Test Set-Up at DTU with Clamp

Test Set-Up TU Vienna

Successful DYNA Grip^® Fatigue Bending Tests

There are several types of vibrations that can occur in stay cables. Until effective dampening measures are installed, cable vibration can result in high amplitudes and large cable rotations at the anchorage zone. There is very little information about the degradation of the fatigue performance of stay cables that were loaded with severe fatigue bending.

The current design code EN 1993-1-11 requires that flexural effects be taken into consideration in the fatigue design for structures with tension components. Protective measures to minimize the bending stresses within the anchorage areas are to be employed and the actual configuration used should preferably be tested. However, there are no clear regulations on how this effectiveness should be demonstrated and how bending stresses should be limited. Furthermore, neither fib Bulletin 30 nor Setra recommendations provide any values for the limitation of bending stress.

1) DYNA Grip^® 2 Million Cycle Fatigue Test at the Technical University of Denmark (DTU)

DTU has designed a new test setup for fatigue bending tests. A test setup with static inclination at the anchorage was chosen to simulate the strand deviation within a full size stay cable anchorage, taking bundling effects and static inclinations into consideration. Following the recommended test setup in fib Bulletin 30, an S-shaped profile was used. Transverse loading at the cable’s mid span was created by a hydraulic actuator to simulate the angular deviation of the cable caused by cable vibration or structural deformation. A simplified model of the new test setup is shown in Fig. 1.
Two series of bending fatigue tests were performed on galvanized, waxed and PE-coated 0.62" 7-wire strands with an ultimate tensile strength of 1860MPa. A first test series was carried out with single strands and without any additional elements such as sealing plates or spacers to reduce the bending stresses (Fig. 2). In a second test series, an anchorage corresponding to the current design of the DSI DYNA Grip^® System was used (Fig. 3). On the test rig, static inclination angles of up to 3.0° were created at the anchorage, and the single strand specimen was stressed to an axial force corresponding to 45%-60% of the guaranteed ultimate tensile strength (GUTS). After strand stressing and power seating of both wedges, the additional angular deviation was varied from +/- 10mrad to +/- 25mrad in fatigue tests with up to 2x106 load cycles. The test results were apparent. In the first test series with the simple anchorage that is normally used in Post-Tensioning applications, wire fractures occurred at a very early stage - mostly between 10.000 and 20.000 cycles. Fatigue failure typically occurred at the tip of the wedge. This area is subject to the highest bending stresses and the strand is notched due to the wedge bite. More importantly, the single strand tests using the anchorage corresponding to the DYNA Grip^® System easily passed the 2x106 cycles without any wire breaks.

2) DYNA Grip^® 10 Million Cycle Fatigue Test at DTU

Following the first test series, a 10x106 cycle test was conducted on the DTU test rig using single strands fitted with a DYNA Grip^® Anchorage including a sealing device without any interruption caused by failures. The static inclination angle of the bearing plates was 3.0°, axial force corresponded to 45% of the guaranteed ultimate tensile strength (GUTS), and the additional angular deviation was +/- 10mrad.
All the test results performed at the DTU show how the DYNA Grip^® sealing device influences the fatigue behavior of 7-wire strands. It has to be noted that tests without sealing device had to be stopped at a very early stage due to wire breaks. Strands with sealing devices did not show any problems at any time during the test. Moreover, the strands passed a much higher value of cycles. The tests showed that the sealing unit of the DYNA Grip^® System, which consists of an HDPE spacer, sealing plates and a compression plate, is most effective in terms of preventing fatigue caused by bending. An additional guide deviator is unnecessary.