Axial–Torsional interactions and wire deformation in 19-wire spiral strand

Tensile tests were performed on a straight steel strand of three layer (12/6/1) construction, having a core wire diameter of 3.66 mm and 3.33 mm diameter helical wires, under conditions of full end-fixity, partial restraint, and ends free from torsional restraint. The torque generated under tensile load was recorded as well as the strand extension and rotation over a 600 mm gauge length. Wire tensions and bending moments in the outer layer of helical wires were determined at the mid-strand position from the outputs of strain gauges in groups of three with parallel grids and mounted parallel to the wire axis on the crown of each wire. The rate of strand extension under tensile load was found to be greater in tests with reduced torsional restraint, the greatest rate occurring in the free-end test. The strand rotation rate was also found to be greatest in the free-end test. The greatest difference from the theoretically predicted rates occurred in a free-end test with increasing load; predicted values of extension and rotation underestimated the test results by 12 and 23 per cent, respectively. Displacement of the load-torque plots occurred in the direction of reducing torque as testing proceeded. This appears to indicate the redistribution of the strand load between the layers of wires. Wire tensions showed a more even sharing of load in the fixed-end condition than in the free-end condition. The increase in rate of tension with strand load was less for most wires in tests with reduced torsional restraint, with the lowest tension rates developing in the free-end condition. For most wires, the rate of bending moment change with strand load was greater (in the sense tending to decrease tensile stress on wire crowns) in tests with reduced torsional restraint. However, the bending moment rates varied greatly between wires, the variation being greater in tests with reduced torsional restraint than in fixed-end tests.