New pier footing in an existing building

[LUGuy]

I'm designing a new pier footing to go inside the basement of an existing building with a concrete floor. It'll probably be about 5' square. How deep does the bottom of the footing need to be? Is there anything wrong with setting the bottom of the footing at the bottom of the slab?

For that matter, would it ever be acceptable to place the footing directly on the slab, provided the slab was sawcut to the outline of the footing to prevent cracking?

The owner doesn't care if the top of the footing extends above the slab to save excavation costs.

 

[Focht3]

How deep does the bottom of the footing need to be?

Generally, it needs to be placed at the same depth as the existing footings.

Is there anything wrong with setting the bottom of the footing at the bottom of the slab?

YES!

Candidly, you need to hire a geotechnical engineer to consult on this project - even if it is a single footing. Otherwise, you may really screw up the building. There may be nothing wrong with your approach. But you need a local expert to advise you.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[gandersen]

MiscMetals:

The question of the depth of the new pier should really consider the performance of the existing foundation system; the loads that the new foundation will need to carry as compared to those carried on the existing footings; and whether or not the new loads will be structurally connected to the old loads (in other words, will the new foundation loads depend upon the performance of the other foundations or visa versa).

I agree with Focht3 that a local practicing geotechnical engineer should look at the project and assess the above issues.

Good Luck

 

[LUGuy]

The existing foundation is stone packed with mortar/cement. Currently there is a continuous stone wall along the middle of the building supporting columns at 15'. Ballpark loadings max at around 30-40k for the existing column locations.

Because the loads are being increased on the second floor and one column is moving several feet, I am requiring that they remove sections of the stone wall to put in a proper pier and footing for at least the new column.

So for the most part, the loadings are already on the soil at this point in time. We won't know the soil until some demo is done. And we have no idea what type of footing is existing, if any.

 

[kmerl]

It sounds like you are rehabilitating an older structure. I would not trust any of the existing structures, if there is any already in place. In order to install the foundation I would suggest trying to get some geotechnical information about the materials directly below the foundation. This could happen while demolition is occurring or before, the later preferably. You can use a tri-pod and complete some STP and augering for samples directly where the foundation is to be located. In an existing structure you can't always be sure what is under the building.

It would be possible to install the foundations in a shallow situation, where it is an interior foundation. But I would require at least the top of the foundation be even with the existing ground (for slipping resistance), where as outside foundations I would require a minimum of 3 feet of soil cover for frost protection.

Based on your foundation size and proposed loading, if I did my math right, your loading should be 1600 psf. This is a pretty low number. You can implement the use of a dial strain gauge to determine the insitu condition at the foundation and soil contact. This monitoring combined with the subsurface information will let you know is a shallow foundation is stable.


Keithe J. Merl

 

[LUGuy]

Thanks!

You actually gave me the type of responses I was hoping for. I'm the first to admit what I don't know about soils, and I'm grateful to have such excellent advice at short hand. And I was sort of hoping for some good arguments to justify to the owner (and to myself) why additional design time and work has to go into this building.

Regards!