The Engineering Contributions of Wendel Bollman Part 2

During the prewar years, Bollman evolved a structural development of most profound importance, which is usually a.s.sociated with the Phoenix Iron Works and its founder, Samuel J. Reeves. In the erection of a high trestlework viaduct for the Havana Railroad, Bollman apparently became concerned with the tensile weakness of cast iron when applied in long, unsupported columns. Although a column is normally subjected to compressive stresses, when the slenderness ratio--that is, the length divided by the radius of gyration of the cross section--becomes great, a secondary bending stress may be produced. If this stress becomes great enough, the value of the tensile stress in one side of the column may actually exceed the princ.i.p.al compressive stress, and a net effect of tension result.

[Ill.u.s.tration: Figure 18.--OHIO RIVER CROSSING of the Baltimore and Ohio at Benwood, West Virginia, completed in 1870. Bollman deck trusses were used in the approaches on both sides. (Photo 693, Baltimore and Ohio Collection, Museum of History and Technology.)]

[Ill.u.s.tration: Figure 19.--PATAPSCO RIVER crossing of the Baltimore and Ohio between Thistle and Ilchester, Maryland. (Photo 695, Baltimore and Ohio Collection, Museum of History and Technology.)]

As already mentioned, the few available rolled-iron shapes were of relatively small area and quite unsuitable for use as columns unless combined and built up in complex fabrications. The normal practice at the time was to use cast compression members in iron bridges and structures, with their sectional area so proportioned to the length that a state of tension could not exist. In the case of long members, this naturally meant that an excessive amount of material was used.

[Ill.u.s.tration: 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 outstanding f.l.a.n.g.es 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.

[Ill.u.s.tration: 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 a.s.sociates 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 pa.s.s 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.

Final Use of the Bollman Truss

The Bollman truss found occasional use elsewhere than on the B. & O.

lines, but generally only when erected on contract by Pataps...o...b..idge and Iron Works. However, the fact that Bollman could profitably erect this bridge in the severely compet.i.tive 1870's indicates that the harsh criticism of the system by authorities of such stature as Whipple was not necessarily justified. Bollman's advertis.e.m.e.nts, in fact, refer to the favorable recommendations of other such renowned engineers as Herman Haupt and M. C. Meigs.

[Ill.u.s.tration: Figure 22.--BOLLMAN DECK TRUSSES in the North River Bridge built in 1873 at Mount Crawford, Virginia, on the Valley Railroad of Virginia (B. & O.). Each end span is 98 ft. 6 in.; the river span is 148 ft. 9 in. (Photo 756, Baltimore and Ohio Collection, Museum of History and Technology.)]

An interesting application of the system was in a drawbridge, formed of two Bollman deck spans, over an arm of the Mississippi at Quincy, Illinois (see fig. 17). The first iron bridge in Mexico was erected by Bollman over the Medellin River about 1864. Another work of this period, which attracted considerable attention, was a pair of bridges that Bollman erected over North Carolina's Cape Fear River in 1867-1868.

These bridges were notable for their foundation on cast-iron cylinders, sunk pneumatically. This was one of the first instances of the use of the process in America, and the depth of 80 feet below the water surface reached by one cylinder was considered remarkable for years afterward.

In the last active decade or so of his career, Bollman produced hundreds of minor bridges and other structures. In 1873 he supplied the castings for the splendid iron dome of Baltimore's City Hall and erected the ingenious water-main truss which carries Lombard Street over Jones Falls in that city. In this structure the top and bottom chords of the central line of trussing are cast-iron water mains, bifurcated at the abutments, and joined by cast- and wrought-iron web members (see fig. 20).

In the mid 1870's Bollman saw his truss pa.s.s into obsolescence. This was due primarily to the generally increasing distrust of cast iron for major structural members due to its brittleness, but advances in structural theory, availability of a greater variety of rolled structural shapes, and the increasing loading patterns of the period all contributed.

[Ill.u.s.tration: Figure 23.--THE ONLY SURVIVING BOLLMAN TRUSS BRIDGE, at Savage, Maryland. The bridge was built elsewhere in 1852 and was moved to this now-abandoned Baltimore and Ohio industrial siding in about 1888.]

Although no Bollman trusses were built by Bollman or the B. & O. after 1875, those in use were only removed as required by heavier motive power. The Harpers Ferry span, as noted, remained in full main-line service until 1894. Bollman trusses on feeder lines were continued in use until much later; a number of them on the Valley Railroad of Virginia (see fig. 22) were not removed until 1923. However, only on the most isolated spurs was the Bollman truss permitted to reach really ripe age. The sole known remaining example (fig. 23) stands on such a branch--ironically, at Savage, over the Little Patuxent, the site of the first Bollman span. This is not the 1850 bridge, but one built in 1852 and moved to the present site 30 years later. The fate of the first span is not known.

[Ill.u.s.tration: Figure 24.--HOT-WATER AND CHOCOLATE PITCHERS of the 10-piece, silver tea service presented to Bollman by his fellow employees when he resigned from the Baltimore and Ohio in 1858. A railroad motif was used throughout, each piece being circled at top and bottom by a track, complete with rail of accurate section and ties.

Spouts are in simulation of hexagonal sheet-iron chimneys, with seams riveted, and the handles are in the form of a surveyor's telescope. On the various pieces are engraved the designs of the more important B. & O. bridges. Throughout is a wonderful profusion of bits and objects of railroadiana in low relief, high relief, and fully modeled. In Board of Directors Room, Baltimore and Ohio Railroad Company, Baltimore, Md.

(_Photo courtesy of Baltimore and Ohio Railroad._)]