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For static gravity loads on a steel truss, I would normally select the
truss configuration that puts the diagonals in tension. This would be a
Pratt Truss. If you have moving loads (such as a bridge) or reverse
loads (such as wind uplift) the diagonals will sometimes be in tension
and sometimes be in compression. Under those conditions, a Warren Truss
is more common. A Scissors Truss is normally used when the ceiling and
roof need to be pitched. It creates an outward thrust on the walls. For
a  high  pitched roof, a Fink Truss might be more appropriate.

I would think that you would want to design for some live load to
account for a hard rain accummlating on the roof before it has time to
drain, workmen on the roof or the occasional hailstorm. I believe in
different parts of the US, they require a minimum live load of 12 psf or
20 psf depending on how much tibutory area the member is supporting.

I hope this helps!

Jim Kestner

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