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H
Hongjie1-Visitor
1-Visitor
August 6, 2014
Question

Calculate internal force in truss

  • August 6, 2014
  • 1 reply
  • 5352 views

Hi I am a fresman in Pro e Mechanisem and Simulate.

I try to simulate enclosed question in Mechanical of Material book, I create a assebly by mechanism, however I really do not know to set it in Simulate. for the beam do not has cross area and masss and do no think about strain now. (sorry i canot upload model which is blocked by office IT system)

In the end I would like to know if ProE can simulte / solve the kind questions which is normally in mechanical engineering book which is not real world situation. thank you.

PS: I use Creo 2.0

Best regards, Hongjie


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1 reply

S
sdensberger2-Explorer
2-Explorer
August 7, 2014

I'm a little confused about what issue(s) you're having in Simulate due to the language barrier. However, this problem can easily be solved in Simulate using spring elements, measures, a point mass, a gravity load, and the proper constraints. With these you'll be able to calculate both the axial force and the axial stress. Getting the sign of the force is a little tricky, but still totally do-able.

H
Hongjie1-VisitorAuthor
1-Visitor
August 7, 2014

Hi Shaun, thank you for your attention.

let explain my question:

there are Mechanism Design Extension (MDX), Mechanism Design Option (MDO) and Mechanica Simulate tools in Creo 2.0, I am learnin them through PTC online trainin course. all the training course usin solide model to simulate practical situation by setting all you mentioned parameters like points mass, constraints and so on.

now I want to represent/ solve questions in my colleague book ( mechanical of material), which can help more deep understand the Creo function.

To sovle above quesiton, I use curve( sketch straight line) with axis to preprent beam/bar and assembly them by pin and slide in Mechanism. however I do not know to add point force in Mechanica Simulate which ask me to set cross area and all material information which like poisson's ratio. the question is very basic in colleague, do not has clearly description of material and detial design.

In brief could you please create a assembly model with right Simulate setting and post here, I can learn from your example, i really apprecate any your instruction.

Best regards, Hongjie

S
sdensberger2-Explorer
2-Explorer
August 7, 2014

There are two analysis tools in Creo: Mechanism and Simulate (formly called Mechanica), and there are core fundemtanial differences between the two.

Mechanism: This tool is used to solve for Rigid Body Dynamics type problems. All parts are rigid (i.e. they cannot deform) and you're solving for quanities such as force, acceleration, velocity, and displacement, much like you would with Lagrangian Mechanics.

Simulate (Mechanica): This is a Finite Element Analysis (FEA) tool, which is used to solve for continuum mechanics problems (e.g. stress analysis). FEA is a very deep field, but typically most problems will one part A and one part B, where:

A: Linear or Non-Linear

B: Static or Dynamic

Simulate can do both linear and non-linear static problems, as well as linear dynamic problems. FEA allows you to solve for quanities such as force, acceleration, velocity, displacement, strain, and stress. Simulate (FEA) is by far the more complex tool between the two.

It's good that you're going through the online resources that PTC offers, but I also suggest that you spend some time reading up on the very basic aspects of FEA (nodes, elements, mesh, h-method, p-method, convergence, stiffness matrix, etc.) to give you a somewhat good foundation. As I said before, FEA is a very complex field and there are tons of pitfalls where the user can make a mistake.

Now, with regards to two problem that you've posted, there are a number of ways to solve this. I suggested spring elements because you can define both the axial and torsional stiffness values for them. Truss systems can, by definition, only carry axial loads (whereas beams can carry axial and moment loads), so it's rather easy to calculate the stiffness value of each truss element and entire this value in the definition of each spring element. For a truss element, the axial stiffness is (AE)/L where:

A = Cross-sectional Area

E = Young's Modulus

L = Length of truss element

Now, the problem you posted doesn't specify a Young's Modulus, but the equation I gave has E in it...so now what?! Simple, we just make up a value. This is because we don't need to solve for the strain in the system, but rather the force and stress, which are completely independent of the Young's Modulus. The high we decide E to be, the lower the peak displacements will be, but the stress and force in the truss system will remain constant.

I created for this problem, but I can't seem to attach it via the form; if you want a copy of it then email me (shaun.densberger@sanmina.com). I've defined parameters (Tool -> Parameters) for the lengths given, the mass of m, the cross-sectional area, the Young's Modulus I made up (I used steel at 200 GPa), and then defined some relations (Tools -> Relations) to calculate the lengths of the diagonal truss members. I then created spring elements with an axial stifness of AE/L where L is the length of the respective truss element. I created some measures (Home -> Measures) to calculate the magnitude of the force in each element and then some Compute Measures to calculate the stress (F/A). Note that both the force and stress values are all positive; this is because the force measure calculates the magnitude. To get the correct sign will require additional measures (which I'll leave for you).

After running the analysis you'll see results for each measure in the Display Study Status (Home -> Analyses and Studies) under Measures.

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