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

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.

Welded moment connections with highly dissipative panel zones for enhanced seismic performance of steel moment frames

Andronikos Skiadopoulos

Research conducted after the 1994 Northridge earthquake in the U.S. and the 1995 Kobe earthquake in Japan led to the development of today's pre-qualified beam-to-column connections for capacity-designed steel moment resisting frames (MRFs). Welded moment connections feature weld backing bars that should be removed after the execution of complete joint penetration groove welds at the bottom beam flange-to-column flange joint. On the other hand, advancements in steel and weld materials, as well as fabrication techniques, allow for the exploitation of simplified weld details at this location. In welded moment connections for seismic applications, the participation of the beam-to-column web panel zones in the energy dissipation is generally limited during an earthquake event. In such a design context, beam local buckling is likely even at modest lateral drift demands, thereby engendering structural repair costs in the aftermath of earthquakes. The primary reasons for such a seismic design practice are twofold. The first one relates to known limitations of panel zone design models that compromise our ability to effectively balance the seismic design of fully restrained beam-to-column connections. The second one relates to the panel zone kinking that could increase the fracture potential of beam-to-column connections. However, recent experiments in the literature provide controversial results regarding the same matter.This doctoral thesis aims at advancing the state-of-knowledge regarding the seismic design and behavior of steel MRFs with highly dissipative panel zones. A new panel zone design model was first developed. This model addresses the limitations of all available panel zone models in the literature. The model was validated thoroughly with about 100 available experiments that encompass a broad range of geometric parameters. The panel zone model can effectively enable a balanced seismic design of welded moment connections with inelastic panel zones. One step further, this thesis revisited the current detailing of welded connections. Simplifications in their fabrication process were proposed by intentionally keeping a customized beveled backing bar in place, without impairing the connection's ductility under cyclic loading. The proposed connection weld detail is substantiated by continuum finite element analyses and full-scale experiments on welded moment connections with highly dissipative panel zones. It is demonstrated that, contrary to the current design paradigm, a stable hysteretic response is achieved up until lateral drift demands of at least 7% rad, thereby diminishing the cyclic deterioration in story shear resistance. As such, the seismic stability of steel MRFs is only governed by global P-Delta effects. Quantitative seismic response characteristics of steel MRFs with highly dissipative panel zones through large-scale systel-level parametric studies were also provided. It is shown that steel MRFs with highly inelastic panel zones, have up to two times lower mean annual frequency of collapse than corresponding results with steel MRFs designed with the current status quo. It is also demonstrated that steel MRFs with inelastic panel zones, enjoy up to 50% reduction in residual story drift ratios at a design-basis earthquake; their beam-to-column connections do not experience fractures due to panel zone kinking; and local buckling in steel beams is very limited even at low probability of occurrence seismic events.

EPFL2022

Out-of-plane instability of thin reinforced concrete walls under seismic loading

Angelica Rosso

Out of plane instability of reinforced concrete walls is a failure mechanism observed rather recently following the earthquakes in Chile (2010) and New Zealand (2011). Global out of plane instability is a phenomenon that may occur in thin walls, subjected to in plane cyclic loading, which develop large out of plane deformations over the storey height. Past experimental efforts to study this phenomenon focussed on thin members with two layers of longitudinal reinforcement. Hence, also the phenomenological models proposed in literature were mainly based on experimental evidence from double layered members. However, a constructive boom in Latin America, together with an elevated cost of materials, has led over the last fifteen years to the massive construction of reinforced concrete buildings in which the shear walls are very thin and have a single layer of reinforcement. Given the lack of experimental data on single layered members made it questionable whether the mechanism develops as in double layered members, and if the existing models are capable to reproduce the response. In view of the above, the present thesis has the two following goals: (i) Provide new experimental insight on the out of plane instability of members with a single layer of reinforcement; (ii) Develop and validate numerical and mechanical tools to simulate and assess the vulnerability of thin walls to out of plane buckling. The experimental findings obtained from two experimental programs, on thin walls and isolated wall boundary elements respectively, are presented. The importance of the maximum (or critical) tensile strain experienced by the member to trigger instability, which was already identified in the past, is supported by the experimental results. Nevertheless, discrepancies from the usual hypotheses assumed for members with two layers of reinforcement are pinpointed, in particular with respect to the buckling height involved in the deformation, and to the magnitude of the axial force at onset of out of plane instability. Moreover, the crucial role played by the crack pattern is discussed, and an innovative failure criteria based on a three hinge mechanism related to the compressive strains attained in the cracks is identified. Finally, also the adverse effect of an imposed out of plane displacement at the top is also addressed. For a more thorough understanding of the phenomenon, and with the intent of developing simple methods to calculate the critical tensile strain triggering out of plane failure, numerical models are developed and validated against experimental results. The simulations allowed identifying important differences between the isolated boundary element and the wall behaviour, in particular with respect to the vertical displacement profile along the height. On the basis of the numerical findings, an improved boundary element model, rather simple to use for assessment and design, is presented. Finally, building on the experimental and numerical findings, a new mechanical is proposed. The model is in principle developed for members with one layer of reinforcement, but it is shown how it can be extended to double layered members. The mechanical model sets apart from existing ones as it assumes different curvature and vertical displacement profiles along the entire storey height, it accounts for axial forces lower than yielding in compression, and it allows to consider various boundary conditions.

EPFL2018

Seismic performance of slender RC U-shaped walls with a single-layer of reinforcement

Katrin Beyer, Ryan Hoult, João Saraiva Esteves Pacheco de Almeida

Reinforced concrete walls are commonly used to resist the lateral loading induced by wind and earthquake actions. While most walls include two vertical reinforcement layers, some regions of the world construct slender, non-rectangular concrete walls with a single vertical layer of reinforcement. The seismic performance of such elements is largely unknown given the paucity of experimental research. This paper presents the results of two slender reinforced concrete U-shaped walls tested at the Earthquake Engineering and Structural Dynamics Laboratory (EESD Lab), École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. Both wall specimens, designed similar to construction practice in Colombia, were tested using quasi-static cyclic loading to observe if out-of-plane instability would develop when deformations were limited to prevent the flange boundary ends crushing. Initial failure of both wall specimens corresponded with local out-of-plane buckling in the boundary ends of the flanges occurring on load reversal. The buckling lengths were approximately 700–800 mm, which corresponded to 44–50 bar diameters. The crack patterns were observed to be steepest in the web of the walls, demonstrating the increased shear demand in comparison to that of a rectangular wall. Both wall specimens reached ultimate drifts larger than 2.5–3.0% before global failure occurring in the web-flange intersection due to crushing. A small twist was subjected to one of the walls when centered and loaded diagonally, which showed that the decay in torsional stiffness is proportional to the decay in translational stiffness.