Seismic design and assessment of resilient post-tensioned steel frames with viscous dampers.

Conventional seismic-resistant structural systems are currently designed to develop a global sway plastic mechanism under strong earthquakes, which is achieved by allowing the development of controlled inelastic deformations in specific locations of main structural members such as beams, bases of columns and braces. Inelastic deformations in structural members result in damage and residual drifts, and therefore, in economic losses such as repair costs and downtime. Moreover, earthquake reconnaissance reports reveal large economic losses related to non-structural damage, e.g. failure of walls due to large storey drifts or failure of acceleration-sensitive equipment due to large peak floor accelerations. These losses highlight the need for resilient structures with the potential to remain intact after frequently occurred earthquakes and return to service within an acceptable short, if not immediate, time after strong rare earthquakes. Moreover, resilient structures should provide a very low probability of collapse (i.e. increased life safety) under very rare maximum considered earthquake. Steel self-centering moment-resisting frames using post-tensioned beam-column connections are a promising class of resilient structures. They exhibit softening force-drift behaviour and eliminate inelastic deformations and residual drifts as the result of gap openings developed in beam-column interfaces and elastic post tensioned bars which clamp beams to columns and provide self-centering capability. Also, post tensioned connections use energy dissipation devices, which are activated when gaps open and can be easily replaced if damaged. Steel frames equipped with passive dampers are another class of resilient structures. Dampers provide supplemental damping to control drifts, and thus, provide an effective means to achieve economical designs with high performance. The main goal of this PhD is to develop a seismic design and assessment procedure for steel self-centering moment-resisting frames (SC-MRFs) with viscous dampers within the framework of Eurocodes 3 and 8. To achieve this goal, nonlinear models of post-tensioned connections, able to capture the strength and stiffness deterioration due to local buckling, are developed. These models enable the assessment of the seismic behaviour of SC-MRFs with viscous dampers up to collapse with the aid of nonlinear dynamic analysis. A seismic design method, which incorporates a robust way to estimate rotation demands in post-tensioned connections, is then formulated. Different SC-MRFs with viscous dampers are designed using the proposed design procedure to study different design scenarios. The accuracy of the design procedure is evaluated through nonlinear dynamic analysis. In addition, the superior collapse resistance of SC-MRFs with viscous dampers is validated through incremental dynamic analysis. The thesis concludes with the implementation of an advanced probabilistic framework for direct economic seismic loss estimation and its application to confirm the potential of SC-MRFs with viscous dampers to significantly reduce economic seismic losses