Déformabilité et capacité portante des colonnes en béton armé

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.

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