Circular concrete pile with Shear supplementary reinforcement requirement 2

[e104909]

Hi,

I am designing a circular concrete pile with a steel column sitting on top of it. 1.5" grout was underneath the base plate. circular concrete pile is 36" in diameter. Factored shear is 24 kips. Vertical load is 53 kips. If i will consider friction using coefficient of 0.5, then friction will overcome the actual shear which make it like shear doesn't exist. BUT, i wanted to design the shear reinforcement since my anchor bolt calculation requires it for concrete breakout. And also, giving some benefit of the doubt in case it slips.

I cannot find an exact literature on how to calculate the shear tie capacity for a circular concrete section. So I just attached my calculation and I would appreciate if any of you can comment on it.

Thanks in advance







Regards,
E104909
Civ-Str P.E.

 

[Agent666]

The usual formula for a circular section will be similar to the following

The A_h is the area of a single bar in your spiral. The pi/2 is 2*pi/4 and is the efficiency of both bars. If you compare to a rectangular section with two legs parallel to the shear force over its full length you get the full capacity of each individual bar (an efficiency of 1.0).

For a circular section you get pi/4 from each bar (an efficiency of 0.785) as the bars are not parallel to the shear force.

It is important to recognise this as a circular section does not get the full strength of the bar used as you've assumed.

 

[r13]

Same as for the regular beam, use equation 11-5 (318-08) to work out Av and s. You can assume d equal to 0.8 times pile diameter. I usually place at least 3 ties on the top most layer, with a spacing equal to 1/3, or 1/2 of the calculated spacing, to provide better confinement.

 

[e104909]

Agent666 and retired13, was this all just about section check? I was hoping you have some insights more on concrete breakout requirement by ACI.

Regards,
E104909
Civ-Str P.E.

 

[Agent666]

Look at the requirements for anchor reinforcement in ACI318, basically you can substitute the concrete breakout mechanism for reinforcement that is appropriately anchored within and beyond the breakout cone. I'd say your stirrup qualifies and your strut/tie model is fairly appropriate.

The point I was making is that circular hoops or spirals should not use the full strength of the bars as you proposed when they are not wholly parallel to the load. Other than that I see nothing wrong with what you're doing if the numbers work. There will obviously be other limit states to check as well, capacity of bolts, pullout, etc.

If capacity was an issue some straight links or rectangular stirrups in line with your bolts around middle column bars would enhance the capacity and loadpath.

 

[Agent666]

Sort of like this (in addition to a circular hoop/spiral, because you sort of want your required reinforcement as close as possible to the top of the breakout surface.

 

[r13]

ACI does not (at least previously) address the effectiveness of rebars/ties in resisting anchorage breakout, whether it is in shear or tension, however it does not meant the reinforcing do not contribute, especially with careful detailing. The linked article provides some ideas. Link

 

[Agent666]

Hi Retired13, ACI318 does address anchorage reinforcement to resist breakout, and has since at least 2008 version if not earlier? Refer to appendix D in earlier versions and chapter 17 in later more recent versions.

 

[r13]

Good, it has finally caught up with most engineer's concerns and believes. In the past, ACI had been very conservative in this regard. Two simple but key requirements for supplementary reinforcement for breakout, 1) across the potential shear failure plane, and 2) has adequate anchorage/development length at the ends.