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Panagiotis Bu

HELP WITH FINITE ELEMENTS ANALYSIS / STRUCTURAL ANALYSIS

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Hello engineer's community !!

I got a problem. If i got a truss frame with 2 bars and an initial nodal displacement for the common node , then how can i calculate the velocity of the oscilation as function of time ? (the 2 bars have different length , same elasticity modulus , same density and different angle with the horizontal line and they are connected to a common node as i said :))

I would be glad if you help me ! 

 

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This looks much like a class of problems I've been studying recently. There are several questions that need to be answered, I think.

1) Are both supports fixed (immovable)?

2) Do you have numerical values for all the data? (This is a very tough problem to address entirely in symbols!)

3) When you speak of "initial nodal displacement for the common node" I presume you mean an initial displacement for the connecting joint; is that correct? You have both magnitude and direction for this displacement?

4) What do you mean when you speak of "velocity of the oscilation as function of time "? Are you looking for an oscillation frequency?

5) How many degrees of freedom do you need in your model? Is bending to be included, or only axial deformation?

Please fill in these gaps, and perhaps we can help you.

DrD

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Thanks for the response , i'm going to answer all the above :

1) Supports are fixed , the bars can only rotate .

2)I dont have numerical values of the data , i try to solve it using variables and create a general formula

3)Yes , the connecting joint has an initial displacement . I dont have any other data about magnitude or direction , i only know that i have displaced the connecting joint and later i let it move freely ( from zero velocity) and an oscillation takes place ( on x , y axis) due to elasticity .

4)If "U" is the horizontal velocity and "V" the vertical velocity of the connecting joint then i want to find how these 2 velocities change during the oscillation/time . I want to find the formulas for U(t),V(t) (t=time) . But yes oscillation frequency is needed for these equations .

5)I consider only axial deformation , and the maximun degrees of freedom for each node should be 2 ( moving vertically , moving horizontally)

 

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I don't know how difficult this problem is with FEA because I don't do FEA (I have no access to an FEA program). Let me suggest that you first try to work the simple statics problem where a load with components (Fx, Fy) is applied to the joint and then you need to compute the displacements of the joint. From this you can compute a stiffness, and you will be on your way to getting the natural frequencies.

If you run into trouble you my consider my help offer in a recent post on my blog.

DrD

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