In capacity-designed steel moment resisting frames (MRFs), column base connections feature a strong-base/weak-column design philosophy. Prior studies on fixed-end wide flange steel columns featuring seismically compact cross sections subjected to coupled axial load and lateral drift demands suggest that columns may experience considerable residual axial shortening due to nonlinear geometric instabilities (e.g., local buckling). Column axial shortening is challenging to be repaired in the aftermath of earthquakes; thus, it may result in building demolition. Fixed end columns are conventionally realized with either exposed or embedded column bases (ECBs). In the latter, that forms the primary focus of this PhD thesis, ECBs are conceived as ideally fixed and non-dissipative. However, they do have an inherent flexibility and often exhibit unintentional inelastic deformations as revealed from field observations after earthquakes. While both phenomena are neglected in seismic design procedures of steel MRFs, they seem to have a beneficial effect on the earthquake response of first story steel MRF columns. This suggests that innovative yet simple concepts should be developed and validated so as to prevent local buckling in steel MRF columns. Within such a context, the primary objectives of this thesis are: 1) to develop a novel ECB connection, termed dissipative ECB that promotes a stable hysteretic behavior during earthquake shaking, thereby minimizing local-buckling induced residual axial shortening in steel columns; and 2) to comprehend by means of simulation-based engineering the steel column-ECB interaction in steel MRFs, which provides the basis of the dissipative ECB connection development.
This thesis first explores the nonlinear interaction between embedded bases and wide flange steel columns by means of extensive continuum finite element simulations. The simulation results facilitate the development of refined design procedures for conventional non-dissipative ECBs. Moreover, the benefits of balancing the inelastic deformation demands between the steel column and the embedded base are highlighted. Based on these findings, a novel dissipative ECB connection is developed that defies the current paradigm in capacity-designed steel MRFs (i.e., weak-base/strong-column). In the proposed concept, a dissipative zone is introduced as part of the embedded portion of the steel column. This zone is decoupled from the concrete foundation with a debonding material layer. The dissipative zone is engineered such that the steel column does not experience nonlinear geometric instabilities at the column base and the reinforced concrete foundation itself remains practically undamaged during earthquake loading. The proposed concept is validated with large-scale quasi-static experiments and supplemental finite element analyses. It is demonstrated that, contrary to its conventional counterpart, the dissipative ECB connection is resilient to local buckling-induced axial shortening. A simple non-degrading mechanics-based model is sufficient to describe the hysteretic response of the dissipative ECB for performance-based design and assessment of steel MRFs under earthquake loading.