Under The Capitol Dome

This article discusses the engineering concepts used in constructing the dome of The Capitol. It is a product of high-grade 19th century engineering. It is really cast iron painted to look like marble, a counterfeiting that provoked controversy before the dome was built. The controversy over the iron dome was a long time in dying. As late as 1904, a movement was still afoot to replace the cast iron with marble. A study was undertaken to explore the idea. The internal structure resembles present-day steel truss work; however, it is really a series of cast iron modules, some as much as 10-feet deep and weighing 10 tons. The castings that were finally adopted show impressive attention to detail and, to a visitor's eye, remarkable dimensional tolerance. Unlike marble, which has corrosion problems of its own, cast iron is prone to rust. In order to support this program, an elegant finite element analysis of the entire structure was performed; it was published in the October 2000 issue of Civil Engineering magazine.