Civil Engineering Reference
In-Depth Information
were obtained, due to the poor correlation between fluctuating wind pressures on the two
surfaces (Ginger and Letchford, 1994).
Appendix F gives an example of the calculation of maximum and minimum lift and
drag on a pitched free roof, and the effective static pressures producing them, based on
data from Ginger and Letchford (1994).
14.4
Attachments to buildings
14.4.1
Canopies, awnings and parapets
Several configurations of horizontal canopy attached to one wall of a low-rise building
have been investigated (Jancauskas and Holmes, 1985). The width of the canopy and the
height of the canopy position on the wall were the investigated variables. A narrow
canopy mounted at the top of the wall behaves similarly to eaves on the roof.
For wind directions normal to the adjacent wall, the peak net force across the canopy
is strongly dependent on the non-dimensional ratios,
h
c
/h,
and
h
c
/w
c
.h
c
is the height of the
canopy above the ground,
h
the total height of the adjacent wall and
w
c
the width of the
canopy.
For the peak vertical uplift force coefficient, based on the mean wind speed at the
height of the canopy, the following conservative relationships were proposed based on
the wind-tunnel measurements:
(14.4)
where,
and
F
z
is the net vertical force on the canopy (positive upwards),
Ū
c
the mean wind speed
at the canopy height and
A
is the plan area of the canopy.
Equation (14.4) can be applied to canopies with pitch angles within 5° of the
horizontal. Appropriate adjustment is required if it is applied with gust wind speeds; such
adjustment has been made for the rule incorporated in the Australian wind loading
standard (Standards Australia, 2002).
The relationships of Equation (14.4) are compared with the experimental data in
Figure 14.11.
The higher values obtained for canopies or awnings near the top of the wall (or eaves)
can be explained by the high flow velocities occurring on the upper side of the canopy
producing significant negative pressures; on the underside of the canopy, stagnation and
hence positive pressures occur. When the canopy is mounted part-way up the wall,
