| Reinforced concrete filled steel tube (RCFST) pile-columns, also known as RCFST drilled shafts, are structural elements used in bridge support systems in regions of high seismic activity. This system consists of a steel tube that is filled with concrete and internal longitudinal and transverse reinforcement. This system has several advantages over traditional systems. Beyond increased strength and deformation capacities, this type of elements serve as a foundation element (pile) inground and as a column abo veground. Moreover, the steel tube serves as permanent formwork, which in turn contributes toward accelerated and cleaner construction processes.;The main goal of the research described in this document was to study the effects of soil stiffness on the performance of RCFST drilled shafts. For that purpose, twelve halfscale, pinned-head, RCFST specimens were tested under cyclic lateral loading and variable soil stiffness levels at the soil-structure interaction facility at North Carolina State University. The main studied variables included: (1) diameter-to-thickness (D/t) ratio, (2) aboveground length-to-diameter ratio (La/D), and (3) soil stiffness. The specimens consisted of steel tubes with outer diameter (D) of either 12 in. or 12.75 in. with D/t ratios of 48, 68, and 95. Aspect ratios (La/D) of 5.5 or 7.5 along with two soil stiffness conditions were considered. The soil utilized in the experimental tests consisted of poorly graded sand (SP) with average unit weight gamma = 90 pcf (14.2 kN/m3), friction angle &phis; = 35°, and relative density Dr ≈ 25 %.;In addition to the experimental component described above, analytical studies were also conducted on both pinned-head and fixed-head RCFST shafts. Variables of such studies included: (1) D/t ratios of 48, 64, and 95; (2) La/D ratios of 4, 8, and 12; (3) three levels of axial load ratio (ALR), namely, 5%, 10%, and 15%; (4) two soil types, namely, sand and clay; and (5) three levels of soil stiffness, namely, flexible, medium, and stiff soil profiles.;Results showed that the D/t ratio has a direct impact on the system strength, with a moderate effect on the effective system stiffness, which ultimately affects the displacement capacity. Regarding the soil stiffness, it certainly affects the effective system stiffness but not as much its strength. Moreover, both D/t ratio and soil stiffness have a small effect on the depth to maximum moment and spread of plasticity. Regarding the La/D ratio, it was found that is the parameter that impacts the most the performance of the system, affecting the displacement capacity, system strength, shear demand, depth to maximum moment, and spread of plasticity. |
