DYWIDAG Systems secure longest Extradosed Bridge in Europe: POVAŽSKÁ BYSTRICA Bridge, Slovakia

The 968m long extradosed bridge No. 206 in Považská Bystrica is the most important element for completing the D1 Slovakian highway between Bratislava and Žilina. The bridge consists of a 30.4m wide single box superstructure supported by eleven piers, seven of which have deviators at intervals of 122m with 8 saddles each to accommodate its extradosed cables.

With this layout, the Považská Bystrica Bridge is by far the longest extradosed bridge in Europe, followed by Puhov Most in Slovenia with four piers and three main spans of 100m.

The bridge was designed by Alfa 04 and Strasky, Husty & Partners, and the general contractor is a consortium of Skanska and Doprastav. The final design and the criteria for the cables were selected from a wide range of different options. In the end, the decision was made to use DSI stay cable systems for the extradosed cables partially because DSI had already been involved in the world’s first extradosed Bridge which was built in Odawara, Japan in 1994.

The cables consist of 37 0.62" waxed and PE coated galvanized 7-wire strands with a nominal ultimate load of 1860N/mm². The corrosion protected strands are guided through a Ø 180mm PE pipe and anchored on both sides within the box girder with DYNA Grip ^® DG-P 37 stressing anchorages.

Above the pier, the cables are deviated in a saddle consisting of a saddle pipe and a recess pipe. At the saddle, the recess pipe is grouted, and different cable forces on both sides of the cables are transmitted reliably to the structure by an anchor groove and pin construction.

The anchorage fatigue and tensile tests, including the wedge-shaped shim plates with an angle of 10mrad underneath the anchorages as well as the leak tightness testing according to fib Bulletin 30, had previously been performed for the design criteria as used for cable stayed bridges. The fatigue tests were performed with two million load cycles at an upper load of 45% GUTS and an axial stress range of 200MPa, while the extradosed cables have maximum service loads of 60% GUTS, but an axial fatigue stress range of less than 30MPa. It is generally known that the stress level only has a minimal influence on the stress range for bare prestressing steels. The correlation between stress range and stress level is described in Smith diagrams. The fatigue behavior of galvanized strands is more favorable than that of bare strands. This is caused by the fact that the zinc layer between the individual wires reduces fretting fatigue. Smith diagrams can be used both for bare strands and for galvanized strands. According to Smith diagrams, the fatigue resistance is decreased only by 10% when increasing the upper stress level from 45% to 60% GUTS. Therefore, no new tests for the cables were required for this bridge.

The extradosed cables were installed using light equipment. First, the PE-sheathing was brought into an inclined position on both sides of the saddle. Then the strands were pushed in using special pushing equipment from one side of the deck through the saddle into the anchorage at the other side of the superstructure.

They were immediately stressed to an initial force with light weight monostrand jacks. Once strand installation had been completed, all strands of one extradosed cable were simultaneously stressed from both ends using the patented ConTen stressing method in two stressing steps. For some of the cables, the individual forces of all 37 strands are monitored using a newly developed EM sensor which is permanently installed at the rear side of the stressing anchorage for long term monitoring. During the stressing operation, the tolerances of the individual strand forces were within the allowed range.

Bridge construction started in the fall of 2008, strand installation for the extradosed cables began in September, 2009 and, due to very fast construction progress, the superstructure was finished in January 2010. The bridge was opened to traffic in the summer of 2010.

Background

The term ‘extradosed’ was coined by Jacques Mathivat in 1988 to appropriately describe an innovative concept he developed for the Arrêt-Darré Viaduct in France, in which external tendons were placed above the deck instead of within the cross-section as would be the case in a girder bridge. To differentiate these shallow external tendons, which define the uppermost surface of the bridge, from the stay cables found in a cable-stayed bridge, Mathivat called them ‘extradosed’ prestressing. Unfortunately, the design of Mathivat was not used for this viaduct.

The first extradosed bridge was the Odawara Blueway Bridge in Japan. This bridge was completed in 1994. It has span lengths of 73 + 122 + 73m, a width of 13m and a tower height above deck of 10.7m. The relation of pylon height to center span length is 1: 12, which is much smaller than the usual ratio of 1: 5 in conventional stay cable bridges.

Thus, the tendon stresses caused by live loads are reduced to nearly one quarter of that of stay cable bridges. An allowable tendon stress of 0.6fpu was chosen for the Odawara Bridge. High durability of the bridge was achieved by triple corrosion protection of the tendons using new technologies:

• Due to the severe climatic conditions of Japan’s coastal region, the tendons consist of epoxy-coated strands. The strands were installed with a specially developed pushing-in device.
• Glass-fibre reinforced plastic tubes were used as sheathing.
• A special crack-free polymer-cement grout was used as filler.

An inexpensive and space saving anchorage solution was provided by deviator saddles, avoiding troublesome end anchorages at the pylon. The Japanese DYWIDAG licensee Sumitomo Electric Industries Ltd. (SEI) supplied 56 t of epoxy-strands and 64 DYWIDAG MC-anchorages 19x0.6” developed for external tendons, which facilitated an easy and quick exchange of the tendons due to the double sheathing.

Before commencement of material supplies, SEI carried out various tests in co-operation with DSI Munich to check the reliability of the applied materials. This included tests with the epoxy-strand and its anchorage, a tensile test, a test on the system behavior and a pushing-in test for the strands. A large-scale test of the deviator saddles and a fatigue test simulating behavior of the structure under wind load have also been carried out by the owner together with the joint-venture companies.