(Load manipulation) Advice/Steer request!

[Studentforlife]

Hello all,

I hope that all my fellow engineers are keeping well and safe. I was looking to get some advice from well-seasoned aircraft stress engineers on this forum.

I have recently come across a problem where I have loads defined for points 1,2 and combined moments at the location where the structure is fixed (points 3,4). I hope you can see the picture which I have tried to attach. I am now assessing Section a-a of the structure for stability and strength (by calculating section properties: moment of inertia, neutral axis etc.).

Would it be acceptable or conservative if I took a 60/40 load and moment split to obtain the loads at Section A-A? So the loading will look something like (Fx1+Fx2)x0.6, (Fy1+Fy2)*0.6 ... Mx x (0.6), My x(0.6) and so on?

Or

Do you suggest I resolve loads from Points 1 and Point 2 onto the neutral axis and obtain loads of Section a-a that way? Please keep in mind that the structure is fixed at points 3,4.

What are your thoughts? Looking forward to hearing from you.

Many Thanks
Regards,
Sam

 

[r13]

Are the forces acting on the solid plate/medium that is fixed to support points 3 & 4? What is the structural rigidity of the plate/medium?

 

[Studentforlife]

Many thanks for your response.

No forces acting at points 3 and 4 (it's fixed at these locations so will only see reaction loads).

The structure is metal (aluminum) to be more precise, so will have high structural rigidity.

 

[r13]

Then, I think you should resolve the forces according to rigid body with fixed edge supports to obtain reactions at points 3 & 4 respectively.

 

[BAretired]

What are the co-ordinates of points 1, 2, 3 and 4? You need to know that before anything can be determined. Even then, the problem is indeterminate because supports 3 and 4 are fixed and the stiffness of the structure is unknown.
Edit: Remove one of the supports and the structure is easily solved.
BA

 

[Studentforlife]

The coordinates of the points and all the necessary mechanical properties for material are known.

Thanks

 

[BAretired]

To you maybe, but not to us. The 60/40 rule is not valid. It could be conservative for some reactions and unconservative for others.

BA

 

[Studentforlife]

I know what you mean! Thanks for your advice. I will have to revisit the boundary conditions, as the points 3,4 are actually bolted onto a structure which is free to move in the z direction. Sorry if my response sounds vague or unclear as the requirements for this test aren't clearly defined, so I'm having to make some assumptions.

 

[r13]

If the rigid plate is loaded forces in all three dimensions, you would need more than two support points to be stable. I think engineering mechanics is the best tool to solve this problem. The next will be a computer program, if rigidity of the plate is questionable, such as a large, but relatively thin metal plate.

 

[Studentforlife]

Thanks for all your responses as it gives me a good steer on how to approach this problem!