Lateral loads affecting Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 10

structural systems increased and the total weight and rigidity of structures decreased.
The effect of wind on tall buildings is explained in Chapter 5 .
Wind speed and pressure increase parabolically according to height, and therefore
wind loads affecting tall buildings become important as the height of the building
increases. In general, structural design begins to be controlled by wind loads in
buildings of more than 40 storeys (ACI SP-97, 1989). Today, thanks to developments in
structural systems and to high-strength materials, tall buildings have increased in their
height to weight ratio but on the other hand reduced in stiffness compared with their
precursors, and so have become greatly affected by wind. With the reduced stiffness,
the sensitivity to lateral drift, and hence the sway under wind loads, increases. The
sway, which cannot be observed outside the building or at the lower floors, can cause
discomfort to occupants at the higher floors of a building. Architectural, structural,
and mechanical design approaches ( Chapter 6 ) are used to control lateral drift in tall
buildings.
In the design of tall buildings, for buildings below 40 storeys with height to width
ratio (the ratio of the structural height of a building to the narrowest structural width
at the ground floor plan, also termed aspect ratio) below 6, the values predicted in
the building design codes can be used to determine wind loads. Because wind loads
can change quickly or even suddenly, unlike live and dead loads, in order to estimate
the wind load in buildings of more than 40 storeys, or that have an aspect ratio of
6 or higher (slender and flexible buildings), or that have unusual forms, dynamic
effect of the wind and dynamic building response must be taken into account. In this
context, dynamic calculation methods, or else wind tunnel tests, are recommended
for estimating the wind loads on such buildings ( Section 5.2 ).
2.2 earthquake loads
Earthquakes are the propagation of energy released as seismic waves in the earth
when the earth's crust cracks, or when sudden slippage occurs along the cracks as
a result of the movement of the earth's tectonic plates relative to one another. With
the cracking of the earth's crust, faults develop. Over time, an accumulation of stress
in the faults results in sudden slippage and the release of energy. The propagation of
waves of energy, formed as a result of seismic movement in the earth's crust, acts upon
the building foundations and becomes the earthquake load of the building. In deter-
mining earthquake loads, the characteristics of the structure and records of previous
earthquakes have great importance. Compared with wind loads, earthquake loads are
more intense but of shorter duration.
Earthquakes can occur almost anywhere, and considering that low, medium and
high severity earthquakes may occur during the life of a structure located in an active
earthquake zone, it is necessary to understand very well the behaviour of a structure
during an earthquake in order to prevent the disastrous collapses that can occur.
An earthquake's effect or power is measured by the “earthquake's intensity” or
“earthquake's magnitude”. Accounting for the effects upon living creatures, structures
and the environment in the measurement of an earthquake gives the “intensity” of
the earthquake, while using earthquake seismographs (seismometers) to measure
the energy released at the centre of an earthquake gives the “magnitude” of the