Experimental Study on Cyclic Behaviors and Restoring Force Characteristics of Concrete Filled Steel Tube Columns Using Ultra-High Strength Steel

H-SA700 is a type of new ultra-high-strength structural steel that is more environmentally friendly and more suitable for mass production than conventional high-strength steel. The objective of this paper is to contribute to the practical application of CFT columns using such ultra-high-strength as a means to provide a significant increase in the seismic capacity of building structures. In this paper, the cyclic behavior of concrete filled steel tubes (CFTs) using H-SA700 was investigated experimentally. H-SA700 offers an approximately three time higher yield strain than that of conventional steel, and therefore, the CFT columns using H-SA700 reached their elastic limit when the infilled concrete sustained compressive strength, while the steel tube remained elastic at the moment. In such a case, it is unsafe to evaluate the strength by using the current design formula based on the superposed strength method. Due to the relatively low ductility of H-SA700, steel is not allowed to yield in the design of CFT columns. Nevertheless, the experimental results revealed that, comparing with CFT columns using conventional steel, the CFT columns using H-SA700 exhibited a substantially large capacity of plastic deformation, smaller axial deformation associated with local buckling, and a similar capacity of energy dissipation.

Interaction sol-structure dans le domaine des tranchées couvertes

Sylvain Plumey

The present research on cut-and-cover tunnels gives a theoretical contribution to the understanding of the behaviour of these structures up to their ultimate limit state. A simplified method inspired on the convergence-confinement method is investigated and applied systematically to several practical cases. This method, based on the failure mode of the structure, gives an approximate solution of the equilibrium between the soil and the structure. It enables a better understanding of the complex soil-structure mechanisms, typical of these constructions, constituting in this way a design tool. The behaviour of the soil which interacts with the structure comprises typically two main phases. The elastic phase is followed by the progressive yielding of parts of the soil until a plastic mechanism is formed (plastic phase). Depending on the geometrical and the mechanical parameters, the structure takes more or less advantage of the soil contribution in carrying the loads. The main conclusion of this work is the demonstration of the existence of different modes of behaviour. Three main modes of practical interest, defined mainly by the phase of the soil governing the structural behaviour, are distinguished : elastic soil, elasto-plastic soil and completely plasticized soil. The identification of the mode then enables an efficient design and analysis of the structure. The theory of plasticity (upper bound) was applied to two soil-structure systems, a surface strip footing submitted to a centred load and a lateral wall of a frame-type cutand- cover tunnel under construction, aiming at studying their behaviour at the ultimate limit state. This study clearly emphasizes the favourable or unfavourable role played by the soil-structure interaction in the collapse of such structural systems. A proper consideration of the structure failure kinematics is thus fundamental to a proper representation of the ultimate limit state. A new safety format is proposed to define the ultimate limit state when the finite element method is used. This safety format is compatible with the new SIA codes of practice and clarifies the structural design procedure. The research also showed that the structure's ductility plays a major role in the development of the structural soil capacity. Several ductility limits are emphasized for cut-and-cover tunnels. For frame structures, the deformation capacity of the top slab is very small if no shear reinforcement is provided in the zones of significant shear forces. Stirrups are thus recommended in these structures. For arch structures, the spalling of the concrete cover can limit the deformation capacity of the structure. Experimental tests performed within this research showed that this phenomenon was affected negatively by plastic deformations of the steel reinforcement and by reinforcement splices and anchorages. SIA 262 (2003) code of practice design criterion is judged insufficient. The structural design of cut-and-cover tunnels with important plastic redistributions is thus possible only if certain ductility conditions are fulfilled.

EPFL2007

Effect of Deformation History on Punching Resistance of Reinforced Concrete Slabs

Robert Koppitz

Deformation-dependent punching shear resistance often constitutes the decisive design criterion for reinforced concrete slabs supported by columns. The increasing number of aging structures exhibiting insufficient punching resistance and detailing deficiencies, and undergoing changes of usage has resulted in a growing demand for strengthening against brittle punching failure. Hence, the research presented in this thesis aims at gaining a better understanding of the identified problems occurring especially in existing structures, such as pre-deformation. A further objective is the experimental and theoretical validation of the performance of a new strengthening concept. A detailed analysis is carried out of the structural behavior of centrically supported rotation-symmetric slabs subjected to punching. In the first part of this research project a literature review is conducted to characterize the available punching models for new slabs and discuss to what extent they are able to consider the problems linked with existing slabs. Benefits and limitations of current strengthening solutions are illustrated, suggesting local prestressing as a promising concept. In the second part such a concept is analyzed that has been developed to improve the system efficiency and punching resistance by immediately activating the post-installed shear reinforcement composed of carbon fiber-reinforced polymer (CFRP) composites, thus partially unloading the slab. An experimental campaign comprising sixteen slabs verifies that the installation of this strengthening concept leads to a more ductile system behavior and significant increase of punching resistance compared to a non-strengthened slab. In the third part the load-deformation behavior of slabs is analyzed and a modification of an analytical model applied to a slab sector is developed, considering the influence of shear on flexural behavior. An overestimation of the flexural capacity and consequently of the punching resistance is thus avoided. The agreement of the modified sector model results with experimental results is confirmed for a large number of experiments from literature. The model is also capable of predicting the load-rotation responses and punching resistances of slabs strengthened with prestressed CFRP straps. The fourth part of the research concerns the effect of pre-deformation, or load history, on the structural behavior of uniaxial members and biaxial slabs. Single unloading and reloading cycles, which reduce tension stiffening and result in additional deformations, leading to decreased concrete punching resistance, are investigated. In slabs prestressed CFRP straps can compensate the additional deformations caused by load history.

EPFL2015

Effect of Deformation History on Punching Resistance of Reinforced Concrete Slabs

Robert Koppitz

EPFL2015

Interaction sol-structure dans le domaine des tranchées couvertes

Sylvain Plumey

EPFL2007