Dynamic Buckling 4

[WARose]

I've got a project where I have to get reacquainted with the concept of dynamic buckling (primarily from axial loads due to a sudden load). Can anyone recommend a resource where I can quickly check sections I have for this? Most of the resources I have (and have found) are really not that useable. Thanks in advance.

 

[MotorCity]

You will rarely find "dynamic" and "quickly" used in the same sentence when you're talking structural. Guess I am not sure what you mean by dynamic buckling as I have not heard of dynamic vs. static buckling (buckling is buckling). I assume you mean buckling due to a dynamic load.

What type of member? Column buckling? Beam buckling? Local effects like web or flange buckling? Typically when we deal with dynamic loads, we treat them as equivalent static loads and apply an impact factor. Is this what you are doing or are you doing some fancy analysis or FEA?

 

[WARose]

I assume you mean buckling due to a dynamic load.

Correct. And we are talking from a short (pulse) load.

What type of member? Column buckling?

Yes, column buckling (as I said).

Typically when we deal with dynamic loads, we treat them as equivalent static loads and apply an impact factor. Is this what you are doing or are you doing some fancy analysis or FEA?

You can typically get a higher dynamic buckling value than static (that is (of course) if the duration is right and localized effects (at the point of application) do not control).

 

[MotorCity]

Not sure what code you are governed by, but if you are using AISC, I am not aware of any allowable increase in load or stress due to short duration (impulse) loads. To me it seems that ultimately, it comes down to the Euler buckling equation as the buckling load is not dependent on the load duration. When the 1/3 stress increase was permitted for short duration loads (wind or seismic), that was an empirical increase to account for such effects. In your case, I would probably use that as an upper bound solution. Are you after how to calculate the actual dynamic load or how to calculate the allowable dynamic load (or both)?

 

[WARose]

Are you after how to calculate the actual dynamic load or how to calculate the allowable dynamic load...

The latter.

To me it seems that ultimately, it comes down to the Euler buckling equation as the buckling load is not dependent on the load duration.

That's my preference as well......but I've got a client who is questioning me on this.

 

[jayrod12]

Well some materials have increased strengths for extremely short load durations, wood being the first that comes to mind. Perhaps he was mis-informed.

 

[BUGGAR]

Its all relative but there are factors (Roark) to reduce dynamic loads from theoretical elastic theory to account for the mass of the object being struck. This is what explains why a cat prancing across the Golden Gate won't cause global vibrations in the bridge. Is this what you are describing?

 

[MotorCity]

For blast design (extremely short duration loads), you may use the expected yield strength. This is typically the nominal yield strength increased by about 10% (the increase varies depending on the material you are checking, but I think its 10% for steel). This is in recognition of the fact that the actual, in situ strength is statistically higher than the nominal strength. I would be comfortable using 110% of the nominal yield strength for short duration impact loads.

 

[WARose]

For blast design (extremely short duration loads), you may use the expected yield strength. This is typically the nominal yield strength increased by about 10% (the increase varies depending on the material you are checking, but I think its 10% for steel). This is in recognition of the fact that the actual, in situ strength is statistically higher than the nominal strength. I would be comfortable using 110% of the nominal yield strength for short duration impact loads.

I hear what you are saying......and like I said: it’s my preference too......but (like I've said) I've been called on the carpet on this, so I'm trying to get an exact idea. The values can greatly exceed the elastic values (under the right conditions).

 

[KootK]

Can anyone recommend a resource where I can quickly check sections I have for this?

I'm afraid that I'm unable to recommend such a resource. As MotorCity intimated, it's a very complex phenomenon and, to my knowledge, can't be boiled down to something even as simple as a monograph without making some significant/neutering simplifications. You're quite right though, dynamic buckling capacity can be greatly in excess of static values.

For those that may be interested, this is an excellent primer on the subject: Little Book of Dynamic Buckling. This is old but also good: Link

Based on the discussion so far, I fear that some are confusing dynamic load impact factors with dynamic buckling. The latter is much, much more complex. Dynamic buckling is influenced by not just imperfections and material characteristics but also by the shape and duration of the applied impulse (rectangular, triangular etc). It's well above my level of theoretical understanding really.

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.

 

[BUGGAR]

I think I understand. Check references on pile driving and pile driving analysis. I've seen some of those buckled shapes in KootK's reference in large O.D. pipe piles (thank you for the reference). We didn't analyze these piles but just turned down the fuel to the hammer.

 

[WARose]

Thanks for the links KootK.

As MotorCity intimated, it's a very complex phenomenon and, to my knowledge, can't be boiled down to something even as simple as a monograph without making some significant/neutering simplifications.

Quite true. I'm finding that duration, slenderness, initial imperfections, and so forth are controlling criteria. (It's as complicated as i remember it!)

ASCE's Journal library has been making some money off of me today.

 

[KootK]

Well, if you find a sensible, simplified method, do report back. I for one would love to hear about it.

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.

 

[appot]

KootK, there have been multiple times I have downloaded files from your Dropbox. Please tell me that your Dropbox password is in your legal will with explicit directions to you attorney to
[ol 1]
[li]make your entire dropbox public[/li]
[li]post the link on eng-tips[/li]
[li]make weekly additions to the dropbox folder from his or her effort of digitizing your hardcopy engineering library.[/li]
[/ol]

 

[KootK]

Thanks for the kind words appot. I can actually do a good deal better than what you've asked: Link. I did that custom build of my filing system specifically for the use of Eng-Tips members but, sadly, nobody took me up on the offer. In retrospect, I get it: we're all real busy. It was always doomed to fail.

make weekly additions to the dropbox folder from his or her effort of digitizing your hardcopy engineering library.

Funny you should mention this. For obvious reasons, folks associate me with my kick-ass digital library. But my physical library is even better in my opinion. Better, even, than any SE firm library that I've seen. And I've currently got two problems in need of solving:

1) When I die/retire, the books need a home. There's some truly collectible stuff in there. I've got a few former proteges that have expressed interest but I'm not sure than any quite reflect my personal level of commitment/insanity.

2) I've got some neat stuff that I plan to dump in the near term because I now have PDF versions. The goal is to lighten my physical load without losing information. That said, I'm loath to actually throw these books away. I would like to find them a suitable home. Being in Canada, shipping to a US applicant is a bit of a problem (serious weight).

On more than one occasion, I've been in a used book store found a heap of good structural books, many bearing the same stamp/signature. It's clear, in those cases, that a good structural engineer bit the dust and the boobs managing his or her estate didn't know what to do with the stash. $1 here, $2 there. I don't want that to happen to me or my babies.

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.

 

[SAIL3]

I am inclined to agree with motor city...many yrs ago I designed axially loaded members for a nuclear blast load....the main considerations were that the dynamic load was really temporary and transient which led to the fact that it was not a "following load" such that the axial member does not have time to react(buckle) before that dynamic load is gone....if the steady state load on the col is significantly lower than the dynamic load then this would also add more confidence in using a higher allowable stress...

 

[MacGruber22]

I've got some neat stuff that I plan to dump in the near term because I now have PDF versions.

I am up for a road trip to Canada to adopt your children...err books. My wife is an English teacher, and I hope to build her a grand library in our next house. Naturally, as an engineer, I no English good, so my collection of books is sad. So, when that time comes, I would love to see a slew of old engineering books being kept safe in a book shelf made of rich mahogany.

On more than one occasion, I've been in a used book store found a heap of good structural books

Our local book exchange (free books!) recently burned down, and it pains me to think there were likely some good engineering texts in there somewhere.

Sorry for the off topic post.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

It wouldn't be a simple adoption process. Credit checks, character references, oral exam... Most years I'm back in Madison, WI the third week of May. I'll pack some treasures and a bunch of stamps for my 2017 visit.

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.

 

[WARose]

While we are on the subject....something I've been meaning to ask: for such a short term load where both the dynamic buckling load and the yielding strength were greatly exceeded, what would happen first: buckling or crushing? Considering the time it takes to crush (versus buckling) I would think buckling would. But I'd be interested in other thoughts.

 

[gravityandinertia]

When I work this problem out from first principles and assuming a half sine impulse load, I get:

P = EI*n^2*pi^2/L^2 + (rho*A*pi^2*x^2/(8*delta(t)^2))*sin(pi*t/delta(t))

rho = density of material
A = cross-sectional area
delta(t) = duration of impulse

I didn't check my work thoroughly, so beware, but I am pretty confident enough of it is right to understand that shorter durations increase allowable loads.

As you can see the second term does add to critical buckling load, P, especially as duration gets shorter due to the delta(t)^2 term in the denominator. So as the impulse load occurs the acceleration load the beam sees changes the maximum buckling load, the shorter the duration the higher the load capacity. This is all theoretical however. I would probably just use the static case unless for some reason you feel this causes a significant amount of over design.