Welding on a Beam 2

[JP20]

I have the below scenario: How do I resolve the force being applied to the welds given a max shear and a max moment (developed from shear-moment diagram).
The orange dots represent locations of the fillet welds. Channels are 15" deep. plate is 36" deep.

 

[CANPRO]

JP20, I like BA's idea of moving the lift points in - also reduces our hook height requirement on the crane. Although, your unbraced length in bending could still be 40' depending on load placement (entire beam could be in negative bending).

 

[BAretired]

BA, well then with no hard feelings, let the debate continue.

Certainly there are no hard feelings. Why would there be? And certainly, the debate should continue when time permits.

EDIT: There is no need...I concede (sigh) that I was wrong and that you were correct.

Regarding your other point, I assumed that load placement would be more or less uniform along the bottom of the beam. If it is possible to place large loads on the cantilever and little or nothing on the span, the geometry of the lift points might change. Or perhaps there could be alternate snatch points when the load is placed near the extremities.

BA

 

[CANPRO]

BA, you should read the other thread started by Johns20188.

I feel sorry for your heavily edited posts - turns out I was wrong. There is shear flow in the welds if it forms a multi-cell closed structure. Others were right here, but for the wrong reasons - but I think that highlights that validity of your original method. It wasn't technically correct, but through "brute force" you ensured there was a valid load path. I didn't necessarily disagree with this approach, I believe it will almost always produce a conservative design - I was just disagreeing with the technical accuracy of the process.

 

[BAretired]

CANPRO,
I haven't read the other thread by Johns20188. I will try to get around to it in due course.

The reason I conceded was that, after thinking about it for a few moments (something I should have done earlier), I reasoned that, even if you bolted the channels to the plate with bolts adequate to transfer the channels share of the applied load, based on stiffness, you wouldn't need a weld on the flange tips (you may need it for weak axis bending or prevention of buckling, but not for strong axis bending).

Maybe there are still aspects of the problem which I am missing, but my synapses were definitely misfiring when I responded to your earlier objections.



BA

 

[CANPRO]

BA, you're going to enjoy this other thread. The bending stress in the section would be the same with or without the weld. The weld is not required for the section to act compositely (all components have common N.A.) - however, in this case, since we create two closed cells in a single section, you develop shear flow in the weld. It doesn't change the bending stress, but the shear flow does exist - and at that point the simple VQ/IT method fails us. The part that really caused confusion is the multi-cell cross-section. If this was (2) channels turned toe-to-toe and and welded to form a box section, there would be no shear flow in the weld.

 

[r13]

BA,

I think we are in line on that shear force is taken by the center plate only, the channels are only the riders. But, the center plate will bend under load, and force the channels to have the same displacement and deflection due to stress and strain compatibility, and weld in the connections is required to make the parts stick together to be compatible.

 

[BAretired]

I decided to just use a W16x77 with lugs welded on top and bottom flanges in line with the webs. Kl/r for weak axis is 196. However this is a lot of determining factors in BTH manual that I still have to check. I did check welds earlier today and they are good being 8” long on both sides of lug - 1/2” fillet.

The choice of WF section will depend on load and span. You will probably need to comply with OSHA requirements for load factors. I am not familiar with them offhand, but I suspect they are higher than normal load factors for beam design for a building.

This beam is going to be rated for 25 Tons of lifting.

If the span is 40' and the load is 25 tons or 50 kips, the maximum moment is 50*40/8 = 250ft-k.
But you told us something different in an earlier post:

The max moment is actually more near 80ft-k .

The part I hate most is that no matter what you do to this type of lifting device, you cannot laterally brace it. If someone knows of a way to, let me know! Otherwise I’ll be using BTH manual to design as 40’ unbraced.

If you use a span of 40', you will require a pretty hefty beam. Why don't you consider reducing the span by having a cantilever at each end? Not only do you reduce the span, but you apply a negative moment at each end which directly reduces the positive moment in the central span.

In a way, you are laterally bracing the beam at the pickup points by applying the load below the beam, preventing rotation about the beam axis. You could brace the top flange between pickup points with a top channel or plate, but it is probably more economical to select a WF which can span the central span without additional bracing.

The cables, chains or slings make an angle with the beam. They put moment and compression into the central span which needs to be taken into account.

Finally, if you feel uncertain about the design, I would strongly recommend that you retain an engineer familiar with this type of beam.


BA

 

[BAretired]

CANPRO,

I still have not read the other thread, but I assure you, I will.

retired13,

We have been all around the mulberry bush on that topic. It is no longer relevant to the OP as he has decided on a different shape for a beam. That does not mean we should not discuss it, but I for one am not up for that discussion at the present time. First, I would like to read the other thread.

BA

 

[r13]

JP20,

I think you know the design criteria well, but rusty in the real design. Your 40' beam length is a problem, not only in design, but you will require ample space, and equipment that capable of handle this operation. I suggest to sort through internally to gather ideas, and refine the plan and method to carry out the task. Also, you'll be definite benefitted from engaging a structural engineer, who is familiar with the nature of your business and the task on hand. My last suggestion, for lateral stability concern, a strong box beam may work, but not for present length.

 

[BAretired]

If total load plus beam weight = W
then W/2 is vertical load at each pickup point.
w = W/40
F = W/2sinA (sling force)
P_span = W/2tanA (compression in span)

M_cant = wa²/2
M_midspan = w(2b)²/8 - wa²/2 = w(b² - a²)/2

If a = 8', b = 12'
M_cant = 32w
M_midspan = 40w


If a = 10', b = 10'
M_cant = 50w
M_midspan = 0



BA

 

[STrctPono]

Wow! After reading through this thread, I can't believe how horribly awry the discussion got at points. In my opinion, EVERYTHING Canpro said in this thread is spot on and was really one of the only consistent voices of reason from the beginning. I applaud him for sticking with it. It's too bad that so many others muddled the conversation with really bad advice and halfhearted responses. Sadly, much of the bad advice came from several rather well respected individuals on this forum. Of course, I am coming into this conversation after the fact, so it's easy for me to criticize with the advantage of hindsight, however, I'm really not sure why it took the OP so long to realize that the original design was trash and to just go with a rolled wide flange with welded lifting lugs.

 

[r13]

STrctPono,

The OP said the best, "I think it will work with 5/8” fillet on both channels, but I want to be super confident. These conflicting statements are concerning."

We all been struggle through the rare/weird shape the OP attempted to use. Pretty much the posts after that comment were focus on checking each others understanding of matters, along the way, I think some good points have been raised, that potentially can benefit people who has patient to read all posts through, and me included, a beneficiary of this lengthy discussion. You shall visit another thread to see the finale of these arguments. Link