roof truss weight assumption

huangyhg

roof truss weight assumption
hello! does anyone knew how to assume the weight of roof truss-for design purpose-in terms of psf unit or i any manner to assume it? i ask this so i could go on with my building analysis and design, and so i wouldnt wait for the reactions from truss analysis.
thanks in advance..
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need more information.  what type of truss (wood, steel, etc.)?  what is the span and loading?
sorry i forgot. steel truss with a typical dead load and live load. i think the commonly loadings for roof.
thank for speedy reply taro.
if you know what the truss profile looks like assume a p.s.f. dead load.  apply that dead load and the live load to the truss profile to estimate the top and bottom chord size.  use that weight to determine a pound/ft truss weight.  
add in an estimate of the web weights and any additional elements.  since you are designing the support structure you can probably be conservative in your dead load.
you could also call a local steel fabricator that does that type of work to see if they have some truss weight estimates based on the type of truss profile.
roughly estimate your chord forces by taking the tributary dead and live loads in klf of truss and use wl^2/8 divided by 0.85 x truss depth. then estimate the top chord size by assuming the unbraced length. estimate bottom chord in a similar fashion. estimate end reaction and component force in end diagonal and then estimate size. add up total estimated truss weight.
another way is to use vulcraft joist girder tables to estimate truss weight.
good luck!
there are equations in some of the old text books that estimate truss weight, some simple, some pretty complex.  for example:
weight of timber roof trusses:
        w = 0.5sl + 0.075sl^2         h.s. jacoby
weight of steel roof trusses:
        w = 0.4sl + 0.04sl^2          c.e. fowler
where: w = approx. weight of the truss in pounds
       s = spacing of the trusses in feet
       l = the span of the truss in feet
ref:  kidder-parker architect's and builder's handbook, 18th edition, 1936
there are lots of others, check in elementary structures books.