One rebar grid or 2 in bridge deck?

[goldilocks]

Building a bridge for my driveway, 30' x 10' concrete over steel beams. Deck will be 8" thick. Is 1 grid of #5 rebar sufficient, or will I need 2? If 2, how far into the concrete should they be? Thanks for any advice!

 

[breaks]

In North Carolina and Virginia, the bridge decks typically use a top and bottom layer of epoxy-coated reinforcement. The top layer should be about 2.5" clear and the bottom should be about 1.25" clear of the edge of concrete according to their standards.

The size and spacing of the rebar depends on the girder spacing and design loads. Rebar running parallel to the girder usually is #4 @ about 18" on the top. Rebar everywhere else should be at least #5 @ 12". I assume this is only a simple span - if it were continuous you would also need to include extra reinforcement over the piers to account for negative moments in these regions.

But also assuming this bridge will be used for personal vehicles (not HS-20 or HS-25 loads), you could probably get by with less reinforcing. It might be worthwhile to get a Structural Engineer to design this slab using AASHTO to fit your bridge's specific load criteria and geometry.

Good luck!

 

[Taro]

breaks,
Good advice on consulting a structural engineer. I would caution against designing for only passenger car loads instead of truck loads though. When the house is on fire and the fire trucks respond, they will not know that the bridge is not designed for truck loads. It is a good idea to always design driveways, garages, plazas, etc. for truck loads unless it is physically impossible for trucks to drive onto the area.

 

[breaks]

Taro, true true. Forgot about those pesky firetrucks!

Consider posting your bridge, too, for the acceptable load.

 

[Zulak]

Totally agree about designing for truck loading. Fire or ambulance vehicles, as well as fuel delivery trucks are quite common. I would go with two layers (#5 @ 12"oc, should be sufficient), since both negative and positive moments can occur in the slab depending on the wheel spacings, and I would highly recommend at least 4000psi concrete as well as a concrete sealer to protect your slab from "undesireables".