hip beam design

[dancristescu]

I have a silly question that goes back to statics.
A simple span roof hip beam's load is a distributed load of triangular shape with load "0" at the corner of the roof and max. at ridge connection. I get confused over the tributary area.

Assuming 34 psf dead and live load, and assuming that the hip is the diagonal of a 9ft X 9ft square (18ft wide roof with equal slopes) and assuming I'm using the true length of the beam -as supposed to the projected length-
What is the load configuration? What is the height of the foot of the triangular load at ridge connection?

Anybody that might help please enlighten me.

PS. I could do this with any of the programs for wood but I believe that one should do this by hand until he or she is completely confortable with it before using any software..

 

[DaveAtkins]

There are no silly questions here--just silly engineers...

The load would be (approximately) 9' times 34 psf. 4.5' of load comes from one sloped section of roof, and 4.5' comes from the other sloped section of roof.

Now, I said approximately, because the dead load portion of the total load would be a little more, because the span of the sloped section of the roof is a little more than 9', because it is sloped. But snow load is a projected load, so the 9' is exact for that.

DaveAtkins

 

[COEngineeer]

I agree with what dave said... I would just use the projected length instead of true length because some of the load will be axial (since you are doing it by hand and assuming this is for residential/small structure). If you
really want to do it right then you need to have live load and dead load perpendicular and parallel to your member (for moment and axial), then you use true length, then use the unity equation. But thats just too complicated especially because the deflection controls most of the time on timber.

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Sea Water Intake and Jetty Construction

 

[ctcray]

This is a good question.

My answer would be the max load on the hip beam is 6.36' x 34psf.

My reasoning: Lets say the 9'x9' section of roof has a 12.73' diagonal (the horizontal span of our hip beam).

All rafters in the roof are supported by one of the 9' walls on one end and the Hip on the other end.

We can say half of this 9'x9' roof area is supported by the hip beam, the other half by the two walls.

The triangular tributary area on one 9' wall would be 0 at the corner to 4.5 ft at 9'. (1/2 x 4.5' x 9' = 20.25 sf)

Since the hip carries the same load as each wall, the triangular load on the hip won't be 0 at the corner and 4.5 at the max because 1/2 x 4.5' x 12.73' = 28.64 sf > 20.25 sf.

To get the same square footage, the correct max load per side of the hip is 9'/12.73' x 4.5'=3.184 ft at the ridge

1/2 x 3.182' x 12.73' = 20.25 sq ft.

That gives 3.182' x 2 sides = 6.36' in this case.

Of course, the other answers will work, but this way can help you sharpen your pencil if you start to get huge beam sizes.

 

[BB27]

Does the ridge (2x) support the hip and valley beams in a hip roof?. I see quite a few high end homes with roofs that start out as a hip roof, and then the roof gets cut up by valleys and ridge beams from dormers and etc. In theory a non structural ridge can support the hip rafters that are connected to the ridge. Once the roof gets cut up what is supporting the ridge. The only way I see to support the ridge is with posts at the intersection of the hip and valleys that are framed into the now structural ridge. of course all the posts require a lot of framing in the ceiling below and then down to the foundation. The connections then are to big for the Simpson hangers and the connections become cutom made. Is this design to conservative?

 

[DaveAtkins]

High end homes (actually most homes these days) would not have rafter framing at the roof, but roof trusses. Roof trusses allow for hips, valleys, you name it--and no thrust to worry about. Just vertical load on bearing walls.

DaveAtkins

 

[COEngineeer]

Im not following you bb27. Ridge beam is always structural. Maybe you are talking about the board which is pretty much a board to nail the top but you have to calculate the thrust at the plates and possible add a strut tie.

I still see mostly stick framed roof on all of the high end homes we are doing. They really want to have that high pitch ceiling and they would not want to settle with scissor trusses.

http://www.swijetty.com

Sea Water Intake and Jetty Construction