Civil Engineering Reference
In-Depth Information
7.6 Modelling of overall loads and response of structures
7.6.1
Base-pivoted model testing of tall buildings
This section describes the procedure for the conducting of aeroelastic wind-tunnel testing
of high-rise buildings, using rigid models.
The use of rigid-body aeroelastic modelling of tall buildings is based on three basic
assumptions:
1. The resonant response of the building to wind loads in torsional (twisting) modes can
be neglected.
2. The response in sway modes higher than the first in each orthogonal direction can be
neglected.
3. The mode shapes of the fundamental sway modes can be assumed to be linear.
With these assumptions, the motion of a rigid model of the building, pivoted at, or near,
ground level and located in a wind tunnel in which an acceptable model of the
atmospheric boundary layer in strong winds has been set up, can be taken to represent the
sway motion of the prototype building. The fact that a scaled reproduction of the building
motion has been obtained means that fluctuating aerodynamic forces that depend upon
that motion have been reproduced in the wind tunnel. This is not the case when fixed
models are used to measure the fluctuating wind pressures or the 'base-balance'
technique is used. In both these cases, the resonant response of the building is not
reproduced.
Even buildings that have a non-linear mode shape can often be modelled by means of
rigid-body rotation, but in these cases it may be appropriate to position the pivot point at
a different level to ground level. For example, a building supported on stiff columns near
ground level might be modelled by a rigid model pivoted at a height above ground level
(e.g. Isyumov
et al.,
1975). The disadvantage of this approach is that the bending moment
at ground level cannot be measured.
There is a direct analogy between the generalized mass of the prototype building,
G
1
,
and the moment of inertia of the model building, including the contributions from the
support shaft and any other moving parts.
Assuming that the mode shape of the building is given by:
(7.5)
the generalized mass is given by:
(7.6)
