B.Backs have a limited lifespan, they are only designed for a certain load. “It’s like a spoon that you bend back and forth too often. At some point it breaks, ”says Matthias Vogler from the Katzenbach engineering office. As an expert, he examined the subsoil of the Salzbachtal bridge. The bridge built in 1963 was designed for a service life of around 80 years. The traffic has increased massively.
A distinction is made between two construction methods for motorway bridges: those made of slack reinforced concrete and those made of prestressed concrete. The Salzbachtal Bridge is a prestressed structure. Built-in pipes with tightly tensioned tension ropes hold the structure up. Vogler compares this to a tense bow. Individual tendons are no longer intact, and the concrete is too old and can no longer take the loads. Several cracks and flakes heralded it.
“The bridge has cracked massively, concrete parts have broken off.” Not only driving over the bridge, which was already closed to traffic, is risky, but also crossing under it, because concrete parts could fall onto the railroad tracks and roads below. The Salzbachtal Bridge consists of two parts: The damage affects the already blocked southern superstructure, the traffic was transferred to the northern bridge, which has now also been blocked for safety reasons.
The fastest possible demolition
The unstable southern structure must be demolished as quickly as possible. If the bridge collapsed in an uncontrolled manner, the northern structure could also be damaged. Vogler compares this to a tree that falls in the forest. “Then he can take more trees with him.” Therefore, blasting the bridge is ruled out. Demolishing such a bridge is almost more difficult than building a new one, says civil engineer Herbert Duda. “That is a demanding engineering task.” According to Vogler, there are basically two methods that come into question: Either you put a complete scaffold under the bridge with a stable catch structure.
Or you start in the middle, work your way slowly from there to the pillars and secure the respective area with formwork. The advantage of this variant: you interfere with the environment the least. The disadvantage is the slow speed. “You only make very slow progress, little by little.” But why are so many bridges in such a bad state?
The renovation can extend the service life. But after reunification, the focus was on the highways in the new federal states. “The motorways in the west were neglected,” says Vogler. Critics also blame German public procurement law with its long deadlines for the delay. Civil engineer Duda also believes that building law, with its many possibilities for legal action, is a reason for the delays in the construction and renovation of bridges: “Many people try to prevent such projects.”