Lifting Load Case 4

[phamENG]

If designing a temporary structure, or a portion of a structure that is going to be lifted during construction, how do you approach capacity checks? I'm specifically speaking about steel here, but insights into other materials could be valuable for posterity/future reference.

I know ASME has their below the hook lifting device manual, but unless I'm actually designing a BTH device or a fixed rigging point I don't like to use those factors of safety - they really punish you when you're designing something that will be moved once, maybe as many as 5 or 6 times before it's either bolted in place or thrown in the scrap pile. I don't need to design for 20,000 load cycles or even a 10 year life cycle.

Do you just use AISC ASD/LRFD factors with static loading and call it good? Good crane operators should keep accelerations close to 1g, so I'm not worried about dynamics. This feels like the most reasonable answer - what does everyone else think? (This would apply to any connections and members in prefabricated assemblies loaded in a manner they may not otherwise experience in service, using standard AISC equations for pins in padeyes fixed to the members, etc.)

 

[r13]

I understand your logic and reason of concerns, but accident investigators wouldn't. Thinking the fat safety factor is the insurance of your professional license, you'll feel better in paying the price.

 

[dhengr]

PhamENG:
Padeyes maybe require a little more special attention, because the lifting load is truly concentrated right there. That is…, load path, padeye parallel with web below not across it, and the like, welding, etc. Once those loads move out into the structure in four different directions, things get a little less critical, but that is part of our regular engineering judgement exercise. I would like to talk with you some time. Years ago, I went through the same work transition that you are going through. I might have some special insight. Try me at rwhaiatcomcastdotnet for starters. You are much smarter on the latest codes and their provisions than I am, but aren’t there some provisions for lower FoS for temporary structures, during construction? Scaffolding, shoring, formwork, etc., are not designed to the same FoS as the primary bldg. components, with longer life and more significant safety implications.

 

[SlideRuleEra]

It's that simple.

http://www.SlideRuleEra.net

 

[Lomarandil]

Sure guys, but where does the lifting device end and the lifted load begin?

Once you are able to make that determination, yes, absolutely, I use the permanent design code for designing the lifted load. For standard picks, no dynamic factor. For complex picks (multi-crane, marine crane, tripping loads, handoffs), I don't have specific thresholds, but typically leave a little more margin before letting the load get to 100% utilization.

----
just call me Lo.

 

[skeletron]

@dhengr has a pretty good summary of how to make the judgement factor work for you. Here are some other thoughts when you need to "sharpen the pencil":
1. Be there on site and supervise the lift.
2. In my experience, I was always directed (by senior engineers) that FS=5 is for plastic design and FS=3 is for elastic design. Use that to your advantage. Can your steel achieve FS=5 if you use the plastic section or distribution based on plastic capacity being reached? Is your steel over the elastic yield point, but still within the plastic region? A specific example of this would be the lifting lug under the eccentric load that causes out-of-plane. We would set up our design calcs to monitor FS under elastic and plastic conditions which helped guide that judgement a bit. Welds, for example, were always checked at plastic forces (again: guidance by a senior engineer that had research experience in welding and work experience with heavy lifts).
3. It sounds like your situation may be beyond lifting lugs and actually incorporate components of the temporary structure. I would advise to still keep a nominal factor of safety beyond the ASD/LRFD factors unless you do #1 and also confirm deflections.

 

[phamENG]

Thanks, everyone. It sounds like Lo is of a similar mind with me. The big question is load vs lifting device. Most rigging guidance is related to to the lifting device. Slings to have a FoS = 5. BTH devices to have FoS between 3 and 5 depending on mode of failure, etc. I've found little specific guidance on the load itself - if any special approach is needed.

dhengr - I agree pad eyes need special attention - I've spent a not inconsiderable time studying them and the various design methods out there (industrial publications, basic rigging guidelines, maritime/offshore technical groups, etc.). I feel like I have decent handle on the specifics of analysis there. I do need to dig deeper into ASCE 37 and see if I can find anything else there.

SRE - I agree with what you posted, but I don't know that it really applies here. I'm not designing a "lifting apparatus." I'm designing a tool stand, or a platform, or an assembly of parts that needs to be moved, or something similar. It could be moved by forklift if there's a clear path, but if not it will be shifted around by a crane. For a below the hook device specifically designed for lifting a load, I would absolutely follow ASME BTH guidelines and ensure OSHA regs are met. But I'm not convinced a load needs to be designed to the same rules as a lifting apparatus.

skeletron - I've used the same 3/5 rule for lifting devices in the past. And yes, it does go beyond lifting lugs. The load can often see stresses during installation/movement it won't see in normal service. In many cases the reversal is trivial, but not always.

 

[r13]

I would apply the real load without load factors, and spell it out on the drawing or spec.

 

[phamENG]

You'll have to expand on that one a bit, retired. I'm not sure how that would ever work. I'm responsible for the design - what's the point of paying me if I'm just going to tell the fabricator how much something weighs and tell him to "figure it out"? This isn't something that can be delegated. I can see in some cases where that might be possible, but for my situation there is no delegated design. And even if I could, I'd still have to be happy with what they came up with. So the same decision would have to be made.

 

[r13]

It is simple - the base load is the real load, you then design the lifting devices for the worst design load case/combination with a safety factors of 3-5, so in the equation, your lifting capacity is 3-5 times of the real load, the safety factors are there to cover all possibilities, so there is no guess work on the contractor's side, just to be honestly follow the rule, not to exceed the specified load during operation.

Note, you did hard work in the front - design the safe devices, the contractor is benefited from your hard work. You do need to specify the operation conditions that meet your design assumptions - lifting points, speed of lifting, lift height, swing angle, wind limit..... etc.

 

[phamENG]

I'm not sure how applying the load with a safety factor of 3-5 is the same as applying the real load without load factors. But I think I get the gist of what you're saying.

 

[r13]

Similar to a DIY person buying product from a manufacture, he/she does need to know how it was designed, only need to know it will work if he/she follows the instruction. But as engineer, we know the product was designed with some safety factor to ensure the product function properly.

 

[SlideRuleEra]

phamENG - Mixing load factors and Factor of Safety (FOS) is like "mixing apples with oranges", they are totally different. For steel, FOS is historically, firmly grounded in "old" ASD (Allowable Stress Design). An example may be the best explanation:

A design with A992 steel (ultimate strength = 65 KSI) is to have a FOS of, say, 5. Stress in the the steel under expected operating conditions should be 13 KSI... the end.

http://www.SlideRuleEra.net

 

[phamENG]

SRE - thanks. I'm aware of the difference between allowable stress, allowable strength, and load and resistance factor design, as well as the factors involved. That's not what I'm trying to discuss. I'm trying to understand how people treat a lifting design load case for a lifted load. Not the spreader bar, not a lifting device, not a below the hook device, but the load itself. Something that may be lifted once and never again, or may be moved around a few times in its design life.

Let's say you design a structure that is predominantly dead load. You design it per Allowable Strength Design and everything checks out. Then, you run a load case to see how it will behave when the contractor picks it up to put it in place. It still works if you check per Allowable Strength based on it now hanging from 4 points at the top rather than resting on 4 points at the bottom, but if you run the lifting case again with a factor of safety of 5, several components exceed this lower allowable stress for "lifting apparatuses." Do you redesign the structure?

 

[r13]

I sometimes got confused between all those factors, and the applied load and reactions for simple mechanics, when I started. Here is another real word application that you might be quite familiar with.

When you rent a high-lift, it will provide you with the limit on the bucket load (a limit on real load), and restriction on boom length and swing angles, with or without extended stabilizing legs. Are you safe if all instructions are followed? Yes. How safe? Maybe in the order of 3-5 (safety factor). Will it fail if there is necessity to double the load? Well, the machine definitely will not fall apart (due to its capacity = FS x load limit), but you might feel bumpy, and shall be worried about the high-lift rolled over, as the overturning stability could be less than 2 under certain conditions (when operation condition violates its design assumptions).

 

[phamENG]

Fair enough, but you're presenting a case about operating a piece of machinery. If you slung straps around that high lift and picked it up with the crane, would you check its frame to support 3-5 times its own weight?

 

[LittleInch]

I think you're splitting hairs here.

The load must be able to withstand the forces involved when it's being lifted. You say that yourself - "The load can often see stresses during installation/movement it won't see in normal service. In many cases the reversal is trivial, but not always."

You don't know several crucial things:

1) How many times this item is going to be lifted.
2) The actual conditions and forces it will experience. So sure a good operator will keep it below 1G, but a bad operator? Or a good operator in a high wind day when he needs to move it faster than he would like?
3) Is it going to be moved on a crawler or a fixed crane?

So do you re-design? I say yes, but choose your FOS as appropriate which looks like 3 to me for elastic. Or add more lift points. Or design a bespoke lifting frame.

I've seen pre packaged buildings designed like this that are only ever lifted once, but you really don't want them to bend in the middle just because the crane operator lowers it a bit fast then buts the brakes on.

I've always hated this idea that just because something is "temporary" or only used "once" it somehow needs to be designed differently than an item which is "permanent" or used more than once.


As a designed you never know 100% how it's actually going to pan out in the real world so you just can't make those assumptions. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

Thanks, LittleInch. The variables you bring up are good points. Where do you get the FoS of 3? Is it judgement, training, code?

I've always hated this idea that just because something is "temporary" or only used "once" it somehow needs to be designed differently than an item which is "permanent" or used more than once.

If you're talking about just ignoring design criteria because it's "temporary" I agree with you. But if you're saying that all structures should have the same rules regardless of temporary or permanent status or number of uses, I disagree. If I design a concert venue in Miami, it'll be designed for the hurricane force winds in the code. But a temporary stage being erected down the street in February that will only be up for 4 days? No. Its foundations and superstructure will be designed differently. Because the statistical probability of it seeing the full hurricane design load is effectively nill. And ASME specifically rates lifting devices based on the expected number of uses and/or desired design life.

 

[r13]

The frame may not, the sling must. So you need to spread out the load to be safely handle the lifting load.

 

[phamENG]

retired13 - the sling strength is a pretty cut and dry requirement in OSHA 1910 (or 1926 if you're talking specifically about construction). I'm not worried about that. Or the crane hook or cable or any of that. I'm just talking about a structure or part of a structure that gets lifted and the stresses it sees during the lift. I'm curious how everyone looks at it and accounts for it - or doesn't.