Buckling behaviour of gusset plates in buckling restrained braced frames.

Abstract

This thesis describes a finite element study of BRB gusset plates under in-plane loading to determine how their strengths compare with those of experiments, and with those of methods in current design standards. Limitations of current standards, such as the calibration of the equivalent column method, with its effective length, Whitmore width and Thornton length, and the lack of explicit considerations for frame action effects are described. In addition, a stiffness based BRB system stability method is developed. It is shown that the finite element modelling replicated behaviour and strengths of gusset plates in BRB system tests. Also, a number of other gusset plate connections were analysed where it was found that current design methods predicted greater strengths than that obtained from the analysis, or from experimental tests, due to limitations in the development of these design methods. The major limitation of the design method is the calibration for the equivalent column strut using the Whitmore length and Thornton length together with a particular buckling curve, and an effective length factor. This effective length factor, K, has recommended values ranging from the fully fixed brace case (i.e. K = 0.5) to the fixed-free sway boundary condition case (i.e. K = 2.0), in the literature. In the NZ standard, it is recommended that K = 0.70. Current design methods for the fixed-free sway boundary condition case (i.e. K = 2.0) rather than 0.70 estimated greater capacities than those from previous experiments and analysis. Also, variations of Whitmore width and Thornton length did not always produce the expected changes in performance indicating that they may not be reliable for determining gusset plate buckling capacity. Finite element models of a BRB frame considering frame action forces acting, and not acting, showed that frame action can decrease gusset plate buckling capacity. A simple stiffness based BRB element stability method was developed to determine how different BRB system elements affected buckling capacity. Flexibilities of the frame beam-column joint and all other elements of the BRB system are considered. Effects of residual stresses and yielding from the column curve are considered. The buckling capacity of the system was determined based on the minimum eigenvalue of the system’s stiffness matrix. A sensitivity study found that buckling capacity significantly decreases if one element of the system has relatively lower stiffness.