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RE: Ridge beam/joist analysis

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Yes, the analysis is a bit against typical residential practice. Re-orienting your vectors will indicate tension at the ridge and compression at the wall. The key is designing for the tension joint at the beam. As long as the rafter (joist) can't pull away from the ridge beam, the deflection of the entire system is limited by the deflection of the ridge beam. Lateral deflection is often taken as zero, assuming an equal w on both sides of the roof (equal and opposite horizontal forces cancel at the ridge), leaving only the trigonometry of determining the lateral deflection of the system due to vertical deflection of the ridge, if small top-of-wall deflections are of concern. Strapping, face hangers, and crossing rafters (above the beam) & nailing together are all ways to tie the system together (though toe-nailing <shiver> is the residential favorite).


You analysis appears to be correct.  Because the load is at an angle to the
joist, you will have a "bending" component and an "axial" component to the
load.

The easiest way to handle it is to tie the joist to the one on the opposite
slope.  Use a strap (Simpson or other) across the top of the joists, or a
bolt through the ridge beam.  The result will be an equal-and-opposite
tension force at the ridge, tension in the member, and no horizontal
reaction at the top of the wall.

---
Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

> -----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...
****** ****** ********



Jordan Truesdell, PE



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