The existing Rader overpass on the A7 autobahn over the Kiel Canal has a total length of approx. 1,500 m and is a 15-span steel girder bridge with orthotropic deck. Due to major load-bearing deficits and a remaining useful life of approx. 12 years, the structure is to be replaced by a new structure. A new replacement structure (demolition of the existing structure and new construction) is planned with the necessary adjustments to the traffic facilities.
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.
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.