Last week I visited Austin, the capital of Texas. For the uninitiated, Austin is best known for its music scene, the University of Texas, and strange bedfellows (this country’s most conservative state government meeting amidst a community whose civic motto is the hippie-inspired “Keep Austin weird”). But where were my eyes turned? Up, of course, to look at the structure that dominates the Austin skyline – the Texas State Capitol building.
The Texas State Capitol is a spectacular marble structure constructed in 1888; the dominant feature of which is its magnificent rotunda. Although at 308 feet in height it is only sixth tallest state capitol in the United States, it does maintain Texas’ tradition (“everything is bigger in Texas”) by being the largest state capitol in terms of interior floor space. And in a twist that could only have happened in Texas (the land of large ranches and low taxes) — the state paid for the building by swapping 3 million acres of land, land which subsequently became the largest cattle ranch in the world.
There is nothing as awe-inspiring as a dome – so it is no surprise that 40 of the 50 US states have chosen to emulate our national capitol by placing a dome of some type atop of their own capitol building. But in 2008, according to an American Institute of Architects poll, only one of them ranked as the “best State Capitol building” – and that was ours.
So I had something to think about during my 3-1/2 hour drive back to Houston – the majesty of the dome as an engineering marvel. It was tough sorting out legend from reality – for example, aren’t eggs considered to be “nature’s perfect food” partially because of their perfect shape?
I thought back on the famous domes that I had seen during my travels – from the Pantheon in Rome (126 AD); to the greatest of the Roman domes, Hagia Sophia, built in 537 AD in then Constantinople; to the domes that Hagia Sophia inspired: Jerusalem’s 7th century Dome of the Rock, as well as the magnificent 16th century domed mosques which the great Mimar Sinan made the centerpiece of Ottoman architecture. From Constantinople/Istanbul the dome spread, to India and the Taj Mahal (built by Sinan’s students), as well as to the onion-shaped domes of St Basil’s Cathedral in Russia, a state which viewed itself as the successor to the Roman/Byzantine Empire.
But then I thought back to the dome visit that had inspired the most anticipation in me – a dome built before the onions of Russia, the Taj Mahal, or the mosques of Sinan: Il Duomo, the Cathedral of Florence, Italy.
One semester in college, I took a class on the Italian Renaissance. There I learned about Filippo Brunelleschi, the goldsmith turned engineer who was responsible for one of the most innovative engineering feats in history.
It seems that in 1296 the people of Florence decided to build a cathedral worthy of that city’s position as the leading power of the newly emerging Italian city states. The cathedral was an apt symbol of the people’s faith in God – since the design called for the cathedral to be topped by a massive dome, something that had not been built on a similar scale since the Hagia Sophia nearly 800 years earlier. Despite the fact that all knowledge of how to engineer large domes had been lost from western culture, the people of Florence had faith that somebody would be divinely inspired by the time construction reached that point.
However 122 years later, in 1418, the cathedral still had no roof. A competition was held to find someone capable of constructing the dome, and it was won by the goldsmith Brunelleschi, largely on the strength of his “secret plans” (it says a lot about the submissions that were not kept secret, since they were judged inferior to the complete unknown).
Once Brunelleschi won the competition, he set to work identifying and surmounting all potential problems. He visited the ruins of Rome and studied the writings of Vitruvius, a Roman civil engineer from the first century BC, in order to learn the theory behind the dome. To him it appeared that the problems were:
- The distribution of hoop stresses was such that the lower section of the dome would be in tension (which the unreinforced concrete and masonry of his day could not resist easily) – so it would require alternate means of reinforcement.
- A dome required a lot of intricate wood scaffolding (called centering) to ensure that it did not collapse during construction; Florence had an insufficient wood supply to meet those needs.
- The construction materials had to be raised to the top of the cathedral in some way, ultimately a distance of 375 feet from the ground.
- And finally, the massive weight of a dome big enough to cover the roofless cathedral would only compound all of the above-mentioned problems.
Brunelleschi came up with ingenious answers to all of these problems. In order to resist the hoop stresses in the lower section of the dome, he built 4 chains out of sandstone beams linked with iron, which wrapped around the exterior of the dome. To preclude the need for wood scaffolding, he devised a herringbone pattern of brick placement, a pattern that ensured that the masonry remained self-supporting during the construction stage. In order to raise all construction materials up to the required height, Brunelleschi (an inventor who had been awarded the world’s first patent) designed and built several complex construction cranes. But his final innovation was possibly his most ingenious.
How to reduce the weight of the dome (besides opting for brick over stone) and how to hide the massive chains encircling the dome? For this, Brunelleschi came up with the idea of building two domes – first, an interior dome, thick enough to support most of the weight, and wrapped with chains to handle the tension stresses. But then he added an external dome, just a few feet thick, covering the first. This assured that most of the volume of the dome was hollow (and so reduced the expected weight); plus he eliminated the need for chains on the outer dome by using tension restraints which tied the outer dome back onto the more robustly supported inner dome. All went well, and Brunelleschi’s dome was finished in 1436.
While sitting in my history class, this solution seemed to me like the perfect example of the creative genius that defined the Italian Renaissance. I had to go see this place for myself!
In 1985 my wife and I got a chance to visit Florence. While she looked at the statue of David and the works of the Italian masters in the Uffizi Gallery, I couldn’t wait to go to the Duomo. I insisted in making her join me in the walk up the stairs inside the dome – a vantage point that prevented her from seeing the dome as anything more than a cramped, claustrophobic, unadorned corridor. I tried to explain my attraction to this thing – how it was the ingenious device that made the beauty of the outside possible. “Well”, she said, “then why don’t we leave this cramped ingenious device and go see that beauty of the outside?” “And then maybe go find some of those panini and Chianti”, I thought, “to celebrate having experienced this work of genius for myself!”
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By Tom Van Laan
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