Devdas Menon

Estimation of along-wind moments in RC chimneys

Abstract This paper reviews the prevailing international codal recommendations to determine the design along-wind moments in reinforced concrete (RC) chimneys and towers. A comparative study reveals significant disparities among the various codal estimates of the design moments, given the same basic wind speed and terrain condition. These disparities are a matter of concern to practising chimney designers and approving authorities in many countries. The relative accuracy of these differing estimates, which are based on different gust factor methods, have been quantitatively assessed with reference to rigorous stochastic dynamic analyses. This study covers a number of linearly tapered RC chimneys with heights in the range 100–400 m, located in different terrain conditions and subject to the range of wind speeds encountered in practice. It is observed that the estimates of the design moments are generally conservative in varying degrees, and are unconservative only in some cases. In very rough terrains, the lack of appropriate design provisions results in extremely conservative designs. With a view to improving the accuracy in the predictions, modifications to the gust factor method have been proposed in this paper.

AXLE LOAD VARIATIONS AND VEHICLE GROWTH PROJECTION MODELS FOR SAFETY ASSESSMENT OF TRANSPORTATION STRUCTURES

Abstract The design formulations of pavements and bridges are significantly influenced by the uncertainties associated with the prevailing vehicle weight characteristics. Accurate modelling of load spectra and the estimation of possible vehicle growth and composition are the main components in the safety assessment of transportation structures. The vehicle weights from the existing traffic form the basis for load spectra development, and the vehicle projections are subjective to the socio‐economic conditions. In this paper, the methodologies of appropriate data consideration for such representation are discussed and demonstrated based on the data from India. The load spectra are also developed for vehicular data from Interstate 95 (I‐95) in New York State. In lieu of the current status of the design codes for pavements and bridges worldwide, most of the developing countries are in need of analogous models for the calibration of the design basis.

Prediction of radial stresses due to prestressing in PSC shells

Abstract Large prestressed concrete (PSC) shells, such as silos, containment structures are prestressed along their principal curvatures. Prestressing cables are provided in the thickness of the shell either in one or two directions. The stresses normal to the middle surface are generally neglected in the design of such shell structures. However, the development of radial tensile stresses at the time of prestressing can lead to possible delamination failure, if these stresses are excessive. This paper presents simple procedures to estimate the radial stress distribution due to prestressing. Two methods are presented: ‘equilibrium of slice’ and ‘modified Lame,’ the latter being more rigorous and applicable to both singly and doubly curved shells. The solutions obtained are in conformity with the results of finite element analysis published earlier. Finally, an attempt is made to determine the local tensile stress distribution at the periphery of the prestressing duct hole, which also contributed to the problem of delamination.

Glass Fibre Reinforced Gypsum Panels for Sustainable Construction

The tremendous housing need of India is causing the depletion of virgin building materials. In an era of scarce resources, sustainable solutions are always preferable. Glass Fibre Reinforced Gypsum (GFRG) technology is one such solution where all the structural members are constructed using hollow panels infilled with reinforced concrete (RC), as per structural design. This paper discusses the sustainability of construction using GFRG panels in terms of embodied energy and indoor thermal comfort. The case study building at IIT Madras campus was found to have lesser embodied energy compared to conventional buildings. The result is significant considering the fact that the building sector consumes 40% of the total energy in world. Also, the inside temperature was found to be lesser by 2 °C which will help in reducing the air conditioning requirement of the building and thereby the operational energy.

Seismic Design Philosophy: From Force-Based to Displacement-Based Design

The Indian subcontinent has experienced several earthquakes, and the major ones have caused significant damage to the buildings and lifeline structures. Researchers have realized the importance of developing a robust design method which can reduce the probability of occurrence of such damages. This paper discusses the currently followed seismic analysis techniques, their applicability and limitations, as well as the newly emerging displacement-based design methods. There is a need for a paradigm shift from the conventional force-based approach to the more rational displacement-based design methods to limit damages and to get uniform risk structures. Researchers are now focusing on the refinement of these new design methods to make them applicable to all types of buildings, bridges, and other structures.

Moment-curvature relation of Reinforced Concrete T-beam sections - Numerical and Experimental studies

A simple algorithm is proposed to develop a nonlinear moment-curvature relation for Reinforced Concrete (RC) T-beam sections using nonlinear material models. The flange in the beam adds to the complexity of analysis compared to rectangular beam sections. This algorithm has been validated by laboratory testing of simply supported flanged beam specimens, subjected to two-point loading. The load- deflection data obtained from the experiments is converted to a moment-curvature relation for the section applying simple bending theory. The results predicted numerically by the proposed algorithm are found to be in good agreement with the experimental data. The algorithm developed can be used to generate the load-deflection curve of an RC T-beam subject to any given loading. A portion of continuous slab integrally connected to the beam serves as flange and the supporting beam as web. The flexural capacity of such sections can be enhanced when the flanges are in compression. The web helps in resisting shear stresses. The moment-curvature relation is obtained by carrying out sectional analysis of RC T-beam section. In this analysis, different cases are considered depending on the position of the neutral axis (6). Testing of three T-beams has been carried out in the laboratory. These beams are loaded symmetrically at two points to simulate pure flexure. Load- deflection data obtained from these tests is used to generate moment-curvature relation using theory of simple bending. The results generated by numerical algorithm are validated with these experimental results. The behavior of the beams is studied for variation in depth of flange and tensile reinforcement. The nonlinear sectional analysis for these RC T-beams is explained in the next section.

Seismic Evaluation of RC Stepped Building Frames

‘Stepped building’ frames, with vertical geometric irregularity, are now increasingly encountered in modern urban construction. This chapter proposes a new method of quantifying irregularity in such building frames, accounting for dynamic characteristics (mass and stiffness). The proposed ‘regularity index’ provides a basis for assessing the degree of irregularities in a stepped building frame. This chapter also proposes a modification of the code-specified empirical formula for estimating fundamental period for regular frames, to estimate the fundamental time period of the stepped building frame. The proposed equation for fundamental time periods is expressed as a function of the regularity index. It has been validated for various types of stepped irregular frames. A new approach to determine the lateral load pattern, considering the contributions from the higher modes, is proposed that is suitable for pushover analysis of stepped buildings. Also, a modification to the displacement coefficient method is proposed, based on time history analysis of 78 stepped frames. When the newly proposed load pattern is combined with the modification of the displacement coefficient method of FEMA 356, the target displacement for the stepped building frame is found to match consistently the displacement demand given by the time history analysis.