Roof truss shear transfer problem
Roof truss shear transfer problem
(OP)
Trusses were laid out for a job site and the truss designed to transfer the drag was placed in the wrong area. What I have decided to do is use the next truss closest to the wall, problem is I don't know what that tolerance ie how far I am allowed to be away from the wall in order for the truss to be "stitch" nailed for shear transfer. I would appreciate some input on this matter. Thank You RDM





RE: Roof truss shear transfer problem
RE: Roof truss shear transfer problem
Sounds like a bit of fuzzy logic to me. What are you trying to do?
As I understand it, you have a batch of timber trusses to form a gable end roof. You are concerned about the effect of wind on the gable end hitting the wall section closing the end of the roof. If that happens how do you resist the end forces, which would tend to collapse all the trusses like a pack of cards. Do I get the picture correctly?
If there were no bracing, the set of trusses would work like dominoes. If one topples over, the rest will follow. To stop this, you have to build in some lateral strength by bracing together one pair of trusses. This could be done in the middle of the roof. It it is a very long roof, it might be better to use two sets of bracing - one set at each end of the set of roof trusses. This is an engineering judgement call - not a code stipulation.
With the "stong braced box" stabilising the roof in the middle, you would have to use the battens, or roof purlins to carry horizontal end loads from the gable wall end to the stabilised section capable of resisting the forces. This is not a major problem in conventional timber construction. It might be more of a problem with skimpy steel purlins of low buckling strength. In that case it is preferable to take the wind end loads in tension where practical. The other option is to stiffen up the purlins with some rod bracing to reduce their slenderness ratio.
The other reason for building the braced box sections is to help during the erection phase. Your tusses are usually so thin and saggy that they need to be tied to something more stable.
On the other hand, if your truss is too short to sit on the wall, the usual option is to strengthen the end gap with steel members - say channel section each side, or steel angle. Your engineer would have to advise you on that one. That is much more dangerous.
Regards - johnp
www.latviantourists.com.au
Johnp.Rz
http://www.mets.net.au