In-Plane Stability Of Eccentrically Loaded Built-Up Columns

Due to structural economy, built-up members are widely used in industrial steel structures. However, the interaction of global and local buckling occurred in built-up members and its detrimental effect on the load carrying capacity have not yet been fully documented. In this thesis the in-plane inelastic stability of laced columns and beam-columns consisting of two identical doubly-symmetrical I sections are investigated.Firstly, the validity of analyzing built-up columns and beam-columns by the general FEA program ANSYS with the Beaml89 element is studied, for the Beaml89 element can consider geometrical non-linearity and the material plasticity. Many numerical results confirm the validity of the method.Secondly, the laced columns in plastic phase are studied with the foregoing method. The overall and local initial curvatures and the residual stresses are considered, also the material plasticity is considered in the solution process. With the program, many members with different section sizes and slenderness ratios are analyzed. A better formula calculating thein-plane stability factor tp of laced columns is achieved by data fitting based on the stabilityof a single branch.Finally, formulae calculating the in-plane stability of built-up columns under compression and bending in various codes for steel structures are compared with numerical results, and it is found that the current formula is unsafe when the slenderness is large. The laced beam-columns worked in elastic-plastic range are studied. The initial curvatures and the residual stress are considered in the solution process. With the program, many members with different section sizes and slenderness ratios are analyzed. It is suggested that the same formula for in-plane stability of I-section beam-column be used for laced beam-columns.