St. Paul's dome in London definitely has a chain around its bottom circumference. The Pantheon dome in Rome definitely does not have a chain, but sits on an absolutely massive wall structure to keep it from spreading outwards. Hagia Sophia's dome in Istanbul definitely did not have a chain when it was built, but has numerous metal reinforcements today. The original Hagia Sophia's dome fell down about 21 years after it was built due to damage from an earthquake, but quickly rebuilt. https://en.wikipedia.org/wiki/Hagia_Sophia Brunelleschi's dome may have some sort of a "chain", but there has been intense controversy about this ever since it was built. https://en.wikipedia.org/wiki/Florence_Cathedral Note that a simple chain which encircles a structure, but isn't embedded into the mortar won't have some of the same problems as a reinforcing bar. Nevertheless, such a chain will still undergo temperature cycles which will cause expansion/contraction of the chain relative to the bricks & mortar, and thus affect the lifetime of the structure. I don't know if a dome-encircling chain would require having its tension adjusted over the years. This would be an interesting question for the people in charge of St. Paul's. --- There is an interesting ancient feature found in some of the domed Islamic mosques: a number of "pillars" which can be easily rotated by hand. These pillars are several feet long, and _measure_, in effect, the distance between two attachment points. If these pillars cannot be easily rotated by hand, then it is clear that the distance between the two attachment points has decreased, and this indicates that the surrounding structure has been compromised and is starting to be crushed. This alerts the maintenance engineers to look for cracks & begin repairs. These rotating pillars seem to be unique to the Arabic domes, and are apparently not a feature of the older Roman domes. At 08:04 PM 12/26/2014, Warren D Smith wrote:
Incidentally it is not known whether Brunelleschi employed metal reinforcements. Apparently at least some plans said they were to be used but he may have decided during construction they were not needed. If he did use them, they are deeply embedded inside his masonry where nobody can detect them. The techniques B used in building the Duomo apparently are disputed and there is no consensus how he did it.