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Re: Ridge beam/joist analysis

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If you cut a notch in the rafter  the board will still slide off, all you
have done is created more friction area that to some extent resists the
sliding. You need a nail to hold the rafter in place. You have to have some
lateral resisting element to even analyze the rafter in the first place. I
think the explaination for Nel's string thing is the string has some
frictional resistance (also two strings connected by a pencil with friction
will not hang perfectly plumb) that will support the string but elevate one
end enough and the pencil will slide off. I answered my own question from
yesterday, even if there is only a vertical net reaction at the support the
horizontal load is nested in the parrallel component to the rafter. That's
the load that must be resisted by the nails into the wall or the frictional
resistance required by the notch. Incidentally IMO the notch in the rafter
is required for bearing stress reasons not stability. i.e. wood fibers would
crush if supported on a knife point.

Rand





----- Original Message ----- 
From: "Jason W. Kilgore" <jkilgore(--nospam--at)leok.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, June 28, 2004 4:51 PM
Subject: RE: Ridge beam/joist analysis


> I had to think about this one a while (in between doing real work) before
I
> figured out the problem.
>
> In the board-on-roller model, the horizontal "force" results from the fact
> that the support reactions aren't vertical - they're normal to the sloped
> board.  Since the load (gravity) is vertical and the reactions have
vertical
> AND horizontal components, you have unbalanced horizontal forces and a
> moving board.
>
> In the original problem, the beam was modeled flat, the reactions were
> vertical, and the load was applied at an angle with horizontal and
vertical
> components.  This requires a horizontal component in one reaction to
> compensate, with a resulting axial force in the beam.
>
> Back to the real world -- vertical load on a sloped joist supported by a
> ridge beam and wall.  If you cut notches in the joist so that a horizontal
> surface rests on top of the ridge beam and wall, you have no horizontal
> force.  This is the model that Nels is talking about.  If you support the
> joist on a hanger with a sloped seat the reaction is no longer perfectly
> vertical and you introduce a horizontal component.  This is the horizontal
> force that must be resisted by the hanger, by a strap, or by nailing.
>
> ---
> Jason Kilgore
> Leigh & O'Kane, LLC
> Kansas City, Missouri
>
> > -----Original Message-----
> > From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
> > Sent: Monday, June 28, 2004 3:57 PM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Re: Ridge beam/joist analysis
> >
> > 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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