Page 85 - Vertical City
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CONSTRUCTION OF THE TOWERS 85
wings of 18 m width, which are successively turned by 90
degrees on the plan, surrounding a central prismatic and
square void of 18 m per side [figure 26].
It can also be realized by bracing the tower using
guy cables as proposed for the small vertical city, in
Chapter V.
Structural dynamics
Although the tower is a static structure, it not only has
to resist but also remain habitable in the face of the
dynamic forces of wind and earthquakes.
The vibrational energy transferred by wind to
the tower increases with the reduction of the gusts’
frequency (the number of gusts per second in Hz). It is
at a maximum for one gust per minute (or a frequency
of 0,0167 Hz) 5 and to take into consideration as from 5
gusts per second (or 5 Hz). Inversely, that transferred by
seismic movement is stronger as its frequency increases;
it is at a maximum between 2 and 6 Hz 6.
Knowing, as a first approximation, that the first
natural frequency of a “normal” tower is 46/H(m), every
tower higher than 20 stories becomes very vulnerable
to the dynamic effects of wind, and this vulnerability
increases with its height.
Inversely, the construction is less vulnerable to
seismic movement the higher it is, as long as its struc-
5 Eurocode 1. partie 1.4. NBN.EN 1991-1-4 (2005).
6 Eurocode 8. partie 1.NBN-EN 1998-1 (2004).
