Devdas Menon

Of the various methods available for the analysis of box-girder bridges subject to vehicular loading, threedimensional finite element analysis (3DFEA) gives accurate results, compared to the other methods. However, 3DFEA is not commonly adopted in practice, and simplified methods of frame analysis are preferred. In this paper, a new simplified method (Modified Fixed Beam Method) is presented for the transverse bending analysis of box-girder bridges without overhangs. The results of extensive three-dimensional finite element analysis have been quantitatively assessed to provide a logical basis for the proposed 'Modified Fixed Beam Method' suitable for design practice. In the proposed method, the top flange of the box-girder bridge is modeled as a beam with fixity at the web-top flange junctions (fixed beam model), and the transverse bending moments so generated are suitably modified to yield the actual distribution of transverse moments. Modification factors are proposed based on extensive numerical studies, to account for the influence of type and location of loads, wheel contact dimensions, spacing of webs and the relative thickness of flange and web. Finally, the application of this proposed simplified method is demonstrated by means of two illustrative examples.

Rectangular column sections, subject to axial compression combined with biaxial bending, are commonly encountered in RC design. As an exact analysis of the strength of such sections involves rigorous calculations, IS 4562 offers a simplified method based on an approximation of the 'load contour' (originally suggested by Bresler4). The normalised load contour is approximated as a straight line at very low axial load levels and as a quarter-circle at very high load levels, and the shape of the curve is linearly interpolated at intermediate load levels. A rigorous analysis on a number of sections with varying aspect ratios and reinforcement patterns reveals significant errors in this codal approximation. It is seen that the resulting designs are either conservative or unconservative to varying degrees at different axial load levels. The present simplified codal procedure can be made considerably more accurate by appropriately modifying the recommended values of the coefficient α (which defines the shape of the normalised load contour), as shown in this paper.

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.

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. © 2007 Taylor & Francis Group, LLC. All rights reserved.

In design practice, the transverse bending analysis of box-girder bridges is commonly done by modeling the cross section as a frame of unit width with imaginary supports at the web locations. The transverse bending moments obtained from simple frame analysis (SFA) is sometimes increased by a small percentage to accommodate the errors in modeling. In this paper, a large number of simply supported box-girder bridges have been analyzed by both SFA and three-dimensional finite element analysis for different load conditions and wheel contact areas, and the errors in SFA have been studied and quantified. The error is found to vary widely at the web-top flange junction as well as under the load (maximum sagging moment), depending on the eccentricity of loading, the wheel contact dimensions and the web-flange thickness ratio. Accordingly, a set of correction factors to the results of SFA have been proposed, which is expected to be of significant use in design practice. The use of the correction factors is demonstrated by means of two illustrative examples. The scope of the study is limited to the simplest case of a single-cell concrete box-girder bridge (simply supported with end diaphragms) without overhanging flanges. ©ASCE.

A study was conducted, to address the problem of estimating short-term deflections in reinforced concrete (RC) two-way slabs, using finite element analysis with appropriate stiffness modifiers. The study proposed a modified Branson's formulation, to account for the complex effects of flexural cracking and tension stiffening, along with a simple modification to account for torsional cracking. The parameters in proposed formulation are determined empirically and the procedure validated by 23 investigative loading-deflection data sets. The proposed formulation stated that the stiffness changes for an RC two-way slab, subjected to bi-directional bending and torsion throughout the slab. It was demonstrated that the stiffness changes throughout the slab, due to cracking, depending on the magnitude of the principal tensile stress. The study also demonstrated that the idea is, top retain the assumed isotropic behavior in the elasticity in the proposed approach.

Safety assessment and rational design of bridge structures requires the uncertainty associated with vehicle loads to be modeled as accurately as possible. This modeling is rendered difficult by the presence of vehicle axle weights that involve different combinations of unimodal and multimodal probability distributions with different dependence structures. In this paper, a transformation invariant approach using copula functions is proposed for the multivariate simulation of dependent axle weights of different vehicle classes. Copula based dependence modeling, which is widely used in the financial risk analysis, is applied to model and simulate three different vehicle cases with different combinations of marginal probability distributions for axle weights. The database of observed vehicle weights is based on the data collected at five locations on national highways in India. The dependence between multimodal distributions of axle weights is accurately considered and simulations are carried out. The simulated axle weights are found to be in very good agreement with the observed data. This type of simulation is useful in carrying out simulation-based reliability analysis of bridges and pavements. © 2006 ASCE.

Simplified methods are generally preferred in practice for the transverse bending analysis of concrete box-girder bridges. The cross-section of the bridge is generally modelled as a frame of unit width with imaginary supports at the web locations and simplified frame analysis (SFA) is carried out. Alternatively, a method (ISM) using influences surfaces for elastic plates (Homberg's or Pitcher's Charts), as recommended by AASHTO, is adopted for simplified analysis. Because of the various assumptions made in the modelling, the SFA and ISM results are found to be erroneous. While the ISM results are generally very conservative, the SFA results are found to be sometimes unconservative in some cases of vehicular leading. In an earlier investigation, the authors have studied and quantifed the nature of the errors in SFA with reference to detailed 3-dimensional finite element analyses (3DFEA), for box-girder bridges (single rectangular cell) without flange overhangs, and a set of correction factors proposed for use in design practice. In the present study, the procedure is extended to box-girder bridges with flange overhangs. Based on extensive numerical studies by both SFA and 3DFEA, a set of correction factors are proposed, and their use demonstrated through an illustrative example. It is established that the use of conventional SFA gives erroneous results which are eliminated by applying the proposed modifications.

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.