Depends on how you mean it fails, as in the bridge falls down or the Inspector rejects it for non-conformance?

Inspector rejects it for non-conformance: The welder. Every welder will put down a turd of a weld every once in awhile. Best ones put down less than the hacks. But the Best ones will oft times see it (if its visible) and fix it before the Inspector gets there.

Bridge Falls Down: You wont find that in a code book, only in a court of law. Too many things can influence where liability lands when something goes wrong.

Was the weld designed properly?
Was the joint designed properly?
Were the proper approved procedures followed?
Did the inspector miss a defect during inspection?
Did the inspector 'pencil-whip' the qualifications?
Were the proper materials used?

All that plus a million other questions would get asked and need to be answered. Everyone involved would be examined, Engineer, Contractor, Inspectors, Welders, etc.

Generally, the welder's employer is liable.

The contractor is responsible for the work produced by their employees. The contractor is expected to provide direction to the employee and to provide oversight to ensure the work is completed in accordance with the drawings and the project specifications.

I had a case a couple of years ago that involved a structural collapse that caused property damage, but no injuries. The contractor tried to lay the blame on the welder and the foreman. It only took seconds to establish the contractor is responsible, no ifs and no buts. It only cost the contractor 2.5 million to learn the lesson.

If the welder is self employed or working as a subcontractor, then he is the contractor.

Al