Structural slab put on top on existing wooden bridge

[wernerfla]

On my property in the NC mountains, I have an existing wooden 15'x 40' bridge which has allowed cars, timber trucks, to cross a creek. It has seen better days, and I would like to use it as a form to pour a reinforced concrete slab on top of it; ideally being able to be standalone later on. I would dig out a substantial footer on both ends, frame around all sides, lay a sheet of polyethylene, add a rebar grid on chairs, pour slab. I need help with the rebar part, and slab thickness. 1" rebar 12"OC in a 6" concrete slab? Overlay two 20' rebar by adding a third centered at their joint? Would a 2'x2'x15' footer on both ends be sufficient? The slab would actually be 6" thick at its minimum and 10" at its maximum, due to spacing of 4x10s running the length, on top of regular surface. Pic of bridge attached.

 

[msquared48]

The permanent dead load from the slab alone will be more than the rated live load for the vehicles crossing the bridge. I have a poroblem with this.

The bridge is a timber tresle with wear strips. Commonly, the wear strips are replaced as they wear. Looks like that needs to be done now. Also, if rot is to occur in the structure, it will most likely occur and hide under the wear stips, attacking the strength of the underlying decking.

You really need to pull in a local structural engineer to help you here. DO NOT attempt to do this yourself.

Your call...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

Please note that this is just a starting point and NOT a design, but for a initial cost I would start at 18" deep and double your footing width, also looking at the rebar at 6" oc. for bridge and footing. If you can live with this cost plus 50%, then you should get a Structural Engineer to finalize a design for you. Please note that the existing bridge may not work to support the concrete and additional members may have to be added to the bridge to use it as a form with additional cost to the project. Also that you will have to add labor into the overall cost. If not, then repair what you have or replace it with something similar.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Teguci]

Have you considered precast? Since the wood deck does not look like its going to work as a form, try calling Tindall Precast and ask them what it would take to drop off a 40+' double tee. You'll still need to provide the abutments though.

 

[MiketheEngineer]

Get help even for a DECENT estimate and the timber truck loads bother me!!