bridge design..first we have to make the general layout of 
a span of a bridge.then cross beams and longitudinal beams 
are placed at suitable spacing and their size is also 
assumed.
steps
1.calculate clear span and dead loads like due to slab and 
wearing coat
2.calculate bending moments.
3.using pigeadus curve calculate moments
4.load dispersion due to wheel of train is calculated.
class A or class AA is used.now a daya class A IS commonly 
used.
5.live load moments calculated.
6.check for shear is also done

First we study about the location, catchment area and 
preliminay survey after that if necessay to provide the 
bridge. we go to soil investigation and study the report, 
afterthat design.