Joist vs Girder clearance to grade requirement

[jdgengineer]

The IBC / IRC both require that joists closer than 18" or girders closer than 12" to grade are required to be pressure treated or naturally durable. Where does the code define a joist and girder? Is a girder just 4x or greater material? For instance some construction consists of 4x6 floor beams @ 4'-0" with 2x6 decking spanning above. There are no joists in this construction. Does this classify as a "girder"? What if there was plywood and the beams were 4x4 @ 16".

If I were to define a girder based on what I typically see referred as one I'd say "4x or wider member spaced greater than 24" oc and used to support other horizontal structural members such as joists or planks.". Does anyone know a definitive definition for one as used in the building code?

 

[JAE]

I don't believe it is the size that defines them but their function.
A joist is a repetitive member - actually a series of repetitive members which are supported by a girder.
A girder supports other members like joists.

But I also think the IBC/IRC have glossaries. Check there to see if there is a similar definition.

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[jdgengineer]

JAE I tend to agree with you. I checked chapter 2 definitions of IBC and did not see girder or joist defined. If a girder does not have a minimum width requirement why do you think they would be allowed to be closer to grade? Hypothetically, if it's the same size as the joist (maybe smaller span) why would it be allowed to be closer to grade? From a for perspective it seems like it would be even more critical of a member.

It's often we see 4x6 girders (no joists) with 2x decking that only has 12" clearance to grade. If this is classified as a "joist" it would violate the 18" requirement.

 

[XR250]

As a general rule, anyone who builds a crawlspace this shallow should be shot. It is a nightmare for the mechanicals and anyone who has to go in there.

 

[jdgengineer]

I agree with you but we don't typically determine the depth of the crawlspace. Anyone know the answer?

 

[dhengr]

Jdgengineer:
I agree with JAE’s definitions. They are both beams, so call em what you want. I also agree with XR250's comments about crawl spaces with low/shallow work space. While someone else might suggest a 12 or 16" high crawl space, you are supposed to be part of the design team and should be able to offer some reasons why this shallow space is not a good idea. More and more, we are becoming second rate citizens (members of the design team) as we allow others to suggest/dictate that we don’t have valid and important opinions on these design issues. Make em all treated, then you are out of trouble, they forced that selection, and they can now be stupid and still meet code.