Geology Reference
In-Depth Information
is mostly oriented towards finding techniques,
which reduce seismic forces, without creating
damage in the structure, or concentrating it in
certain pre-determined points.
In conventional design practice, energy dis-
sipation is intended to occur in specially detailed
critical regions of the structure, usually in the
beams near or adjacent to the beam-column joints.
Inelastic behaviour in these regions often results in
significant damage to the structural members, and
although the regions may be well detailed, their
hysteretic behaviour will degrade with repeated
inelastic cycling. Further, the large interstory drifts
required to achieve significant hysteretic energy
dissipation in critical regions usually result in
substantial damage to non-structural elements such
as in-fill walls, partitions, doorways, and ceilings.
A modern tendency in seismic design is to
avoid damage in the structure by using specific
control techniques.
The control of structures, in fact, was born as
a necessity to reduce the effects of vibrations on
structures and on structural parts of them. From a
broader perspective, control of structures not only
refers to civil structures but also to mechanical
systems and so on.
Some examples where vibrations are important
(not a comprehensive list) are the following:
•
Mechanical engineering: coolers, wash-
ing machines, pressing machines, forging
machines, flexible robots, pipelines and in-
dustrial facilities with dangerous products,
shaking tables
•
Ships, submarines
The most important problems generated by
the vibrations of structures are due to the rela-
tive displacements of the structure, which is the
displacement between two different points of the
structure. As a result, the system is subjected to a
fatigue stress with a reduction of its strength and,
consequently, a reduction of the structural safety.
In addition, the absolute displacements are
cause of problems, as high amplitude could
produce uncomfortable noise. The effects of the
absolute accelerations must be also considered,
as they are especially noticeable on equipment
and machines. The human comfort conditions
are affected too as high accelerations produce an
inertial force on the people preventing them from
acting comfortably.
Seismic Control of Structures
In the seismic design, the structures are considered
as fixed or jointed to the ground to form a single
system with the ground, and the seismic motion
causes stresses and deformations. The objective
of the resistant design is to make the structure
withstand these forces without collapsing. To
this end, it is possible to distinguish two levels:
for earthquakes with a determinate intensity level
(moderate) an absolute absence of damage in
the structure and in the non-structural elements
is required; for other earthquakes with a higher
intensity (and with a less probability of occurrence)
it would be acceptable if there were the possibility
of localised damage that is not dangerous to the
stability of the structure, the possibility of its re-
habilitation and the possibility of causing victims.
On the contrary, the mechanisms for control
of the seismic response attempt to reduce the
•
Flexible structures (slender) or structures
housing sensitive equipment (semiconduc-
tor facilities, ultra-precision machinery,
measuring instruments)
•
Civil engineering: buildings, bridges, off-
shore structures, slender towers and chim-
neys, nuclear power plants, telescopes,
electric energy transportation lines, perfo-
rated elements, cable-ways and high veloc-
ity lifts (elevators)
•
Aerospace engineering: plane wings,
panels, lightweight space structures (free
vibrations)
•
Automotive engineering: panels, vehicle
bodies
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