Ridge Board or Ridge Beam for Hips?

[kmart30]

Ok, really been scratching my head on this one but probably looking way too much into it...

I have a 4/12 hip roof I am designing for a friend, the width on the addition is 34' and the length is about 22 feet, The roof will tie into the existing with a sleeper. I tried convincing him on pre-engineered trusses but he didn't want them. I am using 2x10 roof rafters (24" o.c.) and 2x8 collar ties/ceiling joists, with a bearing wall (near mid span) running the full length perpendicular to the width. I understand the ridge can be designed as a "ridge board" because the ceiling joists will resist the outward thrust and the collar ties will resist wind. However, for some reason I am having a hard time picturing the same case for the hip ridges. To throw a wrench in the whole thing I have a vaulted ceiling under a portion of one of the hip ridges.I believe I have the right design but maybe someone could chime in and help me have a clearer picture of this...

 

[DaveAtkins]

A hip ridge is like a ladder leaned against a wall. At the top of the ladder, where it touches the wall, there is a horizontal reaction, but no vertical reaction. The only vertical reaction with a ladder is at the bottom of the ladder. At the bottom of the ladder, there is also a horizontal reaction, equal in magnitude but opposite in sense to the horizontal reaction at the top of the ladder.

Anyway, that is my long winded way of saying you must resist the horizontal reaction at the top of the hip beam, and the horizontal and vertical reactions at the bottom of the hip beam.

DaveAtkins

 

[dhengr]

Kmart30:
There is a vast difference btwn. “2x8 collar ties/ceiling joists” and you must sort this out to determine how these help resolve the forces in your roof system, they are not one in the same. The concept of the ridge board and opposing rafters is a long tested and used means of a roof structure. It is easy to understand an analyze, when done properly. The hip ridge or valley member is less easy to design, analyze and rationalize, so it is not unreasonable that these members be treated a huskier beam members. These are often ripped on the top edges to match the roof planes. If you have a bearing wall under the ridge, it is not uncommon to provide a column under the ridge, where the two hip ridge members meet. This just adds a little extra safety to the roof framing system with little added cost. As mentioned above, the horiz. trust must still be considered down at the double top plate of the walls, it can still push that corner, those walls, out if not detailed for.

 

[jdgengineer]

DaveAtkins -- I might agree with your ladder analogy if the roof pitch was extremely steep. In reality most pitches are much to shallow for your ladder analogy (in my opinion). We personally, always treat hips as structural beams. In conventional construction a 2x hip at least equal to the depth of the cut end of the rafter and supported at the ridge and top plates is generally accepted. The hip beam works due to folded plate action of the diaphragm amongst other reasons. Except for small hipped areas, we don't personally rely on this mechanism. Housing & Urban Development (HUD) has a pretty thorough Residential Structural Design Guide that can be downloaded for free. They mention this discrepancy in one of their examples.

 

[kmart30]

Thank you all for clarifying this topic for me; I really liked the ladder comparison. My hip ridges are somewhat long for hips and if I treated them as a structural beam I would need pretty beefy LVL's or similar. I designed the hips and ridges as "boards" and added a column under the ridge and hip main intersection at the peak.The rafters above the vaulted ceiling I reinforced extra at the top plate connection to account for the extra thrust force from not having ceiling joists or collar ties.

 

[DaveAtkins]

The only way I would treat a hip beam as a beam (with a vertical reaction at both ends), and not like a ladder, is if there is something supporting the high end of the hip beam. If there is no column at that location, then you are relying on the last pair of rafters, with collar tie, to support both hip beams. The rafters would not be typically designed for this additional load.

DaveAtkins