Devdas Menon

This paper presents details of a nonlinear analysis program capable of tracing the complete load-deflection response of reinforced concrete (RC) beams, frames and grids. The program has been developed in MATLAB using a displacementcontrolled algorithm. The program is capable of handling geometric nonlinearity by using a geometric stiffness matrix in conjunction with a primary stiffness matrix that is constantly updated iteratively. Material nonlinearity is accounted for by making use of prescribed moment-curvature formulations. Stiffness degradation due to cracking of concrete, yielding of steel, tension-stiffening, strain softening and P-A effects is incorporated. The program is computationally efficient and requires only minimal input from the user. The proposed methodology has been validated against experimental data. It is seen that the numerical results generated using the proposed algorithm are in good agreement with the experimental results reported in the literature. Further, the method has been extended to RC grids and RC columns in tension.

At present, there are no recommendations in codes such as ACI 318 and BS 8110 to estimate the critical buckling moment in slender concrete beams. It is assumed that if the slenderness ratios of the beams are limited to the values prescribed by the codes, the failure moment of the beam will be dictated by flexure and not by buckling. Experimental studies carried out as part of the present study, however, show that the specified slenderness limits are not reliable, and failure by lateral instability can occur in slender beams designed according to the code. It is also shown that the existing formulations to predict critical buckling moments in beams, suggested by various researchers, grossly overestimate the capacities in the case of under-reinforced beams. In this paper, a modified formulation is proposed to predict theoretical buckling moment, and it is found to agree very closely with experimental results for both under-reinforced and over-reinforced beams. Copyright © 2006, American Concrete Institute. All rights reserved.

Behaviour of slender reinforced concrete (RC) beams is different from that of normally propotioned beams, on account of slenderness effects which introduce susceptibility to lateral torsional buckling. Highly slender beams are prone to sudden instability mode of failure. Moderately slender beams are also susceptible to slenderness effects and they may undergo flexural failure at moment values less than their flexural capacities corresponding to material failure (Muf). Concrete design codes presently do not provide any procedure to evaluate the critical buckling moment (Mcr), and also do not account for the capacity reduction in moderately slender beams on account of slenderness. The existing recommendations are limited to prescriptions of limiting slenderness ratios, which are semi-empirical in nature. The experimental results carried out in the present study clearly show reduction in moment capacity in beams with slenderness ratios well below the critical values given in various codes and in literature. Based on the experimental and theoretical studies conducted, new design guidelines are proposed for slender rectangular RC beams.

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.

The Department of Civil Engineering, Indian Institute of Technology Madras, has developed a Handbook on Seismic Retrofit of Buildings under the sponsorship of the Central Public Works Department and under the aegis of Indian Buildings Congress. The Handbook contains seventeen chapters covering different aspects of seismic retrofit The first chapter is a stand-alone chapter that explains the concepts of seismic design and retrofit in a language suitable for the lay person. Besides the introductory chapters, there are chapters on evaluation and retrofit of various types of buildings. The types of buildings cover non-engineered, masonry, reinforced concrete, steel buildings and historical and heritage structures. The geotechnical seismic hazards and retrofit of foundations are placed as separate chapters. Retrofit using fibre reinforced polymer composites, energy dissipation and base isolation devices are introduced. A chapter on quality assurance and control is included. This paper presents the content of the Handbook and two case studies briefly.

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.

This paper presents the results of 15 experimental tests on rectangular slender reinforced concrete (RC) beams. The results reveal limitations in existing theoretical formulations to estimate the failure moment capacity and mode of failure. An improved theoretical formulation is proposed here to predict the critical buckling moment including effects related to nonlinearity and cracking of concrete. Also, following the trends in steel design, an improved measure of slenderness ratio is proposed. Based on a study of 72 test results, it is shown that there is an interaction between flexural tension and instability modes of failure in moderately slender beams. To avoid lateral instability failure, it is suggested that the slenderness ratio be limited to unity. A 'moment reduction factor' is also proposed to account for slenderness effects in RC beams. © 2011 Elsevier Ltd.

This paper presents experimental studies on four reinforced concrete (RC) walls subject to sustained axial compressive load for 1 year under ambient environmental conditions. The reinforcement percentage and concrete strength have been varied, and their influence on the time-dependent behavior studied. It is observed that the percentage of steel, concrete grade, and age of loading have a significant influence on the time-dependent strains. The evolution of time-dependent strain was also numerically simulated using six prediction models, in conjunction with analysis accounting for the restraining effect of steel. It is demonstrated that although these models have been empirically derived based on tests done on plain concrete cylinders, they are also applicable to large planar elements such as walls, having low volume to surface area. The ACI 209 model for creep and shrinkage is found to yield good results, when applied to RC walls.

Second-order moments of considerable magnitude arise in tall and slender RC chimneys and towers subject to along-wind loading, on account of eccentricities in the distributed self-weight of the tower in the deflected profile. An accurate solution to this problem of geometric nonlinearity is rendered difficult by the uncertainties in estimating the flexural rigidity of the tower, due to variable cracking of concrete and the 'tension stiffening' effect. This paper presents a rigorous procedure for estimating deflections and second-order moments in wind-loaded RC tubular towers. The procedure is essentially based on a generalised formulation of moment-curvature relationships for RC tubular towers, derived from the experimental and theoretical studies reported by Schlaich et al. 1979 and Menon 1994 respectively. The paper also demonstrates the application of the proposed procedure, and highlights those conditions wherein second-order moments become too significant to be overlooked in design.

Precast concrete pipe racks are an economically advantageous alternative to steel pipe racks in industrial projects like refineries and petrochemical plants. One major reason for this is the elimination of fire proofing needs for the precast element. Optimal design of the concrete elements would further lower the cost. This paper deals with the optimal design of precast concrete pipe racks, considering design variables such as member sizes, grades of concrete and steel, and reinforcement area. Genetic algorithms (GA) have been used for optimisation and a program in VC++ has been developed to arrive at the optimal solution, which corresponds to overall cost, satisfying design criteria for strength and limiting deflection. Proposals for most favourable jointing mechanisms in terms of cost and constructability have also been given.