How do bridges work? I need units to be able to pass on it, and under it.

I tried the blue Protoss one and it wouldn't work.
I tried the metallic one, and it blocks the path under it, but units can move on it. However, a Hunter Killer moving over the bridge will be at ground height (not the bridge's).

I don't think SC2 natively supports multiple heights for the same area of terrain. I could be wrong on this, but you'd probably have to hack it with triggers or validators.

I added a ramp somewhere in my level so that it is at least doable now that bridges are integrated, but it'd be great if someone could find a way.