An efficient method for evaluating the safety of existing girder bridg
es as a function of the load and resistance parameters is presented. T
he bridge capacity is determined using a nonlinear finite element prog
ram in terms of the truck load which is increased until structure coll
apse occurs. It is determined that the reliability of fully prestresse
d bridges is higher than that of partially prestressed bridges, with t
he bridge system reliability is one-and-half to two times the girder r
eliability index. It is also found that the system reliability is more
senstive to accidental or local damage of the exterior girders than t
he interior ones.