Chain/concrete connection anchoring

[confused859]

This may be a bit bizarre but I was given this problem to solve. I need to anchor a 3/4" chain (grade 100 - 113 kips minimum breaking force) to a concrete deadman (6' long, 3' wide, 6' deep). The client wants to wrap the chain around an embedded steel bar (3" in diameter) near the bottom of the deadman (6-12" from the bottom surface).

I realize the deadman is too small to develop the full strength of the chain, but I would like to consider a hypothetical load of 100 kips.

Any advice on calculating the holding force of this type anchor? Also any advice on rebar size and spacing would be great.

 

[JKW05]

For the concrete, I think I would try to correlate the concrete failure with the failure modes suggested in Appendix D.4 of ACI 318.

For the rebar, I think I would determine an effective uniform load over the length of the rod, and consider the length of rebar on each side of the upward force (the chain) as a cantilever. The size of the rod would be determined from this moment and shear. I would also check the effective bearing pressure on the concrete based on the projected area of the rod [upward force divided by (length of bar times bar diameter)].

It should be noted however that the assumption of a uniform load distribution is a simplification, and depends on the stiffness of the rod. The stiffer the rod is, the more realistic the assumption may be. If the rod is relatively flexible, the load distribution way be increasing (linearly?) toward the location of the upward force.

No references for this method; just throwing out a starting point.

 

[JKW05]

One other thought....

You said the chain has a breaking force of 113 kips. I would recommend applying a factor of safety to obtain an allowable working load. For wire rope, a design factor of 5 is not uncommon.

 

[msucog]

here's yet another thought (and forgive me if i'm misunderstanding the scenario)...6'x3'x6' concrete by say 150pcf = ~16kips. what else is holding the deadman down? soil? mighty stiff soil if so...

if it were a magical deadman, then figure the max tensile strength of say 60ksi steel by whatever size it takes to get 100 kips (pulling 100kip requires 100+kips resisting)....without looking i'll guess a #14-16 piece of rebar to hook on to. as far as rebar spacing, i'd say it'll take a lot of rebar with high strength concrete...throw in a touch of luck and it might hold for some time. i don't really know...i'm pulling it out of my arse based on my limited brain power.

i understand that all this is just hypothetical, but i'd say use jkw05's recommendation and keep it in (or at least near) the realm of "realistic" ("realistic" based on something pulled outta my bottom). not trying to be antagonistic with my post but just voicing my "abbrasive" opinion (been told i'm not exactly suave when i disagree with something)...i'm working on being more personable in case you can't tell.

 

[confused859]

Thanks for the ideas and advice. I realize that the scenario sounds far fetched, but that what engineers do - make the impossible possible, right?

 

[JKW05]

Nice catch msucog.

I may have misunderstood the geometry; I assumed the 6'x6' was the plan dimensions with the 3' being the thickness. Thus, the horizontal rod being placed near the bottom. In addition to the rod that the chain is attached to, reinforcing would be required for the deadman (kind of like for a spread column footing, but with a negative load).

I would say if the dead man were buried that the weight of the soil above could be included in resisting the uplift, with perhaps a volume projecting out at ~60 degrees to the horizontal. I would also think that a 1.5 saftey factor would be appropriate for the required resisting dead load (unless the 100 kip uplift is a factored load).

I wouldn't say this problem is far-fetched or impossible. It just needs to be analyzed realistically. If 100 kips is the load, then that's what the dead man should be designed to resist. The contractor will probably jump up and down when he learns he needs a 10x10x10 chunk of concrete, instead of what he proposed. And he'll proclaim that engineers are too conservative. But then I would say he should provide the back-up analysis to prove it. 2+2=4 no matter how much a contractor may want it to =10.