Is there a Prescriptive Method for a slab on grade, single story, ICF wall foundation?

The area frost depth is 18" and I was wondering if there is a Prescriptive Method for a footing for an ICF wall?



The second part to my question is if one is going to do an ICF stem wall off of the footing. What is the best method to make sure the vertical rebar footing is properly placed so that the ICF stem wall will properly go on top of that footing?

Mono pour

Posted By smartwall on 06 Jul 2016 06:49 AM
Mono pour

Sadly nobody does them out here and the county doesn't allow it.

Why?

Posted By smartwall on 06 Jul 2016 12:10 PM
Why?

Nobody does it here, not sure why, just not popular.

If you mention monopour, they will nod their heads in disapproval. Just the way it is. Maybe one can get away with it if they had an engineer stamp off on it but even then, nobody up here knows how to do them.

So getting back to the footing and ICF prescriptive method.

Is there a prescriptive method for ICF footings?

I suggest that the footing doesn't care that the concrete wall sitting on it uses a foam form.

Why yes Wendy, it's table 3.1 in the PM

Footing dimensions are a function of wall construction, wall height and soil load-bearing capability:

Footing Fundamentals

Your local building code will contain a similar table. In summary, heavier, taller walls and lower soil load-bearing capability require wider footings. We typically use 12" thick x 36" wide footings for our ICF buildings (one and two story) where we build. Doing so provisions a one story building to become a two story building in future (i.e., enables the addition of dormers in future to create additional living space). We also use attic trusses for the same reason. Not much additional cost to accomplish this.

As far as getting the ICF dowels in the correct location, your ICF company should provide something like the attached page.

Your county probably has a published design standards for wind, snow load, soil load bearing capacity ect. That is the number you want to apply to the perscriptive table. You can also calculate it pretty easy as a 6" wall weighs 75# per SF and an 8" wall weighs 100# PSF of wall area. You also need to add in the weight of the roof system and the loads if any that it must bear, as it all must be borne by the footing. My county uses the load bearing for sand(1500# Per Square Foot), unless you want to pay for a soil analysis and engineer signoff for a greater LB figure. My buildsite happens to be sand:) As for rebar placement, you need to layout where the blocks will lay, and position the rebar accordingly so it dosn't interfere with the ICF crossties. Keeping it in place is relatively easy with traditional forming. Wood sides with crossed pieces of wood to keep the form tops in line. On top of this you place an upright piece of 2X lumber right alongside where the upright rebar will be placed. You can then measure and mark along the top of the lumber at the rebar spacing. You bend the tails on your uprights, and get a scrap piece of garden hose and cut it into 3" pieces. Slide these over the rebar and run a screw thru the hose, alongside the rebar and into the 2X lumber to screw the rebar to the 2X at the marked locations. The bottom of the bent tail gets wired to one of the lat/long rebar sticks near the bottom of the form. After the pour remove the screws and pull off the hose pieces. I will put up a pic on photobucket of this, works great and dead easy. There will of course need to be some rebar running the length of the footings and some short pieces crossing perpendicular to those at intervals. This last footing we did was 12" tall by 36" wide. Uprights were #5 at 16" O.C.(positioned to miss the ICF crossties) with 10" tails and about 30" protruding above the top of the footing(48" total length). In the form there were 3 pieces(left,right,center) of #5 running continous around the length of the footing about 3" off the bottom. There were 30" sticks of #5 every 2'O.C. laying across/perpendicular to the #5 runs... Most re-enforced footing I have ever done, but also the first one for ICF. Basement 8" and mainfloor6" all the way o the roof.

OK, hopefully these are links to some of our rebar photos. You can just see a little bit of the yellow garden hose holding the rebar. Between that and the wire to the lower bar, it stayed put real well...


http://i270.photobucket.com/albums/...9816eb.jpg

Here is one test fitting a block over the bar. Definitely measure the block to rebar relationships very carefully...

http://i270.photobucket.com/albums/jj85/rmarlett/9351489b7615daa663f9a2ac2e164690.jpg

Here are photos of my ICF footing showing the rebar dowels:

http://www.fab-form.com/fastfoot/fa...nSmith.php

12" deep x 30" wide.  3 x 5/8" rebar footing reinforcement.  5/8" rebar dowels on 16" centers to match my ICF wall reinforcement and tie spacing @ 8" on center.

Posted By sailawayrb on 06 Jul 2016 04:49 PM
Footing dimensions are a function of wall construction, wall height and soil load-bearing capability:

Footing Fundamentals

Your local building code will contain a similar table. In summary, heavier, taller walls and lower soil load-bearing capability require wider footings. We typically use 12" thick x 36" wide footings for our ICF buildings (one and two story) where we build. Doing so provisions a one story building to become a two story building in future (i.e., enables the addition of dormers in future to create additional living space). We also use attic trusses for the same reason. Not much additional cost to accomplish this.

As far as getting the ICF dowels in the correct location, your ICF company should provide something like the attached page.

What type of rebar schedule do you go with on the footings?

12 x 36 is a good sized footing but it's always better to over-engineer the footing since the foundation is key

Lbear, a couple of things to keep in mind with footing design. One, I suggest you make sure to connect appropriate ground wires to the rebar for your electrical service. They're known as UFER grounds. Much better than ground rods.

Second, wherever you have multiple walls supporting a roof and floors coming together in a corner or tee, you need to be sure to take into account the total load on the wall at that point. In my case I had 36" square blocks in a few places while the footings themselves were only 16" wide.

I used 16" wide x 12" deep footings based on my calculations of all the loads resting on them. As I recall (I designed the house 8 years ago!) I calculated the footing width for 1500 psf bearing load but the county prescribed bearing load is 2000 psf. In several sections of the wall the footing at 1500 psf was over 16" wide and, of course, less at 2000 psf, so I went with 16". That also was the standard form size the concrete guy had readily available.

My house is one story but since I built on a sloping lot one gable wall is about 17' high average. Interestingly, it has one of the lower loads on the footing. As I recall from your earlier posts you are planning on a concrete deck for the second floor. That, in addition to the second story wall plus your roof load may be heavy enough you will need more than the 36" wide footing mentioned above.

My recommendation is you have your engineer calculate the loads and required footing widths. Also, find out what your local building code prescribes for the soil bearing load. You may be surprised at what sort of numbers come out of the works.

PM me your email address and I'll send you a copy of my spreadsheet of my footing calcs. You might be able to get something out of it. If nothing more at least you should get an idea of what all needs to be factored into the footing calc. Footings, although seemingly simple, need more attention than what you'll get from a prescription, especially if you have a more complicated design than just a rectangle. Keep in mind that suspended floors, interior walls, roof, live loads, dead loads, etc., are all supported by the footings, either under the outer wall or by basement or crawl space walls or piers. You may need footings under the slab to support interior loads.

Lbear, actual schedule depends on the actual engineering, but four (4) #4 rebar is pretty common for the 36” wide footing. We also always put a keyway in the middle of the footing formed using a 2x4”...so a 1.5” deep and 3.5” wide keyway is formed. The ICF wall pour concrete then flows into the keyway. This commonly done for retaining walls, but we like the additional strength and sealing this provides to ICF buildings. We also put keyways on the backside of our PT lumber door/window bucks for the same reason. Dowel spacing and ICF vertical rebar spacing varies for each block (usually a multiple of either 6" or 8"), but we commonly use a 12” spacing with BuildBlock. We also drop a 1” length of 1.25” diameter PVC pipe on each dowel before building up wall and then use the PVC as the bottom tie point for the vertical rebar. The ICF horizontal rebar spacing is most commonly 16”.

Yes, a Ufer ground is preferable and is often code in dry areas. I also agree with everything else Dmaceld wrote. Having an engineer on your design/build team is often the lowest cost building expense which often results in reduced overall construction expense while ensuring a structurally superior and higher energy performance building.

Also with the UFER ground, you typically need two ground points, one where the panel can connect and one a prescribed distance away from the first for testing. By testing resistance between the two, you can prove that you do indeed have a valid circuit down and thru a certain distance of the footing rebar cage.

So if you use Fastfoot footing bag from Fab Form or something similair, is there any point in grounding to the rebar?
The plastic prevents direct electrical contact with the ground.
The plastic prevents the concrete from absorbing ground moisture, making the concrete non conductive.

ICF rebar spacing in walls is a function of backfill height and length and height of wall, not of the block that is used. That said, yes you need to avoid the webs
Proper dowels usually makes keyways redundant.
While keyways can increase water resistance, that only helps if there is no bypass for the water. Like gravel under the footings or drainage tile or conduits under or through the footings. A lot of work for little gain.
Dowels are usually (but not always) placed in the center of the footing. The vertical bars should be toward the inside face of the concrete wall (tension side) not the middle, so as a rule, the dowel and the vert should not be connected. The tension side is restrained by the basement slab and the subfloor. Bar splices may be contact OR non-contact.

A good structural engineer on an ICF project is money in your pocket. Engineers, like accountants, should save you money and not be an expense. If not, find new ones.

Point loads on top of a concrete wall seldom require enlarged footings. The point load is distributed through the wall which acts as a lintel, so the point load is not carried straight down to the footing but spread to the amount of footing under that amount of connected wall. If the point load actually needs to be transferred to the soils at that particular location you would have to incorporate a pilaster c/w verts and ties at that point to prevent the wall from bursting under the point load.

Footing should be sized for the weight imposed. So all the footings under the house should not be the same sizes. Footings receiving roof loads and floor loads may be several inches wider then say gable end footings which receive neither roof or floor loads. In theory, if all footings are the same width, the loaded footings will sink a bit further into the soil then the non loaded ones, put high tension at the corners that may lead to corner cracking. If you introduce 20% redundancy to one set of footings, you should apply 20% to all footings to keep the ratio the same.

Posted By FBBP on 14 Jul 2016 11:29 PM
So if you use Fastfoot footing bag from Fab Form or something similair, is there any point in grounding to the rebar?
The plastic prevents direct electrical contact with the ground.
The plastic prevents the concrete from absorbing ground moisture, making the concrete non conductive.

That is an interesting question... http://www.fab-form.com/fastfoot/fastfootFiles/electricalEngineeringLetterUferRod.pdf

That is an interesting question. The whole concept of the Ufer ground is that most soil doesn't conduct nearly as well as concrete which has a high pH and lots of ions. You are essentially using the mass and surface area of the concrete footing in lieu of using the soil to create a good low impedance ground. I don't know how dry concrete has to actually become before this would become a significant problem...or if concrete dryness even matters... I would fully expect that this was tested and studied, likely by Herbert Ufer himself during World War II. My guess is that the plastic would not significantly affect the performance of a Ufer ground, but I don't have any actual data to know for sure. One can and should test every Ufer ground for acceptable low impedance before putting it into use.