The effect of wind on Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 153

wind speed. At higher wind speeds, the frequency of vortex shedding may approach
the natural frequency of vibration of the building. When this occurs, the across-wind
loads dominate over the along-wind loads, and the building motion can actually
accentuate the vortex shedding leading to the phenomenon of negative aerodynamic
damping. Aerodynamic architectural design and modifications ( Chapter 6 ) reduce the
strength of vortex shedding on tall buildings.
Even in buildings where the across-wind loads are smaller than the along-wind
loads, the across-wind responses can still be critical for design as they will often still
govern the serviceability accelerations. The serviceability accelerations are important
in ensuring comfort for the occupants as perceptible motion can be disturbing if it is
unexpected or occurs on a regular basis. Since in many tall and flexible buildings,
across-wind building response is more critical than along-wind response (Holmes,
2001; Irwin, 2008; Kareem, 1985; Kwok, 1982), in general the concern to control
across-wind response becomes a major design input. According to Gu and Quan
(2004), “In the case of the Jin Mao Tower (Shanghai, 1999), the maximum accel-
eration in across-wind direction at design wind speed is about 1.2 times of that in
the along-wind direction.” Irwin (2008) states that “It is quite often the case that the
highest overall wind loading on a tall slender building results from across-wind vortex
excitation, which induces a large dynamic response.”
As well as along-wind and across-wind responses, tall buildings may also
experience torsional responses ( Figure 5.1 ). These can occur if the shape of the
building is asymmetric, if the structural system is asymmetric, or if the building is
subjected to asymmetric flows.
The resultant wind force, acting perpendicular to the building face, passes through
the geometric centre of the surface area of the building affected by the wind. On
the other hand, the resultant reaction force passes through the stiffness centre of the
building. When these two forces are not on the same axis, the resulting eccentricity
creates torsional moments, resulting in floor torsion; thus, torsional motion occurs
about the vertical axis of the building.
In general, most wind loading codes provide procedures for estimating the along-
wind forces. Relatively few codes include procedures for across-wind and torsional
responses, which by their nature are a lot less easily codified with accuracy. In the
case of supertall buildings, along-wind, across-wind and torsional building responses,
together with the dynamic effect of the wind, must be taken into account. In this
context, dynamic calculation methods or wind tunnel tests are recommended for
estimating the wind loads on such buildings.
5.2 wind tunnel tests
As the height of buildings increases, wind loads become increasingly important for
efficient and reliable design. While wind loads may influence the structural design
of most tall buildings, for supertall buildings the minimisation of wind loads and
responses can actually influence the architectural design. Shorter and/or less flexible
buildings are generally treated by building codes as static structures, and wind load
can be regarded as a static load on the building. For taller and/or more flexible struc-
tures the static load approach is insufficient, and the wind load on the building is