Concrete duct bank construction

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eleceng09

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Philadelphia, PA
I have an underground concrete duct bank that will be installed 2 feet under a roadway (top of duct bank to grade), in an industrial facility, for a very good distance.

The conduit in the duct bank is Schedule 40 PVC. The standards for this project call for additional rebar, both horizontally and vertically, when running under a roadway (high traffic areas).

Not being a structural engineer, can anyone tell me if there is a standard or guideline for the construction/design of duct banks - especially at different elevations - so that I do not have to worry about the PVC conduits possibly being crushed from vehicle traffic. (I do not want to install rigid galvanized steel - this will increase the cost)

Has anyone seen damaged duct banks due to heavy loads passing over them?

Thanks
 
If there was not any "Specification" document addressing ductbank construction from the engineer of record then you might want to submit a "Request for Information" for additional information. But if I were building this duct bank I would install 1 rebar per conduit (running length wise with the duct bank). I would also install rebar stirups (rebar bent into a square) every 6' or so encircling the coduits and the linear runs of rebar. All of this would be tied together with tie wire at the intersections of the rebar. Be sure that you have some spacers (2" or 3") between the conduits.

The Mix design for the concrete encasement should be at least 3000 psi. for the under road section.
This ductbank design loosely described above is what was "Specified" to be used at an Industrial water treatment plant similar to what you need at your facility.
 
I don't have a good answer for you, but I do know at my plant that our specs call for rigid conduit in all duct banks and an 18" burial depth in most cases, or 36" burial depth under roads. We also call for red concrete since the plant is 120 years old and there is old concrete everywhere underground.

I don't know why those specs are what they are for sure. I have seen references on the forum to using rigid for underground bends and PVC for straight underground sections. The only thing I can figure is that the only PVC we allow in the plant is 1" Sch. 80 as a sleeve for ground wires, and this rule is pretty easy to remember. If we allow PVC in some other cases people might get confused and then get very frustrated when they have to tear it out.
 
eleceng09 said:
I have an underground concrete duct bank that will be installed 2 feet under a roadway (top of duct bank to grade), in an industrial facility, for a very good distance.

The conduit in the duct bank is Schedule 40 PVC. The standards for this project call for additional rebar, both horizontally and vertically, when running under a roadway (high traffic areas).

Not being a structural engineer, can anyone tell me if there is a standard or guideline for the construction/design of duct banks - especially at different elevations - so that I do not have to worry about the PVC conduits possibly being crushed from vehicle traffic. (I do not want to install rigid galvanized steel - this will increase the cost)

Has anyone seen damaged duct banks due to heavy loads passing over them?

Thanks
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I hate to repeat myself but this should be done by a civil/structural engineer and if it is in a public area then it should be stamped by a PE.
 
I agree with Laszlo, this needs the input of a civil or structural engineer. Also, if there is any chance that the PVC raceways will be used for a "isolated" phase installation, then you must make sure that there is no rebar installed between the raceways. It can only go around the outside of the complete duct bank in this specific case.
 
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