Modern Steel Construction Steel Quiz 12

[SteelPE]

I was looking at the steel quiz in the 10/17 issue of MSC. I was able to pick up the problem quickly as I'm sure others have. However, I had a slightly different answer, I came to the conclusions that the framing plan shown had beams that required themselves for support and therefore would not work. However the article simply states that the configuration is difficult to erect. Is the framing plan shown even structurally stable utilizing simple structural theory?

 

[KootK]

Huh. Sounds as though I was incorrect about linear methods not working for this. Which leads me to my next point of interest.

If we drop the pretense of "instantaneous load response", as they discuss in that article of slick's, it seems to me that the question should not be "why is load chased in this situation?" but, rather, "surely load is chased in every situation?".

It has long been my intuition that moment distribution is not just an expedient calculation method but, instead, a true description of how load response flows through an indeterminate structure. The handling of imbalances in moment distribution strikes me as too similar to "load chasing" here to be mere coincidence.

Even in an ostensibly determinate structure such as a simple span beam, I feel that you'd still get this "load chasing" at the atomic level as individual molecules rearrange themselves to produce load resistance (equilibrium) while continuing to obey their own version of material law (atomic repulsion and attraction). Every simple span beam is really a gazoolion interdependent, molecular springs after all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

DETstru, what happens if you have axial springs on the supports? The supporting beams are obviously not relatively stiff in their weak axis flexure and I feel this will change how your beams react.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

Good point. To heck with axial springs though, let's go axial wheels. Should still be stable externally.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Guest262653]

A real life example:

http://www.reciproboo.org/#/coventry-university-105/4584721978

 

[DETstru]

Switching to springs made no difference at all. There are no horizontal reactions, only vertical.

 

[WARose]

One thing that jumped out at me from the start is having a W18 frame into a W16. Not a fan of something like that.

 

[TehMightyEngineer]

Switching to springs made no difference at all. There are no horizontal reactions, only vertical.

Hmmmmm, I was thinking there would be but that makes sense I suppose; each beam is statically stable in the axial direction. Maybe if you had large deflections this would come into play but sticking with the small displacement assumptions then I can see it having no effect.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

Okay, I fully encourage you to make one of these. I'm amazed how sturdy this is.

I'm now wondering what I can make around the house that utilizes this. Wife said she wanted a trellis arch out back, maybe it should have a reciprocal plane frame in the roof...

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

A useful way to think of these systems is as two, crossing beams, each constructed in two pieces that are moment spliced in a peculiar fashion

See it TME? It jumps out all over me when I look at your model.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

See it TME? It jumps out all over me when I look at your model.

I know, right?! I'm going nuts over this, it's simultaneously really hard to wrap my head around but it's just simple span beams! "Peculiar fashion", indeed.

I'm loving this, if I didn't have stuff I should be doing I'd totally fall down the rabbit hole with avscorreia. Used up too much time already.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

Mrs. TME totally deserves a reciprocally framed trellis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DETstru]

Independent peer review incoming:

 

[TehMightyEngineer]

Independent peer review incoming:

Love it. Would you say that is part of a "balanced" breakfast?

I'm just now realizing how interesting these are. With the proper bracing (or otherwise connecting the "peak") these frames become diaphragms! They can provide lateral stability as well as vertical.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dhengr]

DETstru:
‘Pear review,’ what the hell do you mean? Those are just plane ol apples. And, V-8, that doesn’t have any peers in it ether, it’s a vegetable juice. I fiddled around with these structures years ago, they can drive you crazy, and become addictive too.

 

[DETstru]

Well I didn't have any pears so... I did my best!
Unfortunately I now only have 3 apples left...

 

[TehMightyEngineer]

I just realized that everyone makes these frames all the time without realizing it; cardboard boxes! When you fold up the top without taping it you make it into a reciprocal frame!

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Celt83]

I have no substance to add to this thread, I just love how this topic peaked all our interested and led to these small scale models that showed substantially more capacity than we all intuitively thought should be there. Maybe not all @KootK.

Goes to show always something new to learn/experience in our profession. Comes very timely behind the recent post about confidence in calculations as well and how to get comfy with them. TME wasn't comfortable with the results so threw together a small scale real world replica with material on hand and got "performance" in line with DETstru's RISA model, at least thats what I see in the deflected shape of the chopsticks .

As an aside TME need a youtube clip of the erection sequence and any shoring that was needed prior to the system being stable.

 

[dhengr]

DETstru:
Well, you better hurry up then, there is a three legged, apple supported, variation of what you guys built. The center opening is a equal sided triangle.

 

[XR250]

Learn something new everyday!

 

[SteelPE]

So I suppose the answer given in MSC is correct in that the only problem with the framing scheme has to do with erectibility and not my thought if stability. Still not going to do it IRL.