Figure 20.—Two views of Bollman-built “water-pipe truss” that carries Lombard Street over Jones Falls in Baltimore. Built in 1877.
Bollman was conscious of the problem from his experience with the stretchers and struts of his truss, and he must have been aware of the great advantage which would be obtained by a practical method of forming such members in wrought iron, the tensile resistance of which is equivalent to the compressive. He eventually developed the forerunner of what came to be known as the Phoenix form by having special segmental wrought-iron shapes rolled by Morris, Tasker and Company of Philadelphia, these shapes being combined into a circular section with out standing flanges for riveting together. The circular section is theoretically the most efficient to bear compressive loading. A column of any required diameter could be produced by simply increasing the number of segments, the individual size of which never exceeded contemporary rolling mill capacity (see [fig. 16]).
The design exhibits the inspired combination of functional perfection and simplicity that seems to characterize most great inventions.
Figure 21.—The Harpers Ferry bridge toward the end of its career, carrying a common road over the Potomac. The westernmost line of trussing and span no. 1 had been removed long before. View through the Winchester span looking toward Maryland in 1933. (Photo courtesy of Harpers Ferry National Historical Park.)
It may have been because he had no facilities for rolling that Bollman communicated his idea to Reeves, although this seems illogical. At any rate, Reeves and his associates patented the system extensively, and the Phoenix column was eventually employed to the virtual exclusion of cast-iron and other types of wrought-iron columns. By the end of the 19th century it began to pass from use, as mills became capable of producing larger sections with properties relatively favorable to column use and more adaptable to connection with other members.