1920s Commercial building renovation

[tonyiggy]

I am doing renovations to a commercial building from the 1920s. It is 1-story rectangle with a basement. the exterior walls are 8" unreinforced masonry. the floor joists are real 2xs embedded into the masonry wall. The issue I am having is the basement walls have no footings. They are built on a 2" mud slab which is also the basement floor slab. As part of the work we are replacing the old wood floor system with a new wood floor system and raising it 1'-0". The dead load from the 8" wall alone currently exceeds the assumed allowable soil bearing capacity (1500 PSF x 8" width). That is before I add the floor dead and live load. Obviously when you do the math, you need to underpin the wall and add footings. However Since the building has always been used as a commercial building the load path is not changing, Can I justify leaving the basement walls as is? Therefore not underpinning or doing some other remedial foundation work.

 

[JAE]

Assuming you aren't adding any additional dead load or live load from the renovation, and assuming that the existing basement and upper walls don't reveal any significant foundation movement/settlement, I would think you could justify leaving it as is.

 

[RPGs]

If you are truly not changing the loads on the foundation wall, I don't believe I would underpin just for the sake of meeting that assumed soil bearing capacity. Has the building settled considerably and that's why you are raising the floor?

I'd be more worried about the effects of increasing that floor height, thereby increasing the unbraced height of the foundation wall.

 

[tonyiggy]

There is one wall we are providing repair details for with underpinning. However the other walls do not show signs of distress. I was only curious from a code perspective since If I designed this building from scratch now I would have 100 PSF LL and need footings.

 

[XR250]

If there is a change of use that increases the live load, then you may have to do something. If not, I would do nothing. Maybe get a Geotech to evaluate the soil.
I believe the "existing building code" allows you to increase the load by 5% without having to do any modifications.

 

[jerseyshore]

I think we've all worked on existing buildings where we weren't changing the loads and the foundation was "out of sight out of mind".

Now if there is an issue in one area and you (and/ or a geotech) feel that the other walls could have the same issue in the future, that is a bit of a different story. Once you modify the building you are assuming some liability for those other 3 walls in the future if there's a problem.