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Concrete on wood framing

Concrete on wood framing

Concrete on wood framing

Does anyone know of a requirement in either IRC or ACI that mentions support of concrete on wood framing?  A client wants to support the concrete for his front porch on wood framing over an area in his basement.  I know MSJC (ACI 530) states that masonry is not to be supported on combustible framing except for certain circumstances.  I cannot find any similar wording in ACI.
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RE: Concrete on wood framing

I don't know of any specific exclusion... but any lumber material should be PWF quality or similar to reduce the chances for long term deterioration.


RE: Concrete on wood framing

Gypcrete is poured on to wood floor construction all the time--does your client want to use Gypcrete?


RE: Concrete on wood framing

I don't know if Gypcrete is an option.  The concrete will be exposed to weather - Chicago area.  Can you use gypcrete for exterior applications?

RE: Concrete on wood framing

enginerding, I think IBC said that you can only support up to 4" of concrete with wood framing.  If its not very big patio then you can slope the concrete from 4" to 3.5" for drainage.  Or you can frame the joist in the direction of the drain and make one support higher than the other. This way you can pour 4" all over.  Dont forget to add DL and use the higher Snow load or Live load.  I would also design it for L/600 deflection.

RE: Concrete on wood framing

most of Venice is masonry and concrete and is held up by timber.  Keep the stresses low, warn your client of possible cracking and make sure there is enough meat in your timber so that you can get the fire rating you want.

RE: Concrete on wood framing

Thanks to all!

RE: Concrete on wood framing

Wood is subject to creep.  Check your design manual for long term deflection and design accordingly.  Also try to work a waterproof membrane into the system.  Ken

RE: Concrete on wood framing

One provision you might consider is similar to the provision with masonry and that is to isolate the concrete that is supported on wood from any portion that is supported on concrete or steel. That is, you wouldn't want the concter to be supported on two different materials if they would cause differential movement unless the concrete can tolerate it by use of a control joint or something similar.

RE: Concrete on wood framing

Also remember that the duration factor for dead load alone is 0.9.  With the high dead load of concrete this might be significant.

RE: Concrete on wood framing

You asked for IRC or ACI, but in the IBC 2003, section 2304.12, wood can't be used to support a concrete floor, unless it is a nonstructural roof or floor which is 4" or less.  I read that to mean that you have to have studs, floor joists, and plywood which will fully support the concrete covering.

RE: Concrete on wood framing

As a follow up to AggieYank's response, concrete over wood framing is not in the prescriptive provisions of the IRC and the IRC kicks you over to the IBC for compliance.

Don Phillips

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