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IBC 1805.5 "Lateral Support"

thread179-285078

IBC 1805.5 "Lateral Support"

IBC 1805.5 "Lateral Support"

(OP)
Hello everyone,

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.

RE: IBC 1805.5 "Lateral Support"

I am new here, but here is my two cent.  Section 2306.3, Wood Diaphragms, outlines maximum shear loads in pounds per foot of horizontal wood diaphragms (framed floors).  If this section is applied to the design of the floor diaphragm the loads will likely exceed those prescribed in section 2306.3, therefore supporting that case that the wall is not braced at the top.

RE: IBC 1805.5 "Lateral Support"

At 30 psf/ft, the wall generates 320#/ft at the top to the diaphragm, yielding a diaphragm shear of 428#/ft which is very doable if blocked and nailed properly.  Higher loads will give proportionately higher shears.  The chord force is 8.5 kips that can be handled with the wall top steel.

Presently, outside of tolerable diaphragm deflections, I do not see a problem with using the diaphragm for wall restraint.

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

RE: IBC 1805.5 "Lateral Support"

With the foundation factor of .56, the maximum allowable long term shear is about 460 plf, doable, but barely.

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

RE: IBC 1805.5 "Lateral Support"

Not sure where 30 psf/ft came from. ASCE and the IBC prescribe a minimum of 35 psf/ft for washed stone type fill, the footnotes will require an even higher load, but IMO an over design.  The prescriptive design of the IBC (connections) does not transfer the stresses from the foundation wall to the diaphragm adequately nor the loads from the diaphragm to the shear walls.  1/2 anchor bolts at at 6' o.c. For a one story home??  I hate seeing basement wall dseigns with top of wall loads shown and then "by others"

RE: IBC 1805.5 "Lateral Support"

(OP)
I think it would help if the code defined "lateral support" more precisely.  Something like "the prescriptive tables may be used when lateral support of adequate strength and stiffness to resist the applied earth pressure without excessive deflection is in place at the top and bottom of the wall".

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