statically indeterminate vs rigidly indeterminate
statically indeterminate vs rigidly indeterminate
(OP)
The difference between statically determinate and statically indeterminate is whether or not you can determine all the reaction forces and moments from a free bodying diagram without considering strain of the body (i.e. while considering the body rigid).
What does that have to do with static? To me static means not varying with time (opposite of dynamic).
Shouldn't it be called rigidly indeterminate?
What does that have to do with static? To me static means not varying with time (opposite of dynamic).
Shouldn't it be called rigidly indeterminate?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.





RE: statically indeterminate vs rigidly indeterminate
The "static" analysis assumes an instantaneous effect.
RE: statically indeterminate vs rigidly indeterminate
The mechanics of rigid bodies is sub-divided into two areas, statics and dynamics, with dynamics being further subdivided into kinematics and kinetics. Statics is the study of bodies in equilibrium. This means there are no unbalanced forces on the body, thus the body is either at rest or moving at a uniform velocity. Dynamics is the study of bodies which are not in equilibrium, thus there is acceleration. Kinematics is the study of the motion of a body, without regard for how the motion is produced. This is sometimes called the "geometry of motion". Kinematic principles are often applied to the analysis of machine members to determine positions, velocities, or accelerations at various parts of the machines' operation. Kinetics is the study of the forces which cause motion, or the forces which result from motion.
Mechanics of Deformable Bodies: The mechanics of deformable bodies deals with how forces are distributed inside bodies, and with the deformations caused by these internal force distributions. These internal force produce "stresses" in the body, which could ultimately result in the failure of the material itself. Principles of rigid body mechanics often provide the beginning steps in analyzing these internal stresses, and resulting deformations. These will be studied in courses called Strength of Materials or Mechanics of Materials.
RE: statically indeterminate vs rigidly indeterminate
Aren't there many statically-indeterminate problems which can be solved by the methods of statics (where "statics" would include static deformation under static load) ?
For example: find static reaction forces for various loadings of a beam with three supports. This is classified as statically indeterminate but I can solve it using static methods. But I can't solve it if I considered the beam to be rigid (non-bendable). Hence to my mind it would be more logical to call it rigidly indeterminate?
I know none of this is new to you guys. Am I missing something or is this just another one of those ME terminology things that isn't supposed to make sense?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
RE: statically indeterminate vs rigidly indeterminate
"statics" implies static equilibrium, that the thing isn't moving.
"statically determinate" means that you can solve for the external reactions using the equations of equilibrium (sum Forces and sum Moments).
"statically indeterminate" means that you have too many reactions (more reactions than there are equations of equilibrium). This is solved by applying other methods (eg energy, unit force, ...) to assemble enough equations.
RE: statically indeterminate vs rigidly indeterminate
1 – Rigid Body
1A – Statics
1B – Dynamics
2 – Mechanics of deformable bodies.
Statics would under this framework always imply rigid body.
Still I think the terminology is a little misleading. Once again it’s not the limitation of static (vs dynamic) analysis that determines whether or not we can solve this type of problem, but the limitation of rigid body (vs deformable) analysis.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
Although I may be wrong, I believe that when speaking of structures, solutions to static indeterminacy, can sometimes be found by assuming displacements (related to stiffness), not necessarily body deformations from plasticity, rigidity or ductility considerations.
Thus it appears to me that the prevailing semantics still has a logical basis.
RE: statically indeterminate vs rigidly indeterminate
the traditional terminology is still the best (99.99% of people involved understand it). everything deflects, including statically determinate structures but this is still static. as opposed to dynamic which would describe a moving structure, like something vibrating.
RE: statically indeterminate vs rigidly indeterminate
#1 - I don't see what difference it makes whether you call it deformation or displacement. The key thing is that we cannot solve the indeterminate problem if the body is considered rigid (once again the reason for my suggestion rigidly indeterminate).
#2 - "as opposed to dynamic which would describe a moving structure, like something vibrating." You're saying this supports the traditional terminology? I see it exactly the opposite. There is no distinction static vs dynamic involved in this concept of statically-determinate vs statically indeterminate. That was my original point.
One more comment, getting a little further into the realm of semantics.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
RE: statically indeterminate vs rigidly indeterminate
RE: statically indeterminate vs rigidly indeterminate
Can you give an example to help clarify this?
Thx.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
Now, take the same setup and put a point load at one end. If the supports are also perfectly rigid - the load goes straight into the support under the load and NO load goes to the other two. If the two outer supports and equally not rigid and the center one is rigid, then P goes to the support under the load P, 2P goes to the center, and -P goes to the other end support. Messed up, isn't it?
The assumptions made above are the same you make when doing a 4-bar linkage. If those bars aren't rigid, the equations get goofy in a hurry. In the 4-bar case, you assume the bars are rigid to simplify what's going on. The difference is, this assumption actually approaches reality in most cases where my previous examples do not.
We all know this is hogwash in reality, but again, the mathematics work with the constraints given.
Looking back and reading, I have to apologize for this and the preceding post. They're pretty far off topic and useless to boot...
RE: statically indeterminate vs rigidly indeterminate
The case of center rigid support with two equally flexible supports is a little different than what I had considered but the same basic idea that to make the system become determinate you can no longer consider all parts rigid. Before I focused on the "body"... didn't cover the support but the same concept applies.
So I would still submit that to resolve static indeterminacy we need additional relationship which will describe actual movement of some part under the loading.
Still all this is nothing new to you guys I'm sure.
My argument wasn't intended to be mathematical, only semantic. Semantically, one could interpret "statically indeterminate" to mean indeterminate (underspecified) when evaluated under static conditions and requires additional (nonstatic) information to resolve.
That is not the only way to semantically interpret the phrase and obviously not the way it was intended.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
Just to explain where I was coming from to begin with. What if you substituted "underspecified" for "indeterminate".
Would the term "statically underspecified" make sense to you guys?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
It could also be statically overspecified.
RE: statically indeterminate vs rigidly indeterminate
My underspecified analogy isn't perfect.
All in all if it were up to me I'd still call it rigidly indeterminate. I guess it's a good thing it's not up to me!
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
Why is it called "Statically Indeterminate" when the problem has nothing to do with electricity?
RE: statically indeterminate vs rigidly indeterminate
One thing that separate statics from dynamics is time. In statics (actually getting into strength of materials), we can have deflection. I can apply a point load to the end of a beam that is "rigidly" anchored at the opposite end (the classic cantilever beam) and observe a deflection. This is still a statics situation becuase I'm only interested in the beam in it's deflected state (i.e. what the stresses are, etc.) I don't care how long it took to get there and, in this case, the applied load doesn't change with time. Therefore, the situation is one of equilibrium --- static, that is.
For Dynamics, there's going to be a time element involved. i.e., the force applied at the end of the beam goes from 0 lbs to 1000 lbs back to 0 lbs every second. Or, my beam is lying on the ground and I apply a net lateral force of 1000 lbs and the beam starts sliding faster and faster (F = ma, or in this case, a = F/m).
Does that help this conversation, or have I just been babblign to myself this morning?
Edward L. Klein
Pipe Stress Engineer
Houston, Texas
"All the world is a Spring"
All opinions expressed here are my own and not my company's.
RE: statically indeterminate vs rigidly indeterminate
handleman - You will forgive me if I don't agree that electrically-indeterminate describes the situation just as well as rigidly-indeterminate. Bearing in mind that indeterminate is not too far from underspecified. We can add additional non-rigid equations to solve the problem.
Another analogy might be to identify a type of mathematical problem as "analytically unsolveable". That means we need to try another approach such as numerical. The word analytically here describes the bounds within which the problem is not unsolveable...but the problem can be solved by other means outside of those bounds. From semantics alone, one might also expect that the word statically in statically indeterminate describes the bounds within which the problem is indeterminate.
However I acknowledge that is not how it is intended.
I am happy to call this matter closed.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
And further does anyone see that under the semantic interpretation implied through this analogy , rigidly indeterminate would be more descriptive of the condition than statically indeterminate?
Thx.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
The difference between "Statics" and "static" is similar to the difference between "analytically" used in the mathematical sense and "analysis" in the more general sense.
RE: statically indeterminate vs rigidly indeterminate
http://mi
"Analytical Solution An exact solution to a problem that can be calculated symbolically by manipulating equations (unlike a numerical solution). "
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
regarding some other posts, if the structure is determiate then it's stiffness doesn't matter (that will only affect how much the structure deflects) ... the loads will be reacted as required by equilibrium.
if a force is applied at a conventionally pinned support, then the reaction will be at that support. the only way the load will work its way out to the other (redundant) supports is if the support is elastic (rather than rigid), then effectly the applied load is reacted by the stiffness of the support and the stiffness of the structure.
RE: statically indeterminate vs rigidly indeterminate
In the same way, in Engineering terminology the "Statically" part of "Statically indeterminate" refers to the discipline of Statics, not to the fact that the system is not in motion. "Rigidly indeterminate" would make no sense in this application because the term "Statically indeterminate" is from the discipline of statics, which excludes deformation of bodies! You cannot remove "statically indeterminate" from its context of Statics an attempt to make it stand on its own semantically. Otherwise, you must include all definitions of "static", including fields such as electricity, biology, computer science, etc.
Statically indeterminate: Can't solve with Statics alone.
Algebraically indeterminate: Can't solve with Algebra alone.
How are these different?
RE: statically indeterminate vs rigidly indeterminate
Statically indeterminate: Can't solve with Statics alone.
Algebraically indeterminate: Can't solve with Algebra alone.
How are these different?[/quoute]
I think they are very similar. And I think my argument is supported to the extent that we believe they are similar and we accept your words:
"Statically indeterminate: Can't solve with Statics alone."
The other implied half of "Can't solve with Statics alone" is "Can solve by going beyond statics".
Well I can go beyond statics to dynamics and it does nothing to help me solve the problem.
What I need to do is go beyond rigid body mechanics to solve the problem.
So again from this train of logic one would prefer to call it "rigidly indeterminate" (cannot solve by rigid-body analysis).
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
"A Rose by any other name is still a Rose."
RE: statically indeterminate vs rigidly indeterminate
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: statically indeterminate vs rigidly indeterminate
The implied statement here is that dynamics is beyond/above statics. Dynamics is not necessarily "beyond" Statics. It's just different. In fact, it's not even the next logical direction to go once you determine that a system is Statically Indeterminate. Keeping the discipline and logic of Engineering in mind, one would not arrive at the conclusion that a system is Statically Indeterminate (as defined by the discipline of Statics) without first concluding/assuming that the system is not in motion. This rules out Dynamics to begin with. Statically Indeterminate systems are usually (always?) overconstrained rather than underconstrained. If you had a
Also, since "Statically Indeterminate" originates in the discipline of Statics, all that can be said is that Statics cannot solve for the reactions at supports. Statics is not "qualified" to tell the engineer which direction to take. It is up to the experience and knowledge of the engineer to know that deformable body mechanics are needed after that.
RE: statically indeterminate vs rigidly indeterminate
if you have an idealised rigid beam on three rigid supports and load it then ...
1) you can't apply unit load methods (as you've got no deflection to cancel out) so
2) i suspect that energy methods will indicate that the load is reacted by the two adjacent supports; as i think this minimises the internal energy in the beam, by minimising the area of the bending moment diagram.
if you apply the load to a real flexible beam between the supports, the problem is indeterminate (statics won't be able to tell you how much load is reacted by any of the supports). i would typically use the unit load method to the load at the middle support and then the others fall out