Recently there was an article on the planned removal of the Boscawen-Canterbury Bridge. This bridge was designed by John Williams Storrs and constructed in 1907. By 1965 it was decided to close the bridge to traffic as the original design was not compatible to the modern vehicles. This is a two span structure with a total span of about 347 feet. Assuming the spans are symmetrical, the individual spans would be about 174 feet. To the left is a profile view of one of the spans. Note the rounded appearance of the top chords.Storrs is
recognized as one of the first true design engineers in the region. When Storrs is mentioned the first example of his work is normally the Sewell's Falls Bridge currently under redesign and slated for rehabilitation in the next few years, but there is another sister bridge in Hooksett that is also in a similar state of disrepair.
The Lilac or Hooksett Village Bridge in Hooksett is a three-span high Pratt Truss also designed by Storrs. This bridge was opened in 1909 and repaired in 1936. In 1976 this bridge was bypassed and closed. The total span is 490 feet with the longest span about 170 feet. The remaining spans are approximately 160 feet. The horizontal alignment of the upper chords and the different scales on the pictures make these bridges look very different, but looking at other angles definitely shows the family resemblance.
Here is a picture of the front of the Boscawen-Canterbury bridge. The curved shape of the upper chords results in a shorter distance between the horizontal members of the portals, but otherwise the geometry of the truss members appears to be identical, member sizes are close, and the portal bracing is also very similar. I could not get a good view of the connections now that both bridges are closed to pedestrian traffic, but assuming these are simple connections, the spans of the bridges are very similar. The pipe railing is also used on both bridges even though on the Boscawen-Canterbury bridge the pipe railing on the right side is now detached and draped across the deck. Also note the curved molding detail where the portal and truss meet on both bridges. Unfortunately one other thing that both bridges have in common is the passage of time, a number of years being out of service and little or no hope of being rehabilitated before being turned into scrap metal before anyone raises a serious effort to preserve them.
It is unfortunate that when a bridge is tagged with a historic designation it actual further encumbers the engineers that are trying to save it. Bridges not being repaired for lack of funds require even more money when the bridge has been identified as historical. In the end the engineer and historic nature of the structure are put at odds due to the higher costs and longer time lines required for design and construction.
If we truly value our historic bridges, we need to make it an advantage to have a historic bridge designation when setting priorities for funding.
Currently bridges are rated between 0 and 100 with new bridges rated higher and these two bridges rated at or near zero. As bridges deteriorate, they become eligible first for rehabilitation funding and if allowed to deteriorate further then they become eligible for replacement funding. An idea would be to identify bridges that are culturally more significant and make those structures eligible for these funding sources 10 rating points or more prior to other bridges. Since many of our historic structures are already well past their ratings needed to be eligible for funding, the other consideration would be eligibility for funding. Many of these structures are no longer the work horses of our transportation systems and do not rise to the priority of the more heavily traveled structures. This will continue to occur by necessity while we systematically underfund our transportationinfrastructure.
The top deck photo is from Boscawen-Canterbury. The pipe rail is now rolled over onto the deck. Below is the Lilac Bridge deck. Again note the similarity in the truss members and portal framing. A noticeable difference is the sidewalk on the left hand side of the Lilac Bridge. A careful eye will also spot the cradles for what was either a gas or water line that has since been removed. Both decks were wooden, but the Boscawen-Canterbury bridge had longitudinal decking running continuously over what looks like steel lateral stringers. The Lilac Bridge has lateral decking with a steel member every 15 feet. The decking system would be more apparent with a view below the bridge, but it didn't happen this trip.
For both of these bridges I am afraid that time is running or has run out. If you value historic structures, grab your camera and get out for a visit. For many of these structures, recognition of their historic value to the extent where funding will be provided in time to save them will never occur before the only option will be documentation then scrap metal. Engineers are a practical lot. I would be interested to know what John Storrs would think about the historic preservation requirements and attempts to save his bridges, of which I believe only seven are still in existence.
When I wrote the tale of two sister bridges, I did not have compatible photos of the Sewell's Falls Bridge. Today I happen to be in the area so I took photos compatible to the earlier shots. Sewell's Fall Bridge is very similar to the Lilac Bridge in Hooksett. First the top chords form a horizontal line, they are at heart two span structures even though Sewell's Falls has an extended set of approach spans, the main spans are of compatible lengths, and the truss members appear to be identical with the same number of vertical and diagonal members. The Sewell's Falls Bridge was designed by John Storrs and constructed in 1915. It was also rehabilitated in 1936. The face of the structure has very similar features including the pipe rail, horizontal supports of the portals and the rounded connections to the truss members. The portal framing is more open, but generally follows the same geometry. The biggest difference visually is the construction of the deck. Sewell's Falls currently has a steel grate deck where the other two bridges have wooden decks. I have called out to some of the long time residents of Concord to clarify whether the deck was originally wood. I also had a chance to look under the Sewell's Falls bridge. I think I mis-spoke on the earlier post. Visible from the top of both the Sewell's Falls Bridge and the Lilac Bridge are lateral joints every fifteen feet or so. I had previously assumed these were stringers that connected to beams that I had 
not verified from below. In fact it appears that these lateral members are actually the beams and the stringers that support the deck connect into the beams. Not being a bridge engineer by trade I will let my structural counterparts correct my structural vocabulary, but hopefully there articles will spur a few to go and visit these structures before they are gone. It would also be interesting to know more about the properties of the steel used in these bridges. The chemical make up of steel ordered today is of a consistency that engineers using the material can have a very high degree of certainty that the properties attributed to the product will in fact be true within an impressive tolerance. Prior to 1940 those properties had a lot more variance. By changing the chemical composition of steel, such as changing the carbon content, the strength and malleability (brittleness) of the steel can vary significantly. There are design guide books that can provide general guidance on the steel makeup based upon the year made and statistical probabilities, but for greater reliability samples of steel can be cut from the individual bridges being analyzed and tested to determine more accurate material properties. An added twist is that different members of a bridge may be built in different locations or that less critical members may have been made from different steel.
