Geology Reference
In-Depth Information
proaches like isolation or compensation have to
be used. Isolation is realised by the introduction
of efficient intermediate layers at the base of the
edifice. Compensation is done by the installation
of elastically coupled masses along the building's
height.
Compensation is done by introducing fitting
sets of masses, springs and dampers which are
designed to absorb parts of the earthquake energy.
As the impact of an earthquake in terms of ampli-
tude and frequency is not uniquely defined, the
compensating system has to respond to a certain
variety of excitations. On the other hand, the total
mass and space of the compensating system has
to be limited since it reduces the usable space of
the building.
Compensation systems may be active - us-
ing fast control to stimulate accelerations of
compensator masses that counteract the external
earthquake excitation - or passive - absorbing the
energy passed into the structure by an adequate
dynamic response. Semi-active dampers, where
e.g. the viscosity of the damping fluid changes
due to magnetic excitation play an interesting
role between the two designs (Han-Rok Ji, H. et
al. 2005).
Designing an efficient passive compensating
system includes a proposal of position, number
and dimension of compensators. For very high
buildings, this may include up to 10 or more
compensators each defined by mass, stiffness and
damping in two horizontal directions resulting in a
total of up to 60 or more degrees of freedom to be
taken into account for the compensation system.
Finding an optimal set of parameters may be a dif-
ficult task, as the response surface of the building's
loading vs. the parameters of the compensation
system may have a large number of local optima.
Gradient search strategies tend to converge to the
next local optimum, so they are not very efficient.
Evolutionary strategies (Rechenberg 1994, pp.
15-44, Gen 2000, pp. 17-34) may be able to cover
larger regions of the parameter space, avoiding
getting stuck to local maxima.
The application of dynamic loads to simplified
models of high buildings allows us to study the
response of given compensator designs and to per-
form an optimization study. It may contribute to a
significant reduction of the destructive impact the
structure has to withstand during the earthquake.
A short review of the theory and some examples
demonstrate the potential of the method proposed.
As it is relatively easy to implement and to apply,
many variants may be checked, yielding proposals
for more detailed studies.
IMPROVE THE STRUCTURAL
ABILITY TO WITHSTAND
EARTHQUAKE IMPACT
The earthquake loading of a building is understood
as a base excitation of the building (Towhata
2008, pp. 67-71, Bozorgnia and Bertero 2004,
pp. 2-9 - 2-15). Due to the large mass of the
surrounding ground compared to the building's
mass, the excitation may be considered as dis-
placement controlled. The interface between the
surroundings and the building has to follow these
displacements. Any approach to limit or reduce
the impact on the building has to take into account
this displacement history.
The improvement of the static strength by
enforcing the load-carrying elements is not al-
ways feasible in an economic and aesthetic way.
Consequently we need dynamic approaches. One
of the often-used ideas to isolate the building's
base from the excited ground shows very prom-
ising results (Naeim 1999, pp. 93-119, Ordonez
2002). Isolation implies the uncoupling of the
buildings' base from the ground by some less
stiff but damping components. Unfortunately, it is
not easy and sometimes very expensive to design
such an isolating base system for very large and
high structures.
Some earthquake resistance improvement may
be achieved by local dampers or relatively soft
parts in an overall stiff structure (Bozorgnia and
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