Ductility reduction factor formulations for seismic design of RC wall and frame structures

Seismic design of standard structures is typically founded on a force-based design approach. Over the years this approach has proven robust and easily applicable by design engineers and - in combination with capacity design principles - it provides a good protection against premature structural failures. However, it is also known that the force-based design approach as it is implemented in the current generation of seismic design codes suffers from some shortcomings; among these is the fact that the base shear is computed using a pre-defined force reduction factor, which is constant for a given structural system. Thus, for the same design input, structures of an identical type but different geometry are subjected to varying ductility demands and may perform differently during an earthquake. The objective of this research is to present an alternative formulation for computing force reduction factors for RC wall and frame structures, using simple analytical models which only require input data already available at the beginning of the design process. Such analytical models allow to link global to local ductility demands and therefore to compute an estimate of the force ductility reduction factors that lead to equal local ductility demands and expected damage levels. A series of pushover and nonlinear time history analyses are run on simplified numerical models of a set of wall and frame structures. The results show that the proposed alternative formulation yields a more accurate ductility reduction factor than the current Eurocode 8 design approach.

Seismic behaviour and analysis of U-shaped RC walls

Raluca-Tereza Constantin

In many countries of moderate to high seismicity, reinforced concrete (RC) core walls are used as lateral bracing systems in mid- to high-rise buildings where they typically accommodate lift shafts or stair cases. Unlike planar walls which provide horizontal strength and stiffness mainly in the in-plane direction of the wall, core walls provide bidirectional strength and stiffness. This feature complicates their inelastic behaviour which is not yet fully understood as experimental and numerical studies are still rather scarce when compared to planar walls. Therefore, current design codes are based on findings from planar walls while specific guidelines for the design and analysis of core walls are still lacking. In addition, simple analysis tools widely used by design engineers such as the plastic hinge model, have been derived and calibrated for columns, beams or planar walls and their suitability for core walls has been only marginally verified. For these reasons the current study focuses on: (1) improving the knowledge on the inelastic behaviour of U-shaped walls under bidirectional loading and (2) on extending easily applicable engineering type models, such as the plastic hinge model, to the analysis of U-shaped walls. The scope of the thesis is limited to U-shaped walls, which is the simplest type of core wall that still retains the key characteristics of such walls. The first part of the thesis focuses on understanding the behaviour of U-shaped walls under loading along the geometric diagonal of the section. This loading direction is not typically considered in the design process but it was found to determine the shear design of the flanges while the displacement capacity for this direction might be the lowest of all the loading directions. In order to address the behaviour under diagonal loading, two large-scale quasi-static cyclic tests on U-shaped walls were carried out. Failure mechanisms specific to diagonal loading and possible critical design aspects related to these failure modes were identified from the experimental results. In addition, the influence of the longitudinal reinforcement distribution on the wall behaviour was investigated. The second part of the thesis focuses on adapting equations used in plastic hinge model for the analysis of U-shaped walls. Plane section analyses and a shell element model validated against the experimental data were used to perform parametric studies on U-shaped walls. From the results of the parametric studies a new equation for the yield curvature for any direction of horizontal loading was proposed as well as a modified yield displacement equation that accounts for the partially cracked wall height at yield. With this equation, predictions of yield displacements were improved especially for very slender walls. Quantities that rely on the yield displacement, such as the effective stiffness of the wall were also better predicted when the cracked height was accounted for in the prediction. And finally, plastic hinge length equations were also modified to account for the variation with the different loading directions.

EPFL2016

A consistent safety format and design approach for brittle systems and application to textile reinforced concrete structures

Miguel Fernández Ruiz, Aurelio Muttoni, Patrick Lorenzo Valeri, Qianhui Yu

Design and verification of structures in modern codes of practice account for a safety format, ensuring that the probability of failure does not exceed a given threshold. Although specific safety formats are proposed in some cases for special types of structures or analyses, most designs and verifications are currently performed on the basis of the Partial Safety Factor Format (PSFF). This format is applied to cover different materials and structural responses, allowing for a uniform methodology to account for reliability. Such consideration greatly simplifies the design process, but raises concerns on its consistency when different structural responses are observed. In the PSFF as considered in fib Model Code and Eurocodes, no explicit distinction is made on the value of the partial safety factors (for actions or materials) depending on whether a structural system has a brittle or a ductile response. This can be potentially inconsistent, as brittle systems have limited or no redistribution capacity of internal forces (which can give rise to premature failures if action effects are poorly estimated), while ductile systems have large potentials to redistribute internal forces and are thus little sensitive to this issue. In this paper, to investigate on the suitability of PSFF for brittle structures, the most suitable manner to determine internal forces for brittle elements failing in bending and the corresponding model uncertainties of action effects are investigated in detail. The concepts are derived from a theoretical perspective and applied to the case of Textile Reinforced Concrete (TRC). This material is a promising development to reduce the footprint of concrete construction and to build lightweight structures, but exhibits a very brittle response in bending (contrary to ordinary reinforced concrete with usual reinforcement ratios). In this paper, by means of an experimental and theoretical investigation, it is shown that following a suitable approach to estimate internal forces for brittle systems as TRC leads to a low level of model uncertainty of action effects. This leads to the conclusion that, compared to standard design of ductile systems, no additional correction is required for safety issues. Following this outcome, the partial factors for TRC structures are calibrated. In addition, due to the significance of geometrical uncertainties, a method for designing TRC on the basis of a design value of the effective depth (a reduced value accounting for construction tolerances instead of its nominal dimension) is eventually discussed, showing that it allows for a more uniform level of safety.

2021

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