Underpinning existing foundation 1

[jeffhed]

I have a project where an existing structure has settled and we are going to underpin the foundation with helical piers and re level the structure. The existing foundation consists of a 24"x10" continuous footing with a 10" thickened edge slab cast on top of the footing. The j bars between the footing and the slab are #4 bars @ 24" o.c. In the past I have used the thickened slab flexural strength (typically a 10"x10" area with one #4 bar at the top) to resist negative moment and the footing flexural strength to resist the positive moment. I sum the shear strengths of the footing and the thickened slab area to determine my allowable shear strength. With a continuous span and a generally weak concrete slab area on top of the footing, the negative bending typically is what governs. I use the thickened slab area and the footing flexural strengths separately because with a 10" thickened slab and only a straight bar, I can't get full development and therefore can't use shear friction to verify these two separate concrete pours are working together, even though I am sure that they are to some extent but I am not sure how to calculate it. Is there a way to calculate if the exiting J bars are sufficient to tie the two separate concrete pieces together to perform as one piece without shear friction? Maybe the shear strength of the j-bars and the bearing strength of the j-bars in the concrete for the j bar capacity and then comparing those values to the required strength from a shear flow calculation?

 

[dcarr82775]

I realize this doesn't answer your question, but if you jack up the footings, what supports the slab that is not longer on grade?

Onto your question, ACI318 Chapter 17 gives you a way to calculate the strength without what they refer to as ties.

 

[KootK]

I've seen some Eurocode stuff that treated dowel connections truly as dowels, not requiring development.

Any chance the j-bars are wrapped around longitudinal bars?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jeffhed]

dcar82775,
We will be removing the entire slab and pouring a new suspended slab in the interior of the structure. Where I don't have any ties for shear transfer section 17.5.3.1 only talks about surfaces that are intentionally roughened. The tops of the footings wouldn't necesarily be intentionally roughened, but they aren't smooth either. So if I were to used the equation in 17.5.3.1 my bv = 10 and d = (10+10-3" clear-1/2 bar diameter = 16.75". Which makes Phi*Vn = 0.6*80*10*16.75 = 8040 lbs. That is my total force I can use for my horizontal shear force, not a plf, correct?

 

[jeffhed]

KootK,
As far as the j-bars wrapped around the longitudinal bars, I highly doubt it. They put the j-bars in the footing with about 8" or so protruding from the top of the footing. They usually tie a #4 continuous bar to these j-bars. If they were hooked around that continuous bar it would be a first. Most times they complain about the j-bars being at 24" o.c. instead of 48" or 72" o.c.

 

[jeffhed]

Actually, my text books state that shear friction and horizontal shear phi factors are 0.85, but I can't find that stated anywhere in ACI. Is 0.85 the correct phi factor here?

 

[dcarr82775]

That is what I have done in the past. Phi=0.75 for any recent version of ACI318. 0.85 is for the old 1.4D+1.7L load factors.

 

[XR250]

@Dcarr;

Usually the slabs can be mudjacked or pressure foamed in this situation if it is to remain.

 

[dcarr82775]

XR,agreed. 1001 ways to skin that cat

 

[jeffhed]

XR250,
Mudjacking or pressure grouting under the slab is usually the easiest way, but in this situation we have collapsible soils that were not addressed in the original soils report, thus the reason we are removing the existing slab and installing a new suspended slab.

 

[jeffhed]

dcarr,
I stumbled onto that last night. 0.75 it is. This is a two story structure and every little bit will help in maximizing my pier spacing so I wanted to make sure I was using the correct phi factor. Thanks for the reference to chapter 17 also, I didn't know that was in there.