Devdas Menon

The strut-and-tie method (STM) has been widely accepted and used as a rational approach for the design of disturbed regions (‘D’ regions) of reinforced concrete members such as in corbels and deep beams, where traditional flexure theory does not apply. This paper evaluates the applicability of the equilibrium based STM in strength predictions of deep beams (with rectangular and circular cross-section) and corbels using the available experiments in literature. STM is found to give fairly good results for corbel and deep beams. The failure modes of these deep members are also studied, and an optimum amount of distribution reinforcement is suggested to eliminate the premature diagonal splitting failure. A comparison with existing empirical and semi empirical methods also show that STM gives more reliable results. The nonlinear finite element analysis (NLFEA) of 50 deep beams and 20 corbels could capture the complete behaviour of deep members including crack pattern, failure load and failure load accurately.

The present study reports the experimental investigations of the long-term strains in two PSC beams, one bonded and one unbonded, having the same cross-section and concrete mix, subject to the same prestress and environmental conditions. In addition, control specimens in the form of one unloaded prism specimen of the same cross-section, and two standard cylinders, one loaded axially and the other unloaded, were also monitored for the same time duration of 250 days. Electrical strain gauges and fiber optic sensors were used for strain measurement, and the results show that they agree well with each other. The ACI 209 model is found to have the least error in the prediction of axially loaded cylinder creep and shrinkage strains. However, all the prediction models (assuming constant stress) are found, in general, to overestimate the creep and shrinkage strains, and the overall loss in prestress in the two PSC beams. When the loss in prestress is incorporated, using a 3D finite element numerical model, the prediction is found to improve significantly in all the models, and particularly in the case of the GL 2000 and MC2010 models. Alternatively, improved predictions can be obtained by using the compliance from cylinder creep data and the shrinkage strains from a control specimen (having the same volume-to-surface ratio) for predicting the strains in PSC beams; this is found to yield good results.

The current housing shortage problem in the country, especially among the low-income group and the need to address their shelter needs, led to the introduction of GFRG (Glass fibre reinforced gypsum) panels hi India. These are light-weight, load-bearing panels which can resist axial, in-plane and out-of-plane loads and various studies conducted worldwide established the suitability of the panel for the construction of walls and slabs. GFRG buildings consist of GFRG walls and slabs alone and can be constructed up to 5-8 storeys hi low to moderate seismic zones, and lesser height in higher seismic zones. Studies were carried out hi India and other countries to understand various properties of GFRG panels and this paper presents a critical review of the experimental and theoretical mvestigations 011 the structural behaviour of GFRG panels.

A component level experimental study has been carried out on four reinforced concrete (RC) walls subjected to a sustained compressive load for a period of 1 year and shrinkage tests on 10 prisms under ambient environmental conditions. Two extreme longitudinal reinforcement percentages and two grades of concrete were considered and their influence on the time-dependent behavior studied. It was observed that both longitudinal percentage of steel and concrete grade have a significant influence on the time-dependent strains. The evolution of time-dependent strain in RC member was also predicted, adequately accounting for the effect of reinforcement, using a theoretical model which can employ any linear viscoelastic constitutive law for concrete and a linear elastic constitutive law for reinforcing steel. The ACI 209 and fib MC 10 recommendations for creep and shrinkage of plain concrete have been used for the prediction of long-term strains. It is demonstrated that the analysis predicts the time-dependent strains reasonably well (with a statistical mean deviation error of 1.16 and 1.00) for the creep tests on RC walls, when the compliance function proposed by ACI and fib is used. However, in the case of shrinkage tests, the accuracy with both ACI and fib models was limited (1.64 and 1.66, respectively). It is further demonstrated that by suitably recalibrating the compliance and shrinkage parameters in both ACI and fib models, the accuracy of shrinkage prediction in the companion RC specimens improves significantly with mean deviation error of 1.07 and 1.05 for ACI and fib models, respectively.

In the conventional method of strength design of reinforced concrete (RC) beam-slab systems, it is assumed that if the beams are adequately stiff, the slab and beams can be analyzed and designed separately under factored gravity loads. This paper demonstrates, through yield line analysis and load testing of isolated beam-slab systems, that such a design, which tacitly assumes a ‘slab alone failure’ mechanism, is irrational and overconservative (failing at a load level much higher than expected). The actual collapse of the conventionally designed beam-slab system invariably involves a combined beam-slab failure mechanism. It is therefore more rational and economical to design explicitly for such a collapse mechanism, accounting for plastic hinge formation in the beams along with yield lines in the slab. The proposed method suggests provision of minimum slab steel (as prescribed by the design code), and then designing the beams aiming for a combined two-way beam-slab failure. Experimental load testing establishes that the collapse occurs as planned and that the proposed economical design has the desired code-specified safety margins.

Time-dependent strains in reinforced concrete (RC) members are usually estimated using approximate algebraic methods. This paper presents an exact method for estimating the time-dependent strains in RC members subjected to concentric axial compression, using creep compliance and shrinkage strain information for the corresponding plain concrete. The axial strain in concrete is taken to be the sum of shrinkage strain and the creep strain. Shrinkage strain in concrete is modeled as a: "lack-of-fit" problem. Assuming a linear viscoelastic constitutive law for concrete and a linear elastic constitutive relation for reinforcing steel, the corresponding one-dimensional viscoelastic boundary value problem is solved. It is assumed that there is a perfect bond between concrete and steel. The proposed method is validated using tests reported in the literature. The prediction of overall time-dependent axial strains under applied axial loads is found to match the observed test results closely. However, there are discrepancies in the relative magnitudes of creep and shrinkage strains, and the possible reasons for these are also discussed.

In the conventional design of reinforced concrete (RC) rectangular slabs in beam-slab systems subject to gravity loading, it is assumed that the code-specified moment coefficients (derived based on yield line theory, assuming non-deflecting supports at the edges) can be used, provided the beams provided at the edges are adequately stiff. Recent experimental and theoretical studies have established that such designs turn out to be over-conservative, as the assumed yield line mechanism of the slab does not occur. In general, a combined beam-slab collapse mechanism occurs at the limit state of collapse, in which the yield lines in the slab connect to plastic hinges in the supporting beams. With a proper understanding of possible collapse mechanisms and estimation of the lowest collapse load, using yield line theory, a more rational and economical design of the beam-slab system is possible. Considerable savings in steel can be achieved, while fully complying with the strength and serviceability requirements of the code.

This paper attempts to take a fresh look at the behaviour of simple reinforced concrete beam–slab systems, subject to gravity loading. The simplest case of a square slab, integrally connected to edge beams and supported on pillars or columns at the four corners, is considered. It is shown that the usual design procedure of separating the slab analysis and design, from that of the edge beams (proportioned to be adequately stiff), is irrational in terms of expected behaviour at collapse. The expected diagonal yield line formation in the slab is kinematically incompatible with the expected plastic hinge formation in the edge beams. This paper attempts to resolve this dispute in design by showing how the mode of failure depends on a relative beam–slab strength parameter. The yield line theory, which considers the alternative possibility of combined beam–slab failure, is validated by experimental results reported in the literature. It is established that the prevailing design practice, assuming diagonal yield line formation in the slab, turns out to be not only irrational but also uneconomical. The combined beam–slab failure mechanism is more likely to occur in practice, and it would be rational and economical to aim for such a design.

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.