Octagonal Roof Support 1

[jetdraftsman]

Does anyone have any experience in designing a octagonal room that can be built without the use of tie beams from wall to wall supporting a kingpost at the center of the tie beam and supporting the roof planes at the peak.

I was wondering if I could use some metal braces which would be fastened to a triple top plate at each of the eight wall intersections, which would form a continuous ring preventing any outward thrust from the roof eight roof planes. Has anyone ever heard of doing this or something similar? Do you think you think it would work? We would like to do something similar to this to keep the cathedral ceiling in the octagonal room open as much as possible, without any obstructions of tie beams or kingpost. Thanks

 

[Ginger]

A ring beam roof is the way to design the structure without any central support or cross ties.

A tie (tension) ring beam is located at the top of the walls and a compression ring is located close to the top of the structure near the crown. The main rafters will meet at the crown and run down to each corner of the octagon with the purlins running around the roof structure. The ceiling follows the line of the rafters.

The purlins are designed to carry in-plane forces and if the roof is required to provide support to the tops of the walls then bracing within the plane of the roof structure can be added to transfer the loads.

 

[AJUK]

I have to agree with Ginger. Use a ring beam to restrain the horizontal thrust.

 

[jetdraftsman]

Here is what I had in mind in the form of a ring beam. See link below.

http://bsbad.tripod.com/bayshorebuildingdesigninc/id14.html

 

[jetdraftsman]

Actually, the morning room is a decagon (10-sided) structure. Each of the wall sections are 4'6-9/16" in length, 14'-0" across the flats, w/ a 9/12 roof slope, 40# snow load & 15# dead load. We want to frame the roof with 2x12's.

I guess what I am asking for is any input on the design and/or construction of a structure similar to the one I described. I guess the way I see it I only need to support the top half of each of the ten roof planes with the bracing, which from what I can tell, on a polygon hip roof is 25% of the total roof weight, which would be the top half of the roof rafters. By supporting the top half of the roof planes with the use of bracing on the corners of the stud walls should eliminate the outward thrust of the roof rafters, similar to how a load bearing ridge beam works. The other 75% or lower half of the rafter would bear down on the stud wall.