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

 

[Tomfh]

So for the case of a stocky 100kg person that becomes 10kN of force.

Yes. That is the peak load seen in the joints. But it it a very brief load, too brief to do the work required to fail the joint.

There is no way my ankles would be happy with 10kN of force no matter how brief.

If it's brief enough, it doesn't matter. Your ankles would have seen forces of this magnitude at some point.

Well if it briefly experience "10G of force" then I'd be designing it to resist "10G of force" .

If it's a very brief load, you don't need to. It's a question of energy, of work. If the force is too brief to do the work necessary for failure to occur, failure will not occur.

 

[human909]

If it's brief enough, it doesn't matter. Your ankles would have seen forces of this magnitude at some point.

Explain how exactly this occurs. Because repeating the same thing over and over again isn't helping.

Your ankles would have seen forces of this magnitude at some point.

Yes, I have little doubt one has. My tibia and tibia broke and needed surgery.

And as I said in my post above, decelerating my ankle at 10G is a trivial task. Putting a force equal to 10x my body weight through my ankle is not trivial, I will choose not to partake in that experiment voluntarily.

 

[Tomfh]

Explain how exactly this occurs. Because repeating the same thing over and over again isn't helping.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4994968/#bib16

The ankle joint complex bears a force of ...up to thirteen times body weight during activities such as running

 

[human909]

Two points.

One that is the ankle complex. So include the opposition forces by the tendon and joint interaction. That is like computing the axial force in a beams tension or compression flange and claiming the member is resisting that axial force.

(This is a long way from the original discussion. But if this is what you meant then I do concede that the compression part of the ankle sees significantly higher loads than what the net load is.)

That computational study also has the caveat that some of the forces computed exceed the capacity of the tendon determined by other studies.

 

[Tomfh]

One that is the ankle complex. So include the opposition forces by the tendon and joint interaction.

It is not referring to that. It is referring to the compressive forces acting downward through the ankle joint. The biomechanics research indicate these forces are upwards of 10x static body weight.

Likewise the shock loading sticker will detect bumps exceeding 10G. Taking this back to the original discussion, this does not imply that the structure needs to carry a static load of 10 x gravity. You could of course take that approach, and it would be safe, but it's extremely conservative.

 

[human909]

Likewise the shock loading sticker will detect bumps exceeding 10G. Taking this back to the original discussion, this does not imply that the structure needs to carry a static load of 10 x gravity. You could of course take that approach, and it would be safe, but it's extremely conservative.

If the sticker or any accelerometer at all close to the centre of mass of an object registers an acceleration of 10G then by Newtons 2nd law of motion that implies that there exists a force or sum of forces that are acting on the COM given by F=MA.

You can't just ignore a large or reduce a force because it is transient. Transient large forces is just how I can break concrete with a sledge hammer, this is high accelerations of the hammer head producing high forces.

 

[Tomfh]

You can't just ignore a large or reduce a force because it is transient.

It's not a matter of ignoring it. It's simply recognising that a high load acting for milliseconds is not nearly as bad as a sustained load of the same magnitude. To fail a structure you need to both exceed its static capacity AND you need to do a certain amount of work. If a load pulse contains less energy than the amount of work required to fail the structure then the structure wont fail, regardless of the peak intensity.

 

[human909]

I completely agreed with the above statement.

However that same load pulse wouldn't achieve a 10G acceleration of the structure.* Any accelerometer on the structure in or around the COM wouldn't register 10G. Which circles back to the comments that the first few half dozen people posted.

*Unless we are talking of a load pulse that is of the time period more commonly associated with acoustic range range. Then of course you can get very high accelerations, but accelerometers and other devices that are sampling in this range would be better called microphones!

 

[KootK]

Wonderful way of explaining it, Tomfh. Everything has a certain amount of instantaneous energy that it can absorb/release with little to no effect.

I do not believe that statement to be correct or consistent with Tomfh's iron touching analogy (hopefully Tomfh can confirm).

In my opinion, it is safe to briefly touch a hot iron because it imparts energy to your finger at a finite rate. As such, touching the iron for a relatively short period of time means that a small amount of energy is transferred to your finger. So you are unharmed by touching the iron not because your finger can sustain a lot of energy for a short period of time but, rather, because it was never the case that a lot of energy was imparted to your finger to begin with.

In contrast, these two conditions would damage your finger:

1) Touching the iron for a long period of time.

2) Touching the sun, instead of the iron, for a short period of time.

Again, it's all about the integral of the rate of energy transfer over the duration of energy transfer. This is analogous to the application of impulse over the duration of impulse in the case of dynamic loads applied to structures.

Given the same amount of energy being accumulated locally in your finger -- accounting for dissipation rate -- I believe that your finger would experience the same amount of harm whether that energy was transferred slowly or quickly.

I suspect that this supports what I interpret human909's view on the dynamic loading of structures to be. Namely, that a strain induced in a structure will produce the same amount of damage to that structure whether that strain is maintained for a short time, or a long time (hopefully human909 can confirm).

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

FWIW: I've mostly chosen to critique the hot iron thing because it makes for a convenient entry point into this discussion for me. If I'm out to lunch on my understanding of the arguments being made by Tomfh or Human909, I suspect this will tease that out.

 

[Tomfh]

Why do you think westernjeba statement is incorrect, and saying something different to me?

You seem to be saying the same thing - which is that the area under the curve must be a certain size for failure to occur. Peak magnitude alone will not do it.

Human909 seems to be saying that a transient force is just as damaging as a static force, and that the structure must be designed for the peak force, except applied continuously.

 

[human909]

I suspect that this supports what I interpret human909's view on the dynamic loading of structures to be. Namely, that a strain induced in a structure will produce the same amount of damage to that structure whether that strain is maintained for a short time, or a long time (hopefully human909 can confirm).

That is pretty much exactly what I'm saying. Though I don't believe I've always expressed it well in this discussion.

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

Agreed. 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. 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.

Human909 seems to be saying that a transient force is just as damaging as a static force, and that the structure must be designed for the peak force, except applied continuously.

That wasn't what I was saying. I gave numerous examples to elaborate on that, though maybe I wasn't clear. Again another example. If I whack a structure with a sledge hammer then the peak force at the point of impact can be extremely damaging but due to inertial and energy effects this peak force is not transmitted throughout the structure.

THIS is not the case with broad structural accelerations, unless we are talking about high frequency vibration. If a structure is broadly accelerated at 10G (aka the COM accelerates at 10G) then that structure will experience forces and strains in line with that of a 10G x MASS. So it is similar if not equivalent to a force of 10G x MASS.** If a forklift goes over a bump and experiences 10G of accelerations at some location that does not mean the structure experiences that acceleration either. I'd suspect that neither the structure nor the operator of the forklift would experience anything close to 10G. Otherwise you'd have a very unhappy forklift driver.

**Of course many materials experience creep over days/hours/years. I'm aware of this, but for simplicities sake I'm not delving into that complexity.

 

[Tomfh]

That wasn't what I was saying

Hmm. My original comment that you said was wrong was:

A peak load of 10G does NOT mean something needs to be designed for a static load of 10G.

So I’m a little unclear what you’re saying here now?

Do you have to design for the peak load even if it’s very brief? Or don’t you?

 

[human909]

A peak load of 10G does NOT mean something needs to be designed for a static load of 10G.

So I’m a little unclear what you’re saying here now?

Do you have to design for the peak load even if it’s very brief? Or don’t you?

As I keep saying. An acceleration isn't a force.

If the peak load on a structure's centre of mass is 10G then I would be designing all the elements in the load path to the centre of mass to be capable of withstanding a FORCE of 10g x mass. My normal caveats apply. EG vibrations in the sonic range or above which is a different case for most 'structures'.

But this is mostly a non issue because accelerations on a "structure" of 10G is mostly not a realistic event when we are talking about structures that structural engineers design. It likely is a realistic event when I drop my new phone on a concrete pavement, and the associate forces generally result in an unpleasant outcome.

 

[Tomfh]

If the peak load on a structure's centre of mass is 10G then I would be designing all the elements in the load path to the centre of mass to be capable of withstanding a FORCE of 10g x mass.

Yes as discussed before 10G is just shorthand here for the force associated with 10 times gravity applied to the structures mass.

I’m not sure how you can on the one hand agree that a transient force isn’t necessarily as damaging, whilst also saying you need to design for such a force as though it were applied continuously.

 

[rb1957]

has anyone noticed that the OP hasn't replies. I'm guessing he scanned the replies, thought "oh no" and ran for the hills.

a couple points ...
1) analogies can be deceptive and sometimes misleading,
2) these stickers are just witness markers ?
3) placard the structure, "no jolts here",
4) tell the transport company "no", although they are probably transporting your structure inside a cradle/fixture,
5) so have the cradle designed to protect your structure (against impact loads)
6) analyze your structure inside this cradle, apply some sort of load, you may learn something.
7) you may learn that under some jolt load there are parts of the structure that exceed 10g in local dynamic loads. Small weights, lightly sprung, could have high dynamic response.
8) to me using "g" to describe a load is bad practice. Using "g" to describe local dynamic response seems sensible (of course I'd say that ... I just did it !) ...
9) I'd've thought that there were other things to worry about ... like an enforced displacement of one corner of the fixture, distortion of the fixture in some way, and
10) there are a tonne of other things to worry about in transporting something, like protection from environment.


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

 

[human909]

to me using "g" to describe a load is bad practice.

100% agree.

as discussed before 10G is just shorthand here for the force associated with 10 times gravity applied to the structures mass.

And as discussed before it is extremely loose shorthand. 10g on a structure has no meaning if you are actually referring to an extremely brief force. If the force is extremely brief eg a hammer blow, then the acceleration on the structure is far less than 10g. (local accelerations and forces might be orders of magnitude higher!)

If the force persists for a period long enough to cause an acceleration of 10g on the structure then every member in the load path to the object's COM has experienced the strain associated with that 10g force.

I’m not sure how you can on the one hand agree that a transient force isn’t necessarily as damaging, whilst also saying you need to design for such a force as though it were applied continuously.

When you continue to switch between an acceleration on a structure and a transient force on a structure like they are the same thing then we will continue to not be on the same page.

 

[WARose]

has anyone noticed that the OP hasn't replie[d]

Wish he would....curious where this 2.2g criteria comes from. I've worked on items that had to be hauled.....always have been curious what criteria is out there for it.

 

[WARose]

And as discussed before it is extremely loose shorthand. 10g on a structure has no meaning if you are actually referring to an extremely brief force. If the force is extremely brief eg a hammer blow, then the acceleration on the structure is far less than 10g. (local accelerations and forces might be orders of magnitude higher!)

If the force persists for a period long enough to cause an acceleration of 10g on the structure then every member in the load path to the object's COM has experienced the strain associated with that 10g force.

Agreed. Most structural dynamics texts have charts (for simple systems) where the DLF [Dynamic Load Factor] for a short term loading is given. The factor depends on the duration of the load, the natural period of the system, etc, etc. For a suddenly loaded system (of short duration), the DLF can be much less than 1. IIRC, all this is derived from Duhamel's Integral.

 

[Tomfh]

When you continue to switch between an acceleration on a structure and a transient force on a structure like they are the same thing then we will continue to not be on the same page.

“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 to the structure. The same as briefly touching a hot iron is not as bad as holding it continuously.

 

[KootK]

I've added the bits in brackets in the following quote because I feel that they aid in precision here.

You seem to be saying the same thing - which is that the area under the [impulse] curve must be a certain size for failure to occur. Peak [force] magnitude alone will not do it.

Indeed I am saying the same thing. I wholly support your contention that a briefly applied force -- or acceleration for that matter -- will do less to distress a structure than would a static force of the same magnitude. It seems to me that many of your comments have been made from the perspective of the impulse-momentum version of Newton's second law. And that appeals to me as I believe that version to be better suited to short duration dynamics problems than is the F = ma version.

Why do you think westernjeb's statement is incorrect?

I'm afraid that I explained my view that westernjeb's statement was incorrect as well as I'm able to in my previous post. Perhaps you could review that post and quote the specific statements of mine that you disagree with? Having something specific to react to may suggest addition angles that I might take to try to be persuasive on this. Or convince me that I'm out to lunch, one never knows.

Why do you think westernjeb's statement is saying something different to me?

It seems to me that:

1) Many of your comments here speak to the integration of the impulse-momentum equation as a way to demonstrate that a briefly applied force imparts relatively little strain energy to a loaded member. And, as I said above, I agree with that.

2) Westernjeb's statement -- repeated below for convenience -- describes the delivery of strain energy to the loaded member as instantaneous. That's basically the opposite of the integration approach. Instead of implying that the imparted strain energy (damage) is small because the force input is applied over a short duration, it implies that the imparted strain energy is large but does little damage to the member receiving that strain energy because the energy is removed rapidly. Those two perspectives are radically different. In my opinion, the latter is spurious.

Fundamentally, "integration over time" and "instantaneous" just don't play well together.

Everything has a certain amount of instantaneous energy that it can absorb/release with little to no effect.

Human909 seems to be saying that a transient force is just as damaging as a static force, and that the structure must be designed for the peak force, except applied continuously.

I'm not sure that Human909 is saying that but, regardless, I agree that a transient force is definitely not as damaging as and equivalent static force and that a structure need not be designed for the peak force applied continuously.