Seismic Behaviour and Design of Steel Frames Incorporating Tubular Members

Abstract

This thesis deals with the seismic behaviour of steel frames with particular focus on structures that employ tubular members as either columns or bracing elements. It addresses a number of design and assessment issues at the local (connection), frame, and overall (system interaction) levels. At the connection level, two experimental investigations on: (i) blind-bolted and angle connections, and (ii) combined channel/angle connections, are presented. The main behavioural patterns and the effects of key design parameters on the connection performance are examined. Refined mechanical models able to estimate the response of these connecting details are developed. These mechanical models are subsequently employed to perform parametric studies based on which simplified design-oriented expressions for the estimation of stiffness, strength and ductility are suggested. The susceptibility to low-cycle fatigue within critical connection components and the predictions of available fatigue damage models are also assessed. At the frame level, an evaluation of the inelastic demands on moment-resisting, partially-restrained and concentrically-braced steel structures is performed and equivalent linear models for the estimation of peak deformations are proposed. Particular attention is given to the influence of a number of scalar ground-motion frequency content parameters on the estimation of peak displacements. Additionally, simplified models based on rigid-plastic dynamics, and implemented within response history analysis, are proposed. It is shown that such rigid-plastic models can predict global deformations with reasonable accuracy. At the system interaction level, a comparative assessment of the peak response of one-way, two-way and mixed framing configurations under bi-directional earthquake loading is studied by means of idealized 3D simplifications and refined 2D models. This enables a detailed quantification of the contribution of gravity frames to the reduction of seismic risk and highlights the benefits of proper secondary frame design in mitigating the probabilities of dynamic instability. Finally, the findings of the thesis are summarized and future research areas are identified