The role of spiral reinforcement in improving the ductility of concrete and the background to the New Zealand concrete design code equations for spiral reinforcement in potential plastic hinge zones are outlined. Tests and analysis of eleven full scale pretensioned prestressed concrete piles with a 400 mm octagonal cross section are reported. The tests were to determine the available ductility of the piles when subjected to large inelastic cyclic deformations from lateral loading which simulated severe seismic loading. The main variables of the tests were the amount of longitudinal nonprestressed reinforcement present, the applied axial load level, the amount of transverse spiral reinforcement present and the use of either Grade 275 or hard drawn spiral steel. The results showed that when the code specified quantity of spiral steel was present the piles were capable of undergoing very large post-elastic deformations when subjected
to cyclic lateral loading without significant loss in load carrying ability. Piles with about 50% of the code specified quantity of spiral reinforcement were capable of reaching displacement ductility factors of at least ±4 providing the spiral spacing did not exceed about 4 tendon diameters. When the spiral reinforcement was provided by hard drawn wire the pile response was eventually limited by the fracture of the confining steel reinforcement, whereas piles with Grade 275 steel spirals did not suffer confining steel fracture.