A Tale of Two Sisters (Boscawen-Canterbury and Hooksett Lilac Bridges)

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.