Residential Footing Assessment

[Lion06]

I have a family member who owns a ranch home. She would like to add a second story. She knows what I do and asked if there are any problems withthe idea. I told her thatit can certainly be done, it's just a matter of how much work will need to be done. As far as I can tell, she's not required to have an engineer, but I told her I would at least put my eyes on it.

This leads to the question - how do you assess the footing(s)? The structure and even the bearing pressure for the footing is pretty straightforward. Would you get a metal detector and try to locate rebar I'm the footing(s) - both spacing and depth?

I'm sure this kind of thing isn't uncommon. Are there any guidelines out there somewhere? Does this typically require underpinning? What would be the standard of care here, even though it's not a "project" in the typical sense

 

[csd72]

Depends on the type of soil and the depth of the footing.

As long as the concrete footing has enough depth for a 45 degree spread from the edges of the wall and has enough width for the required bearing then I would think it would be only serviceability concerns (e.g. heave e.t.c.) that may cause problems.

These issues would depend on the soil.

 

[ToadJones]

I'm sure I'll be chastised for this...

If the house is older, chances are 99/100 that:

- The footing was never designed in the first place, generic builder variety
- depending on old local codes, there is no bar/minimal bar in the footing
- the exact same footing that is there, would have been used for a two story house.

 

[csd72]

So true - I have seen enough old walls that were founded on surface soil without so much as corbelling.

Lion 06, if the bearing stress is okay and there are no signs of distress than thats 90% of the issue.

 

[Lion06]

It was built in the 60's, so it's older, but not really old. I had a similar thought as you, Toad, but I'm more concerned than normal since this is a family member.

 

[Lion06]

So, do you think a geotech is in order to get an allowable bearing pressure? If I go with IBC values, I'm going to be limited to something very low.

 

[csd72]

Do the investigation and the numbers and see what it turns out like. If that doesnt work then get the geotech involved.

 

[ToadJones]

If you are going to check it, I think IBC says you can use 1500 psf without a soils report, right? (used to be 2000 psf???)

For an 8" block wall
Builders typically use a 8"x16" ftg

12" block, usually 24" wide

I'd be shocked if there was any issues at all.

 

[Lion06]

Good to know. Thanks, guys. Any thoughts on additional settlements?

 

[csd72]

Keep in mind that there has already been an initial 'settlement' under the existing loads which has consolidated the soil to a certain extent.

 

[Lion06]

Right. So additional load shouldn't cause any major problems with additional settlement.

Thanks again, guys!

 

[ToadJones]

It's been a long time since I designed a strip footing, but bending will most certainly no be an issue. You can probably just check shears.
If looking to investigate footing depth & width, excavating a small area in the basement is probably cheapest.

Again, 99% wouldn't even give the footings a thought.

 

[Lion06]

Good to know. Maybe I'm just overthinking it.

 

[JAE]

It isn't necessarily the overall settlement of the house with the new addition - but any differential settlement that might occur.

If you add additional structure above one segment of the house but not the other, you do pose a risk of this differential settling and distress cracking between the two areas.

 

[Lion06]

Good point, JAE. So the idea would be to try to increase the load on all footings by roughly the same %.

 

[ToadJones]

....but the same is true for a new house...depending on the direction of the trusses and floor joists, gable ends are not loaded nearly as much.

Lion- you could orient the floor joists of the addition opposite the first floor (if the walls have the proper headers) ...maybe the same for the roof. That way you could load the basement walls more "evenly"

 

[Ron]

A two-story house will have a perimeter wall load of less than 2k/lf. For a 16-inch wide footing, that results in 1500 psf bearing pressure.

Look at the performance of the existing walls/foundation. If no apparent distress....not likely an issue.

 

[MiketheEngineer]

Use a piece of rebar and from the outside try to find the width of the footing by driving it into the ground and probing. From the inside you could use a concrete drill and probe it.

That will give you a good idea of the footing size. Use the 1500 psf capacity and re-check the calcs. If the house is not so wide and no brick involved - you will probably be OK.

Done all the time.

 

[DWHA]

If the soil is not okay for 1500 psf allowable, you could get a pocket penetrometer and check it youself (which would be much less expensive than hiring a geotech). I know this is not ideal, but it can give you a little information to possibly increase the allowable.

Also realize that the load applied to the top of the basement wall will be somewhat well distributed by the time it reaches the footing. Even on gable ends of a house. The wall is a deep beam that will spread the load over a large area.

 

[a2mfk]

On another tangent, more basic- why does she want to go up? Very expensive lot? Small lot with no option to expand with a one story addition? The reason is I have been involved on several of these, many preliminary, and the end result is it is way less disruptive, cheaper and easier to expand out rather than up. You do not completely disrupt your life and have to move out while they remove your roof, all the MEP, and then build up. Usually these are gutted and start over deals... Even if your addition is two story, that is easier and cheaper.

But there are lots and neighborhoods where it is worth it.