Rock achor to concrete
Rock achor to concrete
(OP)
Hi everyone,
There is this rock anchor design (vertical) for which the contractor wishes to determine the length of the actual anchor sticking out of the rock and embeded in to the concrete foundation to be able to whithstand the uplift load (225kN). The foundation will be poured on top of the rock and the anchor will be embeded. It is a passive anchor (no prestressing), and the foundation is footing with pier on top all reinforced.
So the lengh is function of the concrete-anchor interaction, right? how can I find that value? Can the failure cone be neglected since there are rebars paralel to the anchors and ties for those rebars?
Would be the embedment length 0.45*k*Fy*Diam/sqrt(Fc) from the code be enough, considering there is transverse reinforcing across the potential failure cone (in there is one)?
Any help greatly appreciated.
Regards
There is this rock anchor design (vertical) for which the contractor wishes to determine the length of the actual anchor sticking out of the rock and embeded in to the concrete foundation to be able to whithstand the uplift load (225kN). The foundation will be poured on top of the rock and the anchor will be embeded. It is a passive anchor (no prestressing), and the foundation is footing with pier on top all reinforced.
So the lengh is function of the concrete-anchor interaction, right? how can I find that value? Can the failure cone be neglected since there are rebars paralel to the anchors and ties for those rebars?
Would be the embedment length 0.45*k*Fy*Diam/sqrt(Fc) from the code be enough, considering there is transverse reinforcing across the potential failure cone (in there is one)?
Any help greatly appreciated.
Regards






RE: Rock achor to concrete
RE: Rock achor to concrete
Thanks
RE: Rock achor to concrete
RE: Rock achor to concrete
This anchor bar's Fy is 1035 MPa, as opposed to the 400 for regular rebar. If I calculate with that, the length is way too high, the thing is that the design load for this anchor is 225 kN which divided by 819mm2 of net area, I get a Fy of 276 MPa, no even close to where it starts to elongate, but if I use 276 as Fy to enter the equation to calculate ld then I get a pretty decent ld, on the other hand, I'm not sure if the ratios strain-stress would be the same...
RE: Rock achor to concrete
RE: Rock achor to concrete
I appreciate your input
btw I'm using Canadian A23.3
RE: Rock achor to concrete
RE: Rock achor to concrete
StricturalEIT:even if I use the cb factor of 2.5, the Fy value of 1035Mpa sends the ld value to the roof.
RE: Rock achor to concrete
RE: Rock achor to concrete
The nut or plate would have to be designed for bearing against the against the concrete (using a max bearing stress of 8f'c based on the high local confinement).
Do the deformations on the anchors you're using conform to the spec for rebar deformations? We recently used very high strength bars (over 150 ksi) on a job in a similar application. These bars were threaded rods, not rebar, but the threads conform to ASTM that specs rebar deformations. Based on that, we treated them as rebar for development length. They were almost 3" in diameter and very high strength. Using 8ksi concrete, the development length wasn't completely unreasonable (somewhere around 7'-8', I believe). I agree that is alot if you're going into a footing. What kind of development length are you getting for your bar?
If the threads conform to the rebar deformation spec then the nut (and plate) is (are) really just belt and suspenders. If the threads don't and the nut is necessary I don't think you're going to get away from the thick footing. When the nut/plate load the concrete locally, that will kick you into App. D. Then you'll need rebar (fully developed) on both sides of the failure plane in the footing. This will cause the footing to be very thick.
RE: Rock achor to concrete
Along the lines of what Hokie suggested, I like to use an embed plate at the end of 4 rods for columns with high uplift. Then you design for the tension in the rods and punching shear of the plate. And local stresses also I suppose, but don't skimp on the plate and that should not be a problem. I usually throw a couple of rebar through the punching shear zone and beyond for good measure and cheap insurance.
Maybe that or something similar will work in your situation...?
RE: Rock achor to concrete
but your last sentence
"When the nut/plate load the concrete locally, that will kick you into App. D. Then you'll need rebar (fully developed) on both sides of the failure plane in the footing. This will cause the footing to be very thick."
why rebar needs to be fully developed. you can also reduce it based on Asreq/Asprov for tension right? Also you may add more rebars there to reduce the length right?
RE: Rock achor to concrete
StructuralEIT and hokie66, Thanks for your imput guys, it was valuable and helped me clearly understand how to approach the solution.
Cheers
RE: Rock achor to concrete
a2mfk described what I was suggesting. So if his comment does not apply, neither does mine.
RE: Rock achor to concrete
I ended up calculating the development length of the anchor taking into account the reinforcement around and position of the anchor (dcs+Ktr) and it turned out good, so no need of any other type of afixing.
Thanks a lot guys