The Building Inspector told me to use 1500 psf for the soil load bearing pressure value to design the footings for my ICF house. Soil is pretty loose and sandy where I'm building. When I calculate the load on the footing, i.e. wall weight, dead and live loads for floor, ceiling, and roof, and snow load, all transformed to pounds per lineal foot load on the footing, I find I need right at an 18" wide footing. Now, should I apply a safety factor to that number and increase the width to something on the order of 24", or does the Soil Classification System already have an adequate safety factor built into the soil bearing pressure numbers?

Basement and one story, or two stories?

Dave

One story on a sloped lot. Wall height above top of floor is 9', and varies from 2'-4" to 5' below top of floor. The footing has multiple steps. I label the below floor portion as foundation wall even though the foundation wall and house wall is continuous from footing to roof.

Heaviest calculated load is at the widest part of the house where the foundation wall is 5' (for a total wall height of 14'). The roof span is 28' and the floor rests on a pony wall at midpoint. That load is 2227 #/lineal foot, which equates to a footing 17.81" wide at 1500 psf soil pressure.

I used concrete weight of 80 psf of wall area for 6" thickness.

I'll be happy to post my spread sheet if that would help.

When it comes to footings bigger is always better if you don't know your soil conditions. As you have stated your house size above, on a 1500 psi soil condition doesn't exist in our code book. A 15" wide footing is required on 2000 psi soil but the cost to go 8" x 18" is so insignificant it's worth the peace of mind. 1500 psi soil is non existent in our area.

Dave

I wish! :-) I'm planning on a 12" deep footing. If I go from 18" wide to 24" that calculates out .5' x 1' x 176' = 88 cu ft = 3.25 cu yds. At $100/yd that's $325, a touch more than $100. The garage footing needs to be only 13.5" wide.

I'll give the additional rebar some consideration, although from what I find in the IRC rebar isn't required in the footing at all except in seismic areas.

Look at the rebar holders at quick formz, these coupled with fab-form (fast foot) will be less expensive than 2" x 12" i.e. you use 2" x 4" instead. They come out clean and can be re-used as truss lacing, interior wall plates etc. at no cost to you. As far as the footing goes 12" x 24" would be overkill for the house you specified above. Unless your on blow sand!

Dave

I wouldn't guess or use code minimums for the design of your foundation. I suggest you have your LOT core drilled and then have a structural engineer design your foundation based on the soil report. Thats what I did. :-)

Tony

Excuse me please, Tony. In my first post I said I calculated the load on the footing. That's not guessing. I used the code required numbers. Not all code requirements are minimum. For example, would you expect every square foot of the floor in a house actually to be loaded at 40# live load? But that's what I used! The only concrete number (pun intended!) in my load calculation that isn't a conservative "use this for design" code book number is the weight of the concrete, 160 pcf. And even that's on the high side! The only reason I calculated the load is my wall configuration doesn't fit the charts in the books, i.e. IRC, Prescriptive Method, or block mfr manuals.

Is the 1500 psf bearing capacity of the soil in this particular area a minimum? I doubt it. But that's the crux of my question. Is soil that is classified as capable of holding 1500 psf actually capable of holding, say, 3000 to 3500 psf? If so, then I have a safety factor of 2 already built in by virtue of the code minimum! If the classification system allows soil to be rated 1500 psf when it can barely hold 1510 psf, then I may want to go a little wider. Soil classification numbers aren't arbitrary, either in how they are derived or applied.

I just thought someone here on the forum may know how conservative, or not, the soil classification numbers are. Or on the other hand, when foundation loads are calculated instead of using code minimums, is it normal practice to bump those numbers up?  I've got a call in to my civil engineer cousin who can probably tell me. I didn't even think about calling him until yesterday.

Sorry I didn't mean to offend you. I tried to design my own foundation using similar information and techniques you describe.

A contractor/Engineer reviewed my plans and suggested my foundation was inadequate. After the soil report and structural engineering, I discovered the only thing I got right was the footing width.

My footers were too shallow and I needed many additional interior footers. Also he upgraded the footer and slab reinforcement. And this was in an area with no frost or snow loading.

Tony Wilkey

Tony:
Nice post.
If you don't mind, what were the approximate costs for these core drilling and engineering services?

Kevin

Key points to consider when footing is excavated

1500 psf is pretty low, likely you are okay, however look for issues that may cause differential settlement, which will increase your width requirements, they are

differing colored soils on which you will be placing your footings.
differeing texture soils on which you will be placing your footings.
wet or damp, vrs dry areas on what you will be placing your footings.

The key is to ensure that your substructure rests on consistant soils, to ensure that when, not if, it compresses the soils, the foundation does not "sink" faster or at a greater rate, in one area, compared to another area under the same structure.

Mark Ross

Core Drilling 20' with soil report ~$1000

Foundation Design and Interior/Roof steel framing design details was ~$2500.

Tony

Thanks, apology accepted. Sorry for the sharp retort. I shouldn't have been so sensitive.

When you say your footers were too shallow do you mean not deep enough into the ground, or the footing itself wasn't thick enough? Just curious. One reason  why I'm doing my footings 12" thick is so I can get the bottom of them onto undisturbed soil and below frost without using another 1/2 high course of ICF blocks. When we do the excavation it's quite possible I may have to go a little deeper to be on solid soil. If I have to go close to 8" deeper I'll either add more steps or just drop the entire perimeter footing 8" and add another 1/2 block of wall. I've come to the conclusion buying a sloping lot wasn't the wisest thing I've done on this project!

As for interior footings my loads are well below the threshold for 10" wide x 8" deep. The number and location of them was determined by the allowable span of the I-joists I'm using to keep floor delfection within limits for ceramic tile. When you say number of interior footings are you referring to pier type footings, or runner type? I originally had pier footings to support beams under the joists, but my concrete guy said to forget it. Too much work, make them continuous and use pony walls for floor support.

Thanks for the suggestions. From what I've seen in the subdivision where I'm building I'd be surprised if the color and texture varies much at all. Moisture level could be an issue. The one safeguard I have, for what it's worth, is the BI will have to OK the footing before we pour. He's already made it clear if the soil doesn't look solid he'll require us to go deeper.

The 1500 psf number may be conservative just to be on the safe side. I just looked at the Idaho Mf'd Home Installation Std which I helped write a few years ago. The required size for pier support footings under mf'd homes is based on 1500 psf soil. That's for all of Idaho. My brother, who also worked on the standard, said that number was chosen as the least common denominator for soil strength in Idaho to simplify the standard.

The comment about compressing the soils has me wondering. Should the footing width be varied in different parts of the house according to the weight of the house so that the load on the soil remains nearly constant in terms of psf? Seems like that would be overkill, and something I've never observed in a house. If that's the case, then I should make the footing under the garage walls only 14" wide rather than 18" like the house.

My footers were not deep enough. The engineer added runner type footings criss-crossing between load bearing walls and rooms with large spans.

Tony

Tony:
The core drilling and soils report were a great idea, especially if you are not familiar with the land in question.

We were contracted to do an ICF house in S. Indiana a couple of years ago. Contractor had dug footings which were washed in. We were supposed to clean out the footings and commence vertical accomplishments. The soil didn't seem to "virgin" to us, so we dug some test holes. Nine feet down, we found childrens toys, organic materials and sod, and an old barn.
We ended up overdigging to virign soil and infilled 600 tons of rock to be able to pour a basement floor above 100 year flood levels.

One of our employees stopped by the subdivision last year, it turns out our homeowner was the only one not suing the developer; as lots were supposed to be compacted with structural soils and ready for a structure; most of the other homes in the subdivision were built on the poor soils and had structural issues.

Homeowners ended up being the losers, the developer ran away.....

So Tony, I believe your $1k was well spent.

HaHaHa(not funny really). As a former surveyor whose seen corn fields turned into subdivisions, and 20'+ at the top of a hill moved to the lowest areas of the subdivision, it's buyer beware. I told myself way back then, that I would never build on that crap. Top ok, bottom no way.
 
If there is paving, take note to whether it has settled. That may be an indication that they built on fill.

I remember towers of dirt, where a lath pounded in months before, would take a ladder to reach. Be careful out there folk's.

Fortunately, I have no reason to suspect fill on my lot, but I'll take a good look next time I'm out there just to be doubly sure. The entire subdivision was a farm field only a couple of years ago. Everything is still on original grade.

You guys familiar at all with the New Orleans area? Houses in many areas have to built on pilings, even on virgin ground. It's about as stable as an air mattress floating in a swimming pool. I saw houses on pilings where the ground had settled so much there was daylight under the slab! One coworker had a house built on fill that hadn't been sufficiently compacted. Fortunately he partially recovered the cost of foundation repair. There are companies in Louisiana that specialize in jacking houses back up after the ground has settled under them. One techique is to drive concrete pilings under the foundation, one 18" long cylinder at a time.