35 tons hover over St. Gallen: Lifting the bridge over the Unterer Graben

In a night-time operation, a team of 50 specialists installed the first element of the bridge over the Unterer Graben. The lifting operation gave both the young Basler & Hofmann project team and various spectators a thrill. In this report, we take a look back at the night of September 29.

It is exceptionally quiet on the "Unterer Graben" cantonal road in St. Gallen when Seraina Renggli and Josefine Gerarts arrive late in the evening to discuss the final details together. The cantonal road had to be closed for the upcoming bridge lift. The two of them seem both excited and a little nervous.

"The whole project team has been working towards this one date for the last two or three months. We only got this day for the road closure before the Olma trade fair and the onset of winter."

explains Seraina Renggli, project engineer

Renggli and Gerarts are part of the young project team from Basler & Hofmann, which, together with DGJ Landscapes and Nau2, won the project competition organized by developer and total contractor Senn Resources. The new pedestrian bridge will connect the old town with Unterer Rosenberg. Slowly, the rest of the team arrives, having had dinner together to fortify themselves for the night's work.

Seraina Renggli, project engineer, in conversation with Josefine Gerarts, site manager responsible for the bridge over the Unterer Graben.

Shortly after midnight, the reinforced concrete structure is delivered by heavy-duty transport. Only now do you really realize the dimensions of the component: the bridge element is 32 meters long and weighs an impressive 35 tons. A mobile crane will position the component at the intended location - a "tricky" underpinning, as site manager Josefine Gerarts explains:

"The element has to be lifted in between existing buildings and the trees in St. Mangen Park, which are worth protecting, without damaging them. There's not much space. We checked everything geometrically in advance and are now very excited to see how the lift will work in reality.

And the time has come: attached to four load cables, the bridge element weighing several tons slowly begins to lift from the lower trench.

As the bridge is to have a gradient of six per cent in its final position above the road, it will not be raised in a straight line, but at a slight incline.

After a few minutes, the bridge floats towards its highest point. It is now around 18 meters above the "Unterer Graben" parallel to the Roads in the air. Now it's time for the crane operator's real "feat": completing a quarter turn. This brings the element into the position that it should ultimately have.

The rotation is complete. The project team breathes a sigh of relief along with the media and spectators who have arrived to watch the night-time spectacle over a cup of punch. Now the bridge element is slowly lowered until it finally reaches its destination.

"The bridge has to be set down on the new UG25 multi-storey parking lot with millimeter precision. This requires a lot of dexterity from the crane operator",

says Seraina Renggli.

This operation is also proceeding according to the script. Now the bridge is being fixed to the side of the park - initially with a temporary support structure that will take the heavy load using hydraulics.

Seraina Renggli and Josefine Gerarts check that the position and gradient of the bridge in its final position are correct.

At two o'clock in the morning, the first of a total of three bridge elements is finally in place. The team is delighted. The second and third bridge sections are scheduled to be lifted into place in the coming months. The bridge is due to go into operation in spring 2025. Pedestrians will then be able to walk through St. Mangen Park like on a treetop path.