How is it safe to cut and offset a section of beam?

[Soulconspiracy]

I'm not an engineer, though I appreciate the principals involved and I can imagine how a structure transfers load. I've been asked to draw a structural modification and something tells me it doesn't function like the guy thinks it does.

I'll do a sketch if my explanation isn't clear...

Basically you have a beam simply supported at both ends that's carrying a number of point loads along it's length. The guy wants to cut out a section of the beam, move it horizontally by some distance and connect it to the rest of the beam using more steelwork welded between the ends. There is also a web plate between the original beam and the additional steelwork. He says he's just moved the strength from one place to another.

Is this possible? Maybe it just depends on the strength of all the new connections? The end connections are unchanged. As far as I know, no structural calculations have been made, it's just a case of over-specify everything beyond doubt, I'm just worried the principal of the whole thing is flawed. I am subordinate and cannot question such matters openly unless I know I'm not being ridiculous.

Fortunately no one is going to get hurt if this fails but it would be expensive.

I'd be grateful for any insight on this. I don't need any detailed examination or anything, just wondering if anyone had seen this done.

 

[Agent666]

I'm intrigued, how far is it being moved over?

It sounds suspect, but I love sketches that defy the laws of reality, so please provide one to explain further.

I tend to think of these things one of two ways, guy is an absolute genius and has thought of everything that could go wrong and detailed things accordingly, or he's at the other end of the spectrum and literally has zero idea. I'll reserve my judgement until I see a sketch and a bit more information!

 

[1503-44]

"he's just moved the strength from one place to another." Has the genius moved the load as well? Has the genius thought about excessive deflection? Strength where you don't need it is totally useless. You need it where you need it, which is the usual reason as to why it was placed where it was. If you can document that the original design assumptions no longer fit the present situation, perhaps there is margin for redesign, however any modification without a complete understanding of the original design conditionsl and today's design conditions are never a good idea. Nobody thinks they are ever going to be hurt by these exact kinds of things. Are you an architect? Why are you doing drawings on this? I think you know enough that you already know that something isn't right. Tell your boss that you are not entirely comfortable with doing the drawings.

P.S. While we fully appreciate why you are here, you do need to be an engineer to utilize these forums.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[Soulconspiracy]

"P.S. While we fully appreciate why you are here, you do need to be an engineer to utilize these forums."

What I meant to say is, I'm not a structural engineer.

Here is a sketch-
(left image is existing, right image is proposed)

img

https://res.cloudinary.com/engineer...1589361077/tips/structureal_change_ntzyxr.jpg

]

Just looking at this myself now, it seems obvious that even if the modification was ok the end connections would need to be modified too.

 

[msquared48]

You need to contact a local structural engineer.

Mike McCann, PE, SE (WA, HI)

 

[human909]

M.C. Escher and Dr Seuss would thoroughly approve.

You question can't be answered completely without knowing alot more. But is does seem to be alot wrong with that design and that explanation. It would seem that he isn't at the genius end of the spectrum that Agent666 is describing....

 

[Tomfh]

The new form is plausible (it’s basically a crankshaft), but there are so many things to consider. It is certainty not equivalent to the original. It’s a completely different structure, and is no way near as strong.

 

[LittleInch]

sketch would be great.

So this would be like a Z shape beam? or with a U shaped part half way along?

Yes, this would fundamentally change the calculations and structural design, so you are correct to question this. You are introducing extra bending moments and stresses.

It's not to say it can't be done, but needs complete design and calculation.

I find it difficult to know how you're so clear that there will be no injuries if this fails, but I guess you can see things we can't.




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kingnero]

If there is any significant load, you introduce torsion into the main beam.
This is doable if - as said before - everything has been checked, but this sure seems risky to me, and I'll throw out a guess without knowing sizes or loads but I'd say this won't work.
Another important point to consider is whether or not the beam is a box-beam (as per the drawing) or open sections like I of wide flange beams... And I wonder why there are gusset plates.

 

[LittleInch]

Now seen the sketch.

This will introduce some potentially large torsional moments in the original beam. Depending on the torsional rigidity and the fixing of the beam to the support this might work, but would need a brand new design. At worst the beam will rotate 90 degrees under the middle force and possible bend or fail in the centre or the loads will fall off.....

Those webs do virtually nothing to improve the situation and are there only if there is a force along the original beam. Bit if these are simply supported at either end, there is no force so they are worthless.

Not sure exactly of your position within this or responsibilities, but you need somehow to bring this to the attention of someone more senior and pehaps phrase it int he same way - I've been asked to do this, but something doesn't seem correct - can you have a look please?"

Don't mention you've been on forums, but use it for your own benefit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ron]

It's possible but not probable. The most significant issue I see that you've mentioned is the simple supports and end connections have not changed.....the moment (rotation) at the supports will be significant based on your sketch. Further, the two pieces of the original beam from the supports out will act more like cantilevered sections rather than a simply supported beam. This will cause moment in another plane at the supports.

Ask for the concept to be peer reviewed by a local structural engineer.....otherwise, find a skyhook.

 

[LittleInch]

Or a support under the middle load.... and at the ends of the now cut beam.

We need to know what role this person is playing here. Are they pretending to be an structural engineer or just someone off the street? Or are they related to the owner....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JLNJ]

Crankshafts are supposed to spin. Beams are supposed to stay in one place.

 

[r13]

It looks like a piping or HVAC duct system. As many have pointed out, the offset will produce torsion in the supports, and additional internal stresses at the geometry changes. However it is not difficult to justify the change by an experienced designer, who may have started with a conservative design in anticipating this type of change, which is not rare in industrial projects. Also, unless you work for an one man shop, then, I would say there is not much to worry about, though it will be a good lesson to learn his reasoning/justifications. You shall simply walk up to him, and politely ask for his explanation. As you are not his competitor, I believe he will be glade to do so - a chance to brag about

 

[IRstuff]

Moreover, crankshafts are reasonably balanced and don't intentionally stick out that much, since any eccentricity in loading results in a carload of vibration.

TTFN (ta ta for now)
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[1503-44]

Pretty much what I thought. How does that even stay sitting on top of the columns? Why won't it just rotate and fall off the columns. The ends would need torsional moment resisting connections, thus putting the columns in biaxial bending!

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[Tomfh]

I wasn’t saying it’s literally a crankshaft. Just that it’s the same form. Like a crank shaft the load will translate into a torsional force.

 

[IRstuff]

Yes, but...

A crankshaft is specifically designed to handle the torsional load, whereas, this is obviously not.

Assuming the columns could possibly handle the moment loads, could vertical gusset plates might be more apropos to transfer some of the cantilevered loads back into vertical loads on the columns?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[Tomfh]

I’m not saying it’ll work. Just that it’s shaped like a crank shaft.

 

[rb1957]

now don't you regret that "crankshaft" comment !

Yes, the beam supports (and the beam itself) will need to react the torsion from the off-set load.

Doing this isn't impossible, just needs some attention.
Building it will have a little care too … attention to the obvious.

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