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Maximum Force a Compression Brace can carry just before it "Buckles"
2

Maximum Force a Compression Brace can carry just before it "Buckles"

Maximum Force a Compression Brace can carry just before it "Buckles"

(OP)
A steel braced frame for seismic loads is designed such that any inelastic response occurs in the “braces”. These braces are usually arrange in opposing pairs so that when a brace has buckled in compression, the tensile strength of the other brace is enough to support the earthquake loads.

If these bracing members are designed as per AISC-341, for which the limiting slenderness value is 4.7√(E/Fy), then members strength will most probably lie in the inelastic buckling range.

What I understand about the inelastic buckling is that member buckles before it reaches the yielding point, but the material does not remain elastic anymore.

Where the following snippet is taken from, it more or less says that when L/r lies in the inelastic buckling range, compression member will exceed the yield stress before the applied stress reaches the buckling stress.

What I’m confused about is the word “Buckling”, which is required in the design of compression braces. Do we mean that the member will buckle when applied load P > Pn i.e., Inelastic buckling or capacity of the section OR when applied load P > Pcr i.e., Critical buckling load of the section.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (Blackstar123)

Do we mean that the member will buckle when applied load P > Pn i.e., Inelastic buckling or capacity of the section OR when applied load P > Pcr i.e., Critical buckling load of the section.

The first one. In this context, buckling = instability = usually inelastic instability, as you rightly pointed out. This is the case even if the point of theoretical elastic column buckling (Euler) would not occur until the application of a higher applied load.

Take care that the elastic buckling value (euler) is sometimes used as an intermediate calculation value within broader calculations. This is usually because the ratio of P/Pcr serves as a way to gauge the amount that mid-member bending moments will be amplified by P-Little=Delta effects.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (Blackstar123)

What I understand about the inelastic buckling is that member buckles before it reaches the yielding point, but the material does not remain elastic over the entire cross section anymore.

Just a minor edit to aid in precision and clarity. Some of the cross section usually remains elastic during inelastic buckling.

It might be worthwhile to look into the origins of inelastic buckling theory independent of the seismic stuff. Things like Shanley's theory of inelastic buckling, tangent modulus buckling load, etc:

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

(OP)
Kootk, thank you for the quick response. My knowledge about buckling is limited to what they teach in grad school. I'll surely delve a little deep into the theory as you advised. I keep putting it off in favor of reading about some other topic.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (Black123)

I keep putting it off in favor of reading about some other topic.

What you see below is 80% structural engineering books that I meant to read cover to cover but haven't yet. And that just the hard copies. I've been trying hard to collect PDF exclusively for at least five years now. I get it.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

I like the two span condition you've created.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

I was going to comment on the 2 span as well, I assume those must be books he's already finished.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Not to hijack the thread, but what would be the top 5 books for general structural engineering you would recommend, KootK?

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Euler doesn't account for material nonlinear effects; is uses E as an invariant constant.

To account for plasticity (of stable cross-sections) use Et(stress) … the tangent modulus. this'll change things above yield stress.

To account for unstable sections (like crippling of thin walled flanges), use Johnson-Euler.

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

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (slick)

I like the two span condition you've created.

Thanks. The shelving is a built-in that came with the new house we picked up in May. It's a bit magical in that:

1) It exists in my office and nowhere else.

2) It fits ALL of my structural books into one shelf, albeit barely. It's always been at least two crappy Ikea cases in the past.

3) The shelving is bizarrely sturdy. 5/8" deck and the end ledger that you see in the pic runs all three sides so it's like a high aspect ration, three sided plate. Bitchin'.

The book-column being off center to dodge that vent at the bottom bothers me immensely wrt to the asymmetry of it and the potential beam uplift on the left side. When time permits, I'll do it better, as I should have from the start.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (jayrod12)

I was going to comment on the 2 span as well, I assume those must be books he's already finished.

Sadly, no better than 20% there either. The unfortunate books chosen for the strut were chosen as follows:

1) Books that I thought that I had a very low probability of getting to pre-retirement.

2) Books thin enough to allow for a tight shimming between shelves.

3) It turns out that this works best with some degree of post pre-stressing. The thin paperbacks work well in that regard.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

I recall the image of a brick shelf, for the storage of bricks, built with book supports and boards. Keeps the brick collection easily accessible.

https://external-preview.redd.it/0cI8dBDAO9MhD8J8p...

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Quote (dauwerda)

Not to hijack the thread, but what would be the top 5 books for general structural engineering you would recommend, KootK?

See the attachment. The best that I can filter it down to is a dozen, unordered. My criteria:

1) Rocked my world with some genuine insight on fundamental principles. None of that "how to" BS.

2) Not commonly found on an EIT's bookshelf.

3) Usually hits on some aspect of whole building schematic design, not just elements.

4) Usually, but unintentionally, written by a legend their space.

5) Readable and practical. Not so mathy that it's painful to read or digest.

6) Written by someone that I consider to be a "great explainer". You'll note that Paulay, Galambos, and Lin each have two entries in the dirty dozen. They're that good. I'd read their laundry lists if they'd make them available to me. You know, from beyond the grave.

Sadly, I've yet to find a wood book that's really rocked my world. Maybe we should write one.

One could probably procure most of these for under $100 on Ebay.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Thanks for taking the time to compile the list.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

An impressive collection of books. I think I recognize the one with the yellow back, namely Wood Design Manual at the bottom of the stack between shelf #1 and shelf #2, right where it belongs.

BA

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

I have hoarder problems when it comes to old codes and standards. I never want to throw anything away in case, some day, I need it to figure out how a reno might have been designed originally. A couple of decades on, it's clear that this happens so infrequently as to not be worth the hassle.

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

@KootK - I would anchor a threaded rod with couplers and hang from the ceiling and anchor the free edge of your shelves. Use double nuts for shelf adjustment. :)

RE: Maximum Force a Compression Brace can carry just before it "Buckles"


This is the pine wall unit I designed and built for my computer room. The rear wall and the shelves are solid wood, and the shelves are screwed to the rear. The wall unit is two pieces, make it easier (feasible) for me to move it upstairs from my workshop. The solid rear wall places the centre of mass at the rear of the bookshelf, where it belongs. I had to add steel brackets underneath the bottom shelves, because I did not run the dadoes all the way across. Live and learn. smile

The windows on the doors are UV resistant Lexan. This provides UV protection to my collection of old books.

My textbooks are scattered mostly randomly around the room, but lots are on this shelf. My Machine Design textbook (V.M. Faires) is there, covered in significantly more shipping tape than the 21st edition Machinery's Handbook I bought in college. My 26th edition is in much better shape, since I was on CAD by the time I got it, and I had access to the internet, and a CAD library.

--
JHG

RE: Maximum Force a Compression Brace can carry just before it "Buckles"

Binders taking up valuable shelf space with the real books?? Ew. One of the best small office gifts that I've ever given myself was a decent, document feed scanner. I can PDF a binder in 10-30 minutes depending on whether or not there's tabbing that I care about.

Quote (drawoh)

The windows on the doors are UV resistant Lexan. This provides UV protection to my collection of old books.

Damnn... respect! I hadn't even considered that. I have dust jacket issues. I can't have them on the books because of their inherent frailty and impermanence. At the same time, I can't bring myself to trow them away so they're in a box downstairs pressed like dried flowers. And my hard covers are all eating UV...

Quote (slickdeals)

@KootK - I would anchor a threaded rod with couplers and hang from the ceiling and anchor the free edge of your shelves. Use double nuts for shelf adjustment. :)

I might do that were it not for the fact that the roof over my office is comprised of a bunch of pre-eng trusses with snapped webs and missing plates. And no, the irony of that is not lost on me.

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