Dynamic/Impact loads : Short high-g inertial loads 3

[ecFem]

Hello,

I am analyzing a structure that it's going to be transported by road and by airplane.
Following standards we saw that normally for these transports the structure shall support at maximum 2.2g in all directions.
Hence, the structure was designed to withstand up to 3g (4g in best-case scenario) in all directions.

The thing is that now an enterprise told us that we should put stickers that detect up to 10g acceleration. They told us that this can happen when the structure is in a forklift and it passes through a hole or a bump, or in this kind of operation.
My question then is : Should I be worried about these loads ? I mean the duration is really short and, consequently, I don't think it is relevant to be considered as a potential failure.

Thank you in advance !

Cordially,
EC

 

[KootK]

But if I accelerate a structure's COG at a rate of 10G then I am necessarily imposing at force of 10G x MASS through that structures load path to the COG.

Agree.

And if I'm imposing that force then I'd expect the associated strain through that load path to match the force through the load path.

Disagree. Consider the following and let me know which bits grate on you.

1) Acceleration is not the proximate cause of strain in a member.

2) Force, also, is not the proximate cause of strain in a member. Per general relativity, there's no such thing as a force anyhow. A force is just an accelerative tendency.

3) The proximate cause of strain in a member is differential displacement between parts of the member.

4) Differential displacement within the member can only be brought about by changing the momentum of parts of the member. If momentum doesn't change, then nothing moves and no strain develops. This, fundamentally, is why the impulse-momentum expression of Newton's second law is the right tool for dynamic problems like this.

5) Acceleration / force only affect member strain as upstream causes of momentum changes in a member. Until acceleration or force are integrated with respect to time (impulse) they do not induce momentum change in the member. And, therefore, they do not induce strain in the member.

6) #5 --> the short lived application of force or acceleration in a member will induce less strain in a member than the member would experience were the same force or acceleration applied statically.

The above is really the spirit in which I made this comment:

But, then, an imparted strain is not the same thing as as an imposed force.

 

[wrantler]

fwiw I design large prestressed conc I girders. When shipped they put one end on the semi and strap wheels on the other end. So it becomes a 'trailer'. They def hit pot holes and such at fairly high speeds. Haven't seen any issues so far.

 

[rb1957]

I disagree with the statement that the dynamic 10g is associated in any way with the weight of the structure ... a la ...“10G” was the context of the thread.

"It’s not my terminology.
Say the thing weighs 1000kg, so a force of 1000kg x 10 x 9.81 = 98kN.
All I’m saying is that an extremely brief load of 98kN is not as bad as a load of 98kN applied continuously"

Dynamic loads are felt by pieces of the structure. Some parts are tightly constrained and would feel a much lower acceleration, some parts are very flexible and vibrate under the impact loading. And different parts of the structure respond at different times, The "only" way to fully understand the internal stresses is to do dynamic load case.

That said, you can say that it is horribly conservative to apply 10g as a static load. But the OP thinks this is too conservative (well outside the structure's design)

It is a very tricky part of design ... you want to stiffen the structure (maybe with temporary shipping fixtures) so pieces don't respond with 10g, but you also wany to soften the structure (well it's attachment to the trailer) so that it doesn't impart these high loads into the structure.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[wrantler]

Well I guess my comment is that many structure pieces are transported by road and are not designed for the effects. Probably because the live load is much more than the dead+inertial loads during transport. But I could be wrong.

 

[rb1957]

and I get that. My first thought was "transportation shouldn't be designing the structure. There are so many things you can do to help in this phase of service. And the structure's daytime loads should be sufficient. But these days people ask "prove it" and thee rabbit holes (and warrens) ensue. Someone new walks into the design team, someone without our experience and maybe some contrary experience, and asks a question. Then all of a sudden it is "crisis stations".

And if we're to analyze the question becomes "what loads" ? The trailer goes over what sized step at what speed ?

Depending on the structure, you may look into how NASA (or Boeing or Airbus) take transportation loads into account. Back in the 80s, we had dozens of static load cases for ground transportation (for part of ESA's SpaceLab); none of them were critical for anything.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[human909]

Disagree. Consider the following and let me know which bits grate on you.

I agree with your disagreement. I don't think I expressed myself well there.

My statement is correct only in steady state acceleration and steady state application of force. Read alone it is incorrect. I would say that a "force of 10g" is pretty much undefined unless it is considered as a steady state condition. If it is a dynamic condition then you need to consider the full dynamics.

I actually wrote a fair bit on this but I kinda got frustrated by it all and stopped. I did a few house chores and went for a run.

Dynamic loads are felt by pieces of the structure. Some parts are tightly constrained and would feel a much lower acceleration, some parts are very flexible and vibrate under the impact loading. And different parts of the structure respond at different times, The "only" way to fully understand the internal stresses is to do dynamic load case.

Exactly.

Which is why I always go back to the hammer analogy. I can get a sledge hammer and whack a 1000kg structure and produce an incredible amount of force, 100kN is feasible. But that doesn't mean I'm accelerating the structure at 10G. Even if the structure is unrestrained then I'd expect the peak acceleration felt at the structure's COG to be quite low, but in the area where I'm hitting it I could expect high acceleration, high stress, high strain and plastic deformation.

 

[rb1957]

a "force of 10g" is meaningless. What mass do you associate with the acceleration to create a force. Some random piece of structure is vibrating at 10g (or pedantically, with a waveform that produces peak accelerations of 10g).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[GregLocock]

Yes, but if you hit a solid 10kg steel cylinder with a force of 100N you will measure 10 m/s^2 at the excitation point, and if you do it right, also at the other end. That's how I fully calibrate the instrumentation chain.

Cheers

Greg Locock


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