Valley Beam Reactions 1

[RanAway]

I have a structural valley beam coming in at 45 degrees (horizontal projection down) which is bearing on a corner wall at one end and a ridge board/beam on the other. Of course on the other side of the beam is the other valley beam which is the same configuration.

My expertise is roof trusses not rafter framing and I'm trying to wrap my head around conventional framing.

I'm trying to sort out the bearing of the valley beam on that beam/board. I believe that that valley causes a vertical reaction on the ridge beam/board, but someone else is telling me that the valley beams because they come in to the same point from opposite sides act like the peak of a set of rafters tying into a ridge board. I just don't think it is the same. I believe that the valleys cause a vertical reaction at the wall and on that ridge. But the other individual is saying the vertical reactions are all going to the wall cause the valleys oppose each other (at 45 degrees).

So, is there a vertical reaction on that ridge beam/board or does it all go to the wall?

 

[jdgengineer]

Yes, there is a vertical reaction. Conventional construction would require that the valley beam be supported at the ridge / valley intersection. We always calc valley beams as if they are structural beams. The conventional framing portion of the code treats them somewhere between a structural beam and a ridge board. The analagy of a ridgeboard makes more sense to me for a hip as you have folded plate action. A valley to me should always be designed as a spanning beam. [pre][/pre]

 

[jdgengineer]

With that said, if you can find a good way to tell the bottom of the valley together to form a tension tie I don't think anything is wrong with that. But it would be an engineered design which would need to be properly analyzed and designed.

 

[RanAway]

Thank you so much for your help, I really appreciate it.
I hate these old conventionally framed homes. I had someone recently tell me that sometimes it's now worth it to try and renovate them structurally, but just tear down and start from scratch.

 

[McSEpllc]

Valley and hips are a bit flabbergasting. I think in reality they function jointly with the roof diaphragm, which acts as a beam in itself. The loads will be resisted where to overall roof finds resistance/support. In designing new roofs, I try to stick with simple load paths, designing valley and hips as beams requiring support on each end.

Look at another posting from today:

http://www.eng-tips.com/viewthread.cfm?qid=435528

Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[HouseBoy]

As others have written, valleys and hips can be perplexing.

Draw a free-body diagram. Generally there are two mechanisms that may be available to provide support:
Vertical restraint or thrust resistance. It has to be one or the other but sometimes it is a little of both!

For a cone shaped roof, I think there must be thrust resistance.
In your example of (one or two) valley(s) framing into a ridge, IF the ridge can provide vertical support, that will contribute. IF there s some thrust resistance (such as adjacent roof sheathing) that would contribute too but there has to be at least one or the other. If they are both contributing, the contribution will depend on relative stiffnesses of each.

It is possible to support a valley without vertical support at the ridge. Just think of all of the old stick framed ranch roofs with 1x or 2x ridge boards. Plenty of early 1900's roofs have no ridge board/beam at all. Not saying that's a good idea. Most of those that I look at need "help". I generally try to install redundant systems (like ceiling beams) because it is difficult to determine exact stiffnesses in many situations and I want something that is reliable and that I can quantify. (BTW, I'm not a fan of the argument that "it's been standing for 100 years so it must be OK"). Each situation is distinct though.