Civil Engineering Reference
In-Depth Information
The latter method has advantages that the influence coefficients are not required at the
time of the wind-tunnel testing, and also that the information can be used to determine
equivalent static load distributions, as discussed in Chapter 5. When resonant response is
of significance, as may be the case for the largest stadium roofs, time histories of the
fluctuating pressures can be used to generate a time history of generalized force for each
mode of significance. From the spectral density of the generalized force, the mean square
generalized displacement (modal coordinate) and effective inertial forces acting can be
determined (Section 5.4.4). The application of pressure model studies to large roofs is
discussed in Chapter 10.
Pressure-based methods can also be used for structural loads and response of tall
buildings (ASCE, 1999). Although these methods require a large number of simultaneous
pressure measurements and extensive post-processing of the wind-tunnel data, accurate
account of non-linear resonant mode shapes can be made, and in many cases this method
has replaced the high-frequency base-balance technique. A significant advantage is that
the same building model used to determine local cladding pressures can be used to
determine overall wind loads and response. However, a practical difficulty with this
technique is the installation of a sufficient number of tubes for pressure measurement
within the available cross-section of a model.
7.7 Blockage effects and corrections
In a wind tunnel with a closed test section, the walls and roof of the wind tunnel provide
a constraint on the flow around a model building or group of buildings, which depends on
the blockage ratio. The blockage ratio is the maximum cross-sectional area of the model
at any cross-section divided by the area of the wind-tunnel cross-section. If this ratio is
high enough, there may be significant increases in the flow velocities around, and
pressures on, the model. In the case of an open-test section, the errors are in the opposite
direction, i.e. the velocities around the model are reduced. To deal with the blockage
problem, several approaches are possible:
• Ensure that the blockage ratio is small enough that the errors introduced are small, and
no corrections are required. The usual rule for this approach is that the blockage ratio
should not exceed 5%.
• Accept a higher blockage ratio and attempt to make corrections. The difficulty with this
approach is that the appropriate correction factors may themselves be uncertain.
Although there are well-documented correction methods for drag and base pressure on
stalled airfoils, and other bluff bodies in the centre of a wind tunnel with uniform or
homogeneous turbulent flow, there is very little information for buildings or other
structures mounted on the floor of a wind tunnel in turbulent boundary-layer flow.
McKeon and Melbourne (1971) provided corrections for
mean
windward and leeward
pressures, and total drag force, on simple plates and blocks. However, no corrections
are available for pressures, mean or fluctuating, in separated flow regions, such as
those which occur on roofs or side walls of building models.
• Design the walls and/or roof of the working section in such a way as to minimize the
blockage errors. The most promising method for doing this appears to be the
slotted
wall
concept (Parkinson, 1984; Parkinson and Cook, 1992). In this system, the walls
