university training aid

[teseo.guerra]

hello to all I place a problem as from title.
I ask that I have decided to enter my doubts and my questions in the general directory on fem systems as such questions do not depend on the software used.
the text of this exercise is quoted:
"the plate represented in the figure (attached) is bolted at the lower hole and is loaded at the upper hole, by means of a vertical force facing down and at 5000n.
the thickness of the plate is determined, in order to guarantee a minimum safety coefficient of the structure pairs to 2, regarding the yielding, knowing that the material is a construction steel with yielding voltage of 350 mpa"and now put here below my questions:

1)The constraints to be inserted on my plate of course are located where there is the bolt now I would like to know according to you that this bolt prevents only the translations along the axis the x and the axis y or also the rotation around the axis z?(I assume that I will use a model 2d therefore already to be excluded are the long translations z and rotations around the axis x and y)

2)the method of analysis to find such thickness that I thought is as follows:I construct a model with a plate having a standard thickness of 1mm and determine my voltage for the calculation of the safety coefficient and according to the value obtained of this first safety coefficient (see how far from 2)I make a proportion of the type

first safety coefficient: unit thickness = safety coefficient sought:x (incognita).

I then carry out with the thickness value obtained a new fem analysis to verify my data obtained
in practice to obtain at least the order of the thickness then to blur a bit of numbers carried out before the analysis with unitary thickness and then as described above

 

[cacciatorino]

but the formulation of the problem expressly requires the use of the fem? How it says it looks like the classic problem to solve with beam theory.

 

[teseo.guerra]

the use of the fem is required because the course I am following is incensed exclusively on the use of the fem

 

[cacciatorino]

I'd do that to your place.

1) determination of the necessary dimensions with the tory of the beam

2) verification of the congruence between theoretical results with those deriving from the fem analysis on a model built with the dimensions resulting from point 1.

 

[teseo.guerra]

The problem is that I can't solve the problem with my knowledge, I explain, I can only solve it with the monodimensional analysis and I just don't know how to act on the 2d from the theoretical point of view, can I try, however how about my method? and especially regarding the speech of constraints what do you think?

 

[cacciatorino]

The problem is that I can't solve the problem with my knowledge, I explain, I can only solve it with the monodimensional analysis and I just don't know how to act on the 2d from the theoretical point of view, can I try, however how about my method? and especially regarding the speech of constraints what do you think?

Then, the struttra as it is proposed is labile, in the sense that under the action of force on the upper eyelet, it would tend to rotate attonro to the lower screw. unless the bolt clamping force is such that friction is capable of counteracting rotation. But this is something you can't assess, because you don't know how the screw is and what's behind the plate... .

according to question: the single-dimensional struttra is fine. Once the constraints are cleared, it is very simple to trace the diagrams of the moment and the cut. the section of the beam, once you have imposed a thickness, you have so it is trivial to find the moment of inertia and the resulting sigma.

 

[teseo.guerra]

the problem is precisely this structure if the structure is labile I cannot use the fem theory as such theory is not valid for the labile structures or is it wrong?

 

[cacciatorino]

the problem is precisely this structure if the structure is labile I cannot use the fem theory as such theory is not valid for the labile structures or is it wrong?

no, you can use it but:

you have to consider great deformations (the classical fem theory only applies to small ones).

However if the structure is labile, you can feed it to your fem when it has reached a stable configuration, i.e. cancellation of the moment of force imposed by the load: The force will rotate your team until its action line passes through the center of the bolt. At that point the structure is no longer labile and you can easily calculate it by hand and then verify it with the fem.

 

[Onda]

hello to all I place a problem as from title.
I ask that I have decided to enter my doubts and my questions in the general directory on fem systems as such questions do not depend on the software used.
the text of this exercise is quoted:

"the plate represented in the figure (attached) is bolted at the lower hole and is loaded at the upper hole, by means of a vertical force facing down and at 5000n.
the thickness of the plate is determined, in order to guarantee a minimum safety coefficient of the structure pairs to 2, regarding the yielding, knowing that the material is a construction steel with yielding voltage of 350 mpa"

and now put here below my questions:

1)The constraints to be inserted on my plate of course are located where there is the bolt now I would like to know according to you that this bolt prevents only the translations along the axis the x and the axis y or also the rotation around the axis z?(I assume that I will use a model 2d therefore already to be excluded are the long translations z and rotations around the axis x and y)

2)the method of analysis to find such thickness that I thought is as follows:I construct a model with a plate having a standard thickness of 1mm and determine my voltage for the calculation of the safety coefficient and according to the value obtained of this first safety coefficient (see how far from 2)I make a proportion of the type

first safety coefficient: unit thickness = safety coefficient sought:x (incognita).

I then carry out with the thickness value obtained a new fem analysis to verify my data obtained
in practice to obtain at least the order of the thickness then to blur a bit of numbers carried out before the analysis with unitary thickness and then as described above

You should also tell us that fem uses.

provided that the model makes no sense because it does not bind to rotation on a single bolt, you must:
model with shell model.
binding bolt knots on all 6 degrees of freedom (fem, unless they are only 2d, always behave in 3d)
bind all the other knots of the model on z and on rx,ry (the constraints to stay on the plane) and do the analysis on the plane.
solve with unit thickness of the shell, calculate the increase of thickness as you rightly wrote, change the thickness and turn again to see if everything is correct.

Keep in mind that it is a pure university exercise as a bolt cannot hold the rotation according to its axis!

Hi.

wave

 

[teseo.guerra]

first of all thank you very much for the celerity in the answers, then the software fem that I use is straus7.
Obviously mine is a pure university exercise I do not question the feasibility of this and of course I did not explain well but I know that the elements 2d (shell and plate) behave like three-dimensional.
moral of the fable from how much you tell me the plate of my exercise besides being bound to stand in the x-y plan cannot on this plane neither translating, nor rotate around z?

 

[Onda]

for a fem to function should not be labile, unless you use a nonlinear analysis and large movements and be sure that it arrives at a stable balance condition.
I suppose your professor didn't want to give you an exercise like this, so to solve it, you must also be bound by rotation.
the correct method would be to put a knot in the middle of the hole and create a rigid element that connects to the edges of the hole(rbe2 in nastran) from vicular 1->6.
I don't know how to do strauss.
if you bind the shell knots, you can avoid binding the rotz as by binding the contour knots on x and y already eliminate rotation in z. Moreover the shell elements normally do not like rotation constraints according to normal to the element.
nastran shells for example do not have the degree of rotation freedom according to the z and you have to put a fictitious stiffness, so you avoid binding a rotational shell knot along the normal to the element.
wave