Statically determinate vs indeterminate 1

[duanebw]

I have two components; the first is mounted using statically determinate “rigid” mounts. The second is mounted using statically indeterminate “flexible” mounts. The mounts are attached to a surface, which an impact load is applied to.

I am trying to see how effective using mounts based on the second “flexible” mounts on the first component would be.

I am struggling to figure out the relevance of whether the mounts are statically determinate or indeterminate is. Is a statically determinate design more effective? Can a statically determinate system carry more load? How do you determine if a system should be statically determinate or not?


Many thanks,
Duane.

 

[BAretired]

A statically determinate system is neither better nor worse than a statically indeterminate system. It may be stronger or weaker depending on the details of the two systems.

The main difference is that a statically determinate system can be analyzed using statics alone whereas a statically indeterminate system requires consideration of strain compatibility.

BA

 

[ThomasH]

Hi

I would not say that one system can carry more load than the other regardless of other parameters. But a statically indeterminate system can often redistribute the load more efficiently than the statically determinate system. A simple example is the two span beam with three supports. The max moment is typically at the center support but yielding there does not necessarily mean faliure.

But the statically determinate system can be solved using only equations for equilibrium. Statically indeterminate means that you have to include some type of deformation criteria.

However, in some situations I would say that statically indeterminate systems are more efficient. When you work with different types of accidental loads och dynamic loads like blast loads. Typically, when the deformations are not a big concern. Then the indeterminate systems can often find alternate load paths or supporting systems with a lot of plastic deformation.
But that is a different story.

Regards

Thomas

 

[DCBII]

Statically determinate systems are simple to analyze. Statically indertermindate systems have redundancy built into them, but require more rigorous analysis. One is not necessarily better than the other.

 

[rb1957]

i wonder if the OP is meaning rigid (infinite stiffness) supports vs flexible (finite stiffness) supports ?

comes to mind when i read .."the first is mounted using statically determinate “rigid” mounts. The second is mounted using statically indeterminate “flexible” mounts."

in that light, i doubt there is much difference in the analysis; though if "rigid" means all 6 degrees of freedom then it is indeterminate compared to pinned joints (and, yes, pinned joints could also be indeterminate). the difference between "rigid" and "flexible" supports is that rigid is a mathematical ideal and flexible is a better model of the real structure. although if your supports are determinate, then there's no difference (in the support reactions) !?

another day in paradise, or is paradise one day closer ?

 

[dhengr]

Duanebw:
It seems that you are pretty confused in your terminology. I think BA’s definition of determinate and indeterminate structures is right on the money, and there are many books written on each subject. Why don’t you dig out your Strength of Materials and Structural Analysis text books and do a little self study on these issues? Maybe you shouldn’t be messing with these issues if you can’t muster that effort. Why don’t you show some properly proportioned sketches of your two connection situations, so we can see what we think is fixed and partially fixed. What are they connected to, what are the loads and dimensions, what are they made of, what impact loads, etc., etc. All of these things enter into the picture. You’d be surprised at what judgements an experienced engineer can make from a good sketch or free body diagram. You have to do a better job of explaining what you are really trying to accomplish, because at the moment you are way off base.

 

[glass99]

As a structural glass guy, I can tell you that I always design my glass fins for static determinacy - if not, you never really know where the load is going and therefore your analysis is incorrect, and you break glass!

 

[IRstuff]

"Is a statically determinate design more effective?"

Effective in what sense? What are you trying to achieve, what are the mounts designed for, and what can the components handle? Without understanding these questions, your question cannot be answered. That said, for a component that's sensitive or vulnerable, a rigid mount will be ineffective if the shock level exceeds the capability of the component. A flexible mount could potentially cushion the shock, but only if it's designed for that input. A flexible mount with insufficient design margin will be as ineffective as a rigid mount. For a component that's bulletproof, either mount is "effective." For a vulnerable component with a properly designed flexible mount, the flexible mount will be more effective than a rigid mount, up to the design limit of the flexible mount, at which point, again, neither will be effective.

TTFN
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[civeng80]

That's actually an interesting question.
Sometimes I feel a little apprehensive in designing a statically determinate structure, in that only one member failure may lead to a catastrophic failure of the whole structure.
In theory there is no problems but in reality redundancy may lead to safer structures.

 

[glass99]

civeng80 - Yes and no: Indeterminacy can be worse from a progressive collapse point of view. If you have a long continuous beam over many columns and you lose the end column in a blast event, the beam becomes a cantilever and you lose the next column over as well. On the other hand, if you have a two way slab and you lose one support beam, it will probably safely span the other direction.

 

[civeng80]

Glass99
If you lose support of the end column you have a cantilever which is better than having a simply supported beam losing the end support which will certainly fail.

 

[Struengin]

From what I know in my humble experience, statically indeterminate structures are more robust against accidental or blast loads, meaning the whole structure may not fail due to the failure of any of its elements. While in determinate structures a failure of one element is the failure of the whole structure. Correct me if I am wrong.

 

[charliealphabravo]

@ glass99,

If you have a continuous beam with many supports then you probably have an indeterminate system. If you have a plate (slab/glass) supported on 4 sides then you also have an indeterminate system I think.

In both cases, progressive collapse might be avoided if the system has been designed to survive the loss of a support. My understanding is that determinate systems are more susceptible to progressive collapse due to the limited redundancy and moment redistribution. But your post got me thinking that perhaps determinant systems may have some advantages for the exact opposite reason, namely that collapse moments cannot transfer into the structure.

 

[glass99]

If my continuously supported beam which is designed for WL^2/12 becomes a cantilever with WL^2/2 (i.e. 6X the moment), it will potentially take out the next column over if its moment connected. The best situation is that you have a structure which is both indeterminate and can withstand loss of a column or beam, but at some point your building becomes a bunker. Folks who design embassies worry about such things a lot, and do not hesitate to reach for the 2" thick plate.

 

[rb1957]

ie fail-safe

another day in paradise, or is paradise one day closer ?