Strømmen Theory of Bridge Aerodynamics

1. Auflage 2006
ISBN: 978-3-540-30604-7
Verlag: Springer
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, 251 Seiten, eBook

This text book is intended for studies in wind engineering, with focus on the stochastic theory of wind induced dynamic response calculations for slender bridges or other line ?like civil engineering type of structures. It contains the background assumptions and hypothesis as well as the development of the computational theory that is necessary for the prediction of wind induced fluctuating displacements and cross sectional forces. The simple cases of static and quasi-static structural response calculations are for the sake of completeness also included. The text is at an advanced level in the sense that it requires a fairly comprehensive knowledge of basic structural dynamics, particularly of solution procedures in a modal format. None of the theory related to the determination of eigen ?values and the corresponding eigen ?modes are included in this book, i. e. it is taken for granted that the reader is familiar with this part of the theory of structural dynamics. Otherwise, the reader will find the necessary subjects covered by e. g. Clough & Penzien [2] and Meirovitch [3]. It is also advantageous that the reader has some knowledge of the theory of statistical properties of stationary time series. However, while the theory of structural dynamics is covered in a good number of text books, the theory of time series is not, and therefore, the book contains most of the necessary treatment of stationary time series (chapter 2). The book does not cover special subjects such as rain-wind induced cable vibrations.

Zielgruppe

Research

Autoren/Hrsg.

Strømmen, Einar

Inhaltsverzeichnis

SOME BASIC STATISTICAL CONCEPTS IN WIND ENGINEERING.- STOCHASTIC DESCRIPTION OF TURBULENT WIND.- BASIC THEORY OF STOCHASTIC DYNAMIC RESPONSE CALCULATIONS.- WIND AND MOTION INDUCED LOADS.- WIND INDUCED STATIC AND DYNAMIC RESPONSE CALCULATIONS.- DETERMINATION OF CROSS SECTIONAL FORCES.- MOTION INDUCED INSTABILITIES.

Leseproben

Chapter 1 INTRODUCTION (p. 1)

1.1 General considerations

This text book focuses exclusively on the prediction of wind induced static and dynamic response of slender line-like civil engineering structures. Throughout the main part of the book it is taken for granted that the structure is horizontal, i.e. a bridge, but the theory is generally applicable to any line–like type of structure, and thus, it is equally applicable to e.g. a vertical tower.

It is a general assumption that structural behaviour is linear elastic and that any non-linear part of the relationship between load and structural displacement may be disregarded. It is also taken for granted that the main flow direction throughout the entire span of the structure is perpendicular to the axis in the direction of its span.

The wind velocity vector is split into three fluctuating orthogonal components, U in the main flow along–wind direction, and v and w in the across wind horizontal and vertical directions. For a relevant structural design situation it is assumed that U may be split into a mean value V that only varies with height above ground level and a fluctuating part u, i.e. U =V+ u& . V is the commonly known mean wind velocity, and u, v and w are the zero mean turbulence components, created by friction between the terrain and the flow of the main weather system.

It is taken for granted that the instantaneous wind velocity pressure is given by Bernoulli’s equation If an air flow is met by the obstacle of a more or less solid line-like body, the flow/structure interaction will give raise to forces acting on the body. Unless the body is extremely streamlined and the speed of the flow is very low and smooth, these forces will fluctuate. Firstly, the oncoming flow in which the body is submerged contains turbulence, i.e. it is itself fluctuating in time and space.

Secondly, on the surface of the body additional flow turbulence and vortices are created due to friction, and if the body has sharp edges the flow will separate on these edges and the flow passing the body is unstable in the sense that a variable part of it will alternate from one side to the other, causing vortices to be shed in the wake of the body.

And finally, if the body is flexible the fluctuating forces may cause the body to oscillate, and the alternating flow and the oscillating body may interact and generate further forces. Thus, the nature of wind forces may stem from pressure fluctuations (turbulence) in the oncoming flow, vortices shed on the surface and into the wake of the body, and from the interaction between the flow and the oscillating body itself.

The first of these effects is known as buffeting, the second as vortex shedding, and the third is usually labelled motion induced forces. In literature, the corresponding response calculations are usually treated separately.

The reason for this is that for most civil engineering structures they occur at their strongest in fairly separate wind velocity regions, i.e. vortex shedding is at its strongest at fairly low wind velocities, buffeting occur at stronger wind velocities, while motion induced forces are primarily associated with the highest wind velocities.