Not only what has already been stated but you also have to be insured for bridge work....it is definitely NOT included in any standard welding general liability policy. It is considered a fringe/special risk by every insurer I know of. If you had them sign a waiver of liability on the work to some welding standard with inspection to that standard you could free yourself of it, long as your insurance company would not cancel you for doing it if they found out....or they were ok with it with a written agreement in place. However any job I have done for feds or state had specific insurance requirements to be met just for the privilege of bidding on the work.
Having said all that I have worked for others on bridges that I know for a fact did not have proper coverage, whether insurance or certifications in place. That was back in the day....it is a little harder nowadays to pull it off without paperwork in place. If this is a county road dept asking you to do this, you best force them to pull a spec on the work and any state requirements before you ever strike an arc. If ya go willy nilly at it you may find you spent your money to do a job you will not get paid for and likely have to pony up money on for approved repairs.
just my 2 Cents
Also you can always weld for someone else on the side.
