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Re: Ridge beam/joist analysis

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Good point. I've often struggled with this concept. The system you described
(board on roller) is inherently unstable and so cannot be analyzed using
statics. By virtue of assuming (and providing) a joint  that has the ability
to resist lateral loading (pinned), the net effect becomes zero lateral load
for the inclined board and the supporting member. Where did the load go in
theory? We know there is a force there otherwise the roof would slide off.
For the wood framed house obviously the diaphragm is the only way the top of
the wall can resist the lateral load so that you can assume that the sloped
rafter is pinned at one end. But according to statics no load is in the
diaphragm.Where did that intuitive load go? Perhaps we should add dead load
of the rafters and sheathing to the diaphragm forces to be resisted?

Rand



----- Original Message ----- 
From: "Jordan Truesdell, PE" <seaint(--nospam--at)truesdellengineering.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, June 28, 2004 1:48 PM
Subject: Re: Ridge beam/joist analysis


> 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:
> >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
>
> Jordan Truesdell, PE
>
>
>
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