Post load near edge of basement wall footing 3

[AELLC]

See attached sketch w/ calculations.

Plans Checker is demanding that a spread footing 2' square be provided, I am saying it is not necessary.

Slab, wall footing, basement wall already constructed, he wants slab cut away and new 2' sq ftg doweled into the existing.

In the future, I would call out the 2' sq ftg because it is just a few nickels more, but trying to justify the wall footing alone is OK here.

 

[hetgen]

Hi AELLC,

Unfortunately you will need to widen the footing. With the width of footing you have (1'-4'') your will require a bearing capacity of > 5000PSF to resist the eccentric load for the column.

 

[structSU10]

I agree with hetgen, I get 3500 psf using a 6" eccentricity for the post load over the 3' width. I get that you would need to have a 10' effective width of uniform load to get to 2000 psf, which I doubt is possible.

 

[ztengguy]

Can you bolt the post to the wall to say that it will be distributed down and along the wall this time. Seems it would be easier than cutting and digging and drilling and pouring and patching.

 

[AELLC]

I know it is conservative to assume such eccentricity to the wall footing, but the wall footing is not fully "hinged" where it is doweled to the basement wall.

This is a semi-custom home builder, and they are fairly tight with expenses, I am hoping the glulam "beam" concept makes sense so I don't have to direct them to put in that 2' square ftg. I also could change the (3)- 2x 4 post to 6 x 4 post and bolt to wall as ztengguy suggests, but that may result in very many bolts if the full 6330 lbs is developed thru the bolts alone.

 

[CANPRO]

I think the glulam beam shown in your sketch would have to quite large to get it stiff enough to spread the load somewhat evenly to the concrete below...this isn't based on any numbers, just gut feel.

Can you split the large point load into 2 smaller point loads by placing the glulam up off of the concrete and supporting it by a post on each end?

 

[AELLC]

CANEIT, I did think of that - but a 3-1/8 x 9 GLB x 4'-0" long deflects only .043" theoretically if it was simple-spanning 4'-0".

 

[msquared48]

I assume you are designing to 1500 psf bearing? If so, just the concentric load puts you at over 1300 psf, and you would need to span the 6K load over 40 feet of wall in order not to exceed the 14500 psf value. With an 8 foor high wall, the maximum is 16 feet. You need the footing here, and it needs to be larger than 2 foot square - more like 2'-6" to 3' square considering the weight of the concentric load per foot added to the length of the new footing. footing.

It would be best to underpin the existing strip footing in this circumstance.

Mike McCann
MMC Engineering

 

[hokie66]

ztengguy's idea has merit. The concrete wall will distribute the load, if the load can be taken onto the wall. Can the load be delivered to the wall at the top rather than into the post?

2000 psf bearing seems to be what is being used, which is really not very good material. Any chance the geotech can give a better assessment at this point? My starting figure is usually 150 kPa, or about 3000 PSF.

 

[msquared48]

It's 1500 psf here now Hokie without a soils report. At the 16" width, the strip footing is good for 2000# per lineal foot.

Right now, the wall is seeing 1780 plf (assuming the weight of the concrete wall is included in this figure too), leaving only 220 plf available, and this would have to be spanned out via the wall. At 16 foot max span, that is only 3520#, and you need 6300# or so. No way without a higher allowable bearing pressure or a new footing.

Note that if the wall weight is not included in the 1680# figure, then the footing is overstressed at about 2500#/ft, or 1890 psf.

Mike McCann
MMC Engineering

 

[hokie66]

The OP is using 2000, so he must have a soils report, or else has evaluated the bearing capacity himself. 1500 PSF sounds like something to be used by non-professionals.

 

[msquared48]

Without a soils report, UBC Table 1804.2 governs, based on the type of soil. 1500 is the default in my location without a geotech report. May be different where the OP is.

Mike McCann
MMC Engineering

 

[focuseng]

Following up on what ztengguy said: what about a beam hanger fastened to the conc. wall instead of trying to use the column and bolting it to the wall. Look up simpson "LGUM" or "HGUM" hangers. Totally overlooking the footing and soil bearing discussion for this...

______________
MAP

 

[AELLC]

Yes 1500 psf is the default value, but the plans checkers here have allowed us to use 2000 psf at the basement level, about 10' at least below existing grade. Our soils here are not too good until about 5' down, when they become more of a dense clayey silty sand rather than sandy silty clay as in the upper 5'.

I know I am treading on thin ice with this, but dealing with residential structures and home building contractors, we tend to bend the rules a lot more than heavy commercial structures.