Trussel buckling,question from mechanics of materials

[berdan]

Ok,first of all ,I'm sorry for any further language barrier things,I study in a non-English speaking university,and I might do a mistake in translating some technical terminology.
So,the problem is from mechanics of materials,buckling loads.This subject gives me nightmares as it is,I can barely understand what it is all about.

Homework Statement

http://imageshack.us/photo/my-images/6/krisam.jpg

http://imageshack.us/photo/my-images/6/krisam.jpg

That is the problem basically.Trussel made of 3 beams,two rectangulars,one is round.
First question is what is the minimal force to buckling.
Second is,if we reverse the force (now it acts up instead of down),what is the minimal length of parameter a so that we can use Oiler formula.


My attempt for solution : Non,I don't really know what they want from my life.Mostly,why they don't check if we can use Oiler formula in the first question,and why we have to check in the second?

Thanks in advance.

 

[berdan]

Someone?I'm dying here...

 

[mathmate]

As a starter, I suggest you analyze the structure to find,

step 1: for each member of the truss, calculate maximum force without buckling
- magnitude of the force
- magnitude of the stress
- direction of the force (compression or tension)
- the maximum value of F that will keep the stress of every member below $\rho$y.

step 2: for the compression members,
- calculate the maximum value of F that will not cause Euler buckling.

step 3: if you reverse the force, then the direction of forces and stresses will be reversed.
There will be only one compression member which is longer than the previous case, and it will be relevant to re-calculate the Euler buckling load in terms of F and a.

 

[nvn]

Regarding question 1 (and 2), there are currently infinite solutions. Dimensions a and b are currently not defined; therefore, you can make the truss buckle at any value of applied force F, depending on the values of a and b you choose. Please let me know if I am missing something. Did you omit any information?