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Re: Ridge beam/joist analysis

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Craig,

The ridge beam allows you to assume that the wall and the ridge support the
gravity-loaded rafter with reactions that are vertical only.  If W is a
gravity load, it acts vertically -- no matter how you analyze the rafter,
whether analyzing it with strictly vertical loads, or resolving the vertical
gravity load into components perpendicular and parallel to the plane of the
rafters, the result will be vertical reactions onto the ridge and the wall.
If that is not the answer you get, brush up on statics and keep trying.

If W is a wind load, the analysis becomes a little more complicated and will
involve the two sloping planes of the roof as a two diaphragm planes.  In
determining how the joist reactions are resolved, base your analysis model
on the assumption that the wall and ridge resist only vertical loads and the
each of the diaphragm planes resist loads only in its plane.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net

| > -----Original Message-----
| > From: Craig & April [mailto:csmleko(--nospam--at)bellsouth.net]
| > Sent: Sunday, June 27, 2004 4:42 PM
| > To: seaint(--nospam--at)seaint.org
| > Subject: Ridge beam/joist analysis
| >
| > Any thoughts on the conventional knowledge(?) that a structural ridge
beam
| > eliminates all lateral thrust at the exterior joist bearing walls? I'm
| > designing a timber camp roof with a 10:12 pitch. Looking at the joist
| > member
| > as it will exist in the structure, (i.e. pitched) the loads are
obviously
| > vertical at beam and wall. However, in order to do an exact analylsis of
| > the
| > joist, I applied the 'w' load (reduced for actual longer span) at the
| > appropriate angle to a horizontal joist. An axial load exists as the
| > result
| > of the partial vector. This seems to indicate tension in the joist (if
| > supported from beam) or compression (if supported at walls)...which is
| > contrary to what I thought I knew. Anyhow, I hope I'm wrong because
| > developing the lateral restraint would be tough at 8x8 timber walls...
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