7
$\begingroup$

I am trying to recreate the following model of a wooden roof truss provided to me by a truss manufacturer: enter image description here

My question is twofold:

  1. What is the common/correct way to model the boundary conditions and internal fixity conditions (pin pin?) for the elements as well as the structure itself?

  2. Given generic roof loading in pounds per square foot, how should I distribute the loads to the truss? It seems to me that applying distributed live and dead loads for the top and bottom chords on truss elements poses a stability problem, as it was under my impression "idealized" trusses can only resist axial loading and thus loads can only be applied at the nodes. However this is a real truss, so I expect things may be different.

$\endgroup$
6
$\begingroup$

The most common way to model this structure is as follows (ignore the fact that the proportions are a bit off):

enter image description here

So, all of the diagonals are pinned-pinned. You'll notice in the schematic, however, that the chords (including the diagonals from the supports to the top chord) are not segmented at every point of intersection with the diagonals. Indeed, the chords are probably as long as possible, presenting splices at points E, J and M simply due to constructability demands (usually defined by the size of a standard truck). These splices between beams composing the chords, however, are usually designed so as to behave as a fixed joint, which is why I didn't put pins anywhere along the chords themselves.

Regarding the loads, you can just apply them as distributed loads. Since the individual member spans are quite small, almost no bending will occur and the vast majority of the internal forces will be axial regardless. Some analysis programs do allow you to apply distributed loads but set that they should be transformed into nodal forces, but there's really no need. And hell, the real truss will have that tiny bending, so you're more than welcome to include it in your analysis if you want.

The exception of course are the chords. Since they are not pinned at every joint, they will suffer significant bending, given that the diagonals will "deposit" their axial loads as concentrated nodal loads along the chords' effective spans.

And finally note that obviously the supports are different: one allows for horizontal displacement while the other doesn't.

$\endgroup$
  • 3
    $\begingroup$ I agree, except I'd model the splice in the top chord at E and the splices in the bottom chord at M and K as pinned as well. I'd actually probably model it both the way you show it and as I described and then use the enveloping design forces/reactions. $\endgroup$ – William S. Godfrey- S.E. Mar 17 '16 at 15:37
  • $\begingroup$ What software did you use to draw the above diagram? Can you recommend software for truss analysis? $\endgroup$ – PProteus Aug 2 '17 at 6:48
  • 1
    $\begingroup$ @PProteus, the diagram was created with Ftool, a 2D frame analysis program. $\endgroup$ – Wasabi Aug 2 '17 at 14:56
1
$\begingroup$

It's been a while since I thought about this, so I hope I'm close.

  1. What? 1 fixed in each axis, I think?
    For calculating the truss, you have to treat the pins as.. pins... I believe.
    If you're concerned about any joints, then you can try to do a stress analysis on each joint, I guess.

  2. Distribute the load to the nodes as you normally would for a beam and do the calculation for the truss. Then do the beam analysis for the point load or whatever load you are distributing for analyzing the affected beam.

Sorry I can't comment yet.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.