Bridges are the most complicated structures of today’s road and rail transportation networks, and these are often referred to as a lifeline of the social infrastructure system. Among the different types available, hollow web girder or box girder bridges are best suited nowadays. These bridges are preferred because of their higher torsional and flexural stiffnesses, higher effectiveness, better stability, economy and aesthetics in comparison to other bridge types. The bridges can be constructed by using different materials e.g. composite, steel, reinforced concrete, and prestressed concrete. The reinforced concrete (RC) is a suitable material for short and medium span bridges, and it has the additional benefit of strength against the harsh environmental conditions compared to other materials. Its use became widespread for both road and railway bridges over time, primarily due to high compressive strength, adequate tensile strength, low construction cost, durability, and requirement of less-skilled labour.
Generally, bridge decks are orthogonal to the traffic, termed as straight bridges. But sometimes the bridge deck may not be orthogonal to the traffic due to land acquisition, site consideration, etc. such bridges are called skew bridges, as shown in Fig. 1. Further, with the problem of site restriction, alignment layout, higher traffic and change in the speed limit, many straight bridges are also transformed into curved bridges, as shown in Fig. 2.
Fig. 1 Skew bridge, Located at U.S. 90 in Jefferson Davis Parish in Louisiana
Fig. 2 Curve bridge, Florida bridge
With the rising population growth and rapid development of infrastructure, the need for complicated transport systems has also risen, sometimes leading to the unorthodox, non-collinear configuration of roads and bridges. As per the Caltrans maintenance division’s bridge database, nearly 10,000 br idges in California are skew-curved so that the end abutment is not perpendicular to the curved bridge deck, as shown in Fig. 3.
Fig. 3 Skew-curved bridge, Miami metro rail bridge
The study of skew-curved bridges requires analytical, experimental, or numerical methods. The structural behaviour of the box-girder bridge is very complex and is quite cumbersome to investigate by conventional methods. Nowadays, structural analytical abilities attributed mainly to software development have overcome many limits that occurred in the past, in the analysis of skew-curved bridges. The analysis of girders in a skew-curved bridge can be a challenging task, since the distribution of stress, shear, and moment due to its self-weight may be different. Mostly the analytical methods have been used to predict the individual effects of skewness and curvature. But, for skew-curved bridges, the combined effects of skewness and curvature need to be considered, and the analytical methods become ineffective and generate conservative results. So, the numerical method comes into the picture, in which a skew-curved bridge of different degrees of complexity can be modelled in several ways. The numerical methods also provide precise results in less time and are becoming popular day-by-day for the analysis of various bridges.
Many literatures are available on skew and curve bridges; however, limited literatures are available on skew-curved bridges. Also, the effect of actual Indian loading conditions on such bridges is not properly addressed. There are no specific design procedures and/or recommendations on these bridges in any standards/codes because of their complex behaviour. American Association of State Highway Transportation Officials (AASTHO) has developed the modification factor and preliminary design procedure for skew bridges only, but not for the skew-curved bridges. Further, there are no specifications provided by the Indian road congress (IRC) for the skew-curved bridges. Thus, the analysis of skew-curved box-girder bridges is quite important and some relevant literatures are presented below:
Bakht (1988) analysed the skew bridges assuming them as straight bridges having the skew angle less than 20°. Ebeido and Kennedy (1996) performed the finite element analysis on the continuous (three-span) skew composite steel-concrete bridges subjected to AASHTO truck loading. Sennah and Kennedy (1999) carried out a parametric study on the simply supported curved composite multi-cell bridges under both the truck and dead loads, using the finite element method (FEM). DeSantiago et al. (2005) analysed the horizontally curved bridges using the FEM, under both dead and truck loads.
Jawanjal and Kumar (2006) studied a simply supported RC box-girder bridge curved in plan with skewed supports, using the FEM. Kim et al. (2007) studied the effect of several parameters, i.e., curvature, girder spacing, span length and cross-section, on the maximum total bending moments of curved bridges to develop a new girder distribution factor using the numerical models. Kumar and Samaria (2007) studied the rectangular and trapezoidal reinforced concrete box-girder bridges, keeping the span length and top slab width constant using a 3-D finite element based ANSYS software. Nouri and Ahmadi (2012) investigated the effect of skewness on the continuous composite girder bridges subjected to AASHTO HS20-44 loading using the FEM.
Mohseni and Rashid (2013) analysed the skewed multi-cell box-girder bridges to determine the stress distribution factors and the distribution factor for deflection, using a finite element based software SAP2000. Gupta and Kumar (2017) reviewed the structural behaviour of skew box-girder bridges subjected to static and dynamic loadings, including the seismic effects. Gupta and Kumar (2018) predicted the flexural behaviour of simply supported single-cell skew-curved concrete box-girder bridges under the gravity loads and IRC specified Class-70R track live load considered as a point load using a finite element based CSiBridge software.
Author: Dr. Preeti Agarwal