Cathedral hip roof 2

[purpledragon]

Curious question. I have not worked on hip roofs in a while. I have a cathedral hip roof on a residential two story + attic structure.

Do the hip reactions at the ridge line have to be supported by the ridge beam or do their reactions brace each other back. There is only an elevated ceiling chord at +8'-0 above the bearing elevation. No floor at the bearing walls. The width of the home is 30'

Any input is appreciated.

 

[LowLax]

Do you have a ridge beam or a ridge board? If you're asking about thrust, a ridge beam will take care of that, but a ridge board won't. If you have a ridge board, then your ridge board will act as a column and you would need a way to counteract the thrust at the top of the wall.

But I could be misinterpreting the question.

 

[TTK]

If you are using a ridge beam (not ridge board) AND the ridge beam is supported by columns at each end, then you do not need to worry about thrust as the ridge beam and columns will prevent this.

If you don't have columns at each end of the ridge beam (you didn't mention it) then the problem is much more complicated and controlling roof thrust will be a critical component of the design.

Unless cross ties are allowed at the top plate (say 48" O/C), I would strongly suggest adding the columns to avoid the thrust.

 

[SacreBleu]

If it is a cathedral hip, then how can you have a ridge beam (supported by post at hip to ridge)?
The elevated ceiling chord (collar tie?) would have to bear the brunt of that hip connection load. Perhaps someone could model this in 3-D frame analysis, and see what thrust occurs at the hips to outside corner top of wall.

 

[SacreBleu]

ps...What is the roof pitch? With a steep pitch, the problem may go away.

 

[purpledragon]

Thank you all. Very helpful.

SacreBleu you confirmed my thought of doing a 3d model to analyze the thrust reaction.

I do have collar ties to work with at +8' above the bearing reaction at wall. I do not have any way architecturally to support the end of the ridge beam.

Time to kick it back to the architect.

Thanks again.

 

[AlohaBob]

There's a thrust at the joint along the ridge beam. This is likely counteracted by the hip at the opposite end of the building. The ridge acts like a compression column between the hip beams. Ensure the hips provide the thrust connection at their lower support. The thrust then in turn is resolved in the plate ties on the gable and rake walls. The plate is in tension to counteract the compression in the ridge and the hips. So also make sure the plates are continuous and designed for the tension stress as well as shear and bending from rafter reactions.

 

[SacreBleu]

Also- design/provide a corner strap at the double top plate corner intersection location.