Suspension bridges, engineering marvels that span vast waterways and connect communities, rely on an often-overlooked component for their structural integrity: the main cable. These massive assemblies, composed of thousands of individual steel wires, bear the entire weight of the bridge deck and traffic loads—often exceeding hundreds of thousands of tons—transferring these forces to the bridge towers and anchor points.

Recent research led by Los Alamos National Laboratory has made significant progress in understanding how these cables distribute stress internally. Using neutron diffraction technology, scientists have revealed the crucial role of friction between wires in stress transmission—a finding that challenges conventional engineering assumptions.

Main cables represent sophisticated composite structures rather than simple wire bundles. Their core consists of tightly packed galvanized wire strands arranged in hexagonal patterns for optimal compaction. An outer layer of continuous, pre-tensioned wires wraps around this core, secured at regular intervals by radial clamps that maintain structural integrity and enhance stress transfer to any broken wires.

For perspective, the Manhattan Bridge's 50cm-diameter cables contain approximately 8,500-9,000 wires, while larger structures like the Golden Gate Bridge feature cables nearly one meter in diameter with over 28,000 individual wires. These cables endure tremendous tensile forces from permanent loads (bridge weight), live loads (traffic), and environmental factors like wind and seismic activity.

Analyzing load distribution within cables presents unique theoretical and experimental difficulties. Traditional finite element analysis struggles to accurately model the complex interactions between thousands of wires, particularly at contact points where friction coefficients, local deformation, and contact areas remain difficult to measure or estimate.

The research team overcame these limitations by employing neutron diffraction—a non-destructive technique that measures elastic strain in materials by detecting changes in atomic lattice spacing. Unlike X-rays, neutrons penetrate deep into materials without damaging them, allowing unprecedented examination of internal stresses in operational-scale cable samples.

The experiments demonstrated that individual wire strains depend significantly on boundary conditions at clamping points and radial compression forces. Most notably, inter-wire friction proved far more substantial than previously assumed—a revelation that necessitates reevaluation of conventional cable stress models.

This discovery carries profound implications for bridge safety. By incorporating friction effects into stress analysis, engineers can more accurately predict load distributions, potentially extending cable service life and preventing catastrophic failures. The methodology also enables detection of broken wires and localized defects that compromise structural integrity.

The research, accepted for publication in the Journal of Experimental Mechanics, opens new avenues for bridge engineering. Future work will focus on developing advanced finite element models that account for friction effects and investigating next-generation cable materials. The team also plans to adapt neutron diffraction for real-time structural health monitoring systems that could revolutionize bridge maintenance practices.

This technological breakthrough represents more than academic progress—it offers tangible benefits for infrastructure safety worldwide. As suspension bridges age and traffic loads increase, accurate assessment of cable conditions becomes increasingly vital. The neutron diffraction approach provides engineers with powerful new tools to safeguard these critical transportation links, ensuring their continued service for generations to come.