Isolated Footings Foundation Question

[333OnlyHalfEvil]

Hello everyone,

What does everyone think about foundations consisting entirely of isolated footings? They seem to be typical in rural areas. I've seen a lot of structures where the house itself is built like a giant wood box (with some intermediate walls throughout separating the box into rooms), then the entire wood box is supported by isolated footings arranged in a grid pattern. I'm used to working in major cities where structures at least have spread footings around the perimeter of the structure. Do you think the grid of isolated footing foundation style commonly found in cheap rural communities is not to code or is it acceptable? How would you get the lateral forces to work out when the foundation is a grid of plain concrete blocks sitting on top of the ground (not even buried even a little)? What do you think about this sort of thing?

Thanks in advance for your time/replies.

 

[phamENG]

They can be made to work, but don't expect the contractors to be happy with you. You need to resolve your lateral forces, so at least some of them (if not all) will be little cantilevered piers with the isolated footings to match. You'll also need positive attachment. None of those things are 'the norm'. Lots of houses in my urban area were built like that from the earliest examples of wood framed structures still standing (found one that probably dates to the 1600s) up through the 70s and a few that slipped through into the 90s. That's not to say it's okay...but just to give you an idea of the inertia you'll be up against.

As for plain blocks just sitting on the ground....it sounds like you're talking about mobile home foundations. Those are not permanent foundations at all.

 

[ChorasDen]

It's an interesting concept that I haven't really spent much time considering. I assume very little design goes into this type of construction. First I would want to isolate what my lateral loads are, magnitude, and method of distributing to pad footings. Then I would consider my connections from diaphragm to footing, what's the fixity of such a connection? Are these standard pad footings you buy at home depot, or are they cast in place? Is there a moment resisting post base, or is it just a 4x4 sitting on a shingle?

Isolated footings do have capacity to resist overturning loads (both round and rectangular footings), but as always, devil is in the details, in determining what system is in place to even get that load the footings. I could envision a situation where this could work (what about houses over water on piers, how is this different?) But want to confirm there is a proper load path.

 

[human909]

What is wrong with isolated footings? It is hardly a novel concept, nor is it restricted to rural or residential. A typical portal frame structure with light sheet metal walls is often done with isolated footings. If you have heavy masonry/concrete walls then strip footings might be needed but otherwise the loads don't typically require them.

Isolated footings are not uncommon here for residential construction. Though it really depends on soil conditions as differential movement can lead to big issues.

 

[B-SE]

It is classic pier and beam construction. Have to resolve the lateral load, much like they did in 1900

 

[milkshakelake]

I've never seen one of this type. But speaking theoretically, strip footings provide some kind of water barrier, depending on how well it's constructed. I know these types of isolated footing buildings have been around for a long time. But for a new design, I wouldn't do it because of potential water infiltration. I also don't like crawl spaces for the same reason, but people keep building them. (shrugs) I guess if things are pressure treated like an outside porch or deck, it should be okay.

Putting bracing on, I'm not sure how well that would work compared to legit shear walls on strip foundations. Nail capacity and all of that. But then again, so many old buildings are still standing, so historic empirical design has merit.

 

[phamENG]

I love crawl spaces. As long as they are properly designed, detailed, and constructed, you can't beat them for the ability to conduct maintenance and make modifications down the line.

Once a slab is down, your done. Vapor barrier not taped properly? Punctured by a worker walking on it? Oops. Looks like you may have water issues for the life of the building. Ripping a slab out of a house is typically cost prohibitive, after all. Want to put in a kitchen island or move the sink? Time to get out the jack hammer and concrete saw...

 

[Greenalleycat]

Our standardised code has 3 foundation options, one of which is similar to what you're describing
All footings are isolated concrete piers with timber piles sticking up then a timber subfloor above
Bracing is provided either by deeper piers (cantilever action) or timber braces between piles (braced action)
They work fine but have fallen out of favour in cities mainly due to the labour required to build them - slab-on-grade foundations are typically far faster and cheaper
They're still pretty common on hills though, I don't see anything fundamentally wrong with them and in fact they offer some advantages compared to other foundation types (easy access, basic construction though time consuming, easy to relevel and adjust)

 

[human909]

I wouldn't do it because of potential water infiltration.

Where is the water going to infiltrate if the entire structure is above grade? Water infiltration is mostly an issue when you are sitting close to grade or below grade.

Some of the variance in construction comes down to local requirements. Frost heave is a big deal in frost environments with foundations so deep you might as well throw in a basement. Alternatively you bigger challenges might be storm surges so you end up designing quite differently.

Likewise as soon as you a building a house on a steep slope you will start likely to struggle if you try stick to slabs and strip footings.

Different foundations for different needs.

 

[milkshakelake]

@human909 That structure looks like a reverse pendulum. I've seen those types of houses in flood zones. Not my cup of tea, but they work. And yes, you're right; nowhere for water to infiltrate. I was thinking along the lines of something that had an enclosed crawl space with the post and beam system.

@phamENG True, never thought of it that way. I guess I'm just tired of actually being in crawl spaces, encountering dead squirrels, stuff like that. Yuck.

 

[phamENG]

Put my elbow in a decaying opossum once...or maybe it was a racoon? Long past the ability to tell. That was lovely. No snakes yet, thank goodness. But it's only a matter of time. I do live in the swamp...

I've designed a couple of stilt houses. They are not fun. Down in NC they have a prescriptive design procedure for those. They don't calc out. At all. So it's really fun when you get a builder who's already frustrated that they have to get an engineer to give them a foundation design for a house that's slightly out of the prescriptive scope...and I have to tell them to put in 3 times as many piles as they were expecting...

 

[Enable]

Pardon the slight hijack

phamENG: I might be switching from teaching construction project management to teaching our civil engineering techs sometime in the foreseeable future, and I'm currently ruminating about certain things I'd like to incorporate into the structural classes. Contrasting prescriptive designs with their engineered counterpart where the two are wildly different could make for a great discussion about engineering judgement, empirical design, etc. Would you be able to send me an example of what you're talking about with code references you used for the engineered design & prescriptive comparator by chance?

I had originally thought of doing something like that with our empirical masonry rules and concrete foundation wall prescriptive designs for single family dwellings. But I like the idea of adding piles into the mix, especially if it's as far out as you say!

 

[phamENG]

Enable - I think I can manage that. Won't be quick, though, I'm afraid. But I'll add it to my list and should be able to put together something for you in the next couple of weeks.

 

[Enable]

That's awesome, thanks! No rush. Happy to have it after the snow flies, when things are slower.

 

[phamENG]

It doesn't snow here, so things never really get slower....