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Eccentric wall footer

Eccentric wall footer

Eccentric wall footer

(OP)
Hi Everyone

I am designing a rear addition in dc where the foundation will be built on the property line... We planned to build an L footer which we have done on several project in the past however the reviewer says the footer doesn't account for eccentricity... based on a 2500 Psf soil pressure we would need to offset the 8" cmu 4" from the party line with a 42" footer and build the addition with wood framing instead of cmu all the way up in order to keep the q max under the allowable soil pressure... my question is do you guys know any solutions that would allow me to buld to the property line with no offset either with cmu or wood framing

Building is basmenet plus 2 stories with a roof deck

Any help is greatly appreciated

RE: Eccentric wall footer

Footing, not footer.

You could use perpendicular rectifying beams to take out the torsion and make the soil pressure more uniform.

RE: Eccentric wall footer

Maybe some helical piers under your "L" footer (footing). You are likely going to need a local structural engineer so you might as well start looking.

RE: Eccentric wall footer

I have a great deal to say about eccentric wall footers. Luckily for us both, I've already said it all before, complete with spiffy sketches: Link. What you wish to do is quite possible, you just have to dot all your ies and cross all your tees (or els as the case may be).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Eccentric wall footer

MIKEA,
KootK's link to a previous thread gives a lot of advice, some conflicting, and I don't think any of us would have changed our opinions.

RE: Eccentric wall footer

your link is quite interesting... great discussion... sorry I missed it... I usually treat it as a rectangular uniform block, and suspect that the loading is diminished by rotation at the toe... I never really thought twice about it... (in line with Hokie's comments about simple statics) and have one 4 storey parkade with the one outside wall supported on an offset strip footing.

I usually align the bottom footing reinforcing with the vertical hooked bars if I use bottom reinf in the footing (not often) in addition to the hooked bars... I usually use hooked bars only and if I need the bond, provide a hook at the toe. Bond is not normally an issue for smaller sized bars.

Dik

RE: Eccentric wall footer

You could use an offset footing if the wall is reinforced to resist the resulting eccentric moment and the top of wall is tied to an upper floor which can provide the necessary horizontal resistance.

If the footing is 24" wide, it carries an allowable load of 2500 x 2 = 5000#/' at an eccentricity of 8". If the next floor is, say 96" above the footing, the required reaction at the upper floor is 5000x 8/96 = 417#/'. The moment varies from 3333'# at the footing to 0 at the next floor. The wall must be capable of resisting that moment without excessive rotation at the footing.

Soil pressure on the outside of the wall will help but should not be counted as it is on the neighbor's property and could be removed at his discretion.

BA

RE: Eccentric wall footer

hokie66....you beat me to the footing/footer thing!lol

RE: Eccentric wall footer

I missed that previous discussion too. Rigid body movement causing uniform deformation is the assumption basis for the uniform pressure. In practice it works.

RE: Eccentric wall footer

It works only if the wall is prevented from rotating out. Otherwise, it fails.

BA

RE: Eccentric wall footer

BART: Why would it fail as long as the footing was prevented from translating?

Dik

RE: Eccentric wall footer

Simple statics. Assuming uniform pressure under the footing, the wall and footing rotate as a rigid body unless there is something to prevent that rotation.

BA

RE: Eccentric wall footer

I imagine, but don't know, that most of these offset footings don't result in distress in the structure above because we haven't given the supporting soil enough credit, or the applied loads are overstated. I agree with BA and KootK that if the supported wall and the connection thereto are strong enough, and the propping at the next level is good enough, these can work. But the OP in this case was talking about a CMU wall, and I know from reading these forums that many CMU walls in the US are not even reinforced. So no encouragement from me.

RE: Eccentric wall footer

As long as the footing is prevented from translating either by friction or embedment into the soil or by a slab or something else, then the footing rotates a tad, puts the wall in flexure... and that would be it.

RE: Eccentric wall footer

Quote (hokie66)

But the OP in this case was talking about a CMU wall, and I know from reading these forums that many CMU walls in the US are not even reinforced. So no encouragement from me.
A CMU wall may be sufficient if adequately reinforced. A cast-in-place concrete wall would be better if heavily loaded. An unreinforced wall is unsuitable and should not be used with an offset footing.

Quote (dik)

As long as the footing is prevented from translating either by friction or embedment into the soil or by a slab or something else, then the footing rotates a tad, puts the wall in flexure... and that would be it.
The footing can't put the wall in flexure unless the wall is laterally braced at some point above the footing. A horizontal reaction is needed above the footing to resist the eccentric moment.

BA

RE: Eccentric wall footer

I'm inclined to agree with hokie's concerns regarding the L-shaped footing business when CMU is involved. With 8" CMU, your "d" on the rebar is going to be in the neighborhood of a paltry 3.5". Additionally, the placement tolerance on the starter dowels, which will be doing the hard work within the joint, will be terrible. So now maybe d = 2". Add that all up and you've got a joint that probably ought not be relied upon to transfer mission critical, equilibrium moment.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Eccentric wall footer

I would also be concerned of trying to dump 417 PLF into the floor diaphragm without doing significant dotting of i's and t's.

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