huangyhg

timber roof spread
hi all.
i'm designing a timber cut roof with a lack of triangulation. it typically consists of 2 rafters supported at the feet on block walls. there is a raised collar tie about 1/3 depth from the ridge. there is no ring beam at eaves.
has anybody any experience in what to check for this case?
i run this situation on the computer (i use risa 3d). in my experience, the connection between the collar and the rafter is most critical, followed by the bending-axial interaction in the rafter.
daveatkins
right dave.
and you'd better consider the deflection of the rafters the collar tie is pulling on. might have to make the rafters larger, maybe double them up with a single
thanks guys.
i'm actually more concerned about possible supporting wall spread. what lateral deflection limits do you use for the wall? in the past i've used h/300 as my limit.
another option i've checked before is to use a pinned support at each of the feet of the rafters and applied the lateral load from the rafters to the wall. this will induce an overturning moment about the bottom of the wall. the resisting moment of the wall is the (weight of the wall + dead load) x lever arm. i've never been able to prove this however.
any suggested checks on the supporting walls?
sounds like you need a structural ridge to prevent your rafters from deflecting, thus preventing the spread.
allowable deflection at the top of the wall depends on wall finishes. h/300 would be fine for masonry--i think you could allow h/180 for less brittle finishes. re
thanks guys.
i've actually used daveatkins' solution in the past-i was just looking for some comfort in my logic i suppose!
also daveatkins 2nd point i would agree with, as my wall overturning calculations would suggest.
in some places they call this an a-frame truss due to the shape.
rafters cantilever out from the collar ties.
lateral loads taken by sheathing.
it can all be done pretty easy by hand treating the rafters as a cantilevered continuous beam.
csd
really, there are three ways to control any lateral wall movement here short of adding a ridge beam:
1. increase the rafter size; or
2. increase the e of the rafters; or
3. lower the collar tie.
just pin one end of the collar tie, put the other on rollers, and play with the results. the less bending in the rafters, the less lateral movement.
mike mccann
mccann engineering
or
4. put a load bearing ridge beam in.
as i said csd, short of adding a ridge beam.
mike mccann
mccann engineering

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