 User Friendly Development of the Con-Ten Stressing System for Stay Cables System for Stay CablesToday, stay cables are usually stressed using mono jacks. The advantage of stressing with mono jacks lies in their ease of handling because of their light weight and small size. The challenge is to achieve the same force in each strand of a stay cable once tensioning is completed. Tensioning induces a force into the structure that leads to the structure’s deformation. These structural deformations and changes in the sag of the stay cable lead to a reduction of the stressing force in previously stressed strands. Unless special measures are taken, this leads to an uneven state of stress in the individual cable strands. In 1995, DSI developed the Con-Ten Stressing System for this purpose. Originally, this included a tensioning unit with a first tensioning jack (reference jack) and a second tensioning jack (working jack) connected via a pressure hose and a return hose to form a hydraulically interconnected system. Stressing of the reference strand is initially carried out using only the reference jack. During this procedure, a proximity switch on the reference jack is adjusted. The reference jack remains on the reference strand. The subsequent strand is tensioned using the working jack until the proximity switch of the reference jack interrupts the hydraulic flow to the working jack. This ensures that both strands are equally stressed. By using this technique repeatedly on the remaining strands, all of the strands in a stay cable can be tensioned equally without having to carry out detailed measurements.
The Con-Ten Stressing System has proven itself in many stay cable projects. However, users continually expressed a desire for a system that would be easier, more userfriendly and more economical. In the original Con-Ten system, two equal tensioning jacks and a special electronically operated hydraulic pump were required. The new system only requires one tensioning jack and a slightly modified robust standard Type R 3,0 hydraulic pump. In addition, a common manual hydraulic pump and a mechanical control unit are used. The mechanical control unit is at the core of the new development.
In the case of the new Con-Ten device, the hydraulic system includes a hydraulic pump that is connected to the control unit via a feed line. From here, a connecting hydraulic line leads to the tensioning jack, which is connected to the hydraulic pump via a return line.
The control unit is basically a valve that is driven by the stroke movements of a piston and controls inflow to the tensioning jack. The control unit is positioned on the tensioned reference strand. The tensioning jack is positioned on the strand that is to be tensioned next. As soon as the pressure inside the tensioning jack equals the pressure inside the valve chamber, the tensioning process is interrupted by an automatic closure of the valve. Since the piston area of the tensioning jack equals the piston area of the control unit, both strands contain equal forces after valve closure. All of the strands in a stay cable are successively tensioned using the tensioning jack.
Both Con-Ten Stressing Systems are patented by DSI. The new method has already proven itself in two projects. 7 tensioning units were used during the construction of the Pitt River Bridge, Vancouver, Canada (2008-2009). Two sets of Con-Ten units were used during the construction of the Lech Bridge Bach in Austria (2009). At the moment, 4 tensioning units are being used for the construction site of Povazska Bystrica Bridge in Slovakia (2009-2010). Additional projects are currently underway. The new system will successively replace the previous one.
Advantages of the new generation system: - Robust components – suitable for construction sites due to the elimination of electronic components
- Easy to handle – simple setting of the control unit, light weight
- Lower equipment cost
DSI Patents for the Con-Ten Stressing System: - DE 195 36 701 C2
- DE 10 2008 032 881 B3
|
