NUMERICAL STUDY ON COMPOSITE ACTION IN REINFORCED CONCRETE-FILLED STEEL-TUBE SHAFT FOUNDATIONS

Abstract

Reinforced concrete-filled steel tubes (RCFST) are concrete-filled steel tubes with internal longitudinal reinforcement utilized in substructure systems supporting bridges in seismically active areas. Advantages of RCFST include the steel tube shaft serving as a column above ground and as a foundation pile below ground. However, there is limited research on steel casing tube, inner concrete core, and their composite interaction characteristics. This thesis presents state of art finite element investigation on composite action behavior of the RCFST shaft under lateral loading. Experimental results reported in the literature was used for validation analysis using ANSYS nonlinear finite element software program and further parametric studies were performed on influential parameters such as yield strengths of steel tube, the height of shaft, D/t ratios, embedment length of column extended into the shaft, diameters, and thickness of steel tube to get insight into the behavior of RCFST shaft subjected to lateral loading. Finite element analysis (FEA) results showed high-grade steel tube yield strength and smaller D/t ratio exhibited superior performances. As steel tube yield strength varied from 240 to 520 MPa and the D/t ratio decreased from 150 – 50, flexural stiffness of RCFST due to composite action increased by 36.54% and 66.86%, respectively. Also, results indicated height of the shaft influenced flexural stiffness of RCFST. As the height of the shaft reduced from 8126 mm to 7166 mm, 14% higher flexural stiffness was exhibited, similarly 8.79% flexural stiffness performance gain gained when the height of the shaft was lessened from 7166 mm to 6156 mm. Embedment length parameter has an insignificant effect on flexural stiffness whereas keeping thickness constant and increasing shaft diameter from 550 mm to 600 mm resulted in 43.8% raise in flexural strength of RCFST