Civil Engineering Reference
In-Depth Information
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.
Surely, this is only for convenience as there are really no regions where they exclusively
occur alone. The important question is to what extent they are adequately included in the
mathematical description of the loading process.
In structural engineering the wind induced fluctuating forces and corresponding
response quantities are usually assumed stationary, and thus, response calculations may
be split into a time invariant and a fluctuating part (static and dynamic response). An
illustration of what can be expected is shown in Fig. 1.1.
Fig. 1.1
Typical response behaviour of slender civil engineering structures
For a mathematical description of the process from a fluctuating wind field to a
corresponding load that causes a fluctuating load effect (e.g. displacements or cross
sectional stress resultants) a solution strategy in time domain is possible but demanding.
The reason for this is that the wind field is a complex process that is randomly distributed
in time and space. A far more convenient mathematical model may be established in
