And now the answer to the question of naming the bridge type. As you will recall, in a posting from last Thursday, there was a post card of a bridge that spanned the Wapsipinicon River near Independence in Buchanan County, located in the northeastern part of Iowa. While some people may have found the answer through James Baughn’s website, there are some who are not familiar with that, nor the picture, as it was posted most recently and readers have not yet had a look at the picture until now.
I can tell you that I had written about this bridge type a few years ago as part of an essay for a history class at the university here in Germany, and there are some examples of this bridge type that still exist today, even though there are two different types of this truss type that three bridge builders had used during their days.
The answer: The Thacher Truss. In 1881, Edwin Thacher (1840-1920), an engineering graduate of Renesselaer Polytechnic Institute, invented and patented this unusual truss type. It is a mixture of four truss types: the Warren, Pratt, Whipple and Kellogg. While the Kellogg is a Pratt truss design featuring a subdivided panel supporting the original diagonal beams that connect the vertical beams, the Thacher features two sets of diagonal beams starting at each end of the truss bridge at the upper chord- one creates a panel similar to the Pratt truss, while the other crosses two or three panels before meeting the center panel, which forms an elusive A-frame. The bridge at Independence was the very first bridge that was built using this truss design. It was built in 1881 and was in service for over 40 years. Yet after having the design patented in 1885, Thacher went on to build numerous bridges of this type, most of which were built between 1885 and 1910. He later invented other bridge designs, some of which will be mentioned here later on.
While it was unknown how many of these types were actually built between 1881 and 1920, sources have indicated that Iowa may have been the breeding ground for experimenting with this truss type. Apart from the railroad bridge at Independence, the very first structure that was built using the Thacher, as many as four Thacher truss bridges were reported to have been built in the state. Among them include the longest single span truss bridge ever built in the state, the Philips Mill Bridge, spanning the Winnebago River outside Rockford, in Floyd County. Built in 1891, this 250 foot long bridge, dubbed as one of the most unusual truss bridges built in the country, was the successor to a two-span bowstring through arch bridge and served traffic until it was replaced in 1958. Other Thacher truss bridges built included one over the Shell Rock River north of Northwood (in Worth County), the Ellsworth Ranch Bridge over the Des Moines River in Emmet County and the Okoboji Bridge over the Little Sioux River in Dickinson County. Of which only the Ellsworth Ranch and Okoboji Bridges still exist today.
On a national scale, if one counts the two remaining Iowa bridges, there are five bridges of this kind left, which include the Costilla Bridge in Colorado, Linville Creek Bridge in Virginia, and the Yellow Bank Creek Bridge in Minnesota. Two additional bridges, the Parshallburg Bridge (2009) and the Big Sioux River bridge in Hamlin County (2009) have long since disappeared due to flooding/ice jams and structural instability, respectively. While the majority of the bridges mentioned here were constructed by the Wrought Iron Bridge Company in Canton, Ohio, the King Bridge Company in Cleveland constructed the Ellsworth Ranch, Yellow Bank and Hamlin County bridges, using a different hybrid of Thacher truss that was modified during James King’s reign as president of the bridge company (1892-1922). The Clinton Bridge and Iron Company in Clinton, Iowa built the only Thacher pony truss bridge in the Okoboji Bridge, the bridge that is featured in the next article. While the Ellsworth Ranch Bridge remains closed to traffic and seems to be abandoned, the Yellow Bank Bridge was relocated to Hastings, Minnesota in 2007 to serve as a replica of the Hastings Spiral Bridge at the Little Log Cabin Historic Village.
And that is the answer to the pop quiz, even though for some experts in the field, the answer was obvious. Yet perhaps the next bridge type quiz may be even more challenging than the first one. As for the ones who didn’t know, this one should get you acquainted to the questions that are yet to come that will require some research. So let’s go to the next question, shall we?
Author’s Note: If you know of other Thacher Truss Bridges that existed in Iowa or any part of the US and would like to bring it to his attention (and that of the readers), you know where to reach him: firstname.lastname@example.org or via facebook under The Bridgehunter’s Chronicles. He’ll be happy to add it in any future columns, and for his project on Iowa’s Truss Bridges, it will make an excellent addition.