Use of the Strand Post-Tensioning System for external tendons

Altogether 19 tunnels and 98 bridges, 19 of them major bridges, are being planned on the 64 km long section of the A44 motorway between Kassel and Wommen. The 530 m long Wehretal Bridge is the longest structure in this project.

In autumn 2000, construction of the foundations for the abutments, designed to minimize settlements due to the bridge’s dead load, was carried out. A total of 370 concrete piles were installed in the soil underneath the 11 piers and the abutments to provide the structure's required bearing capacity.

The two separate bridge decks with a length of 530 m and 527 m respectively will be incrementally launched, minimizing the impact of construction on an ecologically sensitive valley. The superstructures are slightly curved and inclined 12 span, single cell hollow box girders. On this project the DYWIDAG MC Strand Post-Tensioning System for external tendons will be used for the very first time. The post-tensioning of the structure is incorporated into the longitudinal direction only. Every week, due to the alternating load cases caused by the launching of the bridge, two external DYWIDAG tendons of type MC 63115 will be installed and prestressed as primary post-tensioning and two external tendons of type 6815 consisting of galvanized strands will be installed and prestressed as temporary post-tensioning over two segments (one span). After final launching the temporary tendons will be distressed and removed.

Subsequently, the secondary post-tensioning, consisting of six up to 201 m long parallel external tendons of type DYWIDAG MC 63115 will be installed. The MC post-tensioning tendons are installed irrespective of the schedule. After the bottom slab and the webs have been poured, the formwork dismantled, and the deck form brought into position, the specialist staff of SUSPA-DSI GmbH install the tendons required for launching. Since the individual construction activities are independent from each other, a four-day cycle can be successfully implemented.

For the substructures 5,500 m3 and for the bridge decks 10,300 m3 of concrete are required. In addition, 2,400 t reinforcement and 235 t prestressing steel are used. If the structural work proceeds according to schedule, the Wehretal Bridge will be completed in late summer 2003. Should also the other work on tunnels and bridges proceed without interruption, the 4.3 km long interstate section near Walburg will open to traffic in 2004.

Installation

Dipl.-Ing. Peter Strauss, Kassel, Germany