Structural Dynamics with Applications in Earthquake and Wind Engineering

This chapter offers an overview of the theoretical foundations and the standard numerical methods for solving structural dynamics problems, with emphasis placed firmly on the latter. Starting with the analysis of single degree of freedom (SDOF) systems both in the time and in the frequency domain, it includes sections on the computation of elastic and inelastic response spectra, filtering in the frequency domain, the analysis of nonlinear SDOF systems and the generation of spectrum compatible ground motion time histories. Discrete multi-degree of freedom (MDOF) systems, condensation techniques and damping models are considered next. Both modal analysis (“response modal analysis”) and direct integration methods are employed, focussing especially on the behaviour of MDOF systems subject to seismic excitations described by response spectra or sets of specific ground motion time histories. Detailed descriptions of the software used for solving the numerous examples presented complete with full input-output parameter lists conclude the chapter.

Since man has been erecting structures, earthquake ground motions have posed a threat to their stability. Only with the advent of and continual improvement to digital measurement, processing, and modeling techniques, have engineering seismologists been able to quantify the spreading of seismic waves in the Earth to facilitate site specific ground motions estimates for potentially damaging (i.e., ‘future’) earthquakes. These earthquake scenarios constitute the boundary conditions for earthquake-resistant design of buildings. The ground motions during an earthquake at a specific site are determined by the characteristics of the earthquake source, the travel path, and the local site conditions. Each of these links along the path of seismic waves influences the amplitudes, frequency content, and duration of the vibrations which ultimately influence the dynamic load of buildings. The chapter gives a brief introduction to the earthquake phenomenon and the spreading of seismic waves and the main characteristics and parameters of strong ground motions are explained. Further discussed are the effects of finite seismic sources and site effects due to the local geology on ground motions and spectra.

Wind loads have great impact on many engineering structures. Wind storms often cause irreparable damage to the buildings which are exposed to it. Along with the earthquakes, wind represents one of the most common environmental load on structures and is relevant for limit state design. Modern wind codes indicate calculation procedures allowing engineers to deal with structural systems, which are susceptible to conduct wind-excited oscillations. In the codes approximate formulas for wind buffeting are specified which relate the dynamic problem to rather abstract parameter functions. The complete theory behind is not visible in order to simplify the applicability of the procedures. This chapter derives the underlying basic relations of the spectral method for wind buffeting and explains the main important applications of it in order to elucidate part of the theoretical background of computations after the new codes. The stochasticity of the wind processes is addressed, and the analysis of analytical as well as measurement based power spectra is outlined. Short MATLAB codes are added to the Appendix 3 which carry out the computation of a single sided auto-spectrum from a statistically stationary, discrete stochastic process. Two examples are presented.

The chapter initially provides a summary of the contents of Eurocode 8, its aim being to offer both to the students and to practising engineers an easy introduction into the calculation and dimensioning procedures of this earthquake code. Specifically, the general rules for earthquake-resistant structures, the definition of design response spectra taking behaviour and importance factors into account, the application of linear and non-linear calculation methods and the structural safety verifications at the serviceability and ultimate limit state are presented. The application of linear and non-linear calculation methods and corresponding seismic design rules is demonstrated on practical examples for reinforced concrete, steel and masonry buildings. Furthermore, the seismic assessment of existing buildings is discussed and illustrated on the example of a typical historical masonry building in Italy. The examples are worked out in detail and each step of the design process, from the preliminary analysis to the final design, is explained in detail.

Industrial units consist of the primary load-carrying structure and various process engineering components, the latter being by far the most important in financial terms. In addition, supply structures such as free-standing tanks and silos are usually required for each plant to ensure the supply of material and product storage. Thus, for the earthquake-proof design of industrial plants, design and construction rules are required for the primary structures, the secondary structures and the supply structures. Within the framework of these rules, possible interactions of primary and secondary structures must also be taken into account. Importance factors are used in seismic design in order to take into account the usually higher risk potential of an industrial unit compared to conventional building structures. Industrial facilities must be able to withstand seismic actions because of possibly wide-ranging damage consequences in addition to losses due to production standstill and the destruction of valuable equipment. The chapter presents an integrated concept for the seismic design of industrial units based on current seismic standards and the latest research results. Special attention is devoted to the seismic design of steel thin-walled silos and tank structures.

During the last 2 decades, new code generations have been introduced which incorporate the achieved scientific and technological state of the art that has been proven its usefulness in practical application. The Eurocodes and the CICIND model codes are examples of this development. The Eurocode applies a modified gust response factor to model in-line wind loading and resonance due to turbulence through increasing the peak velocity pressure by a factor which depends on the individual size and dynamic features of the structure considered. The CICIND model codes for steel and concrete chimneys take account of the particular mechanical behaviour and the specific design requirements of these structures and utilizes the mean and the gust wind force, where the gust load defines an equivalent static load scaled to reproduce the real base bending moment of the chimney induced by wind gustiness. The cross-wind excitation of chimneys by vortex shedding is calculated in the CICIND model applying a negative aerodynamic damping to incorporate the motion induced forces, and a bandwidth factor to account for the reduction of the lift force spectrum caused by wind turbulence. Contrarily, the Eurocode relies on an empirical concept. In addition, it contains a further method, where the aerodynamic damping parameter is given for zero turbulence only. The principal issues of the chapter are to identify the merits and the drawbacks of the different concepts and to identify their dominant fields of application.