Design of bridges for flexure requires determination of maximum bending moment in the deck. For the aseismic design of bridges for gravity loads, traditionally, dead load and live load in conjunction with impact load are considered for the design of the bridge superstructure . For the elastic design of bridge, usually the design moment is obtained by superimposing the maximum moments due to dead load and live load effects. In the concrete bridges, a major portion of loads is due to dead loads which include self-weight of bridge superstructure, railing load, footpath, wearing course, water/electricity/telephone lines etc. and it is considered uniformly distributed throughout the span . For the design of highway bridges, IRC code  specifies two types of live loads namely Class-A and Class-70R/Class-AA. For the two-lane Indian highway bridges, IRC code recommends that the bridges should be designed Class-70R/Class-AA tracked and wheeled vehicles placed on one lane and the bridge need to be checked for Class-A loading on both the lanes. Absolute maximum bending moment due to vehicular live load is usually calculated using the rolling concept . For the IRC Class-70R tracked vehicle, whose wheel contact length is large, but shorter than medium span bridges, the load is assumed uniformly distributed over the wheel contact area. For the uniformly distributed load maximum moment occurs at mid-span, therefore, IRC Class-70R tracked vehicle is placed symmetrically at mid-span to produce maximum moment. Since, the maximum bending moment due to Class-70R tracked vehicle as well as due to self-weight occur at mid span, design moment is simply obtained by superimposing the effect of dead load and live load. However, in case of wheeled vehicular loads consisting of several axels, absolute maximum bending moment in the simply supported bridge occurs under one of the wheel which usually not occurs at mid-span and, therefore, the moments due to dead loads (at mid-span) and due to live loads (under a wheel) cannot be simply added to find design moment. Moreover, in case of Class70-R/Class-A wheeled load, which consists of several axels, the number of axels to be considered over the bridge of given span and their location is tedious to find out and needs several trials to determine the exact number of wheels that should be considered. Shipman examined the effect of lane load on the position of AASHTO HL-93 loading to develop maximum bending moment in the simply supported bridges and developed a simple expression to calculate the position of critical load resulting in maximum moment; however, he did not incorporate the effect of dead load in locating the critical axle . The aim of the present study is to find the number of wheels and their location to produce absolute maximum moment in the bridge considering the effect of dead load and dynamic impact factor used by IRC  for live loads. Finally, in order to enable the designers, the design moments due to Class-70R wheeled and Class-A loading, critical load position from rolling load concept and present approach and shift distance of critical load have been presented in tabular form for simply supported spans ranging from 10 to 50 m.
Bridge is a structure constructed to provide a passage over the obstacle such as road crossing, river crossing, railway crossing, valley etc. Design of bridge structure is depends upon the use of bridge or function of the bridge. It also depends upon the nature of the region where bridge to be constructed. It depends upon the site conditions, construction material used in the bridge construction, construction methods and financial conditions etc. Due to so speedy growth and development of the technology, the traditional bridges are replaced by the cost effective and new designer bridges. There structure designs are designed so that they has a new look or appearance and there cost of the structure is also economical. For the solution of this problem, structural engineers found these two structural systems of reinforced cement concrete. These are
The material as relevant to bridges has been carefully sieved from the author's other eight reference-type practice-oriented books on various aspects of Concrete Bridges and Contract and Construction Management Practices, published by Shroff Publishers & Distributors Pvt. Ltd. (India) and McGraw Hill (New York). 2b1af7f3a8