how do we find the exact load bearing weight of roof trusses

huangyhg

how do we find the exact load bearing weight of roof trusses
we need to fit a monorail and overhead crane.
three ways:
1.  hire a structural engineer to study any original design/construction drawings of the roof trusses and supporting structure and analyze them for the new loads under current codes.  chances are that the original trusses were not designed to take the roof dead loads, the roof live loads and the new crane loads.
2.  hire a structural engineer to design for you a load test that will provide evidence of capacity (in the us - the ibc 2000 code has a section in chapter 17 on this - in-situ load tests)
3.  try to figure it out yourself without having proper structural knowledge and hope for the best.
i don't think no. 3 is really a good idea.
the trusses have to be mathematically analyzed (by hand or computer) for the forces in the various truss members and their connections. those forces then need to be translated into stresses. these actual stresses are then compared to allowable stresses dictated by the governing building code.
if the members or connections are overstressed, then reinforcement of those elements is required to provide adequate safety.
it is recommended that a stuctural engineer perform this type of analysis and design work.
hopefully as built drawings are available, otherwise, quite a bit of field measurements may be required. i would also encourage a site visit by the structural engineer to see the actual truss condition and its current loading.