Development of multi-spiral confinements in rectangular columns for construction automation

Proper design of confinement reinforcement in columns is critically important for buildings that need to survive strong earthquakes. This paper presents the development of an innovative multispiral confinement design for rectangular reinforced concrete columns through testing and evaluation of a large number of concrete column specimens in axial compression and lateral cyclic loadings. In the first phase of this test program, 10 confinement designs, including four rectilinear configurations and six different multispiral configurations, were evaluated in a series of axial compression tests. The test results show that the specimens with multispiral configurations exhibited higher compressive strengths, energy capacities, and ductility than those with rectilinear confinement designs. Among them, two multispiral confinement designs (i.e., combinations of four spirals and five spirals) and a design with a spiral and cross ties exhibited better confinement effectiveness and, therefore, were selected for further study in the Phase II study. In the Phase II study, detailed axial compression tests and lateral cyclic loading tests were carried out for four confinement designs that include one conventional stirrup design, combinations of four spirals and five spirals designs, and a spiral with cross ties. Test variables include various strengths of concrete, spiral pitches, the sizes and yield strengths of the spiral reinforcement, and the volumetric ratio of the confinement reinforcement. The test results clearly indicate that the multispiral reinforced concrete specimens exhibited higher compressive strengths and ductility capacities as compared to the columns with the conventional stirrup design. Among the specimens tested, the five-spiral confinement design (consisting of one large spiral at the center and four small ones at the four corners) shows the highest compressive strength, and ductility capacity.