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Re: Ridge beam/joist analysis

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Nels - It's a poor assumption, as shown by placing a board on two rollers (the kind used to support the free end of a woodworking project). If the two rollers are at different elevations, the board WILL roll towards the lower roller (and off the setup) if not restrained, even through the only force on the board is gravity.

I agree that there will be no NET force on a balanced rafter system (one rafter on each side), but there will be a horizontal reaction force at the rafter ends in a FBD.

One part of the equation we're ignoring is that the roof sheathing will act as a diaphragm, transmitting the horizontal loads to exterior walls. Part of the "luck" (along with glue, drywall, interface friction, etc.) that keeps houses from falling down all the time.

At 11:24 AM 6/28/2004 -0700, you wrote:

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

Jordan Truesdell, PE

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