carry out strength design of reinforced concrete (RC) slab systems is mostly limited to solid slabs without beams. In an earlier paper on isolated rectangular beam-slab systems, the authors had demonstrated that such analysis, considering plastic hinges in the beams along with yield lines in the slabs, can result in rational and economical designs. In this paper, it is shown that such yield line analysis can be further extended to beam-slab systems with secondary beams, and the predictions have been validated by tests carried out on four rectangular RC beam-slab systems (each comprising four symmetric grid units), supported at the four corners on pillars. Six possible collapse mechanisms have been investigated. It is established that the critical collapse mechanism is governed primarily by the beam-slab relative strength. It is shown how an economical and rational design can be achieved, making use of the proposed yield line analysis.

An efficient computational technique for nonlinear analysis of reinforced concrete (RC) beam-slab systems (rectangular grids) using grillage analogy is proposed. A load-controlled algorithm has been adopted for tracing the complete load-deflection response till the ultimate load, under monotonic loading. Material modelling of reinforced concrete in the present study includes nonlinear stress-strain behaviour such as cracking of concrete, yielding of steel and tension-stiffening effect. The proposed methodology has been validated against the test data reported in the literature as well as the experiments conducted for the present study. The numerically generated load-deflection curves are found to be in good agreement with the experimental results. The main advantages of this procedure is that it gives a more accurate estimation of short-term deflections under service loads, and a more realistic distribution of moments and shear forces at the limit state of collapse. Further, the algorithm can be conveniently used in reliability analysis of reinforced concrete beam-slab systems.

This paper revisits the design of simple rectangular reinforced concrete (RC) slabs, integrally connected to edge beams, supported at the four corners, and subject to gravity loads. Typically, the edge beams are made adequately stiff, whereby the slab can be analyzed and designed separately for two-way bending, considering the edges to be simply supported. This paper establishes, through yield line analysis and experimental studies, that the fnal failure is more likely to occur by a combined beam-slab failure-typically by one-way bending along the long-span direction, with plastic hinges forming in the middle of the long-span beams. The conventional yield line pattern (two-way slab-alone failure) will occur only in exceptional cases where the long-span beams are heavily reinforced. It is clearly demonstrated that the actual mode of failure and the collapse load are governed primarily by the relative beamslab strength in all cases, regardless of whether the edge beams are stiff or shallow. The proposed yield theory has also been validated by experiments on square beam-slab systems reported in the literature. These new insights on collapse load estimation of rectangular beam-slab systems can lead to more rational and economic strength design and detailing.