Precast/Tilt-Up lifting anchors - Combined Shear/Tension

[530_3D]

I'll keep it short...half the hardware vendors and consulting engineers say combined shear and tension should be checked for lifting hardware, other half say it doesn't apply. Sometimes it controls.

Assuming its origin is in ACI Ch. 17, there is a disclaimer in the first paragraph of the chapter: "Safety levels specified are intended for in-service conditions rather than for short-term handling and construction conditions." There is also the 4:1 safety factor specified in OSHA 1926.704, which is further straying from ACI Ch. 17.

I feel comfortable not checking it when such a large safety factor is used for tension and shear capacities, and a recent check of an anchor using Ch. 17 calculations showed it passed when maxed out in tension and shear, but the catalog-listed capacities are more conservative.

Any precast or tilt-up folks able to weigh in?

 

[KootK]

I agree with your perception that it is uncommon to see folks checking combined tension and shear in practice in this space.

I can, however, think of no valid technical reason to justify that practice. Be they large or small, I don't believe that it is the intent of safety factors that they should compensate for undercooked design approaches.

I kind of feel as though the catalog purveyors lean into this a bit. "Here are some simplified design values that you are welcome to combine in more complex and penalizing ways if you wish". Wink wink.

 

[530_3D]

So when checking it, would you not use the larger safety factor for allowable, or is that “undercooking”? Some catalog purveyors consider the check with the 4:1 for allowable “overcooking”

 

[KootK]

So when checking it, would you not use the larger safety factor for allowable, or is that “undercooking”?

I would use the larger factor of safety and would not consider that undercooking. By "undercooking", I meant using a design method that is not adequately rigorous. In my mind, safety factors are always about load and material uncertainty and never about compensating for inaccurate design methodologies.

If it is the case that the 4:1 is indeed meant to compensate for not considering combined shear in tension, then I would at least like to see that in writing someplace. Otherwise, how do you know what design cheats are to be covered by it and which aren't?

 

[530_3D]

Unfortunately there is not a single available example of how the capacities are determined, or even a failure mode for more specialized anchors.

The 4:1 safety factor is from OSHA, which doesn't mention combined interaction or ACI Ch. 17. Adopting the most conservative aspects of different codes while disregarding their context and disclaimers could be considered exceeding the standard of adequacy. This is the take of one of our more prudent consultants.

 

[KootK]

This is the take of one of our more prudent consultants.

It's also been my approach at times in the past. That said, when I take that approach, I'm am at least honest with myself about why I've taken that approach. I've done it:

1) Because it was expedient and;

2) Not because I thought that it was technically correct in some nebulous fashion.

The 4:1 safety factor is from OSHA, which doesn't mention combined interaction or ACI Ch. 17

Of course it doesn't mention interaction. Once that can of worms be opened, they would then have to speak to every other possible, combined situation. I feel that the 4:1 is concerned with load and material uncertainty and nothing else.

Adopting the most conservative aspects of different codes while disregarding their context and disclaimers could be considered exceeding the standard of adequacy.

Considering the interaction is not just the most conservative approach, it's the most accurate approach. And it's that latter bit that matters to us. Just because less rigorous approaches may form an "average" that is less than the most accurate one is no rational justification for using something less than the most accurate approach. Shall we include not checking the capacity at all in the "average"? Obviously not.

Unfortunately there is not a single available example of how the capacities are determined, or even a failure mode for more specialized anchors.

You could:

1) Ask the suppliers for back up information be it calculation or testing.

2) Not use products that don't have the back up information.

3) Maybe do your own , DIY interaction based on a some of the components raised to the 5/3 power or something.

4) Ignore interaction and follow what you believe to be the industry standard of care.

I'm trying very hard to be real with you about the situation here. What the industry does, what I've done, whether or not ignoring the interaction has technical merit... That said, if you wish to convince me of the technical merit of ignoring the interaction, expect that to be a protracted slog. And that make sense because this thread exists precisely because you are also struggling to convince yourself of that same thing.

Search your heart Padawan, you know this to be true.