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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


The terrorist threat to U.S. bridges is believed to be very credible, and costs for reconstruction and socioeconomic losses from these threats are potentially in the billions of dollars.(1) The time to address protection measures, especially for new bridges, is during the design stage to produce cost-effective protection, as this is more economical than retrofitting later. Cables of cable-supported bridge designs are subject to extra scrutiny because the cable bundles supporting the bridges can come close to the roadways or pedestrian walkways and may be easily accessible. An Accident and Terrorist Vulnerability Assessment (ATVA) is usually part of the planning and design process for these types of important structures to understand how to best incorporate effective strategies. Part of the ATVA includes establishing performance criteria for protection measures against various threats. However, to date, these performance measures have been developed in an ad hoc fashion based on group consensus with limited uniformity across the nation.

In the summer of 2016, the Federal Highway Administration (FHWA) received numerous mockup cable-stay bundles that were subjected to various threats as part of the qualification of a cable protection system. This report does not discuss the specific bridge project for which this qualification testing was performed, nor does it discuss the design of the protection system that was applied over the cable bundles. However, the qualification testing was performed against the threats of fire, ballistics, blast, and cutting tests. Based on the consensus of subject matter experts and the bridge owner, it was determined that acceptance of these performance tests would be 75 percent survival of wires within a bundle using just visual assessment. After observing the various qualification tests, it became obvious there were some challenges with the agreed-upon acceptance criteria, including the following:


The objective of this project was to conduct tensile testing of individual strands from cable-stay bundle qualification tests to assess the change in mechanical properties that may have occurred as a result of the various types of threats. Additionally, assessments of deformation, hardness, and metallography were conducted to determine if these simpler measurement parameters could be correlated to a change in material properties.

Bundle Description

Qualification tests were completed on 43-strand and 109-strand bundles. Subsequently, the bundles were provided to FHWA for supplemental testing. All bundles were constructed from 0.62-inch-diameter strands meeting the ASTM A416 specification.(2) All the strands were greased and sheathed in high-density polyethylene (HDPE). While multiple threats were considered for the qualification testing, only bundles that were subjected to certain blast and thermal-cutting scenarios were assessed through the supplemental testing described in this report.

Blast-Tested Bundles

Four bundles subjected to blast were used in this project. Each bundle size (43-strand and 109-strand) was tested at two different standoff distances. Three of the bundles were no longer intact after the blast event; thus, only boxes of individual strands were received. Though the last bundle did remain intact, as pictured in figure 1, it was significantly deformed, and numerous wires were severed.

A 43-strand bundle is shown inside a wooden box. The bundle is intact at both ends and resting on the extents of the box. The bundle arcs upward, no longer in contact with the wooden box for what appears to be four bundle widths high at the middle of the bundle. The bundle is no longer intact at the center and is a mix of broken strands, broken wires, and intact strands all jumbled together and expanded out to approximately two original bundle widths in diameter.

Source: FHWA.
Figure 1. Photo. Bundle subjected to a blast charge.

Thermal Lance Cut Bundle

One 43-strand bundle that had three different thermal-cutting tests applied to it was delivered. Two tests were performed with a thermal lance and one with an oxyacetylene torch. The notion behind the qualification test was to see how much damage could be done in a set amount of time. Very little damage was caused by the oxyacetylene torch; thus, only the thermal lance cuts were of interest. Figure 2 shows a picture of the bundle with closeups of the two individual thermal lance cuts, labeled “Cut A” and “Cut B.”

In the background is a picture of the entire bundle comprising 43 strands. On the left side of the bundle, a bracket is drawn showing a designated 36-inch length of the bundle centered around a thermal lance cut. A bubble callout is pointed at the area where there is a cut, and within the bubble is another closeup photo of the actual cut labeled “Cut B.” In the closeup photo of cut B, the individual strands are oriented horizontally, and in the center there appears to be a ½-inch-thick steel plate used to retain the shape of the bundle. A portion of the retaining plate and approximately five of the strands running through it are melted, and the plastic sheathing on most of the bundle has melted away from the retaining plate. Just to the right of the previously described bracket, another bracket indicates a second 36-inch length of bundle, and another cut can be seen faintly in the middle. A bubble callout is pointed at the area where there is a cut, and within the bubble is another closeup photo of the actual cut labeled “Cut A.” In the closeup photo of cut A, the entire bundle is encased in a large plastic pipe, which has been melted away for a couple inches to each side of the thermal cut. It appears that about seven strands have been cut. Near the right side of the bundle is a bubble callout labeled “Acetylene Cut” pointing to a burn mark on the large plastic pipe.

Source: FHWA.
Figure 2. Photo. Bundle subjected to thermal lance tests.

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