English (UK)
Français (FR) 
This article is reporting the magnetic rope inspection of the François Baud pedestrian footbridge achieved on May 2021 in close collaboration with Cordata team of rope access technicians. The suspended footbridge built in 1952 is 175 meters long and is crossing the Dranse river in Morzine, France. The purpose was to assess the rope condition of the bridge suspension as well as of the parabolic tension ropes attached to the girder. To the best of our knowledge, this is a pionneering work mixing both rope disassembly and the MRT inspection of rope section under the hanger’s clamps.
Survey strategy
Hanger's disassembly
After disassembling hangers, hidden rope sections revealed some bleeding of rope lubricant which still exists almost seventy years after their installation. On the first hand, a visual inspection did not revealed any clue of developing damage nearby clamp sections, this notice being later confirmed by the wirelet analysis of magnetic signals. On the other hand, some hanger’s threaded end happened to suffer from serious abrasion due to the lateral motion of the deck.
Cordist operation on bridge stays
Cordata rope access technicians showed their best skills during the inspection to either install a return pulley for the drawing of the MRT device or to insert a sufficient clearance by means of a rope spacer to enable the passage of the SMRT40 magnetic device.
MRT inspection of bridge stays
The SMRT40 magnetic device is pulled along the opened section of suspension rope to reveal rope anomalies along its span and in peculiar at the vicinity of clamp areas. This operation was repeated to inspection ten rope sections following the survey strategy. Upstream parabolic rope used to stabilize the deck laterally was inspecting by pulling the MRT device from the anchoring till the first needle attachment. Strangely, the parabolic rope was the one that suffered the most in seventy years. Nothing critical, don’t worry!
Wirelet analysis of magnetic signals
Magnetic signals were finally analyzed by using Wirelet software to assess rope’s condition . Pictures that follow illustrate :
- The full wirelet scalogram of parabolic rope displays the energy of magnetic signals with respect to the position along the rope and the frequency (or the depth inside the rope). Several events may be distinguished whose signature give some indication about the kind of rope anomaly, and their severity,
- down left picture highlights the signature of a concentrated wear anomaly of internal wire mainly due to the indentation of wires
- down right signature is related to a magnetized lubricant spot with negative energy.
Wirelet software sketched in the carousel allows to make a complete and thorough analysis of rope condition and to edit a report for instance according to the EN12927 standard.
