Trolley Beam Hoist Structure Design

[swearingen]

We have a client with monorail hoists of 1 and 2 ton capacities going in a new industrial structure. The client is insisting that the structure be designed for the weight that will be lifted (1000lb for the 1 ton, 2400lb for the 2 ton), with the weight of the trolley and hoist included, of course. Our position is that the structure should always be designed for the rated capacity of the hoist (plus trolley and hoist).

In their defense, CMAA 74, the only standard we know of that would cover something like this states the following:

"3.3.2.1.3 Lifted Load (LL)

The lifted load consists of the working load and the weight of the lifting devices used for handling and holding the working load such as the load block, lifting beam..."​

The load cases listed under sections 3.3.2.5.1 through 3.3.2.5.3.3 use LL in them. There is a glossary, but no definition of "working load."

This can be read to mean exactly what they're saying - that it only has to be designed for the actual load and not the capacity of the hoist. That said, the industrial environment is often a harsh one and machinery can (and will) be abused. Designing the structure for 50% to 60% of the rated capacity of the hoist itself is folly to me and at the very least, irresponsible.

Am I off base in my position?


-5^2 = -25 ;-)

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[Ron247]

If it is a 1 ton hoist, to me you have to design for 2,000 lbs etc. Even if you change the markings on the hoist to 1/2 ton, they still know it can pick up a ton.

You have to design for the weight of the entire crane and the lifted load.

 

[swearingen]

I agree with your first sentence, but your second sentence agrees with them; the lifted load is less than the hoist capacity.


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[rb1957]

"1000lb for the 1 ton, 2400lb for the 2 ton" … not when I went to school …
1 ton = 2240 lbs = 1000 kg = 1 tonne
1 short (US) ton = 2000 lbs

You are of course being very sensible about this … you want the hoist to fail, not the structure supporting it.
you could always include a structural fuse (in the hoist) to limit (cripple?) its capability.

Who's signing the drawings ?

another day in paradise, or is paradise one day closer ?

 

[rb1957]

oh, I'm sorry … you meant that the 1 ton capable hoist will be used with a 1000 lbs monorail.

I think you're right to design the monorail to the capacity of the hoist.

another day in paradise, or is paradise one day closer ?

 

[bootlegend]

We have designed the beams for a load less than the capacity of the hoist several times. We always de-rate the system to the design load of course. We've even purchased jib crane systems from crane/hoist manufacturers that were rated for less load than the hoist would provide. It usually occurs when you need a long cantilever to lift a relatively light load. We've never had an issue.

 

[Ron247]

I meant lifted load as in the capacity of the hoist not the load the Client says they are going to lift. You can use a stronger hoist/crane than the Client's lifted load but I think you have to do something to insure they cannot pick up more than the design is actually for.

 

[swearingen]

bootlegend: I contacted some hoist manufacturers this morning and they told me they had de-rated hoists for existing beams. I guess I can understand this philosophy.

However, for a new installation in the industrial sector, why in the world would you do that? Say 10 years from now, when they want to upgrade that motor or fan? It seems very short sighted when you consider the cost difference at the present time - these are going in a billion dollar facility, and there's only a handful of them. Not to mention that they are outdoors and are subject to abuse.

Also, if one of these systems were to fail, I would prefer the hoist/trolley/chain/hook/rigging to fail before the structure came down on top of me...


-5^2 = -25 ;-)

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[Shu Jiang PEng SE]

The supporting structure normally be designed to take the marked trolley lifting load plus all related equipment/structure self weight. Appropriate load factors for each load is considered too.

—————————————————————
J&J Structural Consulting Inc.
Structural design, analysis, inspection, drawing review and stamping, and connection design

 

[AUCE98]

My two cents...The hoist, trolley and rigging can all change over the lifetime of the monorail. Design the monorails for the lifted load plus the weight(not capacity) of the hoist and trolley. Have the load capacity of hte monorail welded onto the beam.

 

[JStephen]

I would expect that if a rated capacity is shown on the equipment name plate and on the beam itself, you're good to design for that.
If the nameplate shows one capacity, the beam says another, somebody down the line is likely to "correct" that situation.
With personnel lifts, you have to design for the stall load of the hoist, that might be something to check into as well. If they hook on to something 4 times the rated capacity, what gives?
I don't use hoists myself- but sometimes, stuff like this works better and more reliably if it's not at a 100% of capacity all the time, and that might be their thinking.

 

[ldeem]

I would design for rated hoist load. I recently had a project where someone bought a hoist rated over the marked beam rating and the client had me evaluate the beam for the hoist load.

I wouldn't get into trying to justify designing for the rated load by bring up code requirements. Code's are minimum requirements and as the responsible engineer you are well justified in using the hoist load.

 

[StrucPEng]

I have had to design a few of these and redesign a few for higher loads for plants in the industrial sector and as mentioned before it's pretty foolish to design for the minimum that they need right now. What they want to lift with them will for sure change as the plant evolves and they will be happy to have the capacity in a few years.

Additionally, for those type of loads (1-2 ton) hoists you most likely will be designing them with relatively small section sizes (depending on the span) and I don't really see a cost savings unless we are talking many locations.

I would design to the hoist capacity every time.

 

[Agent666]

Sometimes fatigue governs as well (though unlikely for a monorail hoist), meaning strength is well down the pecking order.

I second what StrucPEng is saying, for industrial clients, paying a little more for a stronger beam up front by far and away outweighs the cost of having to come back to strengthen something at a later date (been there avoided that!). Just make your client aware of it, because they might elect to go with a cheaper cost now (make the decision an informed decision from their perspective).

 

[dhengr]

Swearingen:
My goodness, your client’s direction that you should design for the lesser hoist loads seems short sighted and for a very small percentage cost savings on the entire project. For the 1ton hoist lines the monorail trolley beams, the trolley beam suspension system and the carrying bldg. structural components will be designed for 1kip (not 2k); and the 2ton lines will be designed for 2.4kips (not 4k). Make sure you understand whether the 1k and 2.4k are actual lifted load limits (below the hook, the “LL” in your OP, para. 3) or are these the loads on/at the trolley beams? Can there be 2,3,4or more trolleys in tandem on the same trolley beam? And obviously, if there are a bunch of trolley beams, at close spacing, all supported from the same bldg. structural members, you have a different animal, in your “billion dollar facility.” The only thing they are saving is some small percentage in structural steel weight, assuming the design load change actually changes an available structural member size or hanger dia. and the various hardware. Obviously, it will bump-up the trolley beam size a bit. Otherwise, it doesn’t really change the fab. or erection, or general labor costs appreciably. But, then you say…, there are only a few of the trolley beam lines and they are outside, subject to the environment. This would seem to indicate that their support structure may only be subjected to the trolley system weights and wind and EQ’s. Thus, the load change might indicate a fairly large increase in structure weight depending upon the layout.

I would write them a letter spelling all of this out, the possible small percentage cost savings, and in what other ways that there is no cost change; and the potential advantage of being able to re-rate, fully rate the hoists at a later date for the small $$ increase. Ask them to return a signed copy of the letter, so that in 10 years they have a reminder of what they directed you to do.

 

[EDub24]

In my opinion for a new structure you should design to the rated capacity. The difference in loading isn’t the great so the cost would be negligible. But if the owner wants to de-rate the system for whatever reason that’s his call.

 

[FLCraneBuilder]

Do what the client wants - that's what they're paying you for

Be reminded that many hoists will actually lift 2, 3, 4 or even more times their rated load before the hoist fails
its an oft overlooked "nature of the beast" (of electric hoist design)
so when this kind of debate becomes so tenuous, I sort of laugh at it

larger hoists now a days can have overload limit systems in place.. but even these can be bypassed...
smaller hoists of this range often do not have load limiting systems available- so.....

So, the best way to control this is to place only known loads on the hoist hook
takes us back to the beginning - rate the system for the load being lifted

Lastly, the actual use of the crane or hoist may effect this decision....
is the nature of the load(s) such that it an easily be overloaded?
or
Is this a situation where overloading would require active intent and/or be difficult to do/
The answer to these variables is likely the basis of the conversation you will have with the owner