Cable-Stay Strand Residual Strength Related to Security Threats

Conclusions and Recommendations

This project was undertaken strictly to answer questions pertaining to qualification of cable-stay bundle protection measures and the acceptance criteria used. The specific question addressed was, What residual strength exists for individual strands with all wires intact (not severed) or with only one wire intact to various types of damage? This focus for the testing means the conclusions are only applicable to the qualification tests performed to enable evaluators or quality inspectors to evaluate the satisfactory acceptance of a protection system against prescribed hazards identified from an ATVA. The results were not acquired for the purpose of assessing or load rating entire cable-stay bundles that may be damaged from a hazard event on a real bridge. The following conclusions were derived from the results of this project:

• The virgin strand used to make up the qualification test bundles was in conformance with ASTM A416.⁽²⁾ Since two heats of strand were used in the entire qualification test program, it is believed only one heat was characterized for virgin properties. The average results showed the virgin strand had an average yield strength of 57.53 kip, AUTS of 64.14 kip, and elongation of 3.81 percent.
• The residual mechanical properties of the thermal lance cut strands did not correlate to the proximity of strands with cut or damaged wires. That is, some strands located adjacent to others with obvious thermally cut wires were still able to achieve mechanical properties equivalent to virgin strands. The only reliable indicator of low residual strength was if the grease had been completely burned off the wires. It is recommended that, for future thermal lance cutting qualifications, strands where the grease is completely burned off all seven wires should be considered completely damaged. If strands are not greased, destructive hardness tests could be conducted on wires, and any HV readings less than 450 HV should be considered completely damaged. Low strength results were also attained when a strand had obvious gouges from the thermal lance or even fused slag. Therefore, it is recommended that any wires with thermal gouges or fused slag also be considered completely damaged.
• The residual strength of blast-tested strands had a small reduction in strength. Only strands with FBC and IBC conditions were found to have a statistically significant reduction in strength to virgin strands. This reduction in strength was approximately 2 percent for FBC strands and 1 percent for IBC strands. The reduction is attributed broadly to additional cold work imposed on the strand from the blast event through bending, untwisting, and impacts. However, no strong correlation could be identified between the amount of lateral deformation or maximum strand diameter.
• The prior conclusion was based on testing strands where all seven wires were intact. However, there was a small population of strands where between one and six wires were cut during the blast event. Wires removed from strands with six wires cut showed an average reduction in wire strength of 7 percent.
• Elongation of damaged strands can be significantly degraded while still being able to achieve maximum load requirements per ASTM A416.⁽²⁾ Elongation is currently not considered in cable-stay qualification testing. However, while specifically noted at the beginning of this section that these testing results are not applicable to assess or load rate in-service cable stays that have been damaged, it is inevitable that someone may attempt to apply the results for such a purpose. Therefore, the reduction in elongation must be highlighted, with particular focus assigned to damage states like blast impact gouges, birdcaging, and bending that has introduced additional cold-work.

Recommendations

Future blast qualification testing of cable bundles should abandon careful inspection of individual strands for curvature, birdcaging, and impacts. Rather, it is recommended that damage from all these forms of deformation be lumped together into an overall reduction factor applied to all surviving wires and that acceptance inspection only require counting the number of surviving wires. A reduction factor of 0.95 is recommended to be multiplied against the number of wires surviving the test to define the number of wires then compared to the acceptance criteria. This overall reduction factor accounts for the small reduction in the strength of FBC and IBC strands and the larger reduction in the strength of strands with six wires cut.

The PTI DC-45 committee should consider adding in qualification criteria for security threats to their DC45.1 specification to help unify this type of testing across the country.⁽⁵⁾ Right now, their acceptance testing only considers corrosion resistance, anchorage fatigue, and fire resistance. As far as this work is concerned, only blast and thermal-cutting events can be addressed, and recommendations can be made only for acceptance criteria to qualify hardening systems for cable-stay bundles. The conclusions within this report regarding a mean blast bundle reduction factor and hardness limits for thermal threats are only considered starting points for deliberation.

Future Work

Working toward a national standard for qualification of hardening systems for cable-stay bundles, the following two topic areas are identified as deserving of future work:

• The acceptance criterion used for the particular bridge project that provided the mockup bundles reported herein used a 75-percent survival of wires. More work should be performed to assess if 75 percent is a good number to represent the balance between bridge safety and threat deterrence.
• This work highlighted that elongation of strands and wires can be greatly reduced as part of blast and thermal effects. Elongation is currently not considered in qualification, but additional work should be performed to see if there is a basis for including it and if the 92-percent AUTS requirement used in this report works as an appropriate surrogate.