The Engineering Contributions of Wendel Bollman.

by Robert M. Vogel.

_The development of structural engineering has always been as dependent upon the availability of materials as upon the expansion of theoretical concepts. Perhaps the greatest single step in the history of civil engineering was the introduction of iron as a primary structural material in the 19th century; it quickly released the bridge and the building from the confines of a technology based upon the limited strength of masonry and wood._

_Wendel Bollman, self-taught Baltimore civil engineer, was the first to evolve a system of bridging in iron to be consistently used on an American railroad, becoming one of the pioneers who ushered in the modern period of structural engineering._

THE AUTHOR: _Robert M. Vogel is curator of civil engineering in the Smithsonian Inst.i.tution's Museum of History and Technology._

Wendel Bollman's name survives today solely in a.s.sociation with the Bollman truss, and even in this respect is known only to a few older civil and railroad engineers. The Bollman system of trussing, along with those of Whipple and Fink, may be said to have introduced the great age of the metal bridge, and thus, directly, the modern period of civil engineering.

Bollman's bridge truss, of which the first example was built in 1850, has the very significant distinction of being the first bridging system in the world employing iron in all of its princ.i.p.al structural members that was used consistently on a railroad.

The importance of the transition from wood to iron as a structural and bridge building material is generally recognized, but it may be well to mention certain aspects of this change.

The tradition of masonry bridge construction never attained the great strength in this country which it held in Europe, despite a number of notable exceptions. There were several reasons for this. From the very beginning of colonization, capital was scarce, a condition that prevailed until well into the 19th century and which prohibited the use of masonry because of the extremely high costs of labor and transport.

An even more important economic consideration was the rapidity with which it was necessary to extend the construction of railways during their pioneer years. Unlike the early English and European railways, which invariably traversed areas of dense population and industrial activity, and were thus a.s.sured of a significant financial return almost from the moment that the first rail was down, the Baltimore and Ohio and its contemporaries were launched upon an entirely different commercial prospect. Their princ.i.p.al business consisted not so much in along-the-line transactions as in haulage between princ.i.p.al terminals separated by great and largely desolate expanses. This meant that income was severely limited until the line was virtually complete from end to end, and it meant that commencement of return upon the initial investment was entirely dependent upon the speed of survey, graduation, tunneling, and bridging.

[Ill.u.s.tration: Figure 2.--MODEL OF B. H. LATROBE'S TRUSS, built in 1838, over the Patapsco River at Elysville (now Daniels), Maryland. (_Photo courtesy of Baltimore and Ohio Railroad._)]

The need for speed, the general attenuation of capital, and the simple fact that all the early railroads traversed thickly forested areas rendered wood the most logical material for bridge and other construction, both temporary and permanent.

The use of wood as a bridge material did not, of course, originate with the railroads, or, for that matter, in this country. The heavily wooded European countries--Switzerland in particular--had a strong tradition of bridge construction in timber from the Renaissance on, and naturally a certain amount of this technique found its way to the New World with the colonials and immigrants.

America's highway system was meager until about the time the railroad age itself was beginning. However, by 1812 there were, along the eastern seaboard, a number of fine timber bridges of truly remarkable structural sophistication and workmanship.

It was just previous to the advent of the railroads that the erection of highway bridges in this country began to pa.s.s from an art to a science.

And an art it had been in the hands of the group of skilled but unschooled master carpenters and masons who built largely from an intuitive sense of proportion, stress, and the general "fitness of things." It pa.s.sed into an exact science under the guidance of a small number of men trained at first in the scientific and technical schools of Europe, and, after about 1820, in the few inst.i.tutions then established in America that offered technical instruction.

The increasing number of trained engineers at first affected highway bridge construction not so much in the materials used but in the way they were a.s.sembled. In a bridge designed by a self-taught constructor, the cheapness of wood made it entirely feasible to proportion the members by enlarging them to the point where there could be no question as to their structural adequacy. The trained engineer, on the other hand, could design from the standpoint of determining the entire load and then proportioning each element according to the increment of stress upon it and to the unit capacity of the material.

By the time railroads had started expanding to the West there had been sufficient experience with the half dozen practical timber truss systems by then evolved, that there was little difficulty in translating them into bridges capable of supporting the initial light rail traffic.

In spite of its inherent shortcomings, wood was so adaptable that it met almost perfectly the needs of the railroads during the early decades of their intense expansion, and, in fact, still finds limited use in the Northwest.

Early Career

Wendel Bollman was born in Baltimore of German parents in 1814. His father was a baker, who in the same year had aided in the city's defense against the British. Wendel's education, until about the age of 11, was more or less conventionally gained in public and private schools in Baltimore. He then entered into informal apprenticeship, first to an apothecary in Sheperdstown, Virginia (now West Virginia), and then to one in Harpers Ferry. In 1826 or 1827 he became ill and returned to Baltimore for cure. From that time on his education was entirely self-acquired.

[Ill.u.s.tration: Figure 3.--TRUSSED BEAM.]

It is of interest, in light of his later career, to note that on the Fourth of July 1828, he marched with other boys in a procession that was part of the Baltimore and Ohio Railroad's cornerstone-laying ceremony.

Shortly afterward, he apprenticed himself to a carpenter for a brief time, but when the work slacked off he obtained work with the B. & O.

The right-of-way had been graded for about five miles by that time, but no rail was down. The boy was at first given manual work, but soon advanced to rodman and rapidly rose as he gained facility with the surveying apparatus. In the fall of 1829 he partic.i.p.ated in laying the first track. As his mother was anxious that he continue his education in carpentry, he left the railroad in the spring of 1830 to again enter apprenticeship. He finished, became a journeyman, helped build a planter's mansion in Natchez, and returned to Baltimore in 1837 to commence his own carpentry business. The next year, while building a house in Harpers Ferry, he was asked to rejoin the B. & O. to rebuild parts of its large timber bridge over the Potomac there, which had fallen victim to various defects after about a year's use.

[Ill.u.s.tration: Figure 4.--SIMPLE BEAM of 50-foot span with three independent trussing systems. Bollman's use of this method of support led to the development of his bridge truss. This drawing is of a temporary span used after the timber bridge at Harpers Ferry was destroyed during the Civil War. (In Baltimore and Ohio Collection, Museum of History and Technology.)]

Shortly after the Harpers Ferry bridge reconstruction, Bollman was made foreman of bridges. It is apparent that, on the basis of his practical ability, enhanced by the theoretical knowledge gained by intense self-study, he eventually came to a.s.sist Chief Engineer Benjamin H.

Latrobe in bridge design. He later took this work over entirely as Latrobe's attentions and talents were demanded in the location and extension of the line between c.u.mberland and Wheeling.

[Ill.u.s.tration: Figure 5.--BOLLMAN'S ORIGINAL PATENT DRAWING, 1851. (In National Archives, Washington, D.C.)]

The B. & O. did not reach its logical destination, Ohio (actually Wheeling, West Virginia, on the east bank of the Ohio River) until 1853.

In the years following Bollman's return to the railroad, the design of bridges was an occupation of the engineering staff second in importance only to the location of the line itself. During this time Bollman continued to rise and a.s.sume greater responsibilities, being appointed master of road by Latrobe in 1848. In this position he was responsible for all railroad property that did not move, princ.i.p.ally the right-of-way and its structures, including, of course, bridges.

The recognition of Bollman's abilities was in the well-established tradition of the B. & O., long known as America's first "school of engineering," having sponsored many early experiments in motive power, trackwork, and other fundamental elements of railroad engineering. It furnished the means of expression for such men as Knight, Wright, Whistler, Latrobe, and Winans.

[Ill.u.s.tration: Figure 6.--PLAN OF HARPERS FERRY BRIDGE as built by Latrobe. The second Winchester track was later removed.]

Of these pioneer civil and mechanical engineers, some were formally trained but most were self-taught. Bollman's career on the B. & O. is of particular interest not only because he was perhaps the most successful of the latter cla.s.s but because he was probably also the last. He may be said to be a true representative of the transitional period between intuitive and exact engineering. Actually, his designing was a composite of the two methods. While making consistent use of mathematical a.n.a.lysis, he was at the same time more or less dependent upon empirical methods. For years, B. & O. employees told stories of his sessions in the tin shop of the railroad's main repair facility at Mount Clair in Baltimore, where he built models of bridges from sc.r.a.ps of metal and then tested them to destruction to locate weaknesses. It seems most likely, however, that the empirical studies were used solely as checks against the mathematical.

[Ill.u.s.tration: Figure 7.--RECENT MODEL of Bollman's Winchester span.

Only two of the three lines of trussing are shown. The model is based on Bollman's published description and drawings of the structure. (USNM 318171; Smithsonian photo 46941.)]

In the period when Bollman began designing--about 1840--there were fewer than ten men in the country designing bridges by scientifically correct a.n.a.lytical methods, Whipple and Roebling the most notable of this group.

By 1884, the year of Bollman's death, the age of intuitive design had been dead for a decade or longer.

[Ill.u.s.tration: Figure 8.--THE BALTIMORE AND OHIO RAILROAD'S Potomac River crossing at Harpers Ferry, about 1860. Bollman's iron "Winchester span" of 1851 is seen at the right end of Latrobe's timber structure of 1836, which forms the body of the bridge. (_Photo courtesy of Harpers Ferry National Historical Park._)]

The B. & O. was in every way a truly pioneer enterprise. It was the first practical railroad in America; the first to use an American locomotive; the first to cross the Alleghenies. The spirit of innovation had been encouraged by the railroad's directors from the outset. It could hardly have been otherwise in light of the project's elemental daring.

The first few major bridges beyond the line's starting point on Pratt Street, in Baltimore, were of rather elaborate masonry, but this may be explained by the projectors' consciousness of the railroad's significance and their desire for permanence. However, the aforementioned economic factors shortly made obvious the necessity of departure from this system, and wood was thereafter employed for most long spans on the line as far as Harpers Ferry and beyond. Only the most minor culverts and short spans, and those only in locations near suitable quarries, were built of stone.

In addition to the economic considerations which prompted the company to revert to timber for the major bridges, there were several situations in which masonry construction was unsuitable for practical reasons. If stone arches were used in locations where the grade of the line was a relatively short distance above the surface of the stream to be crossed, a number of short arches would have been necessary to avoid a very flat single arch. In arch construction, the smaller the segment of a circle represented by the arch (that is, the flatter the arch), the greater the stress in the arch ring and the resulting horizontal thrust on the abutments.

[Ill.u.s.tration: Figure 9.--BOLLMAN SKEW BRIDGE at Elysville (now Daniels), Maryland, built in 1853-1854. (_Photo courtesy of Maryland Historical Society._)]

The piers for the numerous arches necessary to permit an optimum amount of rise relative to the span would have presented a dangerous restriction to stream flow in time of flood. By the use of timber trusses such crossings could be made in one or two spans with, at the most, one pier in the stream, thus avoiding the problem.

The princ.i.p.al timber bridges as far west as c.u.mberland were of Latrobe's design. These were good, solid structures of composite construction, in which a certain amount of cast iron was used in joints and wrought iron for certain tension members. They were, however, more empirical than efficient and, for the most part, not only grossly overdesigned but of decidedly difficult fabrication and construction.

What is interesting about the Latrobian timber trusses, however, is the effect they appear to have had upon Bollman's subsequent work in the design of his own truss. This effect is evidenced by the marked a.n.a.logy between the primary structural elements of the two types. The Latrobe truss at Elysville (fig. 2) was only partially a truss, inasmuch as the greater part of the load was not carried from panel to panel, finally to appear at the abutments as a pure vertical reaction, but was carried from each panel (except the four at the center) directly to the bearing points at the piers by heavy diagonal struts, after the fashion of the famous 18th-century Swiss trusses of the Grubenmanns. It was a legitimate structural device, and the simplest means of extending the capacity of a spanning system. However, it was defective in that the struts applied considerable horizontal thrust to the abutments, requiring heavier masonry than would otherwise have been necessary.

It is quite likely that Latrobe did not have absolute confidence in the various pure truss systems already patented by Town, Long, and others, and preferred for such strategic service a structure in which the panel members acted more or less independently of one another. It will be seen that, similarly, the individual panel loads in Bollman's truss were carried to the ends of the frame by members acting independently of one another.

The Bollman Truss

There had never been any question about the many serious inadequacies of wood as a bridge material. Decay and fire risk, always present, were the princ.i.p.al ones, involving continuous expenditure for replacement of defective members and for fire watches. It was, in fact, understood by the management and engineering staff of the B. & O. that their timber bridge superstructures, though considered the finest in the country, were more or less expedient and were eventually to be replaced. In this regard it is not surprising that Latrobe, a man of considerable foresight, had, at an early date, given serious thought to the possible application of iron here.