One of the few regrets I ever had as a kid was not getting to climb the 530 steps to the top of the dome (the Golden Gallery, as they call it). Mother was a little worried about my safety. However I wanted to see the interior structure of the dome itself.
Many people don’t realize, but St Paul’s is one of the weirdest, most idiosyncratic buildings in the world. Designed by Christopher Wren and completed in 1710, the cathedral is a monument to Wren’s ability to “think outside the box” when it came the the structural problems involved in construction.
First, there are the outer walls. As most people know, cathedrals were stunted in height for a while because you could only build a wall so high before it would topple. Think of a stack of blocks; the taller you go, the less stable it becomes. Luckily, someone came up with the buttress, a smaller support wall to hold up the main wall. This then morphed into the “flying buttress” which is the same thing with an arch between the two. Buttresses also mean that the main wall (since it had all this added support) no longer had to do all the work of holding up the roof. And, as a result, you could punch large holes in the wall and fill them up with stained glass.
The only problem with buttresses is that they take up a lot of space on the outside. To Wren’s eyes, this was unseemly. So he created a shell outer wall that mimics the real inside wall, but encloses and hides the buttresses from view. You can see it well on Google Earth. In other words, the wall you see on the outside is about 15 feet or so from the wall you see on the outside, and between them lie the buttresses, completely hidden from view. In a sense, you could say that the entire building is encased within an entire other building.
But for all this, the dome was the real crowning achievement. Nowdays, it would all be done with concrete and rebar, but these were not available to Wren. Building a solid marble dome would be costly and impractical. He wanted to build the dome out of a lightweight metal shell. But there was a problem. On top of the dome was supposed to be a marble “lantern” that weighed hundreds of tons. (Just the gilt cross on top of the lantern weighs 7 tons by itself!) A thin metal shell could never support it. What to do?
Wren solved the problem by creating a dome within a dome within a dome. First there is the dome you can see from the outside. This is the metal shell. Then there’s the dome you can see from the inside. This is clad in marble, smaller and about 60 feet shorter than the outside dome. In between them, holding up the massive lantern, and completely hidden from view, is a third“dome”, actually a giant brick cone. So, just like the walls, what you see on the inside has nothing to do with what you see on the outside. And between the interior and exterior is the real “meat” of the building, vitally important but completely camouflaged.
Fascinating stuff, and I’m happy to report that I finally got the chance to see it all up-close and personal. Let me tell you, the view from the top was fantastic (if a little windy). And the journey there was exciting because as you continued up and up, the stairs (wood ad first, then marble, then iron grating spirals and catwalks) get narrower and ricketier as you go. The space sandwiched between the brick cone and the outer shell felt more like a basement boiler room than a cathedral dome. It was all amazing (especially when you consider its age), and I’m glad I finally got to go.
More info: St. Paul’s Cathedral