Mitigation of Motions of Tall Buildings with Specific Examples of ...

mined and subsequently combined by an empirical combination rule; however, since different
response components may have a different probability structure, requiring different peak factors,
care must be exercised. Further discussions have revealed that the jerkiness of the structural
response may primarily be responsible for perception of motion. Quite simply, while humans are
capable of adjusting to accelerations, any change in the acceleration will require additional adjustments
for equilibrium. As a result, basing perception criteria on a measure of rms jerk, or the rate
of change of acceleration, would better capture the stimulus which defines our perception thresholds
under random motion.
In addition, frequency-dependent motion perception threshold criteria and probabilistic criteria
which take into account the probabilistic distribution of human perception limits are also being
considered. In particular, the frequency dependence of perception thresholds becomes critical,
since there is evidence that, with decreasing frequency of oscillation, there is an increase in perception
levels.
3.0 Structural Systems
In light of human perception and serviceability concerns, a host of techniques have been developed
to mitigate the unnerving motions induced by wind. Above and beyond the rudimentary
design of structural systems to efficiently carry lateral loads in the structure, certain features can
be engineered into the structure to improve its performance under the action of wind. If seismic
effects are not a concern, by increasing the building’s mass, the air/building mass ratio and the
natural frequency will be reduced; however, this modification increases the non-dimensional
windspeed. Therefore, this trade-off relation can occasionally increase the input wind force
energy and increase the displacement, while the acceleration decreases almost in proportion to the
square root of the mass. However, it is very difficult and unrealistic to increase the building’s
mass, considering the resulting amplification of the seismic inertia force.
On the other hand, fundamental dynamics proves that increases in stiffness will provide reductions
in the amplitude of motion, but will not affect accelerations which comprise the stimulus for
motion perception. Furthermore, by stiffening the structure, the jerk component, another contributing
factor to motion stimulus, may increase. Therefore, the selection of an efficient structural
system must include the evaluation of its ability to resist lateral wind loads with minimum jerk
and acceleration levels for the upper floors.
Despite all the considerations, the appropriate selection of an efficient structural system can provide
the most effective means of controlling structural response to wind in the lateral and torsional
directions. This may be accomplished through any number of systems including space
frames, mega frame systems, and the addition of vierendeel frames, belt trusses, super columns,
vierendeel-type bandages and outrigger trusses. A structural system can also benefit from concrete
or composite steel/concrete construction with higher internal damping. For example, the
Petronas Towers in Kuala Lumpur utilized a concrete structural system which aided in improving
the performance of the buildings from a serviceability standpoint. The application of a few of
these strategies are highlighted in the following sections.
Mitigation of Motions of Tall Buildings with Specific Examples of Recent Applications 4