David Guise

The Evolution of the Warren, or Triangular, Truss

The adoption of the Warren truss in America, driven by the need to accommodate ever-increasing span and load requirements, produced a wide variety of subdividing patterns and intriguing variations. Although the use of a truss web composed of contiguous triangles, commonly referred to as a "Warren" truss, traces its beginnings from the use of isosceles triangles in continental Europe to the use of equilateral triangles in Great Britain, it become an ubiquitous part of the American landscape by the early-20th century. The chronological evolution of these American forms is presented with the aid of patent drawings and photographs to demonstrate the wide range of ideas and solutions. The web configuration of a basic Warren truss is composed of a series of alternately sloped diagonal members that produce a distinctive image of contiguous "^^^" shapes, arrayed between a pair of parallel upper and lower horizontal chords (see figure 1). Because of this visual characteristic, examples are also referred to as "triangular" trusses. Ultimately, variations based on this concept of repetitive triangular forms were developed, created by the insertion of verticals subdividing the triangles. Of the literally hundreds of truss configurations proposed during the 19th century, the original Warren configuration is the only one that did not contain verticals and the only one with a precedent in nature. The hollow bones of dinosaurs and modern birds are braced with the same pattern of equilateral triangles (see figure 2).

Figure 1. Warren truss with a triangular pattern of diagonal web members, Worcester, England; 1905 railway bridge over the Severn River. Photo courtesy of Derek Locke.

Figure 2. Section cut through a hollow bird bone, showing a configuration of inclined diagonals. Drawing, courtesy of Institute of Biological Science, University of Wales, Aberystwth, UK.

Early History

The exact origin of a truss web composed only of identical repetitive triangles is obscure. Apparently first used in northern Italy, it eventually migrated to England by way of France and Belgium.¹ Alfred Henry Neville, an English entrepreneur and engineer, built several triangular webbed-truss bridges in France and Belgium during the late 1830s and early 1840s, using a configuration of repetitive isosceles triangles that supported a roadbed at midheight.² Neville, Nash et Compagnie, a firm with addresses in Turin and Paris, obtained a French patent for its design in 1838, and in 1839 William Nash obtained a British patent for this same design (see figure 3).³ It was not until 1848 that James Warren and Willoughby Monzani, who may have had input to some of the earlier uses of this form, obtained a British patent for a configuration of repetitive equilateral triangles that could support a roadbed on either its top or bottom chord (see figure 4). Warren's name became synonymous with this form, as the first major spans using this configuration were built in England, and English construction firms built prefabricated versions for use in the British colonies, especially India. In 1851, a 240-foot, 6-inch span iron Warren truss, Newark Dyke Bridge, was built to carry the Great Northern Railroad tracks over England's Trent River (see figures 5a and 5b).⁴

Figure 3. Neville-Nash 1838 patent (French patent no. 11, 201) for a triangular truss with a web composed of isosceles triangles; bridge deck member at midheight of truss.

Figure 4. Warren-Monzani, 1848 British patent (patent no. 12,242) for a triangular truss with a web composed of equilateral triangles; bridge deck at either top or bottom chord.

Figure 5a. Newark Dyke Bridge, 1852. Drawing by Joseph Cubitt, engineer in charge of construction, found in Minutes of Proceedings, Institution of Civil Engineers 12 (1853): plate 4.

Figure 5b. Newark Dyke Bridge. This 1888 photograph shows a set of three splayed hangers originating at each panel point along the top chord, while the drawing (Figure 5a) indicates a single vertical hanger. Photo courtesy of Carol Morgan, Institution of Civil Engineers, London, England.

The 10-span Crumlin Viaduct, built to carry the Newport Abergavenny & Hereford railway over the Ebbw River valley at Blaenau Gwent, South Wales, was completed in 1857 to worldwide acclaim. The Crumlin truss design incorporated a slight change to Warren's initial patent, making it more efficient. The truss chords, based on the viaduct builder Thomas Kennard's 1853 patent,⁵ were composed of hollow box beams, fabricated from plates and angles, while the Warren-Monzani truss chords consisted of either solid plates or rods. Although the span between the Crumlin's tower supports was a modest 150 feet, compared to the Newark Dyke Bridge's 240 feet, the Crumlin Viaduct was more dramatic. Hovering 200 feet above the valley floor, it created an awesome spectacle that stood for more than a century until it was demolished in 1964 (see figures 6a, 6b, and 6c).

Figure 6a. Crumlin Viaduct. William Humber, Practical Treatise on Cast and Wrought Iron Bridges and Girders (London, England: E. & F. N. Spon, 1857).

Figure 6b. Crumlin Viaduct. Author's collection.

Figure 6c. View of Crumlin Viaduct from underneath the bridge deck, taken during demolition <www.crumlinviaduct.co.uk>. Photo by Colin Bedoe.

Although the concept of a triangular web originated in Europe, it may also have been independently invented in America. In his 1872 book, Squire Whipple states that he built several triangular trusses in 1849 and 1850,⁶ but triangular configured trusses did not see widespread use in America until near the end of the 19th century when bridge construction switched from pin-connected to riveted joints. There were, however, a few early, and sometimes dramatic, exceptions.

In 1858 Albert Briggs obtained a patent for a combination iron and timber bridge (see figure 7).⁷ Although the drawing showed a triangular web, the patent claim was made only for the method of providing blocking at the joints. While not shedding light on what was actually built at that time, the patent clearly establishes that an American awareness of the Warren configuration existed prior to the Civil War. The 1870s saw several American patents issued for "Warren truss" designs. Two were obtained by Thomas Pratt: one in 1871 for an iron truss and a second in 1873 for a timber truss.⁸ Edward Hemberle patented a triangular webbed truss with curved ends in 1874, and in 1876 David Hammond patented a truss that had a triangularly configured web of iron bars that connected parallel chords of tee sections.⁹ Daniel McCallum, a Civil War general also known for his "Inflexible Arched Truss Bridges," built several all-timber Warren-type road bridges with triangular webs (see figure 8).¹⁰ Although none of these early proposals appears to have had an impact on the development of the Warren triangular configuration in America, they clearly establish an awareness of the form.

Figure 7. Albert Briggs U.S. patent no. 20,987 of 27 July 1858.

Figure 8. Daniel McCallum's timber Warren. James Mosse, "American Timber Bridges," Minutes of Proceedings, Institution of Civil Engineers 22 (1863): plate 6.

Albert Fink's Triangular Trusses

Albert Fink's name is most often associated with the unique suspension-truss configuration he patented in 1854.¹¹ It is his early use of triangular truss configurations, however, that had a more lasting impact on the overall development of the American truss bridge.

There is no definitive way to ascertain his familiarity with European use of triangular patterns for the web of truss bridges. It would seem likely, however, that he was aware of them as he graduated from the Polytechnic School in Darmstadt, Germany, in 1848. Fink immigrated to America in 1849 and found employment with the Baltimore and Ohio Railroad.

In 1866, Fink erected a Warren-type truss bridge over the Tennessee River at Decatur, Alabama, as part of the post-Civil War reconstruction of the Memphis & Charleston Railroad (see figure 9). He obtained a patent for this design a year later in 1867 (see figure 10).¹² In his patent description, Fink stated that he was not claiming originality as to the arrangement of the parts but to the connection details for a triangular truss composed primarily of timber. The top chord and all the diagonal web members, with the exception of the first one at each end, were timber. The bottom chord, verticals, and the first diagonal web members were wrought iron. The 1871 combination timber and wrought-iron Blue River Bridge in Indiana for the Jefferson, Madison and Indianapolis Railroad was built to this same design.

Figure 9. Albert Fink's combination Warren over the Tennessee River, Decatur, Alabama, 1866. Decatur Advertiser, 27 July 1900; courtesy John Allison at Morgan County Archives.

Figure 10. Albert Fink, U.S. patent no. 63,714, 9 April 1867.

Between 1868 and 1870, Fink's huge 396-foot bridge, a pin-connected version of a Warren truss, was built over the Ohio River at Louisville, Kentucky, for the Pittsburgh, Cincinnati & St. Louis Railroad (see figure 11a).¹³ This parallel chord truss had a 56-foot, 7 1/2-inch-high triangular web configuration, extending from the bottom to the top chord, containing a secondary triangular truss that braced the top chord. A year after construction was completed he obtained a patent for this unique configuration (see figure 11b).¹⁴

Figure 11a. Albert Fink's 396-foot Louisville, Kentucky, bridge over the Ohio River, 1870. Library of Congress RG 77-HCS-151aF.

Figure 11b. Albert Fink, U.S. patent no. 116,787, 4 July 1871.

A less dramatic but far more typical contemporary use of the Warren truss was the all iron, pin-connected Pennsylvania Railroad span built in 1869 over the Little Juniata River in Pennsylvania (see figure 12).

Figure 12. All-iron Warren truss over the little Juniata River, Pennsylvania. James Dredge, History of the Pennsylvania Railroad (New York, N.Y.: Wiley, 1879).

In 1885, construction was completed on the longest span bridge Fink designed, an enormous 521-foot, 11 1/2-inch-long crossing of the Ohio River at Henderson, Kentucky (see figure 13). At the time, its clear span length was a world record.¹⁵ The configuration of the truss web was not unusual, except for its continuous midheight horizontal bracing.

Figure 13. Albert Fink's 521-foot, 11–1/2-inch Henderson, Kentucky, bridge over the Ohio River, 1885. Author's collection.

Stress Distribution

Under a uniform load, alternate members of the web diagonals are subjected to either tensile or compressive stresses. As a moving load crosses over a Warren truss bridge, the truss diagonals are subjected to stress reversals, thus requiring the web members, especially those near midspan, to be capable of handling either compressive or tensile stresses since there are no counter diagonals such as those found in a Pratt truss. One interesting solution to this problem was to run cables through the hollow compression members in the web to resist the tensile stress the compressive members might be subjected to by moving loads.¹⁶

Since longer spans require greater truss depth, the size of the triangles contained within the truss automatically increases in proportion to increases in truss height. Larger triangles result in successive panel points being spaced further apart along the chords. The increased distance between panel points will cause the framing for the bridge floor to become inefficient. Additionally, as the truss depth increases, so does the length of the compression diagonal struts, lowering their ability to resist buckling. These issues caused engineers to devise ways of subdividing the basic triangular rhythm of the Warren in order to provide intermediate points of support to carry the bridge deck and to brace long compression members. The Warren configuration easily lends itself to the insertion of verticals that connect the midpoints of each triangle's base with the apex of the triangle (see figure 14).

Figure 14. Warren deck truss with a single set of verticals; verticals struts at alternate panel points support the deck carrying top chord. Soo Line railroad bridge over the Mississippi near Royalton Minnesota. Author's collection.

When the cost saved by not increasing the cross-sectional area of the chord is greater than the added cost of the brace, vertical compression web members are used to brace the top chord of through trusses to control buckling. Additionally, verticals at the triangular apexes along either the top or bottom chord tie into and thus augment the horizontal bracing system between the two trusses.

Warren Variations

In response to greater loads and spans, the triangles forming the web of a Warren truss were further subdivided in a variety of increasingly complex ways. A second set of verticals extending the full height of the truss can be added in order to halve the distance between panel points along both the top and bottom chords (see figure 15). For longer spans or very heavy load situations, infilling subdividing members are arranged to brace the diagonals and further decrease the distance between panel points (see figure 16). While many of the subdivided Pratt configurations acquired specific names (such as Baltimore, Pennsylvania, Pettit, and Parker), the Warren variations are referred to only by their generic shape, such as "Warren with verticals."

Figure 15. Warren truss with double set of verticals; vertical hangers support bridge deck; vertical struts brace top chord to prevent buckling, Rockford, North Dakota. Author's collection.

Figure 16. Subdivided Warren truss with double set of verticals, diagonal bracing struts, and secondary set of vertical hangers to support the bridge deck; railroad bridge with a suspended vehicle bridge over the Androscoggin River, Brunswick, Maine. Photo by author.

Still another method of increasing the strength or the spanning capacity of the Warren is to provide a full set of additional diagonals, forming what is known as a double Warren (see figure 17). It is called a triple Warren when two sets of diagonals are added. At some point, usually if three or more extra sets are added to the Warren, the configuration is more commonly referred to as a lattice truss.¹⁷

Figure 17. Double Warren truss vehicle bridge over Delaware River, Washington Crossing, New Jersey. Photo by author.

Additionally, as in the case of the Pratt truss, polygonal curved upper chord variations of the Warren were built, their greater midspan height providing greater strength (see figure 18). The triangular Warren web pattern was also used in a few lenticular trusses (see figure 19).

Figure 18. Polygonal chord Warren truss over the Delaware River, Hancock, New York. Photo by author.

Figure 19. Lenticular truss with a triangular web over Swatara Creek, Waterville, Pennsylvania, 1890. Joseph Elliot, photographer (HAER PA-462).

Fink's 1868 and 1885 bridges over the Ohio River were one-of-a-kind variations on the basic triangular configuration, as was the late-19th-century modest span bridge at Rumford Falls, Maine (see figure 20).

Figure 20. An atypical Warren truss with full-height verticals at all main and secondary panel points, Rumford Falls, Maine. Author's collection.

Tinkering with methods of subdividing the basic Warren continued into the 20th century. In 1922, Claude Turner built a three-span bridge over the Missouri River (see figures 21a and 21b). Each of the 476-foot spans used his unique subdividing configuration. Because Turner's triangulation pattern differed sufficiently from a standard Warren's, he was able to receive a patent for it in 1923.¹⁸

Figure 21a. Claude Turner's Memorial Bridge over the Missouri River, Mandan, North Dakota, 1922. Kent Good, photographer (HAER ND-7).

Figure 21b. Claude Turner, U.S. patent no. 1,441,387, 9 January 1923.

The Warren Configuration vs. the Competition

Complete nationwide statistics as to the number of Warren or Pratt trusses actually built in a given time period do not exist.¹⁹ However, by the start of the 20th century, riveted forms of the Warren truss became competitive with the Pratt for spans under 150 feet. The most common use of the Warren truss appears to be for modest-span railroad bridges and short-span pony trusses for vehicles.²⁰

Full Cycle

The 1992, 800-foot span, Charleston, South Carolina, steel bridge over the Cooper River achieved its clean horizontal line and an unadulterated triangular web pattern by varying the stress grades of the steel used for different segments of the truss (see figure 22). Therefore, the engineers, HNTB Corporation, did not need to either increase the bridge height towards midspan, provide verticals, or subdivide the panels.

Figure 22. 800-foot-span Cooper River Bridge, near Charlestown, South Carolina, 1992; by using a variety of different strength steel, the designers (HNTB Corporation) were able to maintain a uniformity of sizes for the individual truss members and avoid the use of vertical members. Photo courtesy of the HNTB Corporation.

In 2000, a third new bridge was erected over the Trent River at the site of the original Newark Dyke Bridge (see figure 23). Reverting back to the original bridge's web configuration, this new curved-chord steel truss features a web of triangles, thus closing the circle in a two-centuries-long evolutionary development of the triangular webbed truss.

Figure 23. The new Newark Dyke Bridge, erected at the site of the original Newark Dyke Bridge (the third at this location); this crossing illustrates the long evolutionary cycle of the triangular truss that lasted almost two centuries; erected in 2000, this curved-chord truss reverts to the original unadulterated "Warren" web of triangles. Photo by Lucas Engineering, New York, N.Y.

The earliest "Warren" trusses were composed of adjacent triangles, creating a pristine image undiluted by verticals or sloping subdividing members. Longer span and heavier load-carrying requirements resulted in a maze of complex panel configurations. With the onset of high strength steels, engineers have been able to return to the clean lines of the original triangular trusses.

Acknowledgements

This article benefited from the comments of James Stewart, Marvin Lessen, Saul Brody, and Gretchen Grunenfelder who read early drafts. Carol Morgan, archivist, and Annette Ruehlmann, librarian, at the British Institution of Civil Engineers provided invaluable research assistance. John Hooper at the Boston Public Library, Nancy Voyle at the Henderson Public Library, Tom Owen at the Univ. of Louisville Library, Rebecca Rice at the Filson Historical Society, and David Shayt at the Smithsonian were of invaluable assistance.

Notes

¹� For a detailed history of early European examples, see J. (John) G. James, "The Evolution of Iron Bridge Trusses to 1850," Transactions of the Newcomen Society 52 (1980–81): 82–84.

²� Almost all the literature refers to Neville as A. H. Neville. J. G. James in his Overseas Railways and the Spread of Iron Bridges, c. 1850–70 (self-published in 1988) tracked down Neville's origins and full name. Numerous references have incorrectly referred to Neville as French or Belgian. It is interesting to note that Thomas Kennard's 1853 British patent for an improvement to the Warren truss was witnessed by "A. H. Neville, 35 Bridge Street Blackfriars."

³� William Nash obtained British patent no. 7,975 for a triangular truss design developed on the European continent. No record of a bridge built to this design in Great Britain has come to light.

⁴� For a detailed description of the construction of this bridge, see J. (Joseph) Cubitt, "A Discussion of the Newark Dyke Bridge on the Great Northern Railway," Minutes of the Proceedings, Institution of Civil Engineers 12 (1853): 601–07.

⁵� Thomas Kennard, "Improvements in Iron Bridges," U.K. Patent no. 1,613, issued 6 July 1853.

⁶� Squire Whipple, A Practical Treatise on Bridge Building (New York, N.Y.: Wiley, 1872), 253–55, 309; Whipple's claim needs to be treated with a serous amount of skepticism. The claim was made decades after the fact, and no evidence has come to light to substantiate it.

⁷� Albert Briggs, "Truss Bridge," U.S. Patent no. 20,987, issued 27 July 1858.

⁸� Thomas Pratt, "Improvement in Truss-Bridges," U.S. Patent no. 114,039, 25 April 1871; Thomas Pratt, Wooden Truss Bridges," U.S. Patent no. 137,482, filed 16 January 1873, issued 11 April 1873.

⁹� Edward Hemberle, "Iron Truss Bridges," U.S. Patent no. 152,489, filed 28 March 1874, issued 30 June 1874; David Hammond, "Wrought-Iron Girder," U.S. Patent no. 184,522, filed 19 August 1876, issued 21 November 1876.

¹⁰� James Mosse, "American Timber Bridges," Minutes of the Proceedings, Institution of Civil Engineers 22 (1863): 305–07.

¹¹� Albert Fink, "Truss Bridge," U.S. Patent no. 10,887, issued 9 May 1854.

¹²� Albert Fink, "Truss Bridge," U.S. Patent no. 63,714, issued 9 April 1867.

¹³� Shortly afterward, the Pittsburgh, Cincinnati & St. Louis Railroad became part of the Pennsylvania Railroad system.

¹⁴� Albert Fink, "Improvement in Bridge-Trusses," U.S. Patent no. 116,787, issued 4 July 1871, reissued 15 February 1881.

¹⁵� Merriman & Jacoby compiled a careful list of the 50 longest span bridges in America in vol. 1 of Roofs and Bridges (New York, N.Y.: Wiley, 1904), p. 227, and Albert Fink's Henderson Bridge was the only Warren configuration. This bridge was replaced in 1932 by a curved-chord steel Warren truss bridge.

¹⁶� William Merrill, Iron Truss Bridges for Railroads (New York, N.Y.: Van Nostrand 1870), 93. [I was unable to find any records of an executed example of this design.]

¹⁷� An excellent example of an early-American riveted metal lattice (or quadruple-intersecting Warren) truss is the 1887 Boston & Maine Railroad crossing of the Connecticut River near Northampton, Massachusetts (HAER MA-55). The bridge at Slate Run, Pennsylvania, built in 1890 by the Berlin Bridge Company, is an interesting example of a less-common quintuple-intersecting lattice truss (HAER PA-460).

¹⁸� Claude Turner, "Long Span Bridge," U.S. Patent no. 1,441,387, filed 10 July 1913, renewed 21 January 1921, issued 9 January 1923. This 1923 patent is an upgrade to his original 1913 patent application.

¹⁹� Some state highway departments do have such records, but they would have to be combined with railroad bridge records for a complete total. Many states inventoried their bridges in the mid-20th century, but the inventories do not include bridges that were no longer standing at the time.

²⁰�Catalogue of Centenary Exhibition of the Baltimore and Ohio Railroad 1827–1927 (Baltimore, Md: B&O Railroad 1927) p. 30: "The Warren Truss: This truss, as originally introduced, provided for the use of inclined web members only, but subsequently modified by the introduction of vertical members. With the improvement of riveting methods it has, in large measure, superseded the Pratt truss for short spans, and is also extensively used for long span railroad bridges." The largest span on the B&O system was a 434-foot crossing, built over the Alleghany River in 1920.

In 19th-century Europe, new roads and rail lines were built to connect established cities, and bridge sites were located close to population centers with skilled labor pools. Riveting, therefore, was an economically viable connection system, and it produced stiffer structures than those with pin joints. Most of America's new roads and rail lines headed towards unpopulated wilderness areas in the hope their presence would induce development. Crews of unskilled laborers erected America's bridges. Trusses were assembled on the floor of fabricating factories, parts numbered, disassembled, and shipped to the erection site for reassembly over the river or stream. Eventually America developed a skilled labor pool that allowed it to switch from the pin connections to riveting.

A pony truss, as its name implies, is a small short-span truss. The height of a pony truss's top chord is too low to permit horizontal bracing between the trusses on each side of the bridge deck it supports without interfering with traffic. [Information excerpted from Guise, "Historic Bridge Foundation," Bridge News (Fall/ Winter) 2006:5].

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