To continue with the “old skyscraper” theme, one of the great joys of research is seeing an idea taking shape in the historical record. In 1894, Corydon Purdy, one of the first structural engineers in the US to take the kind of rational analysis of steel framing developed for truss bridges and apply it to buildings, gave a paper at the American Society of Civil Engineers titled “Wind Bracing in High Buildings.” Lateral-load design tends to govern the structural engineering of skyscrapers because of a simple fact of geometry: doubling the height of a rectangular-prism-shaped building increases the gravity loads at the base by a factor of two, but increases the overturning moment by a factor of four. As the tallest occupied buildings went from 250 to 500 to 1000 feet and higher, structural form became more and more adapted to resisting lateral loads. Lateral loading meant wind design at first, with more and more of the country adding seismic design as the twentieth century progressed.
Purdy’s paper was not the first discussion of this topic, and Gustave Eiffel’s tower, where the entire structure is designed around wind pressure, was already five years old at the time. The paper was, however, one of the first serious discussions of the topic with reference to ordinarily-occupied buildings (which the Eiffel Tower is very much not), and was widely disseminated in the design and construction world in 1895, when discussion of the paper was heavily excerpted in the Engineering Record.
The paper and discussion are full of great quotes that are the ancestors of ordinary elements of structural frame design today. For example, “…partitions are often omitted in steel-constructed buildings, and in his practice not much dependence is placed on their resistance to wind in regular office buildings. If they are omitted in one floor it is nearly as bad as it they were omitted in several” is both a clear and early statement of the need for top-to-bottom wind bracing (not yet standard in 1894) and also is arguably one of the first engineering statements about the dangers of soft stories.
The diagram above is from an 1893 paper by Henry Quimby on the same topic of wind bracing. Quimby is worth a read if you’re interested in the topic, but his analysis is less thorough than Purdy’s and less useful both in the 1890s and today. The 1889 Tower Building in New York was not a skeleton frame building – it was a very odd hybrid in several ways – but it did have a reasonably well-thought-out set of wind-bracing trusses, as seen in this diagram.
