First, though, I’d like to go back to December 9 for a moment. This was a historic day for the old Sarah Mildred Long Bridge. As you probably know, beneath the road deck, there is a railroad deck. The railroad was quite active until the line between Kittery and South Berwick was pulled up in 1952, and since then has only served the Portsmouth Naval Shipyard (which, for anyone who isn’t local, is actually in Kittery).
Most materials are trucked into and out of the shipyard these days, but the railroad remains the only means of moving nuclear materials off of the island. This primarily consists of spent nuclear fuel (SNF), bound for the Navy’s processing facility in Idaho. SNF is moved in special railcars that each carry a large, reinforced flask, and the trains containing them typically have have caboose carrying guards on the tail end. It’s a sight to see, and I never miss the opportunity to get some photos.
For the past decade or so, rail traffic to Kittery has been fairly predictable. A train brings some empty cars over, they get loaded, and another train retrieves them a couple weeks later. What was special about the outbound move on December 9 was that it was the last train that will ever cross this bridge. The photo below was taken from the NH State Pier, as the 650-ton train crawls across at walking speed with her radioactive payload.
More photos of the inbound and outbound moves can be found on my Flickr page:
The structure has been enclosed for the winter, so it’s a bit hard to see what’s going on. Here’s a look inside. You can see a completed segment, resting just above ground level. Note the red hydraulic jack at the center of the photo. These are on each corner, and allow the segment to be leveled before casting on top of it.
The plastic tubes that run from the top to the bottom create voids in the concrete to accommodate the rod and tendons that will eventually hold the tower together. During assembly (stacking) of the towers, threaded hardened steel rods are inserted two segments deep, and nuts and washers are threaded on and tightened to hold the segments together. Once the entire tower is assemblies, steel tendons (essentially cables) are threaded through sets of tubes that run the entire height of the tower. These are tensioned, and then the void around them is filled with concrete to make them permanent. The temporary rods are also grouted in place, though they are no longer structurally significant.
The other interesting feature is the horizontal steel plate that is visible running along the top of the opening. This is convex on the bottom, and creates a trough in the top of the casting. We’ll look at that below.
The second photo below shows the bottom of a segment, and you can see the feature that is created by the trough below. When the segments are stacked, these will ensure horizontal alignment and increase horizontal stability. Like Legos.
The steel tubes are cut away to just below the low water level, and remain in place permanently below that. Though exposed to salt water, they are engineered to endure for the 100-year lifespan of the bridge.