Lifting lug vs shackle vs impact factors 3

[JStephen]

I'm designing a lifting lug.
The specification calls for a safety factor of 2 and an impact factor of 2.
So fair enough, if I have a 10,000 lb item, I add the impact factor and design for a load of 20,000 lbs, with a safety factor of 2. Or, I need design strength of 40,000 lbs.
Then I'm designing this lifting lug to be lifted by a screw-pin shackle.
The shackle would normally be good for a rated load, but includes a factor of safety of 5 already.
So do I use a shackle rated for 10,000 lbs or one rated for 20,000 lbs?
It seems off to specify a design load and then use a shackle not rated for that specified load.
But then it seems overkill to require the shackle to have a strength ten times what the load weighs, which is what happens with the 20,000 lb shackle.
And surely if the item is just shipped out, the crane operator's not going to look at it and say "I'd best double the weight of this for an impact factor".
Any thoughts?

 

[SlideRuleEra]

For a (static) 10,000 lb. load, design the lifting lug for ultimate 40,000 lb. and use a shackle rated nominal 10,000 lb.

Impact and safety factors used this way apply to the load being handled. If a 10,000 lb. load were subjected to and impact factor of 2, force would be 20,000 lb. Safety factor of 2 for 20,000 lb. is 40,000 lb.

Same with the shackle, the safety factor of 5 means that the shackle would not fail until the applied force is 50,000 lb.

http://www.SlideRuleEra.net

 

[CANPRO]

Agree with SRE. The simplest way to think of this is that you should be able to load test your shackle + lug combo with a static load of 40,000lbs without failure, failure at 40,001 lbs is acceptable. A shackle with a 10,000lbs safe working load and 5:1 FoS meets this criteria.

 

[r13]

I would agree with SRE too. My erroneous response is deleted.

 

[BJI]

It depends on what they mean by safety factor of 2, if it means that the lifting point should be rated for a static load of 20,000 lb, then a 10,000 lb shackle would not be suitable. Essentially, what is your design static sling load? Otherwise you could be undercutting the design margins. For dynamic loading, the GL guidelines provides a limit on dynamic load as the minimum of: a) shackle WLL x DAF, b) Shackle MBL / 3, c) proof load test for shackle. Assuming MBL = 5*WLL, then you would fail a) and b) using this assessment when including the factor of safety of 2, i.e. you actually have a reduced design margin than intended.

 

[r13]

I can see BJI's point. Can you provide the reference "GL guidelines"?

A review on ASME BTH-1 "Design of Below the Hook Lifting Devices", I notice the stated design specification is quite confusion. Important sections from the referred document are quoted below for quick review.

2-2 DESIGN CATEGORY
The design categories defined in paras. 2-2.1 and 2-2.2 provide for different design factors that establish the
stress limits to be used in the design. The design factors are given in para. 3-1.3.

Lifters shall be designed to Design Category B [greater uncertainty], unless a qualified person determines that Design Category A
is appropriate.

3-1.3 Static Design Basis
The static strength design of a below-the-hook lifting device shall be based on the allowable stresses defined in
paras. 3-2 and 3-3. The minimum values of the nominal design factor Nd in the allowable stress equations shall
be as follows:
Nd p 2.00 for Design Category A lifters
Nd p 3.00 for Design Category B lifters

3-5.1 Impact Factors
The design of below-the-hook lifting devices does not normally require the use of an impact factor. The design
factors established in this chapter are based on load spectra in which peak impact loads are equal to 50% of
the maximum lifted load for Design Category A lifters and 100% of the maximum lifted load for Design
Category B lifters. In the event that a lifter is expected to be subjected to impact loading greater than these
values, a qualified person shall include an additional impact factor to account for such loads.

Commentary: The design requirements defined in this chapter are based, in part, on upper bound vertical
impact factors of 50% of the lifted load for Design Category A and 100% for Design Category B. (The loads
used for the development of this standard are discussed in depth in Commentary for para. 3-1.3.) Therefore, the
design of lifting devices made in accordance with this standard will not normally require the use of an impact
factor. The wording of this section permits the use of an additional impact factor at the discretion of a qualified
person if it is anticipated that the device will be used under conditions that may result in unusual dynamic
loading.

 

[dhengr]

JStephen:
Get a copy of ASME BTH-1, “Design of Below-the-Hook Lifting Devices.” You should be able to find it on the internet, next to latest ed. at no cost. It covers a number of things you should look at and check on a lifting lug, and an accepted method of doing this. The customer can spec. almost anything they want to spec., so unless it is completely outlandish, roll with it. The FoS of 2 to yield, on the lug design, and the impact factor of 2 on the lifting lug design are within a fairly common range. The FoS of 5 to failure or ultimate on components like shackles, wire rope slings, hooks, other lifting hardware, etc., all components which are used many times over, and in many cases abused and stored for months btwn. uses, in less than optimal conditions, has been around for a long time, for that type of component. These components should all be inspected by the rigger before each use, as he puts his hardware package together. Otherwise, all the rigger sees a 10k load to be lifted, and will use a 10k (10k plus) shackle, knowing it has a significant FoS, in its own right. His contract may say use all new hardware and an impact factor of 2 on all lifts, otherwise he is not expected to know what the design steps where, from conception of the piece of equip. to be lifted, through the lug design.

The lug should be designed to less than yield (Fy) at a load of 40k, not to ultimate at that load, even though the shackles are designed and tested to ultimate or failure, at their rated load. They are two different animals and need not have FoS’s, and impact factors, etc. added from one item to the next, or to another item. The idea of a 200k shackle {(10k load)(2 design FoS)(2 impact)(5 to ult. for hardware)} to lift a 10k load is just silly. In designing the lifting lug, I would look at this problem by doubling the load (10k)(2) = 20k for the impact factor requirement; and I would us an allowable stress of (Fy)/(2) to account for the design FoS (Factor of Safety). This is essentially the way we would have looked at (thought about) the problem before LRFD added all its constructive confusion to the process, which just doesn’t fit these types of problems.

You also have some control over the shackles which can be used with a pad eye or lifting lug by the size of pin hole you provide and/or the thickness of the lifting lug at the pin hole. Too large a shackle, and the pin won’t fit; too small a shackle and the inside width at the pin of the shackle won’t fit over the lifting lug thickness. Lifting lugs and the-like, are not particularly redundant structural items, so use good clean detailing and load paths into the piece of equip. being lifted, and good clean welding details, so as not to introduce stress raisers in to the system. Don’t weld around the thickness edge (the .5 or .75” thk. pl.) of the lug. The four sharp corners involved almost always leave nicks in the lug at the upper toe of the weld, a serious stress raiser. Obviously, the piece of equip. to be lifted must be designed for the 40k load to make any sense out of the system. Ask the customer if that is the case.

 

[JStephen]

Thanks for the imput, everyone.
I've got ASME BTH (not sure I have the latest). This is not a below-the-hook device, so BTH is not a requirement, but still forms a very handy reference for the application, and I did use it. In this case, I did specify the shackle for the design load + impact (ie, a 20,000 lb shackle with 100,000 lb minimum breaking strength in my example above, for a 10,000 lb item) but couldn't help thinking that was overkill, too. In some previous work, I had factor of safety of 4.0, no impact factor, so exact same lug design for 40,000 lb strength but with 10,000 lb shackle used.

 

[r13]

This is not a below-the-hook device,..

I am puzzled on this statement, but let's jump to other topics.

Your impact factor of 2 seems abnormally high, so I would rather treat it as additional safety factor, thus the design safety factor shall be 4, which is in line with typical practices. So,

Lift weight = Rated load = 10,000 #
Lug design force = 10,000*4 = 40,000 #, impact included.
Nominal shackle capacity = Rated load*Safety factor = 10,000*5 = 50,000 #, assume effect of impact was included in the safety factor.

 

[JStephen]

"I am puzzled on this statement..."
ASME BTH is for "Below the Hook Lifting Devices", such as spreader beams. Specifically, rigging items that are used to lift other items.
I'm designing lifting lugs on a tank, so I'm actually designing the load itself, not the device used to lift it.

 

[r13]

Please have a look on the attached article, which contradict your claim that ASME BTH does not govern your design. Link

The same codes apply to lifting lugs as for trunnions. See ASME BTH-1 and ASME B30.20.The 1,500-ton lifting lug in the photo below had to be tested to 1,800 tons. ASME allows these devices to be rated at 80% of the test load.

 

[JStephen]

ASME B30.20 defines "Below-the-hook lifting device: a device used for attaching a load to a hoist. The device may contain components such as slings, hooks, and rigging hardware addressed
by other ASME B30 volumes or other standards."
In the case of that first picture in the article with the 1,500 ton lifting lug, that lug is a separate piece that bolts to a vessel flange and is removed once the vessel is in place. That piece would qualify as a "below the hook lifting device". The more normal lifting lugs are welded to the tank/vessel and are part of the load, not a separate device. Look through the sketches in ASME B30.20 to get an idea of what is actually covered. Note for example, that below-the-hook lifting devices are required to be labeled with the capacity, are required to be proof-loaded, etc.

 

[r13]

JS,

Are you defining your lug as structural component, accessory, or fitting? What standard apply? I don't think only a removable lug is considered BTH device, the purpose/nature of service defines. Why weld the lug to the tank, because anticipation of repeat lifting requirement, and it shall be inspected each use, and the lifting setups are pretty much the same. I think for design sticking to the industry-wide standard eliminates the need to post the lug capacity, but list the total tank weight.

Not mean to argue here, just curious about your view on this.