Limit state design | EPFL Graph Search

Limit State Design (LSD), also known as Load And Resistance Factor Design (LRFD), refers to a design method used in structural engineering. A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria. The condition may refer to a degree of loading or other actions on the structure, while the criteria refer to structural integrity, fitness for use, durability or other design requirements. A structure designed by LSD is proportioned to sustain all actions likely to occur during its design life, and to remain fit for use, with an appropriate level of reliability for each limit state. Building codes based on LSD implicitly define the appropriate levels of reliability by their prescriptions. The method of limit state design, developed in the USSR and based on research led by Professor N.S. Streletski, was introduced in USSR building regulations in 1955. Criteria Limit state design requires the structure to satisfy two principal cr

Déformabilité et capacité portante des colonnes en béton armé

Serge Dal Busco

For buildings which are stabilized by core or shear walls, the vertical members are generally subjected to compressive normal forces N and certain imposed angles of rotation θ. Similar in this respect are non slender bridge piers. These rotations originate from, on one hand, the length variations of the horizontal elements (beams, slabs) due to shrinkage, temperature and prestressing, while on the other hand the bending of these elements due to the vertical loading. Thus, in this case, it is a problem of imposed deformations. However, the usual methods of calculation do not take this fact into account and bring the computation back to a question of forces. In this case, the design is mostly based on the resistance of individual sections, with the use of moment-axial load interaction diagrams MR-NR. In this thesis, a different method is proposed which takes account of the problem of imposed deformations, which concerns equally well the real behaviour of the structure. This proposal is limited to the cases where the slenderness ratio λcr is less than or equal to 50 (λcr ≤ 50). The analysis of a column is carried out considering the vertical load N and the imposed end rotations θ, due to the interaction of horizontal load carrying members. This method allows one to create structures with a greater performance, thanks to the larger distance between expansion joints and to a reduction in the overall dimensions of the columns. This equally has the consequence that the columns are almost always able to be fixed ended by the horizontal members, thus permitting a reduction in their slenderness λcr and which eliminates the costs of the hinge devices. However, it is mostly necessary to resort to high percentages of reinforcement to transmit the elevated vertical loads. This concept of the association of N and 0 is applied to both the examination of the ultimate limit state (loading capacity and deformability) and to serviceability (aptitude of service). The first verification consists of assuring that the column possesses the required deformability to permit the imposed rotations, while transferring the normal force, with the required security. It is possible to obtain this by using adequate transverse reinforcement at the fixed ends of the column, in such a fashion that plastic hinges would be able to form there. The first part of this work concentrates on the development of a method which allows the determination of the characteristics of such a transverse reinforcement (shape and configuration, spacing and diameter of stirrups), as well as an investigation of the numerous parameters influencing the behaviour of columns at the ultimate limit state. In the second part, an explanation is given on how to control the behaviour of a column, when it is subjected to service loading and imposed rotations, in order to verify its serviceability. This explanation takes into account the nature of the rotations, caused by the variations in temperature or by long term phenomena, such as shrinkage and creep. Its application is easy as it is given in the form of charts. Finally, a method for the practical cases is proposed to the designer. It contains the notes and charts to design and verify reinforced concrete columns with different levels of reinforcement.

EPFL1988

Zur Berechnung von Holzschalen in Brettrippenbauweise mit elastischem Verbundquerschnitt

Claudio Pirazzi

The design of timber rip shells confronts the engineer with major uncertainties during the planning process. The consideration of the elastic compound cross section during structural analysis of curved members by means of conventional calculation methods is possible only in a restricted manner. The shear-analogy provides a seemingly useful method for calculating these structures. However it is only an approximation, particularly with regard to cross sections with high flexibility in terms of shear stress. This thesis treats the limitations of applying of the shearanalogy with regard to the calculation of straight and curved members with elastic compound cross section. The bending of timber lamellas generates initial bending stress. Due to the rheological behaviour of wood and the associated process of plastification within the microstructure of wood, the stress becomes partially reduced as a function of time (stress relaxation). Prior work provides contradictory information about the duration and intensity of this reduction. To shed light on these uncertainties, appropriate experiments have been conducted. To join timber lamellas to a multi-layered cross section, mechanical fasteners like screws, nails and bolts are used. The knowledge of the physical properties, like rigidity and resistance, are of significant importance for the verification of the ultimate limit states. Tests conducted in the past in order to evaluate theoretical models and to determine empirical values refer generally to connections, which differ clearly from the multi-layered connection treated here concerning the deformation and failure mechanisms. Experiments carried out provide estimation in how far the existing values can be applied to structural analysis of the present case. The theoretical model according to Johansen was adapted in an appropriate manner. The aim of the work presented here is to contribute to the clarification of several problems related to the design of spatial timber structures with elastic compound cross section. The results provide further confidence to engineers for planning and realising these challenging lightweight structures.

EPFL2005

Post-punching behavior of reinforced concrete slabs

Yaser Mirzaei

Reinforced concrete flat slabs are extensively used in buildings and parking garages. Their design is governed by deflection at the serviceability limit state and punching shear at the ultimate limit state. When no punching shear reinforcement is provided, failure develops in a brittle manner. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at the top side of the slab are usually not visible. Over the past decades, several structural collapses occurred due to punching shear failure resulting in human casualties and large damages. These collapses revealed some shortcomings in codes of practice and the necessity of reconsidering punching provisions. The investigations of these collapses showed that the collapse initiated from a local punching failure and propagated throughout the structure, in a progressive collapse. The term progressive collapse refers to the spreading of an initial local failure triggered by the loss of one or more load carrying members and leading to partial or total collapse of the structure in a manner analogous to the chain reaction. As a local punching failure can trigger progressive collapse, the study of the post-punching behavior can help adopting constructive solutions to avoid progressive collapse. The post-punching behavior of flat slabs supported by columns has not yet been thoroughly investigated. Therefore, an extensive experimental campaign was performed in the framework of this dissertation to investigate the post-punching behavior of 24 slabs with various reinforcement layouts. The effects of bending reinforcement, integrity reinforcement, bent-up bars, steel type, and anchorage conditions on the post-punching behavior of slab-column connections were investigated. The performance and robustness of the various solutions was investigated to obtain physical explanations of the load-carrying mechanisms. Test results showed that the post-punching strength provided by the top reinforcement is small because the concrete cover is thin and spalling of the concrete cover occurs leaving the reinforcement ineffective. In addition, it was observed that integrity reinforcing bars passing through the column significantly improve the post-punching behavior in terms of strength and deformation capacity. The integrity bars behave as a tensile membrane inclined to the plane of the slab and are able to sustain damaged portions from the column. Thus, one possibility to enhance the robustness of the structure against progressive collapse is to provide well-anchored bottom reinforcing bars passing through the column. A mechanical model capable of predicting the post-punching behavior of slab-column connections without shear reinforcement was developed. The model predicts the contribution of the tensile reinforcement and of the integrity reinforcement to the post-punching strength. The model accounts for possible failure modes including the fracture of the bars and the destruction of the concrete over the integrity bars. The progressive destruction of the concrete within and outside the punching cone is treated by considering the pullout behavior of reinforcement embedded in the concrete. Finally, a parametric study was performed to evaluate the influence of various parameters and their relative importance in order to develop practical proposals for the estimation of the post-punching strength. It showed that the post-punching strength is not only a function of the cross sectional area and yield strength of the integrity reinforcement as it appears in provisions and codes of practices but also of the diameter of the bars, the effective depth, the ductility, and the type of reinforcement.

EPFL2010