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Re: RESIDENTIAL: Discussion of Load Paths

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Bill,
 
My comments are below:
 
----- Original Message -----
From: "Bill Polhemus" <bill(--nospam--at)polhemus.cc>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, September 29, 2004 9:55 AM
Subject: RESIDENTIAL: Discussion of Load Paths

> Belaboring the subject.
>
>  
>
> I want to be clear on some of the concepts regarding transfer of forces,
> particularly in roof framing.
>
>  
>
> As I understand it, we oppose roof rafters at a ridge plate, and do not
> design the ridge plate as a beam (unless the roof pitch is particularly
> shallow; I believe the model codes typically say less than 3:12).
>
>  
>
> However, when we have for example a hip roof, we do design hip beams as
> flexural members because of the way that roof members frame to them-not
> opposing. The same goes for valley beams.
>
>  
>
> In this we explicitly disregard diaphragm action by the roof sheathing
> (which would, it seems to me, act to "oppose" the forces on the hip beams
> just as well as opposing rafters would).
 
You don't need a flexural member at the ridge because you have a ceiling joist not because of the diaph. The diaph is not very useful for out of plane forces.

>
>  
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> I'm not sure if this is simply a CONVENTION, or if it is proven by statics.
> Certainly this isn't your typically "free body diagram" situation.
>
>  
>
> Thrust from opposing rafters is handled by framing rafters at the "heel" of
> the rafter (i.e. at the top plate). Alternatively we can have collar ties
> (with correspondingly larger forces involved) or other means of
> counteracting the thrust.
 
The wall has very little ability to resist thrust unless you have very short distances between cross walls
 

>
>  
>
> I am assuming that this thrust is also present "jack" rafters (framing into
> hip or valley beams).
 
Yes

>
>  
>
> I am also assuming that typically you will need some sort of support at the
> ends of hip or valley beams-a column or post, etc. I would think that, once
> again, diaphragm action by the roof sheathing would actually preclude this
> need, but I assume (again) that this is disregarded.
 
Yes and no because many time two hips oppose each other and a ridge frames in at the same point you could resolve the forces into a self supporting system up to a critical load which would be much lower than the critical load if you put a support at the end of the hip or valley. But again the diaph has very little to do with this.
 
>
>  
>
> Please, someone debunk any of these "assumptions" of mine if they are not
> correct, or point out exceptions where they exist.
>
>  
>
> I know this is elementary stuff, but this "folded plate" business has always
> been a difficult concept for me to grasp intuitively.
>
>
HTH,
 
Rand