Post for Hip Roof 2

[dbryant]

I am in a quandry, and have been here for some time. I have seen many hip roofs, even some with a valley intersecting where the hip hits the ridge, constructed without a post.

I normally calculate all hips as beams and all valleys as beams (the procedue I uses shows that a valley beam receives twice the load of a hip). I always design a post to support the end reaction of the hip and/or valley beam.

This is long-winded, but I am getting to it. Cound someone please tell me, based on the code, when it is OK not to support a hip, or a valley, with a post? A parallel question is: when is a hip a hip board, and not a hip beam?

Thanks,

Don

 

[haynewp]

I have seen hip beams supported from a long spanning steel truss before. I don't know of a code requirement that hips or valleys have to be supported from posts.

How do you get that a valley beam supports twice the load as a hip beam?

 

[dbryant]

Regarding the load on a valley beam being twice that on a hip beam:

Each rafter that connects to a hip beam is supported at both ends: typically by the hip beam at the high end and by the bearing wall at the low end. So, the hip beam supports half of the load of the rafter.

The jack rafters that frame into a valley beam normally frame into a ridge board (not a ridge beam) at the opposite end of the rafter. So, the valley beam carries all of the load of the rafter. This similar to a conventionally framed gable roof: the bearing walls carry all of the rafter loads.

In plan, the tributary area for the hip beam begins at the low bearing point for the hip beam and spreads out at 22.5 degrees from the hip beam on either side, for a combined angle of 45 degrees, projecting until is hits the longest rafter that frames into the hip. The tributery area for a valley beam is a square encompassing all of the area of all of the rafters that attach to it.

Thanks,

Don
(Still in a quandry about the post)

 

[haynewp]

I see, it is based on your situation. I have a building right now where the valley rafters are supported from a ridge beam at the left side of the valley and a hip beam at the right side of the valley, so the valley beam is only taking half the trib at each side.

 

[EG]

dbryant,

To better understand your original question, are you simply asking, "What is holding up the ridge beam that is holding up the hip beam?"

I would think a post is absolutely necessary if there is no truss element involved.

 

[TTK]

I could see a situation where a VERY small hip roof would not require a post to support the ridge beam.

An example might be a small roof with low design loads that produces very little thrust. "System effects" or "diaphragm action" are also popular explanations of why some existing hips roofs appear to perform well without posts for the ridge beam.

How about posting the dimensions of the roof in question and allow others to comment on whether they would provide a post?

 

[dbryant]

TTK et al,

The width of the main part of the house is 28 feet, so the rafters span 14 feet from wall to ridge. The perpendicular wing is 24' deep, so the rafters span 12 feet. The roof slopes are about 8: 12 and 6: 12, so the hip is not quite at a 45. The valley rafter span is approximately 19'.

I know that by conventional engineered analysis, a post is required. However, many houses have been built like this without a post. I guess my biggest question is: Is there a code maximum hip or valley span that does not require a post?

Thanks,

Don

 

[kkoloj]

What I typically do is design a "bent beam" to support the hip(s) and if present the ridge. Typically there are two hips and a ridge meeting at a point. The "bent beam" can be thought of as two rafters--that is, at the point where the two hips meet the ridge, there are also two rafters that also meet at the same point (or there usually is). Anyway, I design this "bent beam" as a solid member that spans from one wall up to the hip/ridge and down to the other wall. Often for large rooms it will be TS. But you could also use wood with steel side plate and bolts. The model I use in RISA is pinned at one end, and free and the other, but CHECK THE LATERAL MOVEMENT OF THE FREE END. If you fix both ends you will have to take care of the thrust (not an easy task sometimes).

I have also used wood "bent beams" with a collar tie about half way up the "bent beam". This gives the architects a vaulted feeling, but it is not truly vaulted.

The "diaphragm action" noted in an earlier post has been used to justify hips without king posts. I have talked at length with some other engineers who have modeled the sheathing/hip/roof system in RISA 3D. They apparently got a roof system to work, although they had to fudge one node (I will expain more if you're interested).

Hope this helps a little

 

[ctcray]

I think it depends on how you "model" your valley. With all roof members in a stick built roof you have some options to make the member work for statics.

Option 1: member has a vertical support at each end and horizontal reaction at top (example: rafter supported on knee wall at bottom - vertical support, ridge BEAM at top - vertical support, horizontal reaction resisted by opposing rafter at top)

Option 2: member has a horizontal support at each end and vertical support at bottom (example: rafter supported on wall at bottom -vertical support, lapped with ceiling joists - horizontal support, ridge board with opposing rafter at top - horizontal support)

....any way you get your 3 basic supports is fine -- as long as they're realistic assumptions. The problem with using Option 2 above to eliminate the post on a Valley Beam is that the horizontal thrust loads get to be a little prohibitive.


I've used RISA to model a simple hip roof on a 12x12 pool house without collar ties and found that the member sizes were reasonable but the connection of the hips to the corners of the wall and tranfering the "tension-ring" forces in the plates at the corners to be the most challenging part of the design.