Why wait for Spring when you can visit a construction site in the middle of November!
Post-Tensioned Concrete Tub Girder Bridge Construction with Launched Formwork
We had the opportunity to visit an active construction site - the Lidingö Bridge, which goes from goes from Hjorthagen to Lidingö and is just slightly Northeast of my school, KTH (labeled "work" on the below map)
The construction site was bustling, even though it was about 3 °C with wind and rain! We got a whole presentation on the design and construction of the bridge. This bridge is a replacement of the old pedestrian bridge. The vehicular traffic will remain on the taller concrete bridge, but the old pedestrian and railway bridge will be replaced with Lilla Lidingönbron.
Substructure
Pile installation, foundation, and all of the substructure construction was completed in 2020. The piles were actually totally gnarly - there were maybe 20 micro-piles (about 20 cm in diameter) in different directions all pile driven through the water, clay later, friction layer, and embedded just enough such that they are bearing on the surrounding rock. Rock is the best type of surface for any structure to bear on, it is incredibly stable. However, it is sloped so attempting to place all of these piles on the sloped surface was tricky. Somehow they managed though, and they were able to place the footings as well. Below is a screenshot from their informational video here: https://www.lidingo.se/toppmeny/byggabo/harplanerarochbyggervi/harbyggervi/lillalidingobron.4.4f1828d515903f133b97a3f.html
At minute 9 the fellow who gave us the tour of the bridge comes on! He's German.
Superstructure
The superstructure is basically defined as everything above the piers. It includes everything from the girders to the deck to all of the appurtenances on the deck like the barriers and lights and pipes. This bridge is a "tub-girder" bridge which is kind of. It kind of goes without saying that there is post tensioning in the bridge, that is incredibly standard for this type of bridge.
The formwork for this bridge is cool because it was a cast-in-place bridge. This means the rebar and any strand used for later post tensioning was placed in position before the concrete was poured. In the video you can see a lot of tubes getting installed between the rebar, and those are for the strands to go through later.
Then the concrete was cast around all of these metal steel bars and tubes. The concrete was allowed to harden and then the post tensioning was applied to any of the strands in the girder. In order to avoid having to build the same formwork over and over again, they built a launched form to be re-used over the span of the bridge.
The engineer told us that during peak summer production a single span would be completed in 6.5 weeks or so.
Overall it was great, if burly, site visit! I am looking forward to visiting this bridge again once construction is complete!
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