Research & Development PDF Print E-mail

During the crisis research phase, October 4 - mid December 2009, leading experts in historic bridge restoration, the State Historic Preservation Office and the Iowa Department of Transportation officials were happy to help, offering valuable leads to citizens, engineers sympathetic to historic preservation, nationally known bridge restoration experts and contractors experienced with bridges and rivers. Nearly 100 volunteers, spread out across the county, raised seed money, spread the word, and gathered signatures, hoping to find an answer that didn’t require destroying the bridge. The Friends of the Skunk River Bridge banded together and convinced the Poweshiek County Supervisors that our goal to save the bridge was feasible. The Friends continue to provide valuable community support for the North Skunk River Greenbelt Association with fundraising, planning, and as volunteers, through learning from Vern Mesler during restoration. A chili supper with music by Rude Dog contributed nearly $1,000 by donation to a community tip jar. Although we are a fledgling corporation, we have been accepted for Fiscal Sponsorship by the Greater Poweshiek Community Foundation, under the guidance of the Ahrens Foundation, for their experience working with big community projects like the Drake Community Library that opened in December 2009 and the Community Safety Building that will open early 2010. Their role will be managing donations but they offer no other administrative support. Bringing Vern Mesler and Nathan Holth from Michigan to a Friends party at the bridge on October 18th when Vern convinced us that we can complete this project with volunteers; solidified our resolve to restore the structure.

An American Society of Civil Engineers Policy Statement, approved by committee and adopted by the Board of Direction on November 11, 2003, supports the maintenance, repair and rehabilitation of historic bridges, preferably those in continued vehicular use, but when that is not possible, in an alternatative use such as a pedestrian or bike bridge.  ASCE feels that bridges are a visible icon of the civil engineer’s art, and are important links to our past; as well, they provide safe and vital transportation routes in the present and into the future. Their rationale is that many historic bridges can still serve the nation’s transportation needs, given appropriate repair, maintenance, and flexibility in the interpretation of transportation standards. Due to many reasons involving funding priority, they argue that historic iron bridges are a heritage at risk, with over half of all historic bridges having been destroyed during the late 1900s and the beginning of this century. Possibilities for bridges when not in vehicular use include: relocation, use for alternative means of transportation for hiking and biking, becoming part of the network of scenic highways and byways. The Society ends its policy statement by suggesting that Civil Engineers as a group need to help lead and support efforts of citizen groups throughout the country now working to save the historic bridges, because there is little chance that historic bridges can be saved without the interest and skills of engineers, so that any effort is doomed unless they become part of the comprehensive policy. What is known legally as the McDowell Bridge*, locally called the Skunk River Bridge, is called an ‘engineering marvel’ by pontist Nathan Holth in an online article documenting the bridge on the website. Along with spotlighting our bridge with great photos, he found:

1. The original patent for the bridge piers has beautiful diagrams, one showing that the piers had wooden panels on the outside between the main i-beams. (Loose bolts along with pack rust and missing rivets were cited by the county engineer as a main reason for tearing down the bridge, but these were actually used to hold wooden cribbing to the outside of the piers.

2. An HAER (Historic American Engineering Record) page for another King bridge had drawings of similar bridge piers mounted on wooden cribbing, probably what McDowell Bridge’s piers rest on, far below. This unique design was patented by Theodore B. Mills, who was president of the King Iron Bridge Company’s branch office in Iola, Kansas. This pier design only appears in King Bridge Company bridges, and surviving examples are very rare, so showcasing the engineering craftsmanship should be taken into account.

3. The star-shaped members on the bridge are referred to as “star-iron” in the Historic American Engineering record, or as “Crucible” members described as wrought iron; in the McDowell bridge’s outriggers, the star/cruciform members are connected by a large variable width lattice.

Not only does Mr. Holth document the rarity of our bowstring truss bridge, he points out that only twenty bowstrings remain in Iowa, with just one in Poweshiek County. His research includes the original and second patent applications, by Zenas King, where the bowstring arches showing significant differences from the original arches of the Squire Whipple design. As a rare surviving bowstring, it is further noted as an example with little or no alterations on the superstructure. As a result, the bridge provides a clear look at exactly what the King bowstring design looked like, including a built-up box beam top chord. Poles/pipes are present on the bridge acting as floor beams on some panel points, and also as sway bracing, which is another unusual detail found only on King bowstrings. Holth stresses that preservation of the patented cast iron piers should receive a very high priority, while at the same time protecting a restored McDowell Bridge from future flooding. Any engineer hired to work with us should be sensitive to the national significance of the piers in designing a solution that will prevent future flooding. One solution he noted could reuse the piers but raise the height of the bridge to prevent future flood or ice damage.

Patented by Squire Whipple in 1841, bowstring arch bridges were very popular during the late 1800s not only because they could cross smaller streams and carry local traffic into the town, but also because they represented a symbol to the community and the people who cherished them. Whipple worked for the Baltimore and Ohio Railroad before finding and developing his only passion in life: civil engineering. With the invention of the steam engine, the expansion of the railroad industry, and the rising popularity of cast and wrought iron during his time, Whipple, beginning in 1840, developed ways to strengthen bridge structures so that they could carry heavier loads from not only horse and buggies, but also from railroads. His concept was that the upper chord, made of cast iron, would curve like an arch, compressing the lower chords, made of wrought iron. This structure would resemble an archer’s bow, with the vertical and diagonal beams supporting the lower chord, but anchored with pinned connections, thus producing an equilibrium between the two chords. Whipple developed and patented this bowstring arch bridge, also known as the Whipple truss bridge. The first one of this type was constructed in 1841 over the Erie Canal near Utica, New York.

Vern Mesler, nationally noted bridge restoration expert, when visiting the McDowell Bridge for the first time in November 2009, offered to haul the parts back to Calhoun County Historic Bridge Park in Lansing Michigan, if we didn’t want it. “Don’t let them tear it down, Julie” he said, followed with “The only acceptable course of action is the preservation of this essential part of Iowa’s rich transportation heritage. Demolition of a bowstring truss bridge is absolutely unthinkable. Even in the U.S., where historic bridge preservation generally receives a very low priority, it is unusual and extremely alarming to hear of plans to demolish one of the rarest, most unique, and most important historic bridges in the country. Indeed, the McDowell Bridge is significant enough, that even just the parts of the bridge are rare enough to stand on their own in a museum and be a noteworthy exhibit.”

In addition, Mr. Mesler argued that the bridge should be restored in such a way that absolutely none of its historic integrity is reduced or compromised. Mesler, after inspecting the McDowell Bridge for hours, noted that the actual restoration work is without a doubt the least difficult part of a successful preservation project. He felt that, unlike many of Iowa’s rural truss bridges, the McDowell Bridge enjoys an extremely high degree of structural integrity, most particularly visible in the relatively small amount of pack rust and nearly non-existent section loss on the bridge, even along the traditionally troublesome bottom chord as the bridge has never been exposed to the severely corrosive effect of de-icing salt during the winter season. In addition, its durable wrought and cast iron design rusts far less than modern steel. The flood damage was mostly limited to isolated areas of bending, and the few cracks and breaks that occurred on members can be repaired.

No organization in Poweshiek County has tried to take on this historic bridge project, but then, no one knew about the bridge as an issue or the County’s decision. Our group, after press coverage, has been contacted by other concerned citizens, both up and down river, from Jasper to Mahaska County, interested in restoring their parts of N. Skunk River and its associated greenbelt. This is one of the few natural stretches of the N. Skunk River, and it takes the brunt of the force of a river much of which has been straightened for agricultural purposes. The current cuts a sharp angle in the bank that forces the river to run faster, with a sharp corner just 100± feet up river from the bridge, near the ruins of the original dam, where erosion continues. Nothing will stop the cyclical flooding of the river here, but a healthy greenbelt and wetlands, working naturally with the river, would help minimize the impact of flooding, still hard armoring (rip rap) will be required on a portion of this river bank. Working with the Environmental Protection Agency and Army Corp of Engineers will be required and banding together across county lines can be beneficial for fundraising efforts, raising awareness and creating a rich river environment in our region.