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New and existing building foundation issues

New and existing building foundation issues

New and existing building foundation issues

I have a new one story structure addition that is two hundred feet wide and three hundred feet long to be constructed against and parallel to face of existing two story structure. The existing structure has a basement at bottom of footing 20 feet below grade. The basement footing heel projects eight feet from outside face of wall into new addition space. The new addition is a slab on grade floor and a steel frame structure with spread footings. I intend to cantilever the roof structure from columns set 16 feet away from existing building wall to avoid surcharging existing footing. However, I still need to support new stud wall that will but against existing brick wall. Slab-on-grade should be okay on this soil which is fill from previous basement construction. Do I need to provide a cantilevered grade beam system to catch new grade beam that runs along exist building face? Thanks

RE: New and existing building foundation issues

I think that before I would go to that level of trouble and significant construction expense resulting from the design, I would investigate whether your additional increase in stress from your building would have any effect on the existing footing.  You might find that it will handle it quite well and you can butt the new to the existing.

Get the orig. geotech firm and engineer involved if you can.  

RE: New and existing building foundation issues

A new stud wall should be fairly light. Can it be supported from the existing structure?

RE: New and existing building foundation issues

I would also see the grade beam/footing that supports the wall as fairly light.  Bring the heavier roof loads down to the footings set away as you describe.  But the wall loads shouldn't do that much to the existing footings.  In fact, you might be able to drop the perimeter footings down to a deeper level if there is concern about basement wall bending from the light added weight of the wall.  

You aren't worried about the floor slab loading, which could be higher than the wall loading.

I'd also agree with Ron to get your, or the original, geotech involved early.

RE: New and existing building foundation issues

I checked the surcharge from the floor load - 100 psf corridor load - JAE's suggestion - and I am finding about a 2% overstress in wall rebar from this. Adding the stud load and footing load shouldn't make much difference beyond this. 2% + should be OK?

RE: New and existing building foundation issues

When you say "butt against", check you have a gap at the roof that will handle the new and existing drift without banging.

RE: New and existing building foundation issues

I would want to be reassured that the backfill has been adequately compacted; as mentioned above, get geotech advice.
To minimise the effect of possible differential settlement the use of a slab edge thickening to support the stud wall should be considered, rather than a separate footing.
I would be comfortable with a 2% rebar overstress, others may disagree. I wouldn't bother checking the effect of the new slab on the footing, the additional load is insignificant (unless you are placing a lot of additional backfill).

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