The pin joint steel bridge the Norfolk Southern now uses at Knoxville (immediately below the Henley Street Bridge)uses the pre-Civil War piers that supported a wooden structure with the center span covered. I have recently found the 1903 newspaper articles stating that the wood was being replaced by steel (a loose term for then) pieces.
So the age of what you see today consists of 1850s piers, and 1903 metal. Also the one beefed-up pier was a repair done in 1993 after a tug pushing a group of barges hit it.
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You're right.. the bridge was indeed built before 1940.. problem is, that's the date it was rehab'ed, and any major rehabilitation to a bridge or tunnel with the Federal Railroad Administration's oversight, is considered a rebuild, and the FRA will list the date of the rehab as its construction date. I discovered that fact on many railroad tunnels and bridges in SW Virginia. If I can locate the old L & N records (which are hard to find), it will have the original construction date of this bridge, which should go back to before the turn of the century.
This bridge is actually the original Louisville & Nashville Railroad bridge over the Tennessee River on the Corbin-to-Etowah Line, which took it to the historic L & N Depot in downtown Knoxville (the new line and its newer river bridge is now a mile and a half further west, and this bridge was sold to the Southern Railroad which had never crossed the river in Knoxville, for them to serve the businesses in Alcoa-Maryville ---the CSX Corbin-to-Etowah line bypasses those cities now).
The L & N and the Southern X-crossed there at Western Avenue downtown--the new X-crossing is now at the Alcoa Highway bridge further west.
Note in Calvin Sneed's second photo that one pier has a larger base. This was the repair needed to put the bridge back into service after it was struck by a group of barges being towed upstream by a diesel powered tugboat in the middle 1960's.
Just to let you know that this bridge is much older than 1940. Note the pin joint and lattice beam construction methods which were used before riveted joints became common in the 1920's. I suspect that some of the components of this bridge, especially the stone piers, are from the original structure which was completed in 1867 by the Knoxville and Charleston Railroad which only got to Maryville, Tennessee. It was then sold to the Knoxville and Augusta Railroad in 1875 and got as far Walland, Tennessee. Since the railroads often moved spans when the routes were expanded from single track to double tracks, it isn't uncommon for railroad bridges to have not been a unique project with everything built at once.
Tony Holmes in his 1992 article, "Knoxville's Ferries and Bridges: 200 Years of Changing a River Town," in the short-lived publication 'Old Knoxville, Vol. 1, No.1, lists this bridge as being completed in 1926. So it is my vote that this bridge built date be pushed back to at least 1926.