Modeling Truss Designs of Yesteryear

Summer 2016

Claudio Malicdem

Claudio Malicdem was born more than a century too late to work on constructing any of the approximately 10,000 covered wooden truss bridges that spanned waterways in America from the mid-1800s to the early 21st century.

Yet over the last year, the 19-year-old civil engineering major has spent close to 500 hours working to replicate four of the most famous wooden truss bridge styles in miniature.

“Before coming to Hopkins, I’d never visited or seen a covered bridge in person,” says Malicdem, as he tinkers with one of the models in a Latrobe Hall basement lab. “One thing I can say for sure is that I now have a new appreciation for the engineering involved.”

Malicdem is a member of a team of civil engineering students constructing the models under the supervision of Rachel Sangree, PhD ’07, the Whiting School’s resident expert on historic covered wooden truss bridges.

Funded by a multiyear National Park Service grant, the project aims to create 3-D models to be used in civil engineering classes at Johns Hopkins, the University of Massachusetts Amherst, Virginia Tech, and several other schools. The models (and others based on them) will be used in a class Sangree co-teaches with Whiting School civil engineering professor Benjamin Schafer, Perspectives on the Evolution of Structures, which delves into, among other things, the important role that covered wooden truss
bridges played in the development of American engineering.

“These models are important because you can touch, feel, sketch, and experience all that these small bridges have to offer. This interaction builds a foundation for intuition that expands one’s thinking when working with the huge abstractions that long-span bridges and tall buildings require of civil engineers,” says Schafer.

Sangree explains that until the early 19th century, most long-span bridges necessitated the use of piers in the waterway to support the horizontal wood beams, which were simply laid parallel to each other to create a bridge structure. Those piers were vulnerable to ice and waterway debris, resulting in frequent damage and necessary replacement.

To increase the resiliency of bridges and improve their economy, early architects, builders, and engineers sought ways to span long distances without the use of piers. One of the earliest truss bridge patents (1806) was for the Burr Truss, which, like earlier European forms, relied on traditional wooden joinery (requiring highly skilled labor) and arches—which, like piers, were perilously close to the waterway, making them vulnerable to damage and decay.

Ithiel Town cited these drawbacks when he patented his Town Lattice in 1820, which did away with arches and traditional joinery, but it used other inefficiencies in its design and was less aesthetically pleasing than traditional panelized trusses.

Stephen Harriman Long followed the Town Lattice with his Long Truss in 1830 that used pre-stressing technology, allowing for long spans without the use of arches or traditional joinery. His model was improved upon by William Howe, who introduced iron as an integral part of his design in 1840, ushering in a new era—that of the iron truss.

“It is not an exaggeration to say that these covered wooden truss bridges signify the birth of structural engineering in America,” says Sangree, who has studied them since 2002. “They represent the first period in American history when the architect/builder/engineer sought to innovate efficient, economic, and—in the best cases—elegant design solutions to a common problem: spanning a wide body of water.”

Christopher Marston, architect and project leader for the Historic American Engineering Record’s National Covered Bridge Recording Project, agrees.

“It is, of course, unrealistic for most engineering students to go out and work on an actual covered bridge project. So working with the models—being able to see them and touch them—shows them how connections worked on the four major truss types,” he says.“The models will help them learn and appreciate the challenges and principles that engineers and builders in the early 19th century faced, and how they solved problems.”