structural dynamics pdf | structural dynamics & vibration by douglas thorby
Structural dynamics pdf | structural dynamics & vibration by Douglas thorby
INTRODUCTION - STRUCTURAL DYNAMICS
The accelerations, velocities and displacements in a system produce forces when multiplied, respectively, by mass, damping and stiffness. These can be considered to be the building blocks of mechanical systems, in much the same way that inductance, capacitance and resistance (L, C and R) are the building blocks of electronic circuits.
The dynamic factor is a useful engineering concept that, at its simplest, compares the maximum dynamic displacement response of a system with the static displacement that would be produced by a steady force with the same magnitude as the peak value of the actual force.
The idea can be extended to responses other than displacement, and inputs other than force, such as base motion. Although the concept can be applied to systems with any number of degrees of freedom, the approach is over-simplistic for multi-DOF systems, and it is usually applied only to single-DOF systems.
The dynamic factor is a fairly crude concept, and should not be used to replace proper dynamic analysis. However, as an aid to engineering judgement, in the early stages of a design, it can be of considerable value
structural dynamics pdf |
Traditionally, structures have been analysed either as continuous or as discretize(‘lumped’) systems. Some structures, such as uniform beams, can still be usefully be treated as continuous systems, but most are now regarded as discrete multi-DOF systems.
The finite element method, in fact, can be said to combine both approaches: it is continuous within the elements, but discrete at the global coordinate level. In this chapter, discussion of continuous systems has been limited to uniform beams, and the classical Rayleigh-Ritz method.
A description of the latter has been included, even though it is now obsolete, to show the historical link with late methods, such as component mode synthesis, the branch mode method and the finite element method, which are still known as ‘Rayleigh–Ritz’ methods.
Structures tend to be characterized by low damping, justifying the use of real eigenvalues and eigenvectors in most cases, and this will be assumed in this chapter.
The basic principles outlined in Chapter 6, such as the use of energy methods and normal coordinates, are timeless concepts. However, the application of these fundamental principles to everyday tasks in structural dynamics has changed considerably over the years, due to the development of modern computers.
Therefore, to put current methods of analysis into perspective, a brief discussion of their historical development is first presented.
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