Accuracy of Applying "Stiff-Girder" Approach in Reinforced Concrete Bridges

It is very common to consider the cross girder of a slab-girder bridge as a stiff-Rigid-girder rested on springs while distributing loads to main girders for simplification. This approach gives suitable results for primary manual analysis for bridges that have cross girder depth almost equal to main girder depth, small spacing between main girders, large spans of the bridge vent, and monolithically casted elements. Two simplified models were studied in this paper: an infinitely rigid beam rested on springs having an applied concentrated load, and an imaginary very flexible beam rested on springs having the same loading condition. These two models were studied to indicate the extreme conditions of the cross girder. An approximate governing equation is proposed to correlate the bridge geometry to the accuracy of applying the stiff girder approach. Concrete sections of main and stiff girders were considered to be cracked sections in either working or ultimate stages. It was obvious from the results that the bridge deformation) which is calculated by the stiff girder approach) is changing dramatically while changing bridge geometry. But the straining action of the bridge may not exceed the resistance values, before or after plastic hinges deformation.