From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 30 Apr 1999 16:45:25 -0400
Thor Tandy writes:
. > I think he is referring to the gravity component of the roof along the
. > axis of the rafter. If you hold a rule between two hands and gradually
. > lower one hand the ruler will ultimately slide off your hands. I think
. > that is the observation.
Yes, the old knife-edge from statics --- but this assumes that the lower
support is rigid, that it can resist horizontal loads and that the reaction
is going to be normal to the rafter.
However, if the support is a wall, pinned at top and bottom, (can't resist
horizontal forces), or if the rafter has a fish-mouth cut into it so that a
horizontal bearing surface is present, or blocking is attached to the bottom
of the rafter to accomplish the same thing, or the Simpson VPA is used, the
reaction at the wall is constrained to be vertical and, for gravity loading,
the reaction at the ridge beam has to be vertical also, and there is no
tension at the ridge beam. In the case that Jim Kestner cited, a sloping
surface was used at the ridge beam to support the rafter, the reaction there,
neglecting friction, would be normal to the rafter, which would impart a
horizontal force at the wall if no strap was used at the ridge beam.
A. Roger Turk, P.E.(Structural)