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Total Load at Point?

jrich
2-Guest

Total Load at Point?


I'm running a static stress analysis where the load is a Moment load about a shaft axis.

For the analysis to run successfully I have to change the load distribution from "Total Load", to "Total Load at Point." I then must pick some arbitrary point somewhere on the model. The location of this point does not affect analysis results.

Now, "total load at point" SOUNDS like, "Oh, gosh, the total moment load is going to all go through that one point."

But, no, after running the analysis we see that that point is ignored. It doesn't matter where the point is chosen.

Rather than calling it "Total Load at Point," PTC should call it, "Arbitrary reference point to be located anywhere on the modal to make Moment loads work."

Tech support couldn't given me a definition of what "Total Load at Point" means, nor could tech support tell me what the arbitrary point is for, other than it's needed to make the analysis work.

Does anyone on planet Earth have any further information on these matters?

Thanks,
Jeff

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6 REPLIES 6

It has a poor name, but it is a very, very useful load



First, it's the best (maybe only?) way to apply moment loads - I just did a
mounting pedestal for a 11,500 ft-lb crane, and created a point 4 feet away
from the center and applied a 2875 lb load. It worked great.



Second, it's also good for measuring how a load on a larger assembly affects
one part - for example, at my old job, I had to analyze a seat pan for an
office chair. The seat pan has an armrest bolted to it. I had to analyze
how a load placed on the arm affected the seat pan. So the point was placed
at the front of the armpad, and the surfaces picked in the load dialog
window were the bolt mounting holes where the arm bolted to the seat pan.



So the point isn't completely arbitrary or ignored - it is good for really
capturing the true load scenario.



--



Lyle Beidler
MGS Inc
178 Muddy Creek Church Rd
Denver PA 17517
717-336-7528
Fax 717-336-0514
-

Jeff,



Total load at point or "TLAP" is a very handy capability that allows one
to "smear out" an applied load over an entire surface (or surfaces).
The primary purpose is to apply equivalent loads on your model from a
point off-body without building geometry to connect them. In your case
the point didn't matter because you were applying a moment, which does
not change value if the point of application is moved. Forces, however,
create a very different result when applied to different points - they
generate both a force and moment. For example, if I have a round rod
oriented along the Z axis and I apply a lateral force in the Y direction
to the center of the end of the rod there is no torsion component;
however if I move that point off the rod a torsion component is created.



Kevin






Kevin Schoonover
PTC Channel Technical Manager
Western North America
coombsd
1-Newbie
(To:jrich)

Jeff,

If you load is a moment, then the location of the point is arbitrary.
This actually makes some sense from a statics perspective. A moment
couple can be placed anywhere in space and still create the same
resulting moment. The only thing that you are controlling is where the
moment is applied, which is the surface you selected.

The scenario where the location of the point does matter, is when you
apply a linear load through the point and it is applied at the selected
surfaces. In this case, the point location determines the moment arm.

By the way, the reason you need to use total load at point to apply a
moment is because you can not apply rotational boundary conditions on
nodes that are part of 3D solid elements. This is not true for shells
and beams. The total load at a point breaks the moment into individual
linear loads at the nodes on the surface that you selected.

I hope that helps.
Dana

e
SPINE
Dana Coombs
Analytical Engineer
1301 Goshen Parkway
West Chester, PA 19380

Dana,

This requirement of an arbitrary point for applying moment loads doesn't make any sense whatsoever. Moments are always about an axis first of all, not a point. Second, how does choosing an arbitrary point ever make sense? Why make me choose something arbitrary just so that it works? Whatever math Creo needs to do in the background in order to do its job is its problem. I don't care, I just want it to do what it says. There should just be one apply moment load tool and it should either do it about an axis, or the center of the surface (as long as it tells me which one it is choosing) and an option to switch between the too would be nice.

Logically if there was a direction specified and a point then that does define an axis. And so having to specify a direction and a point AND NOT having those inputs define the axis of the moment is crazy talk.

Jeff,

You are correct.

PTC,

Please add the option of actually specifying the moment axis somehow as I need it for cyclical FEA simulations with applied torque. Or maybe just put the axis through the axis of cylindrical surfaces automatically even if they aren't a full 360 degree section.

I don't know much about otherFEA systems, but whenever I show TLAP to someone who is 'expert' in another FEA system they tend to be pretty amazed. Very useful and seemingly mindblowing to some



---
David Reid
Optima
davidr@optima-design.co.uk

Since this seemingly simple matter seems to generate much confusion, I thought a sort of 'scientific' () definition of TLAP wouldn't hurt.

'Total Load at Point creates a force distribution over geometry the load is applied to, that is statically equivalent to a given force resultant at a given point' (Tried to make it short and clear, seem to have succeeded with 'short', not so sure about 'clear' though...)

The keyword above are 'statically equivalent', which basically explains what happens if one applies a moment, or a force, etc. As well, since the forces are distributed over geometry, hence we sometimes hear words like 'smearing' etc. The essence is quite simple though, and no magic is happening behind the scenes.

An interesting quiestion noone seems to ask, by the way, might be -- so what is the law governing that distribution? The thing is that the problem of producing a statically equivalent system of forces out of a given resultant load actually has infinite number of solutions, each solution having its unique law of distribution (for instance, linear, or parabolic, or something else). So the question is -- which one is used in Pro/M? That actually might be of interest to many, since type, or 'shape', of force distribution can actually affect your stress results quite a bit. Did anyone try to ask PTC that question? (Though I bet the distribution is of 'bolt pattern' generic type...)

Lastly, I tend to disagree with David that Pro/M stands out in this regard. It may be called differently, or done differently in other systems (e.g. one would use RBE3 to do the same in Nastran, or Distributed Force feature, or a Virtual Part to do that in V5 Elfini), but I'm sure it's quite a common feature nowadays.

Pro/M is actually missing a very useful extension of TLAP, which is -- TLAP distributed over a set of Datum Points rather than over a surface.

Hope some will find the above somewhat helpful.

Yuri

David Reid <davidr@optima-design.co.uk> wrote:
I don't know much about other FEA systems, but whenever I show TLAP to someone who is 'expert' in another FEA system they tend to be pretty amazed.

Very useful and seemingly mindblowing to some



---
David Reid
Optima
davidr@optima-design.co.uk
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