IBC 1805.5 "Lateral Support"

[MCurry]

Hello everyone,
I've cross-posted this from the IBC (Building Officials) forum.

I've been pushed (once again) by our local concrete contractor to allow a basement wall to be built according to IBC minimums (Tables 1805.1(1) through 1805.5 (5).
1805.5 states that the tables may be used when the top and bottom of the wall is laterally supported.
With a building 80 feet long, 30 feet deep and a walk-out basement, I say the floor does not constitute "lateral support" but would love to find a definitive code reference.
Is there an IBC white paper or something that anyone is aware of. I'm tired of having this argument!

Thanks all.

 

[MiketheEngineer]

You might take a "quick" look at designing a floor diaphragm that would (might) work - along with all the connections, fasteners, blocking, etc. Once they get past about 2:1 - seems rather dubious. I have done a few and they get impossible pretty quick.

Once he sees what it takes - I think he might listen.

 

[MCurry]

Thanks Mike,
Wood diaphragms get really gnarly if you reduce the diaphragm capacity by .56 for permanent loads per footnote "h" of the 2006 IBC diaphragm tables.

I'm still looking for a reference that might define "lateral support" as a 'diaphragm of adequate strength' or some such thing so I can point to it in the code.

My client (architect) understands what I'm telling him, but his client (owner) wants to believe the concrete contractor because the walls are cheaper to build.

 

[a2mfk]

Another thing you'd probably want to check is horizontal deflection of your diaphragm in the middle of the 80ft span, and is that compatible with your concrete wall... You may not want an inch of deflection at the top of your basement wall from a sustained load, for example.

 

[hokie66]

If he's a concrete contractor, design a concrete floor for him to build. That will brace the wall.

 

[msquared48]

Or... You might make the diaphragm work laterally if you added an interior concrete shear wall amd drag link to limit the ratio to 2:1 or less.

If there's a pill, there's a way.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MCurry]

Thanks for the response. I guess my question wasn't totally clear - its more of a code interpretation question.
By making the use of the prescriptive tables dependent on having "lateral support" at the top and bottom of the walls, without specifying what constitutes "lateral support", its open to interpretation by anyone - including non-engineers. Then the non-engineers want to argue the point again and again.
It would be great if the ICC has issued a "Technical Opinion" on this to shut down the argument right from the start. Anybody a member of ICC that could check?