Utility anchor for precast structure 2

[DenverStruct]

So the capacity of these anchors in shear is quite high. If you get the appropriate lifting hooks for the weight, the hook actually cannot fit the pocket! Has anyone encountered this before? For example the 8CA18 from CONAC. According to this chart it can handle up to 20,000 in pure shear. I specified 4 of these lifters on a 60,000 lbs structure (15,000 lb each lifter). The contractor rented a 5/8" chains with hook that is capable of lifting up to 18,000 lb. The hook will not fit the lifting pocket. The only chain and hook that can fit in the pocket is rated for only 12,000 lb.

Have you encountered this before? What can you put between the lifter and the hook? Is there some kind of strap or cable you can loop through the lifter?

 

[TehMightyEngineer]

Yep, we've run into that before. For anything larger than about 6,000 lb per lifter we use lift pins or similar.

I'd use a short endless sling to go from the hook to the lifter. Also, unless you're specially rigging it you're likely only going to distribute the load to 2 or 3 lifters so you might actually not be getting the full OSHA safety factor; slings will help with this as they stretch some.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

ALP Lift Pins for reference:

https://www.alpsupply.com/CatSearch/1074/alpt-lifting-pin-anchors

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[DenverStruct]

Those are good, but then it will require the contractor to use a lifting clutch. Your comment about likely to distribute load to 2 or 3 lifters is correct. I think that is why OSHA requires 4:1 factor of safety on embedded lifter.

 

[TehMightyEngineer]

If you were really in a pinch you could carve out a bit of the concrete to fit the hook. It wouldn't be pretty and you'd have to be careful you're not cutting into the capacity of the anchor but it may be possible.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[CANPRO]

I'm not involved in the precast industry, so this question is just purely out of curiosity - the capacities listed for that anchor I assume are based on testing and the concrete strength is listed as 4000 psi. My understanding is that precast elements are often stripped and handled at strengths below 4000 psi...how do you translate the allowable loads based on testing at higher concrete strengths for use with lower values of concrete? Would it be as simple as reducing the capacity by a ratio of (f'_strip^1/2)/(f'_test^1/2)?

 

[TehMightyEngineer]

CANPRO: Maybe other places might do less than 4,000 but we utilize type III cement and we're hitting almost 4,000 after about 12 hours.

Whenever I need to utilize a lower concrete strength than tabulated I do one of these: If I'm in a hurry I'll probably just specify a lifter that has tabulated data for the strength I'm at. If I have the time, I might run the appendix D failure modes, determine which one controls, and find a ratio of the strength change between the two concrete strengths (as you indicated); this is the recommended method by the lifting device manufacturers. Finally, if I'm really backed into a corner I'll just specify that they need to cure it until they hit the required stripping strength.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[msquared48]

Install more inserts for picking the panels to reduce the loads to the lifters.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

Type with both hands, Mike.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[msquared48]

You ought to see me on Fridays!

Thanks..

Mike McCann, PE, SE (WA)

 

[CANPRO]

Thank you for the response TME. Like I said, I'm not in the precast businesses, but I like to keep little bits of info like that in my back pocket.

 

[DenverStruct]

CANPRO, yes that is how you do it. I would not use the higher ratio if you had stronger concrete than the test data though. With the high range water reducer admix precasters use, they have no problem getting the capacity early though.