Transverse reinforcement in bridge columns normally consists of spiral reinforcement in columns with circular cross-sections and tied reinforcement in columns with square or rectangular cross-sections. The circular shape of spiral reinforcement is inherently efficient in providing confinement to the concrete core and restraint of longitudinal bar buckling. In contrast, rectangular columns require cross-ties and/or overlapping ties in addition to the perimeter tie in order to provide adequate confinement and restraint of bar buckling. As an alternative reinforcing scheme, interlocking spiral reinforcement has been used in California for columns with rectangular cross-sections. However, several important design elements are not addressed in the Caltrans specifications, and the performance of columns with interlocking spirals has not been fully established.
This study experimentally investigated the seismic behavior of columns incorporating interlocking spirals under flexural, shear and torsional loadings. The main tests were performed on approximately 1/5-scale column specimens subjected to increasing levels of cycled inelastic displacements under constant axial load. Rectangular and oval cross sections with either two interlocking spirals or conventional ties were investigated. Variables studied included the performance of interlocking spirals compared to ties, the amount of spiral overlap, and the size of longitudinal bars required in the overlap region to maintain spiral interlock.
Columns with interlocking spirals performed as well or better than columns with ties, despite approximately 50% more transverse reinforcement being provided in the tied columns. Test results indicated improved performance when the center-to-center spacing of interlocking spirals was not greater than 0.6 times the spiral diameter. At least four longitudinal bars of approximately the same size as the main longitudinal reinforcement are required in the overlap region to maintain spiral interlock. Procedures were developed for predicting the axial, shear, flexural and torsional strengths of columns with the interlocking spirals.