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]

Another example.

An heavy equipment need to be transported/installed in a mill building, which is surrounded by other facilities that made access difficult. It is determined the best way is to remove a huge part of the roof structure to provide room direct drop. To safe time, the roof beams and roofing are to be lift using crane by one shot. As the engineer responsible to be part of the operation team, other than analyze the stability of the building without part of the roof structure for the anticipated operation period, I'll

1. calculate the structural weight to be lifted;
2. trail selecting lifting points, check stress in the lifting point including dynamic effect, and check deflection and stress in the beams with applied wind load (assume flat lift up);
3. due to flexibility of the roof panel, it is determined that 3 lifting beams should be utilized to support the roof panel, and provide the lift points (6); for the lifting beam design, go to step 2 and using ASD.
4. Inform the contractor with drawing indicating: the lift weight, lifting beam dimension, number and pattern of lift point, lifting speed, limit on wind speed during operation.

I think you can throw more checks/concerns into the short list above. This is just the basic.

 

[phamENG]

I think I'm with you and agree with you on this one in general. But you avoid the primary topic. In step 2, when you check stresses in the lifting point and beams - what is your criteria and why?

 

[dhengr]

PhamENG:
You do not have to design the load you are lifting with a FoS of 5, just because of the ASME BTH-1 stds., that’s why I said the padeye maybe needs some special attention, and meant to imply that that might be the dividing/separation line. You design that load/structure for its normal service conditions and its applicable codes. But, of course, you would be negligent (not meeting the stds. of engineering practice) if you did not also consider the potential that any lifting loads might stress the load/structure in a way that caused unintended overstressed. But, I would not use the BTH FoS of 5 in relation to this over load or over stress. I can’t count the number of reactors, steam generators, large plate girders, and the like, which I’ve been involved with which were not redesigned because of lifting load and stresses, but at the same time, these new loadings were checked so as not to cause problems on the primary structure or component. Again, that’s the engineering judgement thing. I don’t want it to fail or hurt someone, but I don’t need to apply a FoS of 5 to make that a probable outcome either.

 

[r13]

In the lifting point, I check the localized effect on the beam due to the share of concentrate real load, not the hook, sling, or any part of the lifting devices, because I am aware of its much stronger that the lifted subject object. All stress checks will be based on ASD, that would provide the minimum safety considered satisfactory for the short duration of the process. So at the end of the dat, the lifting device is fine, and the lifting beams will be fine (no damages, as it was designed within allowable stress), and the roof panel will be fine if my analysis and design were done properly. Hope we are together (on the same page) now

[ADD] Yes, sometimes you do need to select or check the size of sling/cable to be used. In such case, multiple the reaction at the picking point by the safety factor required for lifting device to be conservative. So simply put, all parts of lifting device shall have the same safety factor required by regulation, and all lifted parts/objects shall have the safety factor implied by ASD. In the end, capacity >> real lift load (same as industry overhead crane).

 

[SlideRuleEra]

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.

IMHO, the type item being discussed is not a structure, a better description is to consider it a piece of equipment. Workers interact with equipment, not so much with structures. In many cases, if a piece of equipment is designed with strict compliance with a fixed factor of safety, no worker would touch it. The components do not "look" right... way too small.

I recall you work in industry, not consulting, these days. Is there some authority demanding the design make "efficient" use of materials? I never encountered anything like that; to the contrary, the goal was for projects that are safe, practical, dependable and well accepted by users.
Also, my experience is that loads applied to equipment are not known with accuracy, as LittleInch mentioned. Different than the codified world of structures.

I'm not saying to ignore engineering design and guess. Instead, if the sensible design of an item (say, welding together two parts) results in a safety factor of twenty, I'm not going to go back and try to make that aspect more efficient.

http://www.SlideRuleEra.net

 

[phamENG]

Thanks again everyone.

dhenger - thank you for that insight.

retired13 - by ASD, are you referring to the AISC 360 spec? In which case the factor of safety is as low as 1.67, depending on the check. Just want to be sure I understand what you mean.

SRE - you are correct. My day job is on an industrial site. I agree wholeheartedly with what you say. And no, nobody is overtly demanding that I maximize efficiency - safety is preached 24/7 and if anyone thought I was sacrificing safety for material efficiency I'd be out in a hurry. This is more conceptual for me (which is why I didn't spell out a specific structure, piece of equipment, etc.). When I approach a conceptual problem like this, I try to take it as close to the edge as possible to really understand the ideas behind it. I want to know why I'm doing something, and why other engineers choose to do what they do. Hope that makes sense. Sorry if I came across as more argumentative - just trying suss out the ideas and experiences that have built the judgement.

 

[r13]

Yes. Use allowable stress design criteria.

 

[EngineeringEric]

I have done these before, doesn't mean correctly. I usually determine the real load, give it FOS of 1.5. Then evaluate it all with ASD at the given omega (1.67,1.5, blah blah blah). So in the end there is still a decent FOS of 2.5-3 pending the failure mode. And i try to make areas that are not redundant to have a 60-70% maximum utilization so they really have a FOS of 3-4... but i honestly don't ever Calc FOS.

So, in summary. I increase all my loads by 1.5, use normal AISC loading, maybe some nice feel-good factors.

Again: this is for the object that is being lifted, like a pre-assembled steel decoration or sign or platform that is not supporting people, it isn't fall arrest, it isn't lifted more than once/twice.

 

[Lomarandil]

Yes, again, I understand that we are talking about items like bridge girders, ductwork, machine components, signs, etc, that are lifted as part of the construction or staging process. Lifted loads (which have conditions of support, bracing or geometry different from their permanent design configuration).

Items like lifting lugs, rigging and lifting hardware, man baskets, fall arrest systems which have elements of repetitive use, life safety, etc. should all be designed to the relevant (BTH/rigging/OSHA) codes.

----
just call me Lo.

 

[SlideRuleEra]

When I approach a conceptual problem like this, I try to take it as close to the edge as possible to really understand the ideas behind it. I want to know why I'm doing something, and why other engineers choose to do what they do.

Ah, I see what you are seeking. An industry that goes to the trouble and expense of having a competent in-house engineering staff, such as you, has different priorities from a consultant. Projects will be performed differently.

The example I can offer is generation of electricity:
Three most important priorities are reliability, cost control (notice I did not say lowest cost), ability of meet established schedule (no mention of "fast").

Carefully review the priorities of your industry and concentrate on performing quality work that meets the corporate goals. Over time, you will establish a reputation throughout the company (from workers to executive management) as the "go-to" engineer. This can lead to behind-the-scenes, by-name requests for you to be assigned to important, unusual, and interesting technical projects anywhere in the company, not just the department where you currently work. You will have the potential to create your own career path.

http://www.SlideRuleEra.net

 

[human909]

It's that simple.

Except it isn't that simple. That FOS of 5 has a whole bunch of hidden factors in it that you as an engineer are not considering. I normally is based off MINIMUM BREAKING STRENGTH which we as engineers are not normally calculating. It assumes that the 'load' is a simple mass, again a calculated load is often MORE than a simple mass number.

I'm hardly an expert. But in my experience it hasn't be "that simple" regarding lifting lugs, lifting beams and the like. Here in AUS we have to get ALL lifting equipment proof tested to 2x WLL or go through a very rigorous engineering control process. Sometimes it has been a pain when we have 20 items to be tested that have a FOS of 50 or so! (Other factors other than dominate the design)

In other cases I've had custom lifting beams proof tested which does give you a good warm and fuzzy feeling when you are lifting heavy loads with it.

 

[SlideRuleEra]

human909 - Turns out, the OP's question has nothing to do lifting devices (I missed that, too, for a while.)

Since you are not an expert, neither am I, suggest reviewing "Safety Factors" on page 6 of the "US Steel Wire Rope Engineering Handbook" on my website: Link

The entire 138 page book on use of wire rope for lifting devices and other applications is dated (1968), but informative.

http://www.SlideRuleEra.net

 

[r13]

SRE,

Thanks for open up your library.

 

[phamENG]

Eric - thanks. Sounds like you're using a sort of modified LRFD approach. Load Factor = 1.5, and a resistance factor equal to 1/FoS. Again - "sort of." That makes sense, and I get the 'feeling it through' idea - it's one I've used often. Your input is especially valuable to me from a standard of care perspective. If I've pieced the puzzle together right, we operate in the same geographic area and, if I ever get back to consulting we may work together or be in competition someday.

SRE - excellent advice. Thank you. I strive to do just that. I do want to keep my mind a little more open than that, for lack of a better phrase. Whether it's through another career shift someday or working on the side, or whatever the future brings, I may well find myself back in the consulting world and I don't want to lose that perspective. I don't want to let it mess up my current pursuits and the unique role I'm filling here, but I don't want the other to happen either. Hope that makes sense.

human909 - thanks for that perspective. I'll be digging into that side of things more in the near future as I look into our rigging test and inspection program. But one thing at a time. SRE is right that I'm currently more interested in the effects of rigging on the load itself, and how engineers consider it the design of structures, structural components, and equipment.

 

[SlideRuleEra]

phamENG - Very good, keep your options open. I didn't want to say this unless you brought it up, which you have:
I don't recall any consulting engineers who joined the utility who were happy with the work. Most left after a year or so. The pace is different (slow), the focus is broad (not all design work), and project involvement is from "cradle to grave", and not just engineering. I found this fascinating, but certainly see why others would not want it.

Since you are making excellent use of your (spare) time by pondering conceptual projects as a learning experience, I expect you'll be moving back to consulting before too long. When the right opportunity comes along... go, you won't regret it. Several of my friends did and were delighted (long term) that they did so.

http://www.SlideRuleEra.net

 

[r13]

SRE,

Agree with you on the boring and slow pace of the agency/governmental works. But when one aged up, the pension and health benefits considerations may change ones mind. One here

 

[phamENG]

Thanks, SRE. I may try to have my cake and eat it too, for a while. If I can find a couple of clients with small but interesting work I can do on the side, I can still enjoy my big-industry benefits package and reliable paycheck while satisfying my engineering itch. At least until my kids get a little older and I have 'enough' socked away in their college funds.

 

[SlideRuleEra]

retired13 - You are right, but I did not say "boring". What could be more interesting than working at the heart of good size business that from time to time has interesting projects such as leading repairs to a hydroelectric dam that "sprang a leak", threatening to potentially flood a mid-size city?

Or, recovering from a 280 MW generating station "flying apart, blowing up, and burning" because of operator error?

Being called in when our brand new main office building virtually collapses because of (legally proven) architectural dereliction of duty, structural design gross negligence, construction contractor incompetence, and one individual's in-house bribery/corruption?

Design responsibility & construction management of green field 500 MW generating station construction in swamps?

Leading field work engineering work to refloat and recover an historic barge in an inaccessible (by land) alligator infested bog?

phamENG - Good combination, I had an unrelated side engineering career, sanctioned by my employer, at your age.

http://www.SlideRuleEra.net

 

[r13]

SRE,

Again, agree with you. The years I'd spent in a steel mill (as contractor's engineer) proven to be both the most challenging and rewarding in my career life. 24/7, always the first contact for fire drills ... most importantly, the trust people put on you.

 

[Settingsun]

I'll post this from Australian Standard AS3850 since this discussion hasn't turned up a US reference yet. This standard is for precast concrete specifically but I'd say the load factoring is transferrable. Not particularly onerous for a stationary crane lift, but the factors jump once the vehicle moves across the ground. It also differentiates between items that are lifted just a few times between the casting yard and final position, and those that are meant to be repeatedly repositioned during their service life.

Here's the load side:

And the capacity side for the concrete being lifted. There are different factors of safety for the lifting system itself (eg lifting clutches and the cast-in anchors that the clutch connects to).

I don't have a reference for steel, but some fairly high proportion of yield stress seems reasonable, say 80%-90%.