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

 

[rb1957]

this seems BS to me. I think I'd tell that enterprise that if they want a 10g "structure" then they'll need to pay extra for it. If they add it to their specification then either charge more or "no bid".

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

 

[WesternJeb]

In structures, we assume that a lot of the energy from impact loads get absorbed/dissipated based on the weight of the structure compared to the weight of what hits it. I believe the same would apply in the forklift applying load to your "structure" when it hits a bump. I would just reinforce the contact areas a little more and not worry about it; if you prove you did your due diligence then the blame would likely go back on the driver if they're being slightly reckless.

At the end of the day you can always tell them it is more cost-effective to tell the drives to be careful than it is to reinforce a building.

 

[canwesteng]

10g would send the forklift flying. The back end is going to come up and tilt over, and then you'll be past the point of equilibrium and down it goes. 3g should be quite conservative for a modular structure, even there I think you'll find whatever you're shipping this on would struggle to get that force onto the structure.

 

[KootK]

Following standards we saw that normally for these transports the structure shall support at maximum 2.2g in all directions.

What standard is that, may I ask?

 

[dik]

10g is quite a hit... your label, if designed for 3g, should stipulate 2.2g or maybe 2.5g... nothing more. Something doesn't seem right...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WARose]

What standard is that, may I ask?

I'd be curious about that myself. I've always had trouble finding a standard that says what loads transported frames should see.

 

[ThomasH]

Since you are dealing with something with a very short duration, there can be very high g-values. But 10g seems a bit excessive. Regardless of that, I would not happily put 10g on a sticker as an allowed value.

 

[Tomfh]

These are PEAK loads. Short duration loads.

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

When you jump and land, your joints experience peak loads of 10+ G. It doesn't mean your body needs to be able to carry a tonne.

 

[GregLocock]

The stickers need to be able to handle more than the design maximum g otherwise they don't tell you anything useful.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[human909]

These are PEAK loads. Short duration loads.

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

Actually it pretty much does mean that. If a body experiences an acceleration of 10G then the force exerted on the body's centre of mass is F=ma so 10G x mass. Though 10G is not in itself a load, it is an acceleration. But finding the force associated with that acceleration is trivial.

The more correct question that should be asking is what is experiencing 10G? As others have pointed out

When you jump and land, your joints experience peak loads of 10+ G. It doesn't mean your body needs to be able to carry a tonne.

That seems a bit misleading. What is a peak load of 10+ G? Should it be a peak acceleration? In which case the next question is what mass is experiencing that acceleration. And it certainly isn't your head, torso or any significant joint!

 

[Tomfh]

Actually it pretty much does mean that

No, it doesn't. Applying the peak dynamic load as a static load is fundamentally incorrect.

What is a peak load of 10+ G?

Say you jump and have a hard landing, your ankle joints will experience a peak load of around 10G, i.e. your ankle will briefly experience the force of 10x your body weight. This is completely different to applying a sustained static load of 10x your body weight to your ankles.

The more correct question that should be asking is what is experiencing 10G

The thing being transported. It's a rough handling sticker. It tells people if the item has been knocked around, treated badly in transit. The sticker will go off if a peak acceleration of 10G is experienced. But that doesn't mean the structure needs to resist a sustained static load of 10G.

 

[human909]

No, it doesn't. Applying the peak dynamic load as a static load is fundamentally incorrect.

your ankle will briefly experience the force of 10x your body weight. This is completely different to applying a sustained static load of 10x your body weight to your ankles.

You are going to need to explain that statement using more precise engineering language.


Also there is a decent amount of research into dynamic heel striking and I've never seen peak forces anywhere near 10x bodyweight.
EG.

https://www.sciencedirect.com/science/article/pii/S1728869X15000027

 

[Tomfh]

An instantaneous shock load of 10G upon a structure, such as what the sticker is trying to detect, imposes far less demand upon a structure than a sustained static load of 10G.

 

[human909]

An instantaneous shock load of 10G upon a structure, such as what the sticker is trying to detect, imposes far less demand upon a structure than a sustained static load of 10G.

It is hard to parse what you are trying to say here but all sensible interpretations sound quite false. For starter if you are treating acceleration as an applied load on a body, then the implicit application is on the mass of the body after all its units are N/kg.

If what you instead mean that if your structure rests on something that briefly reaches a peak acceleration of 10G then I'd agree that it can impose less force on a structure than a static acceleration load of 10G. However that is a different scenario, one that you aren't describing and one that isn't being assessed by measuring the acceleration of the structure.


It is almost like you are suggesting that if hit a building with a hammer and my accelerometer on my hammer measures 1000G then I am applying 1000G on the structure. That is an incorrect framing of the dynamics. I am in applying a localised force on the structure that might even locally exceed local capacity of the point that I am hitting. But at no stage am I applying 1000G to the structure, that is a nonsensical statement.

 

[Tomfh]

When I say a load of 10G I’m talking about force associated with that level of acceleration. Just as 1G of load equates to the static force due to gravity acting on a mass, F= mg, a load of 10G is a force ten times that, F = 10 mg

And I’m talking about the acceleration the thing itself experiences, and the associated internal force, eg when a reckless forklift driver bumps or drops an object, causing it to briefly feel 10G of force, or to use my other example the load your ankles feel for a split second when you have a hard landing, where you can feel up to ten times your normal body weight.

There is a brief load spike of 10G, ie a fleeting force 10x the normal static load due to gravity.

The thing itself feels that high force, but only for millisecond. Same as you can touch a hot iron for a split second, and suffer no harm.

 

[WesternJeb]

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

 

[human909]

Hey Tomfh. I hope you don't find my comments too frustrating or pedantic. I respect you and your contribution to ENG-TIPS.

Maybe I've dug myself a hole of misunderstand and I'm mistaken in this discussion. But at the moment I'm not convinced by your comments and for the sake of clarifying my own understanding I'm keen on further discussion/argument.* While sometimes it can come across as combative, I learn plenty but such interactions. A little if I'm correct, and alot if I'm convinced that I'm wrong.

When I say a load of 10G I’m talking about force associated with that level of acceleration. Just as 1G of load equates to the static force due to gravity acting on a mass, F= mg, a load of 10G is a force ten times that, F = 10 mg

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

or to use my other example the load your ankles feel for a split second when you have a hard landing, where you can feel up to ten times your normal body weight.

There is no way my ankles would be happy with 10kN of force no matter how brief. It is still 10kN regardless of the time frame so you would have stress and strain (no pun intended) associated with 10kN.
(A quick look at the biomechanics and a peak force of 2.5 body weight is typical in running. If you start jumping and landing poorly then I'm sure I can push a 10kN force through my ankle. But it won't be happy about it.)

That said there are PARTS of my body can readily experience very high accelerations. If I flick something hard with my finger then my finger nail would experience extremely high decelerations (probably >10,000G) while my finger tips COG likely >1000G.** And it also experiences some relatively high forces associated with those accelerations, but my finger nail and my finger are low mass so the forces are still not crazy high.

And I’m talking about the acceleration the thing itself experiences, and the associated internal force, eg when a reckless forklift driver bumps or drops an object, causing it to briefly feel 10G of force

Well if it briefly experience "10G of force" then I'd be designing it to resist "10G of force" as that force is REAL and the associate stresses and strains are real too.

But if it is most normal structures it won't feel the force we are talking about because the structure will flex and absorb the energy and thus peak forces associated are much lower than 10G.

 

[GregLocock]

OK, this used to be my job. Hit car with instrumented hammer, measure acceleration at the impact point. 3g would not be unusual. That is not the same as me accelerating the entire car at 3g.

Cheers

Greg Locock


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[human909]

OK, this used to be my job. Hit car with instrumented hammer, measure acceleration at the impact point. 3g would not be unusual. That is not the same as me accelerating the entire car at 3g.

Exactly. It still is my job, I've done the same thing with structures too in the course of writing professional reports for clients.

I've smacked them with large hammers and measured the acceleration and the associated frequencies.


EDIT I decided to do some amateur testing with the accelerometer I have at here. 500hz sampling rate.
Bare foot jumping. Landing was firm. I wouldn't enjoy much firmer.

Ankle PEAK acceleration was 11.6G.
Torso PEAK acceleration was 3.1G.

The latter result is in line with expectations base on available data. The former result I had no data available. So sure my ankle and forefoot decelerates at 10G. But the mass associate with that deceleration is NOT the total body mass. The appropriate mass to use would be the unprung mass. You'd also have to combine it with the forces imparted by the sprung masses of the upper leg and torso and their associated accelerations.