Aluminum Rigging Beams 1

[mikek396]

First project I'm taking on myself and totally having imposter syndrome. Project is to provide load ratings for some aluminum (6061-T6) beams that will span between A-frame legs to be used for rigging. Beam sizes S6x5.1, S8x8.81, S10x12.1, lengths in 8'-15'.
I have a previous engineer's calculations - he took a very simplistic Sigma_y = M/S approach to calculate max loads. He also used Omega = 1.67 (from AISC).

This seems too simple to me as this method has absolutely no consideration for LTB, and also uses the incorrect omega based on the Aluminum Design manual. However, using the Aluminum Design Manual, I am calculating what seem to be extremely conservative values - less than half of those previously calculated. Am I missing something here? Can this really be as simple as M/S? Am I totally overthinking this, or did the previous engineer oversimplify?

Additionally, can somebody point me to a reference for any FOS I should be applying to these beams? I assume there is an additional factor to be added for rigging beams above the 1.95 or 1.65 in the Aluminum manual for flexure.

 

[phamENG]

I hate to say it, but if you don't know what standard you should be using the imposter syndrome may not be too far off. Make sure you have somebody familiar with rigging design review your work. Your description doesn't make a lot of sense to me, either - if it's between legs of an A-frame I would expect a considerable axial load as well as moment, but only moment if you have suspended loads attached to the beam along its length. If it's just a spreader, it would be axial with a touch of secondary moment from self weight deflection.

You need ASME's BTH-1, Design of Below the Hook Lifting Devices.

 

[mikek396]

@phamENG - thanks for the quick reply. Maybe I did a poor job of explaining the situation (specifically the mention of A-Frame legs) - consider this a simply supported beam with a point load either from a rigging strap or beam trolley. This is an above the hook beam.
I have used CMAA 74 for beam trolleys and monorail design, I was just wondering if OSHA or another standards body requires additional FOS for rigging setups specifically, or if the standard values of Omega for building type structures found in the Aluminum manual would apply.

The imposter syndrome I am talking about comes from the fact that I believe I just discovered a condition whereby I will need to challenge work done by a much more senior engineer, who I believe totally oversimplified the analysis and created potentially many unsafe conditions. Proving this to the manufacturer may require an actual load test and comparison to the provided ratings. I would reach out to the previous engineer, but unfortunately they're no longer around.

Am I crazy, or is sigma = M/S not a valid way to design a 12' aluminum beam for a point load? This seems to ignore LTB completely...

 

[RenHen]

Re: the FOS, for rigging I add an impact factor to the loads. The best guidance I have found on that is in the CMAA 74.

Unless it's braced to prevent buckling, I don't know how you'd get away with ignoring LTB for an aluminum trolley beam... Even the local web/flange buckling likely controls over the flexural stress.

Only other thing I'll add is that if you're using a trolley, local bending of the flange to transfer those loads into the section may be a concern as well.

 

[mikek396]

@RenHen - in my analysis I have provisions for the local flange buckling and load impact factor. Ill refer back to CMAA to see if I missed anything else on FOS.

Beam is definitely not braced against LTB - thank you for confirming the prior analysis doesn't make much sense - really just needed to hear that from someone else

 

[human909]

I won't attempt to comment too much on the problem at hand except to agree with RenHen than if a trolley is being used then local flange bending will be significant and you will need to combine it with the overall bending stresses.

The imposter syndrome I am talking about comes from the fact that I believe I just discovered a condition whereby I will need to challenge work done by a much more senior engineer, who I believe totally oversimplified the analysis and created potentially many unsafe conditions.

There are plenty of incompetent people out there in all professions and engineering isn't an exception.** You'll need to balance backing your own judgment, doubting others and doubting yourself.

**For example a company recently came to me at the 11th hour of their project, initially just to review and rerate the design life of a new construction designed by others. The design was a mess, highly indeterminant and didn't account for a torsional eccentricity at a critical load point. It was over designed in most areas but with a few questionable load paths and likely deflection issues. A quick redesign involved a week of work from me, and saved in excess of $100k of steel.

Yet the CV's of the original engineers particularly the director of this small company are very impressive. These engineers have 2-3x my experience in years. But they clearly had no idea in this field. Their expertise (assuming it exists) is elsewhere and certain not in structural steel.

 

[dvd]

OSHA standard interpretation regarding load testing on engineered lifting equipment-

https://www.osha.gov/laws-regs/stan...s must not be,load sustained during the test.

 

[phamENG]

Got it. My apologies, then. From the description in the OP it sounded like some sort of adjustable spreader bar.

While the approach you mentioned is very basic, it has some basis. Looking only at the vertical load, LTB wouldn't be much of a concern for an underslung trolley. With the load applied below the neutral axis, any tendency of the beam to twist will be immediately resisted by the applied load. So as long as the beam is kept within the elastic range and the ends are properly braced against rotation, you're unlikely to have a stability failure. This is just a fundamental mechanics view of one load case, not considering any unique code requirements.

Now...in terms of code requirements, you have to determine what governs for this situation. CMAA 74 is certainly a good place to start, though I'm not sure if they get into allowable stresses for the rails. I've typically used ASCE 7. ASCE 7-16 has a section in chapter 4 for crane loads. It essentially says to use the rated capacity of the crane plus the weight of the crane itself plus multipliers for vertical impact, lateral force, and longitudinal forces. These would be used in conjunction with the ADM.

 

[mikek396]

@dvd I'm not sure this applies to the design of these beams specifically, but this is certainly useful information regarding load testing requirements by the end user - thank you for the reference

 

[dvd]

Be aware of when to require load testing - here is a little further info -

https://www.osha.gov/laws-regs/standardinterpretations/2003-07-07-0

 

[canwesteng]

For local flange bending stresses - those are elastic stresses, and it's pretty conservative to check them at ULS. See Flange Bending in Single Curvature by Bo Dowsell

 

[mikek396]

@canwesteng - great article. Most of these items are already incorporated into my monorail spreadsheet, but a great read nonetheless. Thank you

 

[Struct123ure]

mikek396 your calculations are probably right and the previous engineer is most likely being unconservative.
I find this type of construction related/temporary works has the most cowboy engineers around. That FS is the only reason that they get away with it. Probably due to the pressure from contractors that have no idea what they are doing, except they have been "doing it for it way for 40 years".

I have had a debate with another engineer much more experienced that he was using the incorrect y when calculating Sx. Since extruded aluminum section are often unsymmetrical you have y bot and y top. Never managed to convince him otherwise, I think he just wanted to believe what he wanted to believe.