Stiffness and Load

[AAllbbeerriicc]

Hello everybody,

I'm fairy new to structural engineering in aerospace, and I try to get a better understanding and good "sense" of prediction of the structure behaviour.

So I've got a couple of questions related to strength and stiffness.

Can someone explain me why a stiffer structure will attract more load?
Does that mean that a thin structure will undergo less stress and be more resistant?

In the case the structure is very flexible, will it be an issue in terms of dynamics?

I'm a bit confused now about what should be a good design!

Thanks a lot for the help you can provide me!

Al

 

[JAE]

Can someone explain me why a stiffer structure will attract more load?
[blue] Force follows stiffness - If you push or pull on two things with different stiffnesses, the stiffer element will attract more of the force.
Think about a rigid bar which has two springs attached to each end, with the springs attached to an infinitely rigid base.
If the two springs are equal in stiffness - when you pull on the bar, keeping the bar parallel with the rigid wall, the two springs will each take 50% of the pull force.
Now if you change this such that one spring is much stiffer - say 100 kips/inch on one with 1 kip/inch on the other, and then pull the bar 1 inch, the deflection of each spring is the same - 1". But the force required to stretch the 100 kip/inch spring will be....100 kips. The other will be 1 kip. More force in the stiffer element.
This applies to shears, moments and axial forces in more complex assemblies of structures as well. [/blue]

Does that mean that a thin structure will undergo less stress and be more resistant?
[blue] The stress in anything depends on the force vs. the element's sectional properties. The "Force-follows-stiffness" concept only dictates WHERE the force goes, not how it develops stress in that structure.[/blue]

In the case the structure is very flexible, will it be an issue in terms of dynamics?
[blue] More flexible structures are more susceptible to deflections, vibrations, instability, buckling, etc.[/blue]


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[AAllbbeerriicc]

Hi JAE,

Thanks for the quick reply and the infos! it's much clearer now, so if I got the point, a stiffer part of the structure will attract more internal force, and then the resulting stress at each location will depend on the geometry.

So a stiff member will attract more force but not necessarily the highest stress regading the geometry (thick structure)

In general having a very stif structure is what we try to achieve or is there any benefit having a flexible one? Maybe for pipework?

Thanks!

 

[Tomfh]

Can someone explain me why a stiffer structure will attract more load?

If there is only one structure (e.g. a stiff structure, or a flexible structure), then it "attracts" the whole load. When there are structures acting together, e.g. a heavy beam connected to the slab, then the stiff element "attracts" the load. It's worth realising that it doesn't literally attract the load. It's simply that for a given deflection a stiffer structure carries more load than a flexdible structure, and thus for a given deflection, a stiff element will be resisting a lot more load than a flexible element it is attached.

 

[klaus.]

well it depends what you want to do or what you want to achieve
If you need to 'shift' around the forces for what ever reason modifying the stiffness of some parts is a very good and common practice to do this

well JAE explained this very good in the posting above ...
but of course if your structure is statically determined ( internally and/or externally) then stiffness does not matter




best regards
Klaus

 

[AAllbbeerriicc]

Thanks for these additional info, very helpful.

About the statically determined structure, I know what the criteria is, regarding the number of unknown vs number of equations. But what I can't understand is how can we "SEE" an indeterminate structure?

What are the consequences of such a structure? Is it something beneficial? wanted?

Is there any example to illustrate this?

I can understand the equations and calcs, but at the end I don't really have engineering judgment and true understanding of the behaviour of structures unfortunately.

So help help!!

Thanaaaks

 

[msquared48]

The stiffer element will deflect less than one that is not as stiff. So, if the elements are linked, and they both deflect equally, then the stiffer element 2will control the deflection and suck up most of the load.

Think deflection rather than stiffness here and it may help you get a better feel for what is going on. They are directly related.

Mike McCann, PE, SE (WA)

 

[bowlingdanish]

On a fundamental level I feel like it's a optimisation of energy type deal. I think in all forms of physics, chemistry, etc. the processes that take place do so because they are the most efficient and consume the least energy. If you have a tall building a put a big load on the top of it, that loads wants to get to the ground. It will therefore find the 'quickest' way to get there. There are many loadpaths from the top to the bottom, but the load will always follow the quickest, most efficient one - which is through the stiffest parts. This causes the least deformation in the structure and uses the least energy.

That's grade A punting but I like the sound of it.

 

[MikeHalloran]

Phil Irving's book, "Motorcycle Engineering", has a most entertaining chapter on how the Greeves motorcycle frame came to have a massive 'frontbone'.




Mike Halloran
Pembroke Pines, FL, USA

 

[klaus.]

yes....for example Temperature load case :
for a statically determined structure a change of temperature does not cause any force (stress) because the structure can move (deform) without any force
So Yes... there are many good reasons to design a statically determined structure




best regards
Klaus

 

[XR250]

Phil Irving's book, "Motorcycle Engineering", has a most entertaining chapter on how the Greeves motorcycle frame came to have a massive 'frontbone'.

I have not read that book but I learned about this concept when I was designing the frame for the Formula SAE car in college. The load on certain members kept going up so I kept making them bigger. Turns out making the OTHER members bigger to attract more load solved the problem.