Truss - compression in top boom not matching tension in bottom boom! 1

[Naggud]

Hi all,

I would appreciate your help in explaining to me why the tension in the bottom chord is not the same value as the compression in the top chord?

See attached for a screen grab of the model (disregard that the load values don't match on both sides (the program picks out the forces in braces in some places, the boom/verticals almost at random on the screen shot). The values do actually match on both sides!

I should not that the support conditions on the bottom cord provide support vertically only - the members can role horizontally.

I look forward to hearing from you, thank you.

 

[dik]

Nothing attached.

Dik

 

[Naggud]

Thanks Dik, hope it works now!

 

[dhengr]

Naggud:
The bottom end supports are not particularly effective on this truss, since there is no vert. chord member. Have you checked the sum of the moments and forces (M, H & V) at the nodes, and do they check properly? At each vert. chord member, note that the diag. chords come in from opposite sides and their magnitudes are also different at the top or bottom nodes (joints), thus the top and bot. chords could not have the same value for the node forces to sum properly. The assumption that the top and bot. chord forces are about the same is usually pretty good first approx. at the center of the truss and are a reasonable gross approx. indicator of the/a moment cap’y. of the truss.

 

[Triangled]

Two tension elements at node 2 is my first clue
I'd suggest
a) decide whether you have a bottom chord bearing truss or a top chord bearing truss
b) revise model according to (a)
c) looks like you have fixed end conditions at both top chord end supports. revise to one end pinned and one end roller

 

[JoshPlumSE]

c) looks like you have fixed end conditions at both top chord end supports. revise to one end pinned and one end roller

I completely agree. That's why the top chord is going into compression near the supports.

 

[racookpe1978]

If the neutral axis is not equidistance between his two chords, the top and bottom force would not necessarily be the same, right?

 

[oldestguy]

Wow, a simple problem for freshman students. Make a cut down through any bay and show all forces as sums of vertical and also horizontal.

 

[Naggud]

Thanks for your responses.

I figured out what was up with the model, it was a simple fix - I needed to make one of the pinned supports a roller so as not to induce unwanted horizontal forces in the booms. I've attached the updated screen grab of the axial force diagram for anyone who would like to see it!

Again thank you to those who tried to help.

 

[Naggud]

.

 

[JoshPlumSE]

Yeah, I think Oldestguy may have been a little harsh. This is a simple problem from a statics point of view. And, can be easily solved with a hand calc. So, I understand where he's coming from.

However, in this case the OP did understand that the results were not correct. So, he did exhibit a greater understanding of statics than a mere student. He just didn't understand what he had done wrong in his model. Therefore, to me at least, the question was more of a structural modeling question.

Then again, I'm always more forgiving of this sort of thing than others.... I've seen a lot of really smart SE's and PhD's make similar silly modeling mistakes over the years.

 

[Naggud]

JoshPlum, absolutely agree. Very simple to calculate by hand.

I modeled it because it is a long truss and the loads in it are quite high. I wanted to get the specific loads in every node quickly so that individual connections could be rationalized. The problem was related to modelling.

 

[rb1957]

why bars 61 and 71 ?

your picture load internal reactions confuses me. "yellow is tension" but it seems you've given the lower chord compression loads ? from the colours I'd expect to see an down load on the truss (tension in the lower chord), then up reactions at the grounds, tension in the first diagonal, compression in the upper chord ??? so maybe the (axial) constraints are messing with the upper chord loads ?

I agree with Josh ... marks for realising you have a problem with your model. Over-constraining models is an endemic problem, often unnoticed, often labelled "fake news" when you try to tell people the error of their ways.

I also agree with Oldestguy (being possibly an Olderguy here) in that seeing the model misbehave you should have been able to solve the simple truss problem and then tried to understand why the FEM differs from your hand calc (and FBD). Then the question here is possibly "why the difference?" showing the two solutions if you didn't get to the problem (of over-constraint). btw, I think your biggest hint of over-constraint is redundant (self-cancelling) reactions.

another day in paradise, or is paradise one day closer ?