Actual load test on concrete foundation

[PostFrameSE]

I am considering setting a new building on top of an existing foundation. It is 18" in the ground, 12" above ground, 6" wide, and has no footing underneath it. The overall dimensions then are 6" x 30" per lineal foot. Using 150psf, I calculate my dead weight at around 190plf. Conservatively, the current building setting on this foundation may weigh approximately 400plf. So I have around 600plf dead weight per lineal foot.

I took my load cell, screwed in a Simpson Titan HD screw and yanked on this thing with a piece of equipment. I did this twice and got around 6.0kips.

How can this be, without moving the entire foundation? What I don't know is if this thing is tied to the slab on the inside. Even if it was, under those loads I would expect some cracking, and there was none on the floor.

The fastener failed by pulling the screw right out of the hole.....the threads couldn't hold........not a cone failure.

What am I missing? I suspect there is some reinforcing in this concrete. How many lineal feet would it be reasonable to suspect is contributing to this uplift such that I can get so much more uplift resistance than I expected? And then.....what reasons do you give a building owner that despite not being below frost level (building hasn't heaved in 40+years) and the uplift resistance doesn't "calc out" that I'm not comfortable sitting on that foundation even though we did a "live" test and it carried three times what the design loads would be?

Would anybody set on something like this that you've actually load tested to be 2-3 times the design load?

Thanks.

 

[Ron]

First of all, you're using the foundation as a reaction, so you are only testing the pullout capability of the anchor, not the uplift of the foundation.

If you test it such that you are trying to lift the entire foundation, then you are testing the beam action of the foundation. If pullout failure occurs, it is likely that the beam action is performing properly.

Here's the issue with loading the foundation as you've described....You have a narrow, long foundation. You plan to load it to a bearing pressure of about 1200 psf. Not that great; however, with such a narrow foundation, failure is likely to be a shear failure in the soil, not a bearing capacity failure (though shear is part of a bearing capacity failure)...in this case it is more of a punching shear in the soil.

Further you probably have a code requirement for a minimum footing width (at least twice the wall width)...you can't comply with code for your premise.

 

[apsix]

The reason you give your client is that it doesn't comply with current codes.

 

[TXStructural]

Are you certain there is nothing underneath, like small diameter piers or a wider base below?
How about suction from damp/wet soil resulting in higher resistance to lifting?

Attempting to lift the foundation with a single post installed anchor sounds like a disaster in the making, but a sudden "yanking" is not a valid test of weight.

A proper load test would be used to indicate performance, but you would probably only use the foundation for loads in the range of 1/6 to 1/4 of the test load, and then only after a geotech approves it, since sustained loads and soil moisture changes confound the results.

To reinforce what apsix says, in most cases, if a new structure is placed on a old foundation, you probably trigger mandatory compliance as for new buildings under IBC Chapter 34.

 

[PostFrameSE]

Thanks for your thoughts. I appreciate them.