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Re: Design of Valley Beams

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John G.,

I don't believe there is anything special about designing a valley beam, if
it is truly a beam with a span between reactions.  The tributary load
pattern is interesting the first time you figure it out, but otherwise, it's
nothing special.

If the valley is part of a conventional framed roof with a slope not less
than 3:12, valleys (like hips and ridge boards) need not be designed,
providing the rafters are tied by ceiling joists, and rafter ties.  The
valley, then becomes a part of a three-dimensional space truss.  I know of
no analytical rules for design of that kind of system, but I think you can
figure out the load paths by using statics.  I've stopped trying to make a
conventionally framed roof system "figure" by following the load paths -- my
experience is that the connections in a roof designed by the conventional
light-framed provisions will be overstressed if the rules for loads and
allowable stresses of other parts of the Building Code are applied.  But
that's O.K. if the building's structural system fits within the narrow
limits imposed by the Conventional Light-frame Construction Design
Provisions -- the system has stood the test of time "pretty well".

Nels Roselund
Structural Engineer



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