Chain hoist on PEMB frame

[Thoughtclaw]

I've been asked to evaluate whether a customer can hoist an empty 4,000-pound tank on a beam that is part of a pre-engineered metal building frame. This would be a one-time maneuver (well, a three-time maneuver, as they have three tanks) as they put the tanks in place, and would be done later this spring. My initial thought is that the building was designed to carry a 30 psf snow load that it obviously will not be carrying at the time of the hoist, and my quick analysis indicates this would put considerably less moment or shear on the beam than the snow load it was designed for. Does this seem like a reasonable analysis? Is there something else I should consider?

 

[WARose]

I'd be mighty cautious with a PEMB. Just because you are getting less moment than it was designed for.....shouldn't be 100% reassuring. For one thing their basis for design (i.e. the Cb used or if they used a plastic hinge mechanism) could be different. I'd also be very cautious about bracing. Looking at existing PEMB systems (especially older ones), they tend to have issues as far as lateral bracing for flexural members. (Doing it by current codes.)

You may want to do a straight up design check (including localized checks from the hoist attachment)....rather than a force to force comparison.

The bottom line with PEMB: you touch it.....you own it.

 

[Ron247]

The localized load noted above is a definite "look into". A lot of PEMBs have a one-sided web to flange weld that could easily be affected by someone throwing a chain around the beam and hoisting. Definitely do not let them attach a chain hoist to a bottom flange without checking it out.

As far as the comparison of design load to lifted load moments and shears, while the concept is not completely off base, I would at most use what has it been subjected to in the past rather than design. Design for 30 psf but never been subjected to more than 20 psf does not tell you if the PEMB design was good or not. Like WARose said, touch you own it. I do not know how old the PEMB is nor do I know the manufacturer but that would make a big difference to me. Older codes were more forgiving and some suppliers/designers took a lot of liberties in design years ago.

 

[Thoughtclaw]

Thanks for the thoughts, guys. It's actually a brand-new Butler building, if that makes any difference. But I see what you're saying. I'll take a closer look at it.

 

[JAE]

If it is that new... have Butler check it for you and then you report the results to your client.

You may not even know the correct grade of steel they are using.

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[Thoughtclaw]

I thought about that. I'll do that. Thanks, JAE.

 

[Ron247]

I would feel more comfortable with new and a reputable PEMB supplier. Since you have both, your load moment and shear comparison becomes more reasonable but still a risk you take to some degree. Having Butler check it is still worth the price of admission. The load comparison method would not require you to know the yield strength of the steel but any analysis method would require it. Butler used to use 50 ksi plate material years ago, but I am not sure what they use now.

The localized loading is still an issue and Butler will probably require you add stiffeners at load points. As stated, throwing a chain around the top flange and loading it may cause the flanges to bend or other localized defects.

 

[Thoughtclaw]

Makes sense. Thanks, Ron.

 

[MotorCity]

Even if the beam can handle the vertical load, chances are the tank will not be hoisted up perfectly vertical. The lift may have a horizontal component. Imposing even a slight, unintended lateral load or torsion on a PEMB member that was not designed for it could spell disaster.

 

[JStephen]

Depending on how it is handled, you may get some impact loading as well.
If you give the customer a "safe if there isn't any snow" maximum load, that may get translated into a "safe for any time in the future" load, especially if they go posting that maximum load on the beam/hoist. The building manufacturer might be leery of the situation for that same reason.

 

[Thoughtclaw]

Just to wrap it up, Butler has given the OK as long as it's done when there is no snow or any kind of live load on the roof.