Urban Planning

Creativity in Cast Iron: Strickland Kneass’s Chestnut Street Bridge

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For Strickland Kneass (1821-1884) engineering was not about letting tradition dictate uninspired designs nor did the profession thrive in clannish fiefdoms of expertise. Trained in the era before formal engineering curricula, Kneass saw engineering as an organic profession whose rules, though important, were always secondary to imaginative solutions. In his nearly half century of work in the private sector and his seventeen years of service to the City of Philadelphia as Chief Engineer and Surveyor, as a sewer and drain expert, as a bridge builder, and as Fairmount Park Commissioner, Kneass distinguished himself as a polymath designer and organizer who deftly navigated between the shoals of tradition and innovation.

The remains of Kneass’s boldest design, a bridge whose scale and use of cast iron made it singular in the United States and throughout the world, stands ignored by hundreds of thousands of motorists, pedestrians, and joggers who pass it. A vestige of his Chestnut Street Bridge (1861-66): the eastern granite abutment and the central pier, though ignobly incorporated into the current highway overpass, still testifies to the creativity and vision of one of the city’s most talented builders.

The son of a Philadelphia engraver, Kneass attended the Rensselaer Institute, later Polytechnic Institute, when that institution began developing its own idiosyncratic approach to training civil engineers. For a young Kneass attending the Troy, NY school in the late 1830s, the curriculum—which still included geology, law, Biblical history, and surveying—was far from a dry inculcation of mathematical formulae. Despite a rigorous schedule that roused students at dawn, “there was considerable flexibility, informality, and probably even laxity in the actual operation” of the school. Students were taught to work through problems, develop their own conclusions, and report on their findings during examinations. A Rensselaer engineer, an advertisement touted, “are taught all these things (23 subjects of civil engineering) and many others, with the appropriate instruments in their hand, accompanied by short lectures of their own.”

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Graduating in 1839 at the age of 18 with full honors, Kneass reaped the benefits of this diverse technological education and soon lived up to the reputation of the multifaceted Rensselaer engineer. In 1847 he assisted the Pennsylvania Railroad in laying out a portion of their Harrisburg to Pittsburgh segment. He also worked as a draftsman at the Naval Bureau of Engineering and as a topographer for the British Commission mapping the U.S.-Canadian border. Later, in 1869, he assisted James Worrall in surveying the famous 12 Mile Arc border between Delaware, New Jersey and Pennsylvania.

In 1855, Kneass resigned as chief engineer for the North Pennsylvania Railroad to become Chief Engineer and Surveyor of the City of Philadelphia.  He found the bureaucracy of the recently-consolidated city in shambles. He soon concentrated all the operations of the seven survey districts and standardized grade plans, weights and measures, and designs for sewerage. In 1865 he organized a Registry Bureau as the central repository for property data and building plans. All the while he made recommendations for the improvement of the city’s sewer and storm water systems.

Undoubtedly, Kneass stayed abreast of the new developments in bridge construction; he knew of popular ornamentation then in vogue in Europe and construction management methods. He probably followed the reorganization of his alma mater on the pattern of the progressive French Ecole Polytechnique. He certainly knew of the iron bridge at Colebrookdale, England over the Severn River—the often-reproduced icon of the Industrial Revolution built in 1791. Perhaps closer to home, he knew of William Strickland’s use of iron members at the Chestnut Street Theater. And he may have recalled the work of two innovative Army engineers at Dunlap’s Creek in Brownsville, Pennsylvania. There, Captain Richard Delafield and Lieutenant George W. Cass constructed the country’s first metal arch bridge in 1836 as part of the Cumberland Road.

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The success of these projects, and a willingness to try new materials, may have influenced Kneass’s design for the Chestnut Street Bridge in 1857 which included an unprecedented amount of cast iron. Though it is unclear who supplied the cast iron, two features made iron attractive.  One was the adaptability of the casting process.  Artistically and practically, cast iron offered designers great flexibility.  Bridges made of a multitude of smaller, mass-produced components could be assembled easier and were inherently safer.    This, coupled with the proximity of Philadelphia’s cast iron suppliers, led Kneass to build a bridge around two sweeping 184’ arches using six cast iron ribs. Yet Kneass was no bare functionalist and his line and watercolor drawings abound with Gothic arches in stone and iron. And though Kneass was applying an untested material to a major arterial bridge, he still followed an important standard practice: overbuilding the bridge to ensure safety redundancy. Each rib, he estimated, could sustain a transient load of 486,000 lbs. Anticipating increasing traffic, Kneass wanted wider approaches—a detail the shortsighted city councils wrong-headedly vetoed as the bridge had to be widened in 1911.

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Construction began in 1861 and by 1864 the center pier was completed as evidenced by the date “1864” etched into the stone shield on the central pier’s southern side. Two years later the bridge opened to the public at a cost of $500,000. For most of the latter half of the nineteenth century, the bridge remained a point of pride for American civil engineers. “As far as known, with the exception of the Chestnut-street bridge, Philadelphia,” wrote engineer Malverd Abijah Howe in 1897, “there are no cast-iron arch bridges of any magnitude in the United States.” Despite its apparent stolidity Strickland Kneass’s Chestnut Street Bridge did not last a century and it was demolished in 1958, perhaps because its massive western abutment sat right in the path of the Schuylkill Expressway.

Works Cited:

  • Gilchrist, Agnes.  “Chestnut Street Bridge,” Historic American Engineering Survey, (Washington: National Park Service, 1958), 2.
  • Graff, Frederic.  Obituary Notice of Strickland Kneass, Proceedings of the American Philosophical Society, Vol. 21, No. 115. (Apr., 1884), pp. 451-455.
  • Howe, Malverd A.  A Treatise On Arches: Designed for the Use of Engineers and Students in Technical Schools (New York: John Wiley and Sons, 1897), xvii.
  • Rezneck, Samuel.  An Education for a Technological Society: A Sesquicentennial History of Rensselaer Polytechnic Institute (Troy: RPI, 1968).