Cable-Stay Strand Residual Strength Related to Security Threats

Introduction

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 evaluation of visual damage. A wire was considered ineffective if, in addition to being completely severed, it had any abrasions, nicks, or gouges. Strands that had started to untwist were considered completely damaged. This was based on conservative assumptions, but it was recognized that wires or strands in these partially damaged states may have some reserve strength.
• The evaluation of heat damage. The fire testing subjected cable bundles to heat without a tension load in the bundle. Visual inspection of the cables does not identify the possible changes to material properties that may have occurred due to heat exposure.

Objective

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.

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.”

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