Cable Safety Factors for Four Suspension Bridge

Abstract

Four suspension bridges are studied: the Williamsburg (1903), Bear Mountain (1924), Triborough (1936), and Golden Gate (1937). Safety factors against failure are calculated for the main cables from original specifications as well as from actual cable wire tests performed at the time of construction. Using recent inspection reports, current safety factors against failure are computed for the main cables using a Type I Extreme Value Distribution. Two wire models are used: a more conservative Ductile-Brittle Wire Model (where wires with less than 0.6% elongation were considered brittle and are discounted) and a Ductile Wire Model (where only fractured wires are discounted). The number of fractured cable wires is estimated from the limited number of actual fractures observed; the Type I Distribution is used to estimate the true number of brittle wires. The most conservative current safety factors against failure range from 2.0 for the Triborough Bridge to 3.0 for the Williamsburg Bridge. Additional wire sampling and testing should be performed on the Triborough Bridge's cables.