Displacement in external redundant truss

[fonseh]

Homework Statement

In this question , the author doesn't stated which direction ( either horozintal support of the pinned E or the vertical support force at pinned E to be selected as redundant ) . So , I tried this problem by applying the vertical force support at E as redundant .

Homework Equations

The Attempt at a Solution

I have done the step 1 , which is to find all the member force in the truss . ( The working is not shown here) .

However , I am having problem in part 2 , which is to find the member force by applying 1 unit load (virtual foce) ... By doing so , all the external force is removed and the new member forces are to be found .

I used ALTERNATIVE method to find the forces in the member . However , i can't balance the forces at joint C . What's wrong with my working ?

 

[PhanthomJay]

You cannot take the vertical force at E as the redundant force, because then the truss becomes unstable, since there must be a vertical force at E to balance moments about A.

 

[fonseh]

You cannot take the vertical force at E as the redundant force, because then the truss becomes unstable, since there must be a vertical force at E to balance moments about A.

So , the only way is to take the horizontal support force at E as redundant ?

 

[PhanthomJay]

So , the only way is to take the horizontal support force at E as redundant ?

That is correct. It is also worthy to note that the loading and truss geometry are both very symmetrical, so one would expect equal vertical reactions at each support, and equal but opposite horizontal reactions, if non zero, at the supports.

 

[fonseh]

That is correct. It is also worthy to note that the loading and truss geometry are both very symmetrical, so one would expect equal vertical reactions at each support, and equal but opposite horizontal reactions, if non zero, at the supports.

Thanks for your effort . Btw , can you help me with this question ?
https://www.physicsforums.com/threads/moments-sign-convention-in-beam.910959/