Vaulted ceilings are created using curves or arches called ribs. Arches transmit their load directly to the load bearing walls or are hung from the rafters. When the vaulted ceiling is hung from the rafters, the rafters carry the load to the supports.
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Supported By Walls
"Roof Watching" by Edmund Jupp states: "When the load comes on to the arch, the outer ends of this curve tend to spread, increasing the horizontal forces on the supports more and more as the load increases." Taller spans add greater horizontal load on the supporting walls. In load calculations, the ridge board carries the load of the vaulted ceiling unless the vaulted ceiling is hung from the rafters.
Hanging from the Ceiling
The weight of the vaulted ceiling is added to the roof weight for a total force W. The total force W is multiplied by the cosine of the ceiling or rafter angle perpendicular to determine the axial force in the rafters. If the vaulted ceiling hangs from the centre span, the load is equally distributed to the supporting walls.
Broader arches and heavier loads require thicker arch ribs to support the weight. Yet arches can be made of multiple sections connected together. The effective length of an arch is a factor determined by its stress multiplied by the true arch length divided by two. The factor ranges from 1.02 to 1.25. According to "Steel Structures" by Hassan Al Nageim and Thomas Joseph MacGinley: "Higher factors apply to high rise arches." Effective length is used as vaulted ceiling rib length in roof load calculations.
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