Click for next page ( 2

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
INSPECTION AND MAINTENANCE OF BRIDGE STAY CABLE SYSTEMS SUMMARY In this synthesis effort, a worldwide search of information on inspection, repair, testing, and design of stay cable, was undertaken. On-line sources of information as well as engineering databases were examined. Contacts were made with a number of knowledgeable individuals for information. A questionnaire was prepared and distributed to all state and provincial depart- ments of transportation in the United States and Canada. Completed questionnaires were received from 75% (27 of 36) of all known U.S. cable-stayed bridges and 81% (13 of 16) known cable-stayed bridges in Canada. Based on this information, various methods, approaches, and practices are explained in detail and their strengths and weaknesses identified. Specific approaches to inspection and repair are presented and discussed. Challenges in the inspection and maintenance of cable-stayed bridges are significant. The main tension elements (MTEs) within a cable bundle are, in most cases, hidden from the view of inspectors. Access to cables for visual inspections or nondestructive testing is generally dif- ficult and, in the case of the anchorage zones, nearly impossible. Those who are responsible for the inspection and maintenance of stay cables are faced with challenges for which proven and accepted methodologies and tools are limited and, in many cases, very costly. There are 36 cable-stayed bridges in the United States and 16 such bridges in Canada. As of 2005, the average age of cable-stayed bridges in the United States was 11.4 years. As these bridges age, the need for effective inspection and maintenance methods and tools becomes more acute. The following list cites some of the issues involved in the inspection, maintenance, and re- pair of stay cables, presents methods identified in the literature to address these issues, and briefly summarizes their known pros and cons and other factors. Detailed descriptions and discussions of these methods are given throughout this report. General inspections (visual)--Visual inspections are, in the great majority of cases, the only method used for cable-stayed bridges. Assessment of MTE condition in free length (magnetic flux leakage)--This system has a long history in the inspection of industrial cables and ropes. Assessment of MTE condition (cable force measurements)--This approach is the most widely used, and sometimes misunderstood, nondestructive evaluation method. Assessment of MTE condition (ultrasonic testing)--This method has been used on a few bridges to evaluate the condition of MTEs in Hi-Am-type anchorages. Assessment of MTE condition (radiography)--Theoretically, this method has the poten- tial to successfully assess conditions of cable anchorages where there is access to the perimeter. Detection of wire breaks as they happen (acoustic monitoring)--Test laboratories per- forming qualification fatigue tests of stay cables have long used this method to detect wire breaks in the cable specimens as they happen. Detection of grout voids inside high-density polyethylene (HDPE) pipe sheathing (impulse radar)--Hand-held impulse radar equipment can be placed over the cable and moved longitudinally to identify potential grout voids inside the cable sheathing.

OCR for page 1
2 Repair of large grout voids (vacuum grouting)--This method has long been used in post- tensioning tendon applications. Cable force measurements (vibration-based using laser vibrometer)--A laser vibrometer is used to measure small vibrations of the cable from a large distance. Cable force measurements (vibration-based using accelerometer)--Similar to the laser- based method described previously. Cable force measurements (based on measurement of cable sag)--Although the tension in a cable is related to the square of the fundamental frequency, it is also inversely proportional to the cable sag; therefore, measurements of the cable sag can also be used to estimate cable tension. Detection of hidden splits in HDPE (infrared thermography)--Hand-held infrared thermo- graphy equipment can be used to detect splits in HDPE pipes that are hidden under the protective tape. Detection of damage to polyvinyl fluoride tape (infrared thermography)--Similar to the method discussed for the detection of hidden splits in HDPE. Assessment of cable vibrations (long-term monitoring using accelerometers)--When cable vibration problems are suspected, sensors (accelerometers) can be mounted on select cables to monitor vibrations over a period of several weeks, months, or years. Assessment of cable vibrations (video cameras on bridge)--There are no known instances of using video cameras to monitor vibrations on cable-stayed bridges. This option was discussed for two bridges in the path of hurricanes, but was not implemented. Assessment of cable damping (vibration decay method)--There are different approaches to measuring cable damping. In one, an accelerometer is first attached on the cable. There is no single method that would provide answers for all the questions regarding the con- dition of stay cables. In most cases, it is the combination of nondestructive testing techniques together with the experience, knowledge, and judgment of engineers, inspectors, and techni- cians that could lead to the appropriate answer. The effectiveness and accuracy of many of the methods might be significantly enhanced if baseline comparative measurements are available when the bridge is known to be defect free.