Civil Engineering Reference
In-Depth Information
Chapter 5
WIND AND MOTION INDUCED LOADS
5.1 The buffeting theory
The buffeting wind load on structures includes the part of the total load that may be
ascribed
to
the
velocity
fluctuations
in
the
oncoming
flow,
(
)
(
)
(
)
(
)
(
)
wx z t
, as well as any
motion induced contributions. The theory presented below was first developed by A.G.
Davenport [13, 14]. In the following it is a line like horizontal bridge type of structure
that is considered. It is taken for granted that its
z
-position in the flow prior to any
loading is constant along the entire span, that the wind field is stationary and
homogeneous and that the main flow direction is perpendicular to the span-wise
x
-axis
of the structure, in which case
x
is constant and
y
may be exchanged by
x
. It is then
only the velocity fluctuations in the along wind and the across wind vertical directions
expressed in structural axis that are of interest, i.e. the components
,
vx z t
and
Uxyzt Vxyz
,,,
=
,,
+
uxyzt
,,,
,,
,,
f
f
f
f
f
f
f
f
f
f
f
f
f
(
)
(
)
Uxt
,
V uxt
,
=+
(
)
and
wxt
. The theory may readily be applied to a vertical (tower) type of structure, in
which case any
z
-variation needs to be included and the
w
component must be replaced
by the
v
component (but maintaining all other notations shown in Fig. 5.1 below). The
basic assumptions behind the buffeting theory are that the load may be calculated from
the instantaneous velocity pressure and the appropriate load coefficients that have been
obtained from static tests, and that linearization of any fluctuating parts will render
results with sufficient accuracy. Thus, the load may be calculated from an interpretation
of the instantaneous relative velocity vector and the corresponding flow incidence
dependent drag, lift and moment coefficients that are usually applied to calculate mean
static load effects. It is taken for granted that structural displacements and cross sectional
rotations are small. Furthermore, it is a requirement for linearization of load components
that
,
(
)
(
)
uxt
and
,
wxt
are small as compared to
V
. The situation is illustrated in
,
Fig. 5.1.
