Trench Footer w/ICF Wall Steel Building Foundation Design 1

[Midwayman]

Hello to all,

Over the last year, I have been trying to educate myself on a proper foundation for a steel building I want to build. I wanted to share the design as it sits to see if if there any gross oversights, and to ask a few questions.

The building is all steel (open-web welded trusses, steel girts, steel purlins), 50' (endwall) x80' (sidewall) , with a full mezzanine (bar joist supports on the steel trusses). Bay spacing is 13' and 14'. The trusses are built to potentially support a 12' lean-to on either side at some later date.

Soil is clay and the building will be on a slope (about 4' of drop across the 50' endwall). After some deliberation, it seemed the best measure to go with a frost depth (36") trench footing, with a 54' stem wall on top built from ICFs. At each truss location, a pilaster would be built. I planned to mount the endwall columns on top of the wall (no pilaster).

Here is a drawing of the foundation cross section for the footer and wall:

The pilasters which would reside at the base of each truss:

So here are some questions I have:

[ol 1]
[li]Is it a concern that the center of the ICF perimeter wall is toward the outside of the footer, while the pilaster extends in to the very edge of the pilaster. I had a hard time tracking down definitive rules for how these should be centered on a footer.[/li]
[li]Is the fact that the building will be supported by a pilaster/wall with a cold joint to the footer (right at grade in some cases) a concern with respect to the non-axial forces from the truss leg?[/li]
[li]You will notice the use of foam on the sides of the trench footing. Is this acceptable? Does it need to be of any specific PSI rating?
[/li]
[li]I happened across a significant number of pre-made grade 60 #4 stirrups that are 12" wide by 30" tall. The width seems about right, but the height would violate the 3" cover rule if I kept the footer at 30" deep. Then I came across an article discussing the advantages of angled stirrups Link which if I angled (@53°) would fit perfectly . However, in reading, this almost seemed more appropriate for suspended beams for overpasses than in a footer.

a. Is angling the stirrups beneficial or detrimental in this instance?

b. If not, would there be an advantage to making the trench footer 36" deep (using 6" forms above grade) and letting the slab pour against the footer instead of between the ICF walls?[/li]

[/ol]

I appreciate any help with answering these questions, or general suggestions you may have.

Best Regards,
Midwayman

 

[Ron]

Several comments:
1. You need to consider lateral thrust of the columns. ICF stemwalls are not ideal for that.
2. Don't compromise the rebar cover
3. It is a footing, not a footer.

 

[Lomarandil]

1. This is only a minor concern. As Ron pointed out, the steel truss frames will want to push laterally (usually but not always out) on the pilasters. If the ICF wall was further inward, this would help balance the net overturning effect. But that usually takes a back seat to concerns like keeping the forms or envelope material simpler.

2. Not a significant concern.

3. Not my area of knowledge, sorry.

4a. If you know which direction the stirrups need to be angled, it is beneficial. If the angle is opposite, they will be nearly useless.

For suspended beams, the correct direction is often obvious, and it becomes an economic question. For a continuous foundation, the correct direction is not always clear (depends on your soil and loading parameters).

Now, it might be a moot point if your foundation doesn't actually require stirrups for strength (and only for incidental benefits like holding the main bars in place).But if your design really does need them, I would keep the stirrups vertical, and increase the depth if you want to reuse that material.

----
just call me Lo.

 

[Midwayman]

I appreciate the feedback so far. Couple questions:

1. The ICF build was only to allow me to build this without renting/buying forms with the added bonus of a nice R value for the stem wall. Is there a significant quality factor difference in going to a traditional lumber form?

2. I will keep the stirrups vertical. Question: Is there a reason not to add a 2x6 lumber form above the footing (above excavated grade) and then pour the 6" slab up to that footing? This would allow me to use the 30" stirrups with correct cover and also allow me to retain my 48" stem wall using conventional forms (see 1.). In my mind, this would also alleviate a cold joint below the slab to hopefully mitigate bulk water entry under the slab?

3. Is the lateral force both of you mentioned the "kickout" force of the truss leg trying to push toward the outside of the building? What are the changes you would suggest to aid in this? Lomarandil - you mentioned moving the stem wall inward, is this simply to center it better with the pilaster as opposed to having the outside of the wall flush with the outside of the pilaster?

Thanks again!

 

[Lomarandil]

1. I wouldn't expect much quality difference between an ICF form and an internally braced (tied) lumber form. When externally braced (or one-sided), a lumber form requires attention to keep the wall surfaces straight and plumb -- very achievable, but that's the only difference I can think of.

2. I believe you are suggesting to pour the footing (36" tall), then pour the slab against the side of the footing? Yes, that's absolutely a viable plan (and is a detail I often choose for other reasons -- slab shrinkage joints).

3. Correct, the lateral force is the truss leg kicking toward the outside of the building. Depending on your soils (and building loading), there might not be any change needed. Or it might be significant. You'd want a local structural engineer to check that out.

When the lateral force pushes outward on the top of the pilaster, it is going to want to rotate your entire foundation counter-clockwise. Moving the stemwall toward the inside of the building will put some weight at the inside edge, which will want to rotate the foundation clockwise. So it might be useful to help balance a marginally-stable foundation. But without more numbers (and with a relatively small stemwall), I doubt it is a critical design choice for you.

----
just call me Lo.

 

[Midwayman]

Thank you for the feedback.

Another question if I may:

I am considering conventional forms instead of the ICF. The height of 4' forms would necessitate what I mentioned in my last post of putting a 6" form above the trench footing and then pouring the slab up against the top footing top. The stem wall would then rest on top

I was instructed that doing the slab this way would circumvent the slab resting on top of the footing so therefore connection dowels would be needed at 12" intervals around the perimeter of the slab. See this picture:

This concerns me as I was under the impression that there were benefits to "floating" the slab to allow for expansion and contraction and avoid cracking from things like shrinkage. Anyone care to comment?

 

[CarlB]

There are pros & cons for the 2 methods. Has been discussed various times in the Structural Engineering Misc. forum. Here's one: thread507-437865

 

[Midwayman]

I wanted to ask a quick question without a new thread because this may be silly. I see a lot of pictures of how to install the continuous rebar around corners. This is from a "Detailing Corner" drawing and has been discussed on this site:

However, they seem to justify these by saying they are easier for prefab and setting, not that these techniques are "better" for the final result.

Since I building these in sections with stirrups every 2', I was wondering if it is just as acceptable to keep all of these rebar runs continuous through a corner. Like this:

I just want to make sure there wasn't more than just convenience to these designs

Thanks,
Midwayman

 

[Lomarandil]

No, there is a performance concern for the bars as you have shown, specifically that as tension force tries to wrap around the corner in the inside bar, it will push inward on the corner concrete, potentially causing it to spall.

In your application, you might get away with it, but the detailer's corner detail is objectively better.

----
just call me Lo.

 

[Midwayman]

Very interesting.

So mind if there are any tips on the correct way to reinforce corners when the footing has 3 continuous bars like mine?

I can only assume that all of these rules apply to walls as well as footings no?

Thanks again!

 

[Lomarandil]

You'll probably end up looking like the second detail "C" in that screenshot.

What happens with the middle bars varies. They can be taken to the far face and hooked, like the inside bars. If there's plenty of space, that's the solution that will give the best strength (and given the dimensions you've shown, you should have plenty of space). But if the rebar is getting congested, it is sometimes better just to carry them straight to the far face and end them there.

----
just call me Lo.

 

[Midwayman]

Due to the way the sections have been built, It seems the "hairpin" style connections (double bar, example e.) would be the easiest.

Know that the footing reinforcement has changed a little to this:

Would a connection like this be appropriate for the inner bar.