Dynamic Behavior of Concrete and Seismic Engineering

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As distinct from the term “static”, “dynamic” implies the influence of time. A test is said to be “quasi-static” when the effects of time are present, but can be neglected. For a structure test, and for any real test, the effects of time are typically expressed in two ways: – by forces of inertia resulting from the not equal to zero acceleration to which the elements of structures are submitted; – by the behavior of each elementary volume of the material depending on the evolution in time of the elementary mechanical values (stress and strain) and possibly of their time derivatives. This dependence is described by the generic name of “viscosity”. This distinction is strictly linked to the notion of elementary volume underlying the definition of behavior. Actually, the fact that viscosity effects might be the manifestation of inertial microscopic phenomena cannot be excluded. This remark is important in the case of concrete, as considerations about material homogenity The static and quasi-static behaviors of concrete have been the subject of so many works that we often consider that they are quite well known and mastered as far as modeling with a view to structure calculations is concerned. However, the same is not true of concrete’s dynamic behavior, because of the complexity of the tests needed to reach pertinent loading rates. The subject matter of Chapter 1 is divided into two parts: it presents the most
widely used experimental techniques to study the dynamic behavior of concrete, drawing attention to the difficulties in interpreting the results of tests designed to identify its intrinsic parameters. It also offers a synthesis of properties that have been published in the literature dealing with concrete (chiefly its traction and simple compression strengths), as well as values for reinforced or fiber-reinforced composites. An extensive bibliography enables the reader to refer to the relevant original articles. Dynamic loadings can generate non-linearities and a range of deteriorations in concrete (failure from bending and/or shear, traction, mechanical spalling, tearing, compression, compaction and hole perforation, etc.), all of which have to be carefully modeled to enable prediction of the behavior of a specific structure under a violent action. The variety of responses has generated several unique modeling approaches. Depending on the phenomenon under consideration, we use either the damage approach for cracking, the plasticity or viscoplasticity approach for shear,
or the still volume-pressure influence approach for compaction. The theoretical contexts are discussed in Chapter 2, before the essential elements of several
“conventional” models are described, along with their strengths and weaknesses. In Chapter 3, the subject matter turns to the particular category of dynamic
oscillations associated with earthquakes. As an introduction, Chapter 3 deals with the way seismic movement measurements – which generate the data used  for structure reaction calculations – are made. Besides presenting the addresses of databases of signals measured in different countries, this chapter also introduces the concept of spectral representation, which plays a key role in engineering practice. A geophysical interpretation of seismic movements in connection with subjacent phenomena is proposed, which integrates the contributory effects of the site and the topography of the environment around the structure. Though typical practice involves calculating the reaction of a structure submitted to an earthquake by considering its base to be totally embedded, the nature of some soils, coupled with the exceptional character of some structures (like dams and nuclear reactors), demands that the behavior of the structure is modeled in a particular environment. This problem is called structure-soil interaction, and forms the subject of Chapter 4. To solve this problem, it is necessary to have a model of the soil’s behavior under cyclic loading. Different models exist, depending on the nature and amplitude of the loading. After modeling, the interaction problem can either be treated by superposition, by considering the soil and the structure separately for linear cases, or globally for non-linear situations.

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