Truss - ned someone to look through my calculations.

[kaffekjele]

Homework Statement

Original figure is here: http://tinypic.com/r/svrm39/6. The task is to calculate forces in N1, N2 and N5 with the correct sign.(+ or -)

The Attempt at a Solution

Figures and calculations attached. I fear it might be messy - especially when it comes to use of angles, but I'm hoping at least some of it is correct.

 

[voko]

I do not see how you account for the reaction force of the wall at A and C.

Note you can determine these forces by assuming the truss is one rigid body in equilibrium with the wall.

 

[kaffekjele]

Yes, I was thinking about that.But if I did add x and y force in C and D I assume the correct thing to do would be to start by looking at the truss as a whole and do moment and force calculations around one of the joints? Should I start in A or C?

 

[voko]

You can choose any point, including A and C, about which to compute moments. It does not matter which one to choose. I would suggest D, because the sum of the moments of active forces about D is zero - can you see why?

 

[pongo38]

I am not disagreeing with voko but in this case (a cantlilever truss) you can start the analysis at any joint such as E where there are not more than two unknowns. Then D, B, A, C in that order. It is then worth checking the result by looking at overall equilibrium. kaffekjele, these problems are all self-checking. You don't need us to do it for you.

 

[kaffekjele]

kaffekjele, these problems are all self-checking.

Perhaps a stupid question, but is that based on the fact that sum of forces and moment should always be 0? (I'm doing physics for the first time in 14 years, so I fear a lot of what I learned in high school might have been forgotten.)

 

[voko]

I am not disagreeing with voko but in this case (a cantlilever truss) you can start the analysis at any joint such as E where there are not more than two unknowns.

Indeed. The power of this approach is that you never have to compute any moments at all, you just solve a 2x2 system for each joint. So you end up solving 4 2x2 systems.

Yet, in the approach I suggested, one first solves a 3x3 system, then the reactions sought are obtained by a simple projection, so it may be quicker for this particular problem.

I think it will be instructive for kaffekjele to solve the problem using both methods.

 

[kaffekjele]

I would suggest D, because the sum of the moments of active forces about D is zero - can you see why?

The two F/2 cancel each other out regardless of whether you do the moment calculations clockwise or counter clockwise?

 

[voko]

The two F/2 cancel each other out regardless of whether you do the moment calculations clockwise or counter clockwise?

Correct!

 

[pongo38]

quote "is that based on the fact that sum of forces and moment should always be 0?"
Yes. Introductory courses generally only use statically determinate examples, and there is always a spare equation to use for checking, whichever way you chose to do it. The ability to check is useful in exams to find silly mistakes, and it is useful in practical cases where there is no 'answer in the book'.