Behind The Scenes

Two views of the same church, starting with a look at the ceiling – flat, done, and cupola – from the sanctuary:

followed by Ellen¹ taking measurements in the attic as part of an investigation:

She’s standing on the framing that supports the flat ceiling and next to the framing that supports the dome. (What you see at the dome is the wood lath – the dark brown horizontal lines – and the plaster keys that hold the inner face – the light tan lines – and the wood ribs that support the lath.) The big timbers on the left are part of the truss system that supports the roof, allowing it to span across the sanctuary. The curved framing of the ceiling dome carries no weight but its own: the dome is a decorative element, not part of the roof structure. Despite that, the ribs are sizable pieces of wood, not the 2x4s or 1x3s you might expect. If you look closely, you’ll see that (a) all of the ribs are spliced (because they were cut curved from straight pieces of wood) and (b) the ribs alternate between single pieces of wood and built-up double or triple pieces.

If you only think about the built-up ribs for a moment, they can be considered more-or-less continuous, as the splice points in the different pieces don’t align. I haven’t been part of our analysis effort but it certainly wouldn’t surprise me to hear that the built-up ribs are strong enough to carry the dead load of and small amount of internal wind pressure on the dome. The single-member ribs, even though discontinuous, can act as local stiffeners for the lath and plaster.

That last thought leads to a different path. The dome is a composite shell structure made of continuous plaster and discontinuous lath, meaning that it can carry some degree of compression overall and local bending or tension. It may well be that shell is stiffer than the ribs and carries the weight of the dome to the supporting beams in the flat area at the edge of the dome. There is no question that the builders thought the ribs were doing the all the work – and as I said above, they are probably strong enough to do so – but that relatively stiff plaster shell carrying only its own weight is a structural element that we can see and they didn’t.

1. Wearing her stylish OSE jacket, of course. ↩︎