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Publication Number:  FHWA-HRT-17-109    Date:  January 2018
Publication Number: FHWA-HRT-17-109
Date: January 2018


Cable-Stay Strand Residual Strength Related to Security Threats

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This report documents tensile testing of selected seven-wire strands and individual wires of strands that were damaged during cable-stay bundle protection system qualification. The qualification testing was performed to assess the adequacy of protection systems applied over the stay bundle against terroristic threats of blast and thermal cutting. The qualification used a primary acceptance criterion of 75 percent survival of wires. After the testing was performed, it was questioned if certain wires survived sufficiently despite being intact and, in particular, whether wires with nicks, gouges, kinks, or untwisted strands should be considered fully or partially damaged. The tensile testing assessed the residual capacity of strands and wires in various states of damage, attempting to answer these questions while evaluating the qualification results. The results showed that, in terms of blast, the residual strength was not correlated to magnitude of damage (e.g., degree of curvature, impact gouges, or untwisting), and rather the overall strength of the strands uniformly decreased by 5 percent. In terms of thermal cutting, the residual strength was greatly affected by the amount of heat to which the strand was subjected.

The results attained were useful to the bridge owner who performed the qualification testing, and it is expected these results will be very beneficial to other bridge owners who must define protection scheme qualification acceptance criteria for future cable-supported structures. This report will benefit those who oversee qualification testing of cables used on cable-supported bridges, including State transportation departments, bridge design consultants, and cable suppliers who manufacture suspension cables and stays and their protection measures.

Cheryl Allen Richter, Ph.D., P.E.
Director, Office of Infrastructure
Research and Development


This document is disseminated under the sponsorship of the U.S. Department of Transportation (USDOT) in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

Technical Report Documentation Page
1. Report No.
2. Government Accession No. 3. Recipient’s Catalog No.
4. Title and Subtitle
Cable-Stay Strand Residual Strength Related to Security Threats
5. Report Date
January 2018
6. Performing Organization Code:
7. Author(s)
Justin Ocel, Ph.D., P.E.; Jason Provines, P.E.; Vince Charito, P.E.
8. Performing Organization Report No.
9. Performing Organization Name and Address
Bridge and Foundation Engineering Team
Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296
10. Work Unit No.
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296
13. Type of Report and Period Covered
Final Report; June 2016–May 2017
14. Sponsoring Agency Code
15. Supplementary Notes
This project was undertaken as part of an Accident and Terrorist Vulnerability Assessment (ATVA) of a particular bridge partially paid for under the Federal-Aid Program. The U.S. Army Corps of Engineers was a consultant to the bridge owner and devised this research project in support of the ATVA. The testing was conducted in Federal Highway Administration’s Turner–Fairbank Highway Research Center Structures Laboratory. The Contracting Officer’s Representative for work performed by the contractors was Fassil Beshah (HRDI-40).
16. Abstract
Cable-stay bridge designs are an economical bridge type for long spans. However, long-span bridges also tend to be signature spans and often serve as critical freight corridors and/or critical lifeline structures. As such, it is prudent to consider terroristic threats against these assets and strengthen cable-stay bridges to minimize vulnerabilities. The stays are a critical element in the cable-stay design that should be protected if they are easily accessible from roadways or walkways. The protection schemes applied over cable stays must be qualified based on performance criteria and judged to pass or fail based on agreed-upon acceptance criteria. This project was conducted in support of a particular bridge project where questions were raised after qualification testing regarding the acceptance criteria.

One of the criteria was 75 percent survival of wires against blast and thermal-cutting threats. After the qualification testing, it became questionable whether a wire that was gouged, nicked, or bent should be considered to have survived. The testing reported in this report assessed the residual strength of individual wires and strands in various states of damage. For blast, no correlation in strength could be identified against any of the damage metrics, though generally the entire population tested had an overall reduction in strength of 5 percent over new strands. As for the thermal cutting, gouges were quite detrimental, along with exposure to heat. The exposure to heat could easily be visually inspected based on the presence of the polyethylene coating or grease on the strands, though this was a coarse measure. A better means of assessing heat damage was through destructive hardness testing. It was found that strength did not degrade unless the strands were exposed to temperatures of more than 900 °F.
17. Key Words
Cable bundles, cable-stay bridge, terrorist threats, thermal cutting, blast design, threat protection
18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.
19. Security Classif. (of this report)
20. Security Classif. (of this page)
21. No. of Pages
22. Price
Form DOT F 1700.7 (8–72) Reproduction of completed page authorized.

SI* (Modern Metric) Conversion Factors

Table of Contents

List of Figures

List of Tables

List of Abbreviations

ATVA Accident and Terrorist Vulnerability Assessment
AUTS actual ultimate tensile strength
COV coefficient of variation
DIC digital image correlation
FBC full birdcage
FHWA Federal Highway Administration
HAZ heat-affected zone
HDPE high-density polyethylene
HV Vickers hardness
IBC incipient birdcage
PTI Post-Tensioning Institute
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101