Civil Engineering Reference
In-Depth Information
The Silsoe Structures Building was a larger steel portal-framed structure, 24 m long,
12.9 m span and 4 m to the eaves, with a 10° roof pitch, located in open country. As well
as 70 pressure tapping points on the building roof and walls, the building was equipped
with 12 strain gauge positions on the central portal frame to enable measurements of
structural response to be made (Robertson, 1992).
The building could be fitted with both curved and sharp eaves. The curved eaves were
found to give lower mean negative pressures immediately downwind of the windward
wall, than those produced by the sharp eaves. Measurements of strain in the portal frame
were found to be predicted quite well by a structural analysis computer program when the
correct column fixity was applied. Spectral densities of the strains were also measured—
these showed the effects of Helmholtz resonance (Section 6.2.3) on the internal pressures,
when there was an opening in the end wall of the building. Generally these measurements
justified a
quasi-steady
approach to wind loads on low-rise buildings (Section 4.6.2).
8.3
General characteristics of wind loads on low-rise buildings
Full-scale measurements of wind pressures on low-rise buildings, such as those described
in Section 8.2.2, show the highly fluctuating nature of wind pressures, area-averaged
wind loads and load effects, or responses, on these structures. The fluctuations with time
can be attributed to two sources (see also Section 4.6.1):
1. Pressure fluctuations induced by upwind turbulent velocity fluctuations (see Chapter
3). In an urban situation, the turbulence may arise from the wakes of upwind
buildings.
2. Unsteady pressures produced by local vortex shedding and other unsteady flow
phenomena, in the separated flow regions near sharp corners, roof eaves and ridges
(see Chapter 4).
These two phenomena may interact with each other to further complicate the situation.
It should be noted that, as well as a variation with time, as shown for a single point on
a building in Figure 8.4, there is a variation with space, i.e. the same pressure or response
variation with time may not occur simultaneously at different points separated from each
other on a building.
8.3.1
Pressure coefficients
The basic definition of a pressure coefficient for a bluff body was given in Section 4.2.1,
and the rms fluctuating (standard deviation) pressure coefficient was defined in Section
4.6.4. A general time-varying pressure coefficient,
C
p
(t),
for buildings in stationary, or
synoptic, wind storms is as follows:
(8.1)
where
p
0
is a static reference pressure (normally atmospheric pressure measured at a
convenient location near the building, but not affected by the flow around the building),
