How do I shim a steel structure?
Steel shims if shimming steel and concrete - either with holes that you bolt through, or tack weld them in place. To the extent that you can use "fewer, thicker" shims rather than more thinner ones, do that. Paint, galvanizing, or "cold galvanizing" (zinc-loaded paint) may be advisable to limit/reduce rusting over time.

The other solution, though it's not usually referred to as "shimming" would be to use mortar/grout to correct the concrete beam surface for good bearing of the steel in correct orientation.