Do bridge floorbeams follow the same depth rules as stringers?

[Enginerdad]

I have a bridge over a railroad that has limited underclearance, so I'm trying to minimize the structure depth under the road as much as possible. One thought was to use a truss to move the primary structure up above the road. But my truss is roughly square (the width is close to the span). Would my floor beams have to be basically as deep as the beams would be if I went with a conventional beam-slab bridge? If so, then this makes the whole exercise pointless and I'll abandon it now. It's a multilane highway, and there's no possibility of using an intermediate truss (I'm already planning separate trusses for each direction of traffic).

 

[BridgeSmith]

If the floorbeams are the main load-carrying members, I think the same limitations for depth to span would apply. I don't have my AASHTO 9th Ed. LRFD spec accessible at the moment, but I think the depth-to-span limts are optional. I know the live load deflection limits are optional.

In any case, there's really no advantage to using a truss and floor beams if the width is about the same as the length. Longitudinal steel girders, composite with the deck, or prestressed concrete girders will minimize the depth most effectively.

If the span is in the 60' range or less, you could consider precast prestressed slab girders. We're finalizing a standard design for a 56' slab bridge with 4' wide by 20" deep prestressed slab sections that are laterally post-tensioned.

 

[bridgebuster]

Hopefully, I'm reading your question correctly. I'm not aware of any provision in AASHTO that truss floorbeams are subject to the minimum span length /depth ratio as stringers. I know some agencies will waive the L/D ratio provided that the live load deflection limitation is met. Sometimes truss floorbeams tend to be deeper than the bottom chord but the bottom of the FB will be flush with the bottom chord. However, the depth could affect your roadway profile, more so if you run the stringers over the floorbeams.