Structure et séisme : l'influence des matériaux

#earthquake #seismic #seismicdesign Interview with Gérald Hivin, Professor at the University Institute of Technology (IUT) of Civil Engineering, Grenoble Alpes University. "Describe your building to me, and I'll tell you how it would react to an earthquake." This, in short, is the method used by specialists to understand the behavior of structures during a seismic event. Small, large, wide, made of concrete, brick, or wood—all these factors determine the frequency at which your building begins to oscillate when the earth shakes. For example, a slender structure oscillates slowly, while a low-rise house vibrates more rapidly. This number of oscillations per second, called the resonant frequency, is a determining factor in a structure's resistance and must be taken into account in its design, particularly by following these principles. First, the structure must have a single resonant frequency. Why? When you set an object in motion, it vibrates at its own rhythm. Now imagine that each part of the building oscillates as it pleases: the elements become detached, collide with each other, or cause significant deformation. This general cacophony leads to extensive damage or even the complete destruction of the building. Conversely, if you manage to ensure that your entire structure vibrates in unison, the risk of damage decreases. To achieve this, it is essential, among other things, to distribute the mass properly: a flexible level (typically with large glass facades) should not be topped by a rigid one. Indeed, the movements of the more flexible level are different from those of the other floors, so the structure does not move smoothly, and the more flexible level eventually gives way. Secondly, the structure must not have the same frequency as the ground. When two similar frequencies meet, the amplitude of the movements increases. If your building vibrates in the same way as the ground, it resonates: the movement is amplified, and the structure is weakened. The ideal scenario is therefore to build flexible buildings on rigid ground and rigid buildings on soft ground. This is one of the reasons why the subsoil is surveyed before construction. Knowing the resonant frequency of a building is very practical but also very complex. To determine it, seismic sensors would need to be installed in each building, and the data then processed. This has been done for a few structures in Grenoble (City Hall, Vercors Tower, and one of the Arpej Towers), but it is clear that these measurements cannot be taken on all buildings, including single-family homes. Therefore, simplified methods are used to determine the frequency range in which the movements would be amplified. These methods can, for example, be based on the number of stories: a flexible building has, roughly speaking, a resonance frequency equal to 10 divided by the number of stories (f = 10/N), while a rigid structure has a frequency given by 20 divided by the number of stories (f = 20/N). Despite its fundamental nature, the resonance frequency does not entirely govern a structure's resistance. Structures must also withstand inertial forces and the duration of the earthquake. Inertial forces are the forces that oppose movement and hold the structure in its original position. Materials must therefore be able to deform sufficiently to absorb the lag between the top of the structure and the bottom, which follows the ground movement. Furthermore, once materials deform irreversibly, the duration of the vibrations will worsen the situation. Produced and directed by: Institute for Major Risks With the support of: The Auvergne-Rhône-Alpes Region The Auvergne-Rhône-Alpes Regional Directorate for the Environment, Planning and Housing (DREAL) The Isère Departmental Council Follow the latest news from the Institute for Major Risks: Website: http://www.irma-grenoble.com/ Facebook:   / institut.des.  . Twitter:   / risquesmajeurs   Instagram:   / irma_grenoble