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RESIDENTIAL: Discussion of Load Paths

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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).

 

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.

 

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

 

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.

 

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.