Strand jacks supplement movable scaffolding for demolition of Unterrieden viaduct

The approximately 642-meter-long Unterrieden viaduct on the A6 autobahn at Altdorf near Nuremberg spans slopes of the Middle Franconian Alb and the Raschbach valley.

Built in 1967, the bridge was to be demolished and replaced by a new structure. Dismantling is being carried out both in and counter to the direction of construction, which places special demands on structural analysis. At the same time, the close positioning of the piers in cross-section had to be taken into account, which made moving the scaffold a challenge due to the confined space. The bridge over the deep valley has piers up to 40 meters high, spaced 42 meters apart in longitudinal direction.

The conditions on site limited the lifting equipment that could be used to install the support structures in the pier area. thyssenkrupp Infrastructure devised an alternative solution using strand jacking to suspend the pier scaffolding. Two strand jacks each are placed on the bridge so that the strands reach down into the valley to the steel structure to be lifted. Once a positive connection is made between the strand ends and the steel structure, the pier scaffolding, weighing a total of 110 tons, can be lifted in hoists of 50 cm and suspended from the pier head. Thanks to the technology used, this method was particularly economical for the client and independent of mobile cranes.

The movable scaffolding is located under the span to be dismantled and forms a kind of dividable trough. After the demolition of each span, the movable scaffolding is shifted hydraulically from pier scaffold to pier scaffold in the next demolition section. Debris arising during demolition is collected by the trough, allowing controlled removal so the valley below is not affected. The röro movable scaffolding system reduces cycle times per span to two weeks, saving construction time for our client.