Inspection and Maintenance of Bridge Stay Cable Systems (2005) / Chapter Skim
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Pages 37-49
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From page 37... ...
Visual Inspections Visual inspections are the most common approach used on stay cables. Surveys completed by a number of respondents indicated a preference for visual inspections (when feasible)
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From page 38... ...
General visual inspections of stay cables typically involve the following: • Identification of longitudinal or transverse cracking or excessive bulging in the sheathing, as well as damage at connections to dampers or cross cables, if any. • Inspection for cable alignment irregularities including waviness or excessive sag.
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Stay cables have bending stiffness, T L f m= 4 2 2 Eq. 2 FIGURE 55 Trolley used for inspection of Luling Bridge cables (Elliott and Heymsfield 2003)
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Unless and until wire breaks result in global stiffness changes in the cable, section loss could not be inferred from cable force measurements. The following comments related to vibration-based force measurements were provided by the respondents to the survey: • "Laser-based force measurements will give results that will indicate if a cable is deviating from the trending valFIGURE 56 Rolling device for inspection of the stay cables on the Dame Point Bridge ("B&N Creates Custom Device .
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It is relatively easy and inexpensive to perform." • "TxDOT has employed vibration-based force measurements to refine the model used for designing viscous dampers on each of the cable stay bridges. The technique seemed to give good correlation cable dimensions and damping requirements.
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Figure 59 shows an ultrasonic test record indicating a wire break. However, it is not clear if the test record shown in the figure is indicative of the clarity and definiteness of a typical ultrasonic test record or perhaps a representation of one of the better results.
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A dual system would include the two different sensor types and would measure LF and LMA at the same time. They reported that EM methods had been used in Germany for bridge stay cables for 25 years (the publication date of the paper was 1995)
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It is believed that the multiple materials (steel, grout, steel strand) which comprise the cable stays combined with the changing geometry would make interpretation of the image difficult and would not allow for an accurate understanding of the conditions.
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FIGURE 62 Schematic diagram of MsS sensors (Bartels et al.
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Acoustic Monitoring Acoustic monitoring is a passive method for detection of wire breaks in stay cables. It "listens" for shock waves emanating from wire breaks.
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From page 47... ...
. Impulse response Relative Cost Relative Inspection Rate Adaptability to Environmental Conditions Unfilled Split in Sheathing EpoxyFilled Split in Sheathing Damage to UV Tape Grout Void or Damage Effectiveness low high high none none none none low medium high none none none good low medium low good none good none high low high good fair good good NDT Method Impulse radar Infrared thermography Low energy X-ray TABLE 8 COMPARISONS OF SOME NDT METHODS FOR DEFECTS IN PE SHEATHING, GROUT, AND TAPE
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From page 48... ...
provides an accurate method for monitoring wire breaks due to fatigue damage in grouted stay cables. The system was able to identify the number of wire breaks accurately.
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From page 49... ...
(1998) reported on the testing of distributed fiber optic strain sensors for stay cables.
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