Devdas Menon

The prevailing version of IS 1343 (1980) needs improvement with respect to the procedure for estimating the ultimate moment capacity of prestressed concrete (PSC) sections. There is a lack of clarity in the specified typical stress-strain curves for two different types of tendons specified in the code especially with regard to the manner in which the specified partial safety factor has to be applied. The code also suggests a simplified method in Appendix B, as an alternative to more rigorous strain compatibility method for estimating the ultimate moment capacity. However, it is seen that the IS 1343 simplified procedure invariably fails to satisfy the condition of force equilibrium and also overestimates the moment capacity. These shortcomings in the code are examined and reviewed in this paper, and suitable modifications are proposed in the form of modified stress-strain curves and simplified tables for incorporation in the forthcoming revision of the code.

The incresead cover specifications in the recently-revised IS 456 will serve the desired objective of enhancing durability, only if the cracking of the cover concrete (under tension) is effectively controlled. Increased cover in flexural members such as RC slabs will lead to increased crack-width, which in turn, will adversely affect durability by permitting ingress of moisture and chemcial attack. The need for the designer to necessarily check for the limit state of serviceability (cracking) has not been sufficiently emphasised in the revised code. Instead, it is mentioned that such a check need not be carried out if the prescribed detailing (bar spacing) is followed. This paper attempts to establish that this assumption is generally not satisfied in RC slabs. Crack-width control is most effectively achieved by reducing the tensile stress in the steel and by minimising bar spacing. Results of a parametric study bring out the difficulties in maintaining the crack-width below 0.3 mm when large cover is provided; such cases may necessitate an incresed in the slab thickness.

Prestressed concrete masts provide an economically viable alternative to steel masts for lighting and communication towers. The tubular section of the mast has to be optimally designed to resist the bending moment due to lateral load (predominantly wind) in combination with axial compression due to self weight. To facilitate this, interaction curves need to be generated at service and ultimate loads, as indicated in the paper. A software has been developed, in which the design program is linked with an optimisation program to optimise the basic cost of the mast. A genetic algorithm procedure has been used for this purpose. Application of the software to single-tube and three-tube PSC masts reveals that the single-tube mast is economical for heights up to 50 m, beyond which the three-tube mast provides a more economical alternative. Some practically useful results of single-tube optimal mast sections are presented. Additional details regarding the connections in the three-tube masts and cost comparison with steel masts have not been included in the study.

Use of rapid techniques for timely delivery of construction projects by adopting fast methodologies and alternate building materials is necessary in India, given the tremendous housing shortage. Further, the construction has to be affordable and also sustainable. The construction technique proposed in this paper, by making use of Glass Fibre Reinforced Gypsum (GFRG) panels (with reinforced concrete infilled cavities) to build homes, promises such a solution for rapid affordable mass housing in India. GFRG building system is a rapid building technology composed of prefabricated wall panel load bearing system. GFRG is also an eco-friendly building material which can be manufactured out of either natural or industrial gypsum. In India, these panels have been manufactured from the fertilizer by-product waste. As an outcome of the research done at IIT Madras on the feasibility of using these panels for affordable mass housing in India, a two-storeyed GFRG building of 184 square meter area was built at IIT Madras campus, as a demonstration of the suitability of this technology. This is described in this paper.

In reinforced concrete (RC) liquid retaining structures cracking of concrete is a major design criteria. The current IS:3370 (Part II) bears little relationship to actual crack widths that can be directly predicted and there is a need to revise the prevailing codal provisions. In this paper, the direct method of crack control advocated in ACI and BS codes are reviewed, and suggestions made for improvement of IS:3370. Suggestions are also made for dealing with situations involving two-way flexure and direct/eccentric tension (for which reliable crackwidth formulae do not currently exist), using appropriate correlation between load factors and limiting crackwidths.

Slenderness effects in reinforced concrete beams are not comprehensively accounted for in the prevailing design codes of concrete. Existing recommendations are prescriptions of limiting slenderness ratios, which are semi-empirical in nature. No reduction in flexural moment capacity is recommended for beams with moderate slenderness. Experimental studies on 15 RC beams reveal that, the existing design guidelines are not sufficient to ensure gradual failure and that a reduced ultimate flexural moment capacity results in beams of moderate slenderness. A simplified formulation for evaluating critical buckling moment in reinforced concrete beams is proposed in this paper. Authors highlight a theoretical basis for limiting slenderness and based on the experimemtal results, propose a conservative reduction factor for evaluating the ultimate flexural moment capacity of RC slender beams.

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 presents a reliability-based analysis of reinforced concrete slab culverts constructed as per the design recommendation of the Ministry of Road Transport and Highways (MORTH) at the flexural limit states. Hasofer and Lind's technique is employed to evaluate reliability index, β, which is a measure of probability of failure. The results indicate that the nominal probability of failure of slab culverts designed as per MORTH's recommendations at the limit state of collapse in flexure lies in the order of 10 -5 to 10 -8, which reveal that the designs are conservative (in comparison with AASHTO's recommendations), and the degree of conservatism is not uniform for all spans.

The present study attempts to model the load-deflection behaviour of slender rectangular reinforced concrete beams, accounting for deflections in both the vertical and horizontal planes under gravity loading. The lateral deflection depends on imperfections and proximity of the applied moment to the critical buckling moment of the beam. A simplified formulation is proposed here, based on an extension of existing theories that model the reduction in flexural stiffness and torsional stiffness with increasing load, for the purpose of predicting the complete load-deflection response of slender beams. This formulation is found to generate load-deflection plots that match reasonably with the experimental results.