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.
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.
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