Wire tensions and bending moments in axially loaded seven wire strand

Carefully instrumented tensile tests were performed on a straight single steel strand of seven‐wire construction having core and helical wire diameters of 3–66 mm and 3–33 mm, respectively, and a lay angle of 14‐6d̀. Strain gauges in groups of three with their grid axes parallel and having a common backing were attached to all six helical wires at the mid‐strand position. For loading within the linear range of the load‐extension response (all tests) and load‐rotation response (in tests with free and partial torsional restraint of the ends) the outputs from these strain gauges were used to compute wire tensions and bending moments. Strand extension, rotation and torque generated under axial load (fixed‐end and partial torsional restraint) were also recorded. Test results showed that the share of the total strand load taken by the helical wires decreased as the torsional restraint on the strand was reduced. The mean tension in the helical wires was found to be 27‐5% smaller in the free‐end tests than in fixed‐end tests for a given strand load. Theoretical predictions underestimate this reduction in tension by up to 80%. Bending moments in the wires were found to be about 8% less than predicted in the fixed‐end condition and over 20% smaller in the free‐end condition. As expected, the slope of the linear regions of the load‐extension and load‐rotation plots decreased as the torsional restraint was reduced. However, the rate of decrease in the case of load‐extension plots was found to be less than predicted theoretically and the rotation measured was generally less than predicted theoretically. This is consistent with the helical wire tensions being smaller than the predicted values since the tangential components of these reduced tensions would have less effect in their unwrapping action on the strand under increasing axial load.