Abstract: The useful life and carrying capacity of an existing steel truss bridge are increased by adding a support arch assembly secured to the truss structure. In the preferred embodiment the support arch structure assembly includes arch members running along each side of the bridge. Each arch member includes two spaced arch sections, each arch section including end-to-end beams, preferably channel beams, forming an arcuate path. The web portions of the arch beams are secured to hangers of the truss structure, one arch section extending on the inboard side of the hangers, the other section extending on the outboard side. Additional floor beams may be added, along with corresponding additional hangers which can be secured to the arch beams. The support arch assemblies may be post-tensioned by tension bars connected between opposite ends thereof.