lability problem (frame 3d)

[Domenico Garofalo]

Good evening to all, I am a student of ing.meccanica and I am preparing the examination on ansys (fem analysis).
I can't understand why I'm not here; I've blocked the lability of the archer (rotz=0 in all the trave-arcareccio intersection points) but when I solve the structure I have a mz moment of 60000 n*mm (I would have to find a value almost null, as if the bond was not there).
I have attached the exam track to you; the problem is related to the combination of load 5, i.e. when on the structure only the gravitic load (i.e. that of the structure) and the weight of the panels acts.
the element used is the beam 188.
I thank those who will answer me in advance.

 

[Stan9411]

“I’ve blocked lability” is one of those phrases that is not unique and you don’t understand what it means. But then write rotz = 0 ... I suppose then you have transformed the hinges into recesses in the plan that it has as director z ... and watch case the disaster produces the moment as a reaction.

If you want zero-resistant time, you have to give the rotations.

 

[volaff]

static-cinematic duality: basic construction science stuff.
If you want the null moment there must be the possibility to rotate.

think about the zipper bond: the zipper can rotate for which it has no bending moment.

 

[Domenico Garofalo]

thanks guys, having inserted a constraint on the rotation z is normal to find a bending moment resulting z different from zero

 

[Onda]

if the only load is the proper weight and the structure was balanced compared to the zy plane, the resulting z moment should be null. the bond is necessary to remove lability, otherwise the system becomes a cinematism. It is necessary to see if the length of the oblique rods is the same on one side and on the other of the mounts. If it wasn't, the moment is explained.

 

[Domenico Garofalo]

if the only load is the proper weight and the structure was balanced compared to the zy plane, the resulting z moment should be null. the bond is necessary to remove lability, otherwise the system becomes a cinematism. It is necessary to see if the length of the oblique rods is the same on one side and on the other of the mounts. If it wasn't, the moment is explained.

Can you explain yourself better?. for oblique rods I think you refer to the "traves" (see track), you are all 3 of the same length. yes the load would be the weight of the structure. I gave him only acceleration and density of the material

 

[Onda]

the length from the negative x part of the oblique beam is the same from the positive x part. otherwise you create a moment because the center of gravity is not on the same floor where you placed the constraints.
to verify, plot the center of the structure and verify that it is on the yz plane, if so not, a moment is generated due to the mass product for the distance of the center of the above floor

 

[Stan9411]

the bond is necessary to remove lability, otherwise the system becomes a cinematism

If I have understood correctly, the thread author refers to the constraints between each of the two purple long beams and the 3 water green beams. also because the specific text that the tilted beam bond - vertical column is a welding = ink.
What planet is a triple hinge?

n.b : from the design I do not understand where the z axis is, so something could escape me.

 

[Onda]

I consider your offensive tone and therefore not suitable for discussion.
If the system did not have a constraint on the z rotation, it would rotate around this axis, being the three points of support aligned. This is about cinematism. the coordinate system is well detected by looking at the measurements.
Moreover, the initial request was to understand why the system sees 60knmm of moment in mz, then discharged to the ground, as from the results of the fem. my reflection is that if there is a moment downloaded to the ground, it means that the center of the structure is located outside the floor of the supports and therefore a moment is generated in the constraints.

 

[Domenico Garofalo]

If I have understood correctly, the thread author refers to the constraints between each of the two purple long beams and the 3 water green beams. also because the specific text that the tilted beam bond - vertical column is a welding = ink.
What planet is a triple hinge?

n.b : from the design I do not understand where the z axis is, so something could escape me.

I attached you more photos, maybe so you better understand the orientation of the axes.

 

[Domenico Garofalo]

I consider your offensive tone and therefore not suitable for discussion.
If the system did not have a constraint on the z rotation, it would rotate around this axis, being the three points of support aligned. This is about cinematism. the coordinate system is well detected by looking at the measurements.
Moreover, the initial request was to understand why the system sees 60knmm of moment in mz, then discharged to the ground, as from the results of the fem. my reflection is that if there is a moment downloaded to the ground, it means that the center of the structure is located outside the floor of the supports and therefore a moment is generated in the constraints.

Did I plot the coordinates of the mass center ("lsum"), there is a way to see it on the structure?? I don't know how

 

[Onda]

the mass center is located practically on the yz plane, so it is obvious that this does not create a moment.
At this point, you have to better review the output file of forces to knots. You should understand what the knots put on the ground and evaluate the loads only on these. I have impression that the output file also takes into consideration the high knots, those of connection with the transversal beams, where you obviously have a moment mz.

if you consider only the knots: 1; 252; 503. see that these dump only a result in y. as it is correct that it is. So I think that the 60knmm are the internal moments, but they are perfectly balanced and do not discharge to the ground.

 

[Domenico Garofalo]

the mass center is located practically on the yz plane, so it is obvious that this does not create a moment.
At this point, you have to better review the output file of forces to knots. You should understand what the knots put on the ground and evaluate the loads only on these. I have impression that the output file also takes into consideration the high knots, those of connection with the transversal beams, where you obviously have a moment mz.

if you consider only the knots: 1; 252; 503. see that these dump only a result in y. as it is correct that it is. So I think that the 60knmm are the internal moments, but they are perfectly balanced and do not discharge to the ground.

Thank you. @onda for availability; That's exactly what I didn't understand, I thought the 60kn*mm weren't balanced, but they're balanced by my constraints. I don't know how to plot the agents' forces on the individual nodes, because I've only given an acceleration (gravity), I only read that software.
I have selected only the knots on the base of the pillars (1,252 and 503) and I have a result y due to the weight force of the structure and a small mx bending moment.
ps I have attached photos

 

[Stan9411]

@onda I never offend anyone.. I just don’t like to talk like a printed book and occasionally I give the phrases a little more colorful.

Domenico said he had blocked all the rotations in the trave-arcareccio intersection points, which I suppose are the purple-blue contact points. for this reason I deduced that the moment different from zero that you are talking about was measured at that point, not on the ground. If those contacts are hinges you don't have to have reagent moments.

I assumed that on the ground a point must be blocked by force in rotations, otherwise it becomes all a ride ...

(Is it okay or even here I've given myself a license too much? )

 

[Domenico Garofalo]

si @stan9411 are the purple-blue contact points.
thanks guys for the explanations.