Geotech: Surcharge load on existing wall footing at building addition (sketch) 2

[TroyD]

I could use some input from a geotechnical engineer on this topic. The attached sketch shows a foundation section at a commercial building addition. The existing building is from 2003, and the original plans indicate the allowable soil bearing capacity is 2,500 psf. (The original geotech report is not available). There was no soils investigation for this addition, so the new footings have been designed for 1,500 psf (assumed). The existing foundation is to frost depth (SE Minnesota, 42 inches). I proposed a slab with thickened edge along the existing wall. The building official structural plans reviewer is requiring a geotechnical engineer verify that the existing lower footing will not be overloaded with surcharge loads. Note that the original footing is back-filled on both sides; there is no retained soil condition. Is this a valid concern? I think the wall load would distribute outward into the soil over a greater width.

Any input is greatly appreciated.

 

[BridgeSmith]

I'm no geotech, but I can see that, even if the addition is not tied to the existing, there would be some increase in bearing load for the footing due to the vertical surcharging of the top of the footing from the loading of the addition.

A substantial increase in load on the footing would occur if the addition is attached to the existing, which I thought would be the typical way. If not, then I'm confident the geotech guys here can help you quantify the vertical surcharge load to the footing.

 

[CANPRO]

The existing footing will see an increase in loading from the new footing. If not attached directly to the existing, it would be hard to quantify how much load goes to the existing footing and how much gets dispersed to the soil.

Maybe I'm over simplifying this, but this is my take - according to your sketch, you have an existing load of 1000 plf and a new load of 1500 plf = total load of 2500 plf. Existing footing is 2'-0" wide, so average bearing stress of 1250 psf, which is well below the original allowable of 2500 psf and still below your very low assumed 1500 psf. You might want to throw a few dowels in between the existing and new to transfer the 1500 plf and eliminate any differential settlement.

 

[GeoEnvGuy]

Looking at the sketch you are going to be adding additional load onto the footing and the column, making it loaded off center. You need to hire a local geotechnical engineer to figure out how much additional loading on that side of the footing and if it could handle it.

To illustrate the additional stress below is a stress distribution for a circular footing and strip (your new foundation). Essentially if you make a 1 foot wide strip foundation you will be adding 20% stress at just over 3 ft down. Given that there is a column also restricting part of the stress distribution in your section this could be more. The result may be the existing footing fails or settles more. Your new foundation may also settle less at the old footing locations.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[TroyD]

Thanks for the great replies and info. Perhaps I will revise the detail to a frost depth foundation wall and scab onto the existing footing, reinforced with some dowels and epoxy...

 

[oldestguy]

Have the new geotech look at having the new slab reinforced to span some zone out from the old wall. Assume no significant pressure on soil say within a distance out from old equal to height above old footing. In the extreme case place the new slab on a blanket of compressible material in that zone. In line with the comments below, that backfill to the old wall likely is loose. Transferring load to competent soil below the loose stuff should be considered, or use the cantilever of the new slab over that loose stuff.

 

[GeoPaveTraffic]

The actual problem is not overloading the existing footing. Yes some load will get applied to the existing footing, some a lateral load will also be applied to the stem wall between the floor slab and the footing. Neither of these loads appear to be significant, but they are there. The vertical load will tend to make the existing footing want to rotate counterclockwise while the lateral load will tend to make it want to rotate clockwise. I wouldn't worry about either given the light loads.

What really should be checked is how much settlement will the new addition experience and how much settlement will the addition cause to the existing building? The only way to determine this is to do a settlement analysis and that requires soil data and a geotechnical engineer who understands the soils in the area. You can't get either of those here.

In short,bearing pressure is rarely the problem, settlement is almost always the limiting factor in shallow foundation design.

Good luck.

Mike Lambert

 

[civeng80]

My concern with your detail would be that the founding soil of the addition may be backfill (compacted or uncompacted) for the wall and perhaps not good foundation material.
Is there any way you can go down to existing footing level say with bored piers and then cantilever the edge footing ?