In-plane buckling analysis of a steel arc 1

[nivoo_boss]

Hey everyone!

Perhaps you can give some advice on how to check if this steel arc will buckle under the specified ULS load. It's created in SCIA Engineer software. It has a span of 45 m. It's like a 1 m high steel truss with U-sections for chords and L-sections for brace members.

If you can point me to some hand calculation methods for this analysis, I would be grateful as well. But perhaps you can give me some advice on how to approach it in software? I can make stability combinations in SCIA and then it calculates me any amount of buckling modes I specify with a factor alpha_cr after them - basically it should mean that the structure will buckle under my load factored by that alpha critical factor.

A picture from my calculation model:

 

[Tomfh]

That makes thing easier!

 

[dik]

are responsible for stability during erection/building.]

I'm cautious about CYA notes... in particular if the courts decide the problem was formidable enough... remembering he's just a 'dumb' contractor... one of my concerns when I'm the only engineer 'in the room'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tomfh]

I'm cautious about CYA notes... in particular if the courts decide the problem was formidable enough... remembering he's just a 'dumb' contractor... one of my concerns when I'm the only engineer 'in the room'.

Spot on.

Be careful playing lawyer and thinking a couple of notes will save your ass if things go bad.

 

[Settingsun]

 It's the overall buckling (or moment magnification) of the arch that will get captured by a 2nd order analysis that includes initial imperfections which mimic the global buckling mode.

I had thought that falls between the direct analysis and advanced analysis methods, so not strictly to code. Is that also an option for straight columns? (Albeit the relative practicality is different than for an arch.)

What about if the arch were just a curved channel rather than a truss, so there's no obvious subdivision for checking localised buckling?

I went to buckling analysis because direct analysis is less useful when the effective length for non-sway isn't obvious for applying the K=1.0 check. But it's a different matter if that can be sidestepped.

 

[JoshPlumSE]

Buckland's law states that the engineer must be able to envision at least one way - not necessarily an economical way - in which the structure they designed could be erected. If the designer did not do so and the structure could not be erected safely, that's on the engineer. If it could be, but the contractor didn't bid it that way / couldn't conjure up another way to do it safely, that's on them.

I love it. That's the perfect way to think about it. It's easy to remember and it really crystalizes the ethics of it.... Thank you!

 

[JoshPlumSE]

I had thought that falls between the direct analysis and advanced analysis methods, so not strictly to code. Is that also an option for straight columns? (Albeit the relative practicality is different than for an arch.)

What about if the arch were just a curved channel rather than a truss, so there's no obvious subdivision for checking localised buckling?

I went to buckling analysis because direct analysis is less useful when the effective length for non-sway isn't obvious for applying the K=1.0 check. But it's a different matter if that can be sidestepped.

What I described is the Direct Analysis Method.... just a more advanced application of it. The DAM tells you how to use initial imperfections for the simple structures (like a mult-story building). That's all the method I described does.... We just use the buckling mode as the basis of our initial imperfections.

You can use this same method to capture many of other types of moment magnification / buckling. But, it can be tricky:
[ul]
[li]It only works for elastic buckling, to approximate the inelastic effects, you have to use those DAM stiffness reductions or such.[/li]
[li]Individual member flexural buckling can be captured this way, but not local buckling. I think it probably works for an arched individual member.... provided torsion isn't a factor.[/li]
[li]If your member is likely to buckle via torsional buckling (cruciform?) or flexural torsional buckling (un-symmetric or singly symmetric shapes including channels) then this will probably not be captured. Maybe it could be captured if you added some initial "twist" type of imperfection to the member.[/li]
[li]Be careful with plate element models. Most of the verification problems related to the DAM demonstrate the ability to capture P-delta and P-little Delta for members. But, you'd have to come up with your own way to verify that this is sufficiently captured for plate element models. [/li]
[/ul]

 

[WARose]

I'm cautious about CYA notes... in particular if the courts decide the problem was formidable enough... remembering he's just a 'dumb' contractor... one of my concerns when I'm the only engineer 'in the room'.

Something beats nothing. I like my chances better in court with a note rather than without one. That being said, if I thinking lifting is going to be a problem (for something odd).....I do try to address it. (And have hours accounted for it.)

 

[dik]

I still have the bruises from my 50 years... but, memory is still good.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Not always... it like using the statement, "I don't recall", too often in court. Pretty soon your credibility is shot... It's best if you don't have to...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[WARose]

Not always... it like using the statement, "I don't recall", too often in court. Pretty soon your credibility is shot... It's best if you don't have to...

Sorry but it is just not feasible to address every lifting scenario for job. This "Buckland's law" is utterly preposterous. How exactly can you anticipate "at least one way - not necessarily an economical way" of lifting every piece? Impossible. There are a million different ways to rig a piece. (And I've seen just about every one of them on job sites.)

And what about bracing the overall building during construction? (My CYA notes typically cover that too.) How am I to know how much of [this or that] will need to be braced at the various stages of construction?

Someone trying to do all that on a project doesn't need a engineer....they need Miss Cleo (i.e. a psychic).