Load Rating of steel bridge with straight and curved girders

If the spans are independent simple spans, they can be separated for your rating calculations. If they are continuous, of course, they cannot.

If the curvature is significant, rating them as straight girders will overlook stress effects. I wouldn't recommend it in most cases.

You may find NCHRP 725 to be valuable reading

If the curvature effects can be approximated using the V-load method, you can model the bridge with straight girders and manually add the V-loads to the curved spans.

Alternately, you could enter it with all spans curved, and if the controlling ratings are in one the curved spans, you'd report those ratings. If one of the straight girder spans controls the ratings, you could rate it with straight girders and compare the ratings for the controlling straight span modeled as straight and the curved span modeled as curved, and see which controls.

If the controlling rating is at an interior support between a straight girder span and a curved girder span, getting an accurate rating may be difficult, but a slightly conservative rating easy to obtain by using the model of the bridge with curved girders in all spans.