29 Jan 04 7:16
I did one similar a few years back. Hopefully, I did it right!
If your top and bottom chord are the same, it's a bit easier. Unfortunately, mine were not. You know, for all intensive purposes, the deflection of the top chord is the same as the deflection of the bottom chord. Therefore, your loads will be distributed rationally. If the two chords are the same, half the load goes into the top chord and half into the bottom. If not, it's ratioed by their moment of inertias. For the proportion in the top chord, Itop/(Itop + Ibott.)* the uniform load and, obviously, the bottom chord is taking the remaining load.
I calculated the maximum bending moment and shear in the top chord by placing the total uniform load across it, with vertical point loads acting upward at the uprights. The vertical point loads are the tributary width between the uprights * the load for which the bottom chord is responsible. I aslo calculated the maximum bending moment and shear in the bottom chord by placing the vertical point loads down on it. Check both chords for shear capacity. If the point loads are high, which they probably will be, you'll need to check for crippling of the webs of the chords.
I then calculated what the total moment of the truss would be which, for me, was just (Uniform load * Length^2)/8. Then I took this number and divided by the distance between the neutral axis of the top and bottom chord to get my axial force in the members (F=M/D). Once you have the axial forces check the top and bottom chord for combined axial and flexure.
Check the verticals for axial capacity.
FYI the top chord of the one I analyzed was overstressed by about 5%, so I just added additional bracing.
Hope this helps,