Sonotube

[penpe]

We often use Sonotube drilled piers for pipe support foundations. Generally the pipes are near grade, and lateral loads are not a significant design factor. The diameter of the pier is typically chosen for the relationship between the load, the soil's allowable compressive strength, and the "footprint" area of the foundation, (also anchor bolts not interfering with vertical re-bar). Sometimes the load is small compared to the pier's bearing capacity. ACI 10.9 defines some limits for compression members, but is aimed more at columns, not necessarily foundations. Any suggestions for how to determine adequate vertical reinforcement? ACI 10.9 suggests reinforcement steel area 1/2% to 1% of footprint area for columns.

 

[JAE]

Not sure what you mean by "Sonotube" piers. Sonotube is a cardboard column form typically used for above-grade columns, not piers.
I suppose you could use them as liners in pier holes but haven't seen that done - usually I've seen temporary steel pipe liners for holes that might slough in.

But if the pier is in the ground fully, we've typically used 0.75Ag for the vertical bars. Sometimes higher if there is expansive clays and fears of uplift tension forces in the shaft.

If above ground - I'd call it a column and use minimum 1%.





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[penpe]

I think Sonotube forms are used frequently for shallow pier type foundations. We do it all the time. If we were to use 0.75Ag (75% of footprint area) that would be As=339 square inches of vertical reinforcement for a 24in diameter pier. Did you misplace the decimal? That's more steel than concrete.

 

[AskTooMuch]

I'm more concerned with how you take into account expansive clay soil. It would look weird if you got 2" dia or small pipe and about 8 ft long drilled pier because pier needs to be outside active zone of expansive clay.

In drilled pier supporting lightly loaded pipes, it's expansive clay and/or uplift that needs to be addressed more.

 

[jayrod12]

I would bet JAE meant 0.75%Ag. We typically use 0.5%Ag for drilled piers unless there's a reason to increase.

 

[JAE]

Yes, sorry - 0.75% Ag

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[penpe]

AskTooMuch: Assume no expansive clay concerns, and no uplift. JAE: Did you mean to suggest 0.0075Ag? - which would be halfway between the 1/2% and 1% that I mentioned from ACI 318-11: Chapter 10, part 9 - Limits for reinforcement of compression members.

 

[penpe]

jayrod12: Can you refer me to and ACI 318 passage to back up that 0.5%Ag? That's the steel to concrete ratio that has been used frequently here.

 

[AskTooMuch]

You already posted in your OP where the 0.5% come from ACI. I also use 0.5% for drilled pier.

 

[EZBuilding]

Check out TABLE 1810.3.2.6 of the 2017 FBC and section 1810.9.3.2

 

[jayrod12]

I practice north of the border, sorry not familiar with the ACI clauses specifically. However for anyone's interest the CSA A23 code handles drilled piers separately from columns. clause 15.9.2.1 mandates 0.005Ag minimum.

 

[TLHS]

Okay, so what I do for long piles is (referencing Canadian codes but I'm pretty sure these clauses all have ACI equivalents):

Minimum steel is 0.005Ag. I then reduce compressive capacity due to the reduction in min steel from standard 0.01Ag for columns. I've based this on 10.10.5 of A23.3, which doesn't seem to technically apply to piles.

However, I do this because I'm generally in seismic areas where reinforcement is required to hold everything together in case of ground motions, and this is a pretty typical looking pile for western Canada even when in lower seismic areas.

I am also aware that this is not standard practice all throughout North America. Some areas have partial cages to some distance past the zero moment point, or have a single bar down the center for uplift. I'm assuming that some people do pure compression piles with no reinforcement at all.

You're looking at a post foundation, which is a /bit/ different. You definitely want reinforcement regardless of where you are (with one exception I'll note below). I'd personally still try to stick with 0.005Ag and maybe a higher ratio for smaller posts where it won't look silly. Then I'd prove compressive capacity as per above and waive bending minimum steel using clause 10.5.1.3 of A23.3 which allows you to waive minimum bending steel if your factored resistance is at least one third greater than the factored moment.

For small pipe supports, you have an alternate methodology that was more typical in the past than it seems to be now. It's basically fence post style construction. If you're using a T-Post or a Frame, extend your steel posts into the foundation so that they terminate maybe 4-6" from the bottom of the hole (some people seem to just have it bear on the bottom of the hole, but that's always seemed like a corrosion problem). Then you just pour plain concrete in the hole. The post carries any moment and the concrete just acts to spread load via bearing. It doesn't really matter if the concrete cracks a bit.

Jayrod, technically that clause is just a requirement in the interface zone with the cap or structure, by my read. I generally use 0.005Ag as well, but I'm mostly just trying to comply with the spirit of 10.10.5, which doesn't technically govern either. I /think/ there's space in the Canadian code for partial height reinforcing cages or center bars. I don't think I've seen it in a long time though.