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RE: Valley Rafters

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Gary,

I agree with you, with the following reservation.  If I judge a roof's
configuration to be conventional, and thus under the jurisdiction of the
Conventional Light Frame Construction provisions of the UBC [that's still
the adopted code in CA], I will use the provisions of Chapter 23 of the UBC,
which is pretty liberal in what it allows.  This procedure can result in
roof structures that could not be rationalized by Chapter 16 of the Code if
it were not for the exception of Section 1601.1.  I agree that excessive
lack of symmetry, as well as other conditions, could lead me to decide that
a given configuration is not conventional and needs structural design.

The potential dilemma is this: as structural engineers, our goals in our
clients behalf are generally to comply with the building code, to maximize
effectiveness and to minimize costs with our structural designs.  My design
should, at least, comply with the code, as well as provide a more effective
structure, or a less costly structure, or ideally, both, than one designed
by a non-engineer.

However, for residential construction, I cannot argue that a roof with
structurally designed hips and rafters is more effective than one built to
comply with Chapter 23, because in CA below snow level, the conventional
framing rules have proven very effective.  A roof built to comply with
Chapter 23 will generally be judged to be "effective enough", but it will
cost less than an engineered roof.

Many potential clients will think, "Why should I hire and engineer?  It's
just going to cost me more".  Perhaps I can explain why a structurally
designed roof is better, but I'll also have to deal with the contractor's
charge of "over-engineering" when costs are considered.  So, before I take
on a project that is principally Conventional Light Frame Construction, I
may tell the client, "You don't need me for this".  If I take the project,
I'll need to be able to justify where I've drawn the line between
conventional and non-conventional.

Nels Roselund, SE
South San Gabriel, CA
njineer(--nospam--at)att.net 
-----Original Message-----
From: Gary Hodgson & Associates [mailto:ghodgson(--nospam--at)bellnet.ca] 
Sent: Wednesday, September 14, 2005 4:30 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Valley Rafters

Nels,
I think that what yoy say is true if there is a certain degree of 
symmetry, i.e. loads on both side are approx equal.  However without 
that, the valley must be designed taking into account the unequal 
loadings and support.
Gary


On 10 Sep 2005 at 11:02, Nels Roselund wrote:

> Thor,
> 
> 
> 
> The design of a valley has more to do with the connections than with
> design of the member itself.  And it has to do with the roof structure
> as a system. The components of the system include the rafters, the
> ridges, valleys and hips, and the continuous double top plate.
> 
> 
> 
> In a rational and properly designed roof system, only the rafters need
> to be designed for flexure.  If the rafters are tied in each
> direction, their axial components of force resolve into an axial force
> in the valley or hip, so that neither the valley nor the hip acts as a
> beam.  However, as you follow the axial forces through the system, the
> forces at the connections can become pretty large, and even
> unmanageable.  And it can become impractical to tie rafters in each
> direction.  If you can't follow the axial forces through the system
> and resolve them all with designed connections, you should not
> hesitate to design ridges, valleys and/or hips as beams.
> 
> 
> 
> The conventional light-frame construction rules for framing can result
> in roof systems that cannot be rationalized.  Nevertheless, they seem
> to work O.K..  If they are not magical, perhaps, as Barry suggested,
> it has something to do with the buttressing or diaphragm action of the
> sheathing. I have not figured out how to take advantage of the action
> of the sheathing in a gravity-load design of a roof-framing system.  
> 
> 
> 
> Nels Roselund, SE
> 
> South San Gabriel, CA
> 
> njineer(--nospam--at)att.net
> 
>   _____  
> 
> From: Avicpeng [mailto:vicpeng(--nospam--at)telus.net] 
> Sent: Friday, September 09, 2005 3:04 PM
> To: SEAINT
> Subject: Valley Rafters
> 
> 
> 
> And just to start ... does anyone design the valley rafter of a roof,
> or do you write it off as "standard framing"?  If not, why not?  If
> so, why?
> 
> 
> Thor Tandy P.Eng MIStructE
> Victoria, BC
> Canada
> vicpeng(--nospam--at)telus.net
> 
> 



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