Route Planning

An essential boundary condition for the new replacement construction is to maintain traffic flow over the entire period of construction (4+0 traffic). When selecting the routing, which was closely linked to considerations for the replacement construction, the near east variant (axis clearance 1 bridge superstructure) 3,250 m emerged as having the most advantages and the lowest costs.

Replacement Construction

During the preliminary design, only cross-sections with two separate superstructures were considered. This resulted in the following construction phases:

• Production of the eastern superstructure
• Traffic rerouting (4+0 traffic)
• Demolition of the existing 1-part bridge
• Production of the western superstructure

For the support positions in the longitudinal direction, statically economical spans were optimised with the least possible interaction with the existing foundations. In the area of the canal piers, however, it was unavoidable to overlap the bored piles of the new foundation with the existing caissons. The following cross-sections were chosen after completing variant studies:

• "Classic" (single-cell hollow box girders)
• "Box with classic supports"
• "Cross-section with two box girders"
• "3-cell box girder cross-section"

In reaching a final decision, other key design parameters were considered in addition to the usual criteria of overall economic viability, durability and sustainability. The following options and consequences arise for the maintenance and inspection inside the hollow box girders:

• Standard solution: walkable, ventilated, full corrosion protection (230 μm), full testing
• Scandinavian solution: sealed, with dehumidification, reduced corrosion protection (70 μm), full testing
• Reduced solution: sealed, modified box girder design to dispense with corrosion protection and accessibility

After a detailed analysis, the Scandinavian solution appears particularly interesting. The basis is that no corrosion takes place at a maximum relative humidity of ~40 %. In Germany, there are currently only six known bridge projects in which corrosion protection was carried out in one form or another using dehumidified air. After a review of operating costs for air dehumidification for the Werratal Bridge and corresponding conclusions were reached for the cross-sections of the Rader overpass, there were clear economic advantages for considering its service life. Consequently, the capitalised costs for renewing the internal corrosion protection according to Ri-Wi-Brü (Guideline for the performance of economic feasibility studies within the scope of repair/renewal measures for road bridges) are clearly higher than the capitalised operating costs for dehumidification.