Civil Engineering Reference
In-Depth Information
Chapter 1
INTRODUCTION
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 topic 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.
UVu
.
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
=+
1
2
2
()
()
U
qt
⎣ ⎦
(1.1)
Ut
=
ρ
⎡ ⎤
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
flexiblethe fluctuating forces may cause the body to oscillate, and the alternating flow
and the oscillating body may interact and generate further forces.
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