How to get reactions from given structure

[prelude1234]

Can anyone tell me how to find the reactions of this structure? If not by summing forces and moments, then how? I'm thinking it's indeterminate and assumptions have to be made.

 

[berkeman]

Can anyone tell me how to find the reactions of this structure? If not by summing forces and moments, then how? I'm thinking it's indeterminate and assumptions have to be made.

Welcome to the PF.

What is the context of the question? Is it for school?

 

[prelude1234]

No, this is a problem I'm having at work (aerospace) that I can't figure out.

 

[nvn]

prelude1234: Your structure is currently unstable, because it has no supports (also called constraints). A static structure must be constrained to its surroundings. In your diagram, please show the http://upload.wikimedia.org/wikipedia/en/c/cb/Simple_cantilevers_%28comparison%29.svg" (supports, boundary conditions) for your structure.

 

[prelude1234]

New image attached.

 

[nvn]

prelude1234: It currently seems your structure is pinned to ground at every joint, although we are not 100 % sure, because you did not use the standard (clear) constraint http://upload.wikimedia.org/wikipedia/en/c/cb/Simple_cantilevers_%28comparison%29.svg" for connections pinned to ground. Also, one normally labels joints with capital letters, so they can be identified. And dimensions are usually necessary.

If all of your joints are pinned to ground, then you have four independent structures; and I think each structure is statically indeterminate, unless you assume one of the two pins on each member is a roller (?).

 

[prelude1234]

For now i think I've figured it out with a FEM software.
I think i needed to remove the two lower pinned connections.
will try and verify through hand calcs. Thanks for the help...

 

[Shockman]

Hi
If Joint 1 and 3 not rigid, then the structure is not overconstrained so you do not need to remove pins.

 

[Studiot]

Hello prelude, what company do you work for?

 

[prelude1234]

I am a contract Design Engineer with a major aerospace company. Why do you ask?

 

[Studiot]

Are other details in this company to the same standard?

I am bearing in mind that the application has potential public safety (airworthyness) involved.

You have stated that the structure is both rigid and pinned. This is incompatible.

nvn asked for a proper description of the problem, I do not see this in your second post.

Please re-present it using appropriate mechanics terms; it is impossible to determine anything from what you have presented so far.

 

[AlephZero]

Are other details in this company to the same standard?

Why do you assume it is a "detail"? For all you know, it might be the first sketch of a conceptual idea.

I am bearing in mind that the application has potential public safety (airworthyness) involved.

Aerospace companies design lots of items that have nothing to do airworthiness, or public safety.

You have stated that the structure is both rigid and pinned. This is incompatible.

There is nothing incompatible in having an (effectively) rigid structure pinned to something else (which may or may not be rigid). There may be good reasons for having an apparently redundant arrangement of pins - for example to deal with failure scenarios.

The way I interpreted the OP was that this was a "rigid block" connected to something else with several pins, with a load applied at the CG of the block. The lines were just to define the geometric location of the pins. (But hey, what do I know, I just work in the aerospace industry...)

But given the later posts, I'm less sure about what it really means.

 

[prelude1234]

The 3 rods are also pinned to the plate as shown and this is what it looks like in the image.

 

[Studiot]

Alephzero, the floor ( or roof? ) is all yours.

 

[Shockman]

Hi
This (your last drawing) is exactly the system like your drawing in the FEM system. No overconstrains.
First you calculate the forces in rod 1-2 and rod 1-3 (take joint 3 as pinned and remove rod 3-4 and 3-5.), so you get the forces in joint 3.
Secound you can calulate forces in rod 3-4 and 3-5 . (Remove rod 1-2 and rod 1-3, joint 3 is now not pinned)

If you are unsure, i can drop for you the detailed calculations.

 

[prelude1234]

Thanks for the replies. I finally figured out the reactions by hand and simplifying the structure further, i.e. a simple dual triangle truss.