LOAD DISTRIBUTION AND THERMAL RESPONSE OF COMPOSITE CELLULAR BOX GIRDER BRIDGES

Abstract

The use of composite bridges in interchanges of modern highway systems has become increasingly popular for economic as well as aesthetic considerations. Cellular steel section composite with a concrete deck is one of the most suitable in resisting torsional and warping effects induced by highway loading. This type of structure has inherently created new design problems for engineers in estimating its load distribution when subjected to moving vehicles as well as the level of thermal response to environmental loads. Indian designers face a daunting task as the formulation of design codes and guidelines for composite cellular bridges are not even in the infancy stage. The current Indian code of practice does not specify any guidelines for the design or method of analysis of composite cellular bridges. It also does not specify any design thermal loads and recommends the use of specialized available literature. In absence of any indigenous research and national codal provisions, the bridge designers are forced to take recourse to state-of-art research and the codes of practice of European or North American countries. The current design practices in relevant literature, recommends few analytical methods such as grillage-analogy method, folded-plate method, finite-strip method, and finite-element method for the analysis of composite multicell box girder bridges. To meet the practical requirements arising during the design process, a simple design method is needed for composite multi-cell bridges in the form of load distribution factors for moment and shear. As far as the design thermal loads are concerned, the bridge designers are extensively adopting the practices of British code, BS 5400 by and large. However, the applicability of such provisions is yet to be established in Indian context, where a wide range of daily and seasonal variation occurs in ambient temperature and solar radiation throughout the country. The intention of this thesis is to provide a simplified design method and realistic design thermal gradients for composite cellular box girder bridges to be constructed in different parts of the country. The prediction of the elastic and the ultimate behaviour as well as thermal response of composite multi-cell box girder bridges is presented. For this purpose temperature measurement in an outdoor type model of a composite bridge section, instrumented with thermocouples, is conducted for over a year. 11 An automated instrumentation system, using sensors and remote data acquisition system modules, is evolved for the measurement and record of the temperature distribution. The analysis of long-term measured temperature data has been carried out for accessing the realistic trend of design thermal gradients applicable to composite box girder bridges. A two-dimensional analytical model is described to estimate the temperature distributions and their effects in bridges. It is based on the finite element formulation using existing heat transfer and solar engineering theory. This analytical model takes into account the geometry of the bridge cross section, the location of the bridge, and the climatic conditions. The validity of this model is tested against the results obtained from experimental study on a laboratory model of a composite bridge section as well as against published temperature data obtained from a bridge in service. The present study is aimed to predict the temperature distribution and its effects on a composite box girder bridge located in different parts of the country. For this purpose, the country has been divided into twenty-five zones to start with, however, the numerical implementation suggested the adequacy of classifying into five zones and in turn, an attempt has been made to put forward thermal design recommendations for each zone. To this end, first, a detailed parametric study has been carried out to compute the thermal gradients and induce stresses in composite bridges due to variations in environmental and geometrical parameters for one location i.e. Delhi, to finalize the critical values of the key parameters for the probable thermal response under most adverse conditions. The parametric study is then carried out for different geographic locations by selecting one representative city from each of the five identified zones, to decide the design thermal gradients. A finite-element analytical model, based on the commercially available "ANSYS" software, was used for the load distribution analyses. The analytical model was verified by published results from tests on composite concrete deck-steel three-cell bridge model [Sennah, 1999]. Extensive parametric study, using the finite-element modelling, was conducted, in which 70 composite multi-cell bridge prototypes were analyzed to evaluate their load distribution factors for moment and shear under dead load and IRC live loading conditions. The key parameters considered in this study were: end-diaphragm thickness, span-to-depth ratio, number of cross-bracing and top-chord systems, number of cells, number of lanes, bridge spans, bridge aspect ratio, and loading conditions. Based on the in data generated from the parametric study, expressions for load distribution factors for moment and shear were deduced. Recommendations to enhance the torsional resistance are made. Mechanical shear connectors in composite steel and concrete beams require slip to transmit shear. However, most composite bridges are designed using full-interaction theory assuming no slip at steel concrete interface because of the complexities of partialinteraction analysis techniques. In the assessment of ultimate load capacity of composite bridges this simplification may not be warranted as it is often necessary to extract the greatest capacity from the structure. For this purpose an analytical model using partialinteraction theory is described, in which the finite stiffness of shear connectors is modelled as nonlinear spring elements. Stiffness perpendicular to longitudinal axis of the stud, are defined by the load-slip relation adopted from relevant literature. Material nonlinearity is incorporated in the analysis using nonlinear material model available in ANSYS software. For concrete Drucker-Prager failure criterion is used while for steel bilinear isotropic hardening is used as yielding criterion. Results from published literature are used to substantiate the analytical modeling. A nonlinear analysis of two-lane three-cell bridge of 30m span with incremental loads for IRC Class 70R wheeled vehicle loading and dead load is carried out to determine its ultimate load capacity. It is revealed that the ultimate load is appreciably decreased due to the interface slip. The serviceability of the bridge which otherwise seems to be satisfactory on the basis full interaction analysis may adversely affected when interface slip is properly accounted, The results from this practical-design-oriented dissertation would enable the bridge engineerto design composite cellular bridges more reliably and economically.