Development of Dissipative Steel Column-Base Connections for Steel Moment-Resisting Frames under Seismic Loading

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.

The Role of the Composite Floor System and Framing Action in the Seismic Performance of Composite Steel Moment-Resisting Frames

Hammad El Jisr

With the advent of performance-based earthquake engineering (PBEE), the need for reliable prediction of earthquake-induced collapse of structures is essential. Despite the significant progress that has been made towards this goal, there are several hurdles that still need to be overcome. Deterioration models, which are currently used for simulating structural collapse, are largely based on available test data that featured subassemblies with overly simplified boundary conditions. These experiments did not ac-count for the force redistribution occurring within structural systems after the onset of nonlinear geo-metric instabilities under cyclic loading. For the case of composite steel moment resisting frames (MRFs), which is the primary focus of this thesis, the influence of the axial restraint provided by the slab continuity and framing action on the seismic behavior of composite steel MRFs has been recognized in prior work. Nevertheless, these effects have neither been thoroughly investigated nor quantified at lateral drift demands associated with dynamic instability of composite steel MRFs under seismic loading.In this doctoral thesis, a unique experimental program of a two-bay composite steel MRF subsystem was conducted at full-scale. The experimental program, which was corroborated by continuum finite element analyses, aimed at comprehending the role of the underlying physical mechanisms, associated with the slab continuity and framing action, on the hysteretic behavior of composite steel MRFs with a particular emphasis at large deformations associated with collapse. The research findings suggest that the slab continuity limits the extent of local buckling and beam shortening within the dissipative zones of composite steel MRFs. While nonlinear geometric instabilities attributable to local buckling and concrete crushing are pronounced in composite steel beams at large inelastic deformations, the framing action and slab continuity preserve the overall stability of the structural system even in cases that ductile cracks form within a dissipative zone. The test results suggest that the presence of the transverse beams within the floor system should be factored in capacity design principles of composite steel MRFs. The experimental findings also suggest that methods for computing the effective slab width in composite steel MRFs should be reassessed. The available data along with information from prior experiments informed the development of practice-oriented engineering models for the seismic assessment of composite steel MRFs. Moreover, a macro-model was proposed for simulating the hysteretic behavior of composite steel beams under cyclic loading. The model, which was validated thoroughly with available experimental data, simulates explicitly the cyclic deterioration in strength and stiffness of composite steel beams and their respective beam-slab connections. The proposed macro-model was employed to benchmark the seismic col-lapse risk of prototype buildings with composite steel MRFs, as well as the repairability of beam-slab connections within the framework of PBEE.

EPFL2022

Experimental Behavior of Concrete-Filled Steel Tube Columns Using Ultrahigh-Strength Steel

Hiroyuki Inamasu

Concrete-filled steel tube (CFT) columns offer significant advantages over columns made of either steel or concrete alone, such as large energy dissipation and increased strength and stiffness. To further improve the seismic performance of these columns, an experimental investigation was conducted into CFT columns using ultrahigh-strength steel. More specifically, seven square and circular specimens made with high-strength and conventional steel were subjected to constant compressive axial load and cyclic flexural load protocols with two and 20 cycles imposed at each drift level. Based on the test results, the influence on the CFT’s cyclic behavior of the high-strength steel, crosssectional shape, axial load, and number of cycles in lateral loading history was studied. In comparison with the conventional steel specimens, larger elastic deformation, higher strength, and delay of local buckling were observed in the high-strength steel specimens, while compared with the circular specimens, the square specimens sustained larger drift angles without fracture of their steel tubes because of the development and progress of serious local buckling. Furthermore, a simple analytical model based on the concept of the superposed strength method was proposed. The accuracy of this model was confirmed with the experimental results.

2016

Influence of embedded steel column base strength on earthquake-induced residual deformations

Hiroyuki Inamasu, Dimitrios Lignos

Recent experiments on steel columns under multi-axis cyclic loading suggest that a steel column may experience appreciable axial shortening due to local buckling. Reconnaissance reports from recent earthquakes indicate that a number of embedded column base (ECB) connections in steel frame buildings were found to be lightly damaged although they are expected to remain elastic. As a result, the amount of inelastic damage concentrated in the respective steel columns was much less than it was expected to be. This paper investigates the influence of the ECB strength on the residual deformations of first story interior steel columns in steel moment resisting frames (MRFs) under multi-axial cyclic loading. The evaluation is conducted with rigorous finite element (FE) simulations. The developed FE model is able to simulate the pinching hysteretic behavior of typical ECBs. The parametric study involves two main cases. In the first one the ECB remains in elastic. In the second case the inelastic behavior is spread to the ECB and the steel column itself. The simulation results suggest that the resultant plastic deformation within the steel column is much smaller in the latter. The onset of column local buckling is delayed; therefore, the residual column axial shortening becomes fairly minimal. In particular, it is reduced by 50% at lateral drift demands associated with low-probability of occurrence earthquakes.

2018

The Role of the Composite Floor System and Framing Action in the Seismic Performance of Composite Steel Moment-Resisting Frames

Hammad El Jisr

EPFL2022