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Stability of frame
2

thread194-470042

Stability of frame

Stability of frame

(OP)
Hello everyone, I have a question regarding the stability of a frame:

I may be wrong but from what I have learned to check for stability given a determinant system you must control two things:
1) Not all reactions forces are parallel or concurrent. (This frame satisfies this rule)
2) Rigid bodies are connected by 3 non-parallel and non-concurrent constraints. (I am not sure about this, from what I see the frame is composed of two rigid bodies connected by a hinge and the hinge provides 2 constraints (one in x and one in y direction)

So I would conclude this frame is determinant and NOT STABLE. Im I wrong?

Thank you very much I am attaching a picture for your review

RE: Stability of frame

What about roller support? If the roller support provides only compression resistance , it is unstable...

RE: Stability of frame

(OP)
The roller support is providing a horizontal reaction force as shown that's it

RE: Stability of frame

As per your assumption 2, if the elements are rigid , yes..still stable.

RE: Stability of frame

The horizontal member with gravity load will fall from the roller, which does not provide vertical support. The falling member will cause rotation about the hinge, which has no rotational restraint capacity. So, is it stable?

RE: Stability of frame

(OP)
Thank you for your reply but the two rigid bodies are only connected by two non parallel constraints (ie. the hinge) where is the third one? For a stable system the two rigid bodies should be connected by three constraints

RE: Stability of frame

You need to replace the vertical roller to a pin, or horizontal roller, then it is stable. Note the former is a statically indeterminate system.

RE: Stability of frame


Below find a solution with selected loading as per ur sketch..

RE: Stability of frame

(OP)
Thank you very much for your input, appreciate it.

RE: Stability of frame

I've modified my last response to include the condition below.

RE: Stability of frame

Quote (retired13 (Civil/Environmental))


I've modified my last response to include the condition below.


Dear retired13, the deflected shape is not applicable since both elements are rigid as per assumption 2.

RE: Stability of frame

The tip of L fame will deflect an amount due to gravity load, which will initiate the rotation about the hinge. The rigid bent won't help, as it is standing on a support that is free to rotate. The only chance for this system to survive is the horizontal load is huge, that push the horizontal member tightly against the wall and produces shear friction against sliding. But, the shear friction is usually ignored, as it is unreliable.

Interestingly I see another problem associated to my suggestion to make horizontal roller support - if the horizontal load reverses direction, then again, it is not stable. So, I'll stick to my original suggestion, the roller shall be replaced by a pin. Sorry for the confusion.

Mechanically speaking, the system can stand still with favorable conditions, but it's so called "limiting equilibrium", that is a neither-nor phenomenon - a marble stay on top of a dome.

RE: Stability of frame

(OP)
Thank you very much for your replies

RE: Stability of frame

You are welcome. This is a seemly simple, but really tricky problem. When you do design, always keep stability in mind, be conservative.

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