Note: Descriptions are shown in the official language in which they were submitted.
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Title: Seismic dissipation module made up of compression-resistant
spheres immersed in a variable low density material.
Description
Field of invention
The present invention concerns the industry for making seismic isolators,
namely devices used for isolating the load-bearing structure of buildings from
the effects of an earthquake and consists of a seismic dissipation and
isolation
panel or module made up of compression-resistant spheres, made of sintered
alumina, bound by variable low density substances, polyurethane foams or
polystyrene or other similar material, to be used in new buildings by placing
it between a reinforced concrete bed to be made on the ground and the
foundation structures of the building, so that, in the event of an earthquake,
there can be movements of the building independent from those of the ground
on which it is built, so absorbing and isolating the seismic wave and
therefore
reducing the effects on the structures until, in theory, they cancel them.
Background
Seismic events are the cause of considerable damage to both concrete and
masonry buildings, with well-known consequences on the life of many
people. Buildings are anchored to the ground by various types of foundations;
they are consequently totally affected by the seismic wave that propagates
through the ground to the foundations and therefore to the building, so
producing forces that cause considerable stresses to the structural masses,
which it is attempted to remedy by laying considerable dimensions of
structures and of metal reinforcements that can withstand these forces as
much as possible.
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To contain the uncertainties due to the uncertainty of the determination of
the
structural modelling parameters and guarantee good behaviour of the
structures under seismic actions, specific measures must be adopted, which
are listed below, aiming at ensuring ductility characteristics to the
structural
elements and to the building as a whole.
When an earthquake occurs, the base of the isolated building can move in all
horizontal directions compared to the foundations; therefore, after a
movement, it is necessary for the building to return to its original position,
if
the residual movements are not of small magnitude compared to the building.
To that end, the base of the building must be provided with suitable re-
centring systems, also called auxiliary devices, whose function is to
dissipate
energy and/or re-centre the system and/or to provide lateral constraint of the
structure.
Devices that can re-centre the structure and also dissipate energy may include
hydraulic devices or devices based on the particular mechanical properties of
Shape Memory Alloys (SMA). These materials, typically made up of nickel-
titanium alloys, have the ability to "remember" their original shape, which is
unusual for other types of materials.
In general, movements experienced by the isolated structures as a
consequence of seismic action must fall within tolerable values to contain the
dimensions of the structural joints and not cause problems to the connections
of the installations/systems. For isolated structures, flexible connections
should in fact be envisaged for all the installations/systems that, from
ground
level, are connected to the superstructure.
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Foundation structures must withstand the effects resulting from the response
of the ground and the structures above, without permanent movements that
are incompatible with the reference extreme state.
Buildings must be provided with structural systems that will guarantee
stiffness and resistance to the two orthogonal-horizontal components of
seismic actions.
The foundation system must be provided with high extensional stiffness in
the horizontal plane and with sufficient flexural stiffness.
The structural elements of the foundations, which must be sized on the basis
of the stresses transmitted to them from the structure above, must have non-
dissipative behaviour, irrespective of the structural behaviour attributed to
the
structure bearing down on them.
By inserting isolators between the foundations and the elevation structures,
the frequencies of the earthquake are uncoupled from the frequencies of the
elevation structure and so the development of resonance phenomena is
prevented.
In the event of seismic stress, the insertion of isolators allows the proper
period of vibration of the structure to be increased so moving it away from
the area of the response spectrum with greater accelerations.
This effectively causes a dynamic uncoupling of the building in relation to
the ground ("filter" effect), so as to reduce transmission of the energy
supplied by the seismic action to the superstructure. As a consequence of this
last, the foundation-isolators-structure system can dissipate the seismic
energy of the ground: the dissipation is almost exclusively concentrated in
the isolation devices, which dissipate the seismic energy transmitted to them
from the foundations at the expense of large plastic deformations, through
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wide hysteresis cycles. This allows the superstructure to have a response
practically in elastic field by remaining almost immobile compared to the
motion of the ground. This considerably changes the seismic input, since, by
reducing the accelerations transmitted to the building, the response capacity
of the structure to the ultimate collapse strength and to the extreme state of
damage is considerably raised.
In addition to protecting the load-bearing structure, these devices also allow
the non-structural parts and all it contains to be protected. In fact, as a
consequence of the almost total absence of intermediate landing deformations
(drift), this technology allows the prevention of cracks or damage to infills,
partition walls, installations/systems or to goods inside buildings, such as
museums or libraries, data processing centres, etc. This allows the damage
caused to the structures by an earthquake to be minimized or completely
eliminated, so maintaining unchanged the activity carried on in it, even after
the occurrence of a severe telluric event.
During an earthquake, the isolated structure behaves almost like a rigid body
that tends to remain still compared to the vibrations of the ground.
Using seismic isolators, a structure is designed that remains in elastic field
even during the most violent earthquakes and keeps intact the energy
dissipative capacity given by ductility.
Currently, in the field of seismic engineering, there are three categories of
isolators and various types for each category.
Isolators made of elastomeric material and steel are made up of layers of
elastomeric material (natural rubber or suitable artificial materials)
alternated
with steel plates, having the predominant function of confining the elastomer,
and are arranged in the structure so as to withstand the rated horizontal
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actions and deformations through actions parallel to the position of the
layers
and of the vertical loads through actions perpendicular to the layers.
They are usually of circular design, but can also be made with square or
rectangular section. They are characterized by reduced horizontal stiffness,
high vertical stiffness and appropriate dissipative capacity.
Elasto-plastic isolators are made up of elements that stay elastic when there
are just vertical loads but plasticize when there are horizontal actions
higher
than a set threshold.
Thanks to their high dissipation capacity, elasto-plastic isolators have the
task
of limiting the transfer of stresses to the substructures, and so guarantee a
better response of the entire construction to a seismic event.
Sliding or rolling isolators, made up respectively of steel and Teflon
supports and of supports on roller or spheres, are all characterized by low
friction resistance values. Therefore, whereas for elasto-plastic isolators
and
for those made of elastomeric material and steel, the damping needed to
contain the relative movements of the two separate structures is ensured by
the strongly hysteretic behaviour of the material with which they are made,
for sliding isolators and for rolling isolators, it is necessary to place
suitable
energy dissipators in parallel.
The dynamics of these types of isolators is complex, since the sliding process
is inherently non-linear.
Some dissipators, called viscoelastic dissipators, exploit the viscous
behaviour of materials such as plastics, mineral oils and silicone. Other
dissipators, called elasto-plastic dissipators, exploit the plasticization of
metallic materials to dissipate energy in hysteresis cycles. Finally, so-
called
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friction dissipators exploit the friction between suitably treated metal
surfaces
that slide against each other.
In addition to the aging of elastomers (rubbers) and thermoplastic polymers
(Teflon), the physical and chemical properties of the adhesives used to glue
the steel sheets to the rubber, as well as those of the linear chain
organosilicon polymers (silicone oils and greases) used in viscoelastic
dissipators, possibly arranged in parallel to sliding or rolling isolators,
are
also important for the purposes of durability.
Moreover, elasto-plastic isolators and those made of elastomeric material and
steel are particularly vulnerable in the event of fire and must be suitably
protected from such an eventuality or used together with devices that can
replace them if they are destroyed.
In the current art, there are also ENEA's aseismic marble bases for the Bronzi
di Riace.
These belong to the family of seismic isolators developed by ENEA for the
protection of delicate instruments. These are passive and/or semi-passive
non-invasive seismic protection devices made up of two superimposed blocks
of marble on the internal surfaces of which, in a specular way to the two
blocks, four bowls have been hollowed out whose geometry is a rotation
ellipsoid where four marble spheres are placed, which, with their rolling,
give
the requirements of large movements, low stiffness and low friction required
to maximize seismic isolation.
When there is an earthquake, it will be the part beneath the base to be
subjected to the seismic action and this will be able to move with the ground
without transmitting the stresses to the upper part, since they are completely
absorbed by the movement of the spheres inside the cavities hollowed out in
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the marble. The movement of the spheres makes the protection system not
very rigid with very low friction, characteristics that minimize the stresses
of
the earthquake or render them almost null. The new aseismic marble base is
particularly suitable for statues vertically developed that have a very small
base and are therefore particularly vulnerable to horizontal seismic actions,
which can compromise their balance and cause them to tip over.
This type of seismic isolation cannot be used for houses and flats since the
materials used for making this device, if used for large areas, become very
expensive in terms of both raw material and installation, and therefore the
use
of these aseismic bases with marble spheres is limited to works of art.
The different types of seismic dissipators or isolators on the market are very
costly and made with a highly specialized technology, and even their
installation cannot be carried out by an ordinary construction firm.
The rolling mechanical isolators described in patent "MICALI" No. 1146596
are made entirely of steel or other suitable rigid material and each made up
of
a pair of circular concave elements with a sphere interposed of diameter not
less than the sum of the heights of the two concavities. According to this
patent, by setting the concave elements in two reinforced concrete beds or
net-like structures with one resting directly on the ground and the other on
the spheres, only the lower bed or net-like structure is forced to undergo any
horizontal seismic movements of the ground whereas the upper one, thanks to
the rolling of the spheres beneath, can follow the calm inertia of the
building
and stay nearly immobile because it is forced only to undergo brief upward
traverses due to the momentary movement of the lower concave elements
compared to the upper ones set in it.
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The limit of these rolling mechanical isolators regards the compressive
strength of the spheres because of the small contact with the relevant concave
rolling seats and the consequent need for large numbers or large dimensions.
WO 99/07966 discloses a friction ball, made of either plastically deformable
homogeneous material (lead, aluminium, brass, iron, steel, etc.) or
elastomeric material, which is deformed when it supports a weight; said
deformation generating a frictional force which resists rolling motion in the
deformed ball.
The limit of these friction balls regards both the expensive costs of the
materials they are made of and the low durability over time in terms of
resistance thereof.
Disclosure of invention
It is an object of the present invention to make a seismic isolator made up of
raw materials that are easy to find on the market, that have very low costs
but
have technical characteristics, in terms of density and strength, that allow
an
identical response for each event to be had in case of repeated earthquakes,
and also involve no maintenance costs, since the components do not need to
be replaced. Moreover, the isolator will form an excellent insulation from
rising damp given the quality of aggregate of the sintered alumina spheres.
It is another object of the present invention to make a seismic isolator that
can withstand a multidirectional seismic input so that, in the event of an
earthquake, there can be movements of the building independent from those
of the ground on which it is built, so absorbing and isolating the seismic
wave and therefore reducing the effects on the structures until, in theory,
they
cancel them.
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It is a further object of the present invention to make a seismic isolator
that
can avoid transmitting the induced seismic forces on the building, so giving a
reduction in structural dimensioning and, at the same time, maintenance of
the functionality of the building.
These and other objects are obtained with the present invention that concerns
a panel or module to be used in new buildings, to be installed between a
reinforced concrete bed to be made on the ground and the foundation
structures of the building, such as for example a reinforced concrete beam or
foundation bed, so that, in the event of an earthquake, there can be
movements of the building independent from those of the ground on which it
is built, so absorbing and isolating the seismic wave and therefore reducing
the effects on the structures until, in theory, they cancel them.
Further features and advantages of the invention will be more readily
apparent from the description of a preferred, but not exclusive, embodiment
of the product that is the subject of the present patent application,
illustrated
by way of non-limiting example in the drawing units in which:
Fig. 1 shows a plan view and a cross-sectional view of a prefabricated panel
or module (1) made up of spheres (2), with a pre-set centre-to-centre distance
between them that changes depending on the point load (load acting on a
single point of the sphere) that it is wished make the sphere support, made of
sintered alumina, bound by variable low density substances, polyurethane
foams or polystyrene or other similar material (3);
Fig. 2 is a cross-section:
= of the prefabricated panel or module (1) made up of spheres (2) made of
sintered alumina and bound by variable low density substances,
polyurethane foams or polystyrene or other similar material (3);
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= of the reinforced concrete bed (4) of the foundation resting the ground
(5)
and bounded on the upper part by the "isolation interface", by "isolation
interface" meaning the separation surface on which the isolation system is
active;
= of the reinforced concrete beam or foundation bed (6);
= of the building (7) with column (8).
Figures 3 and 4 show an alternative embodiment of the prefabricated panel or
module (1) in which:
Fig. 3 shows a plan view and a cross-sectional view of a prefabricated panel
or module (1) made up of spheres (2), whose movement capacity is localized
inside a circular area (9) and having a pre-set centre-to-centre distance
between them that changes depending on the point load (load acting on a
single point of the sphere) that it is wished make the sphere support, made of
sintered alumina, bound by variable low density substances, polyurethane
foams or polystyrene or other similar material (3);
Fig. 4 is a cross-section:
= of the prefabricated panel or module (1) made up of spheres (2) made of
sintered alumina, with capacity to move inside a localized area (9) and
bound by variable low density substances, polyurethane foams or
polystyrene or other similar material (3);
= of the reinforced concrete bed (4) of the foundation resting the ground
(5)
and bounded on the upper part by the "isolation interface", with "isolation
interface" meaning the separation surface on which the isolation system is
active;
= of the reinforced concrete beam or foundation bed (6);
= of the building (7) with column (8).
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Detailed description of invention
In the description below, the following terms will be used, of which the
definition is given:
-
"substructure" or "first foundation", the part of the structure situated
below the interface of the isolation system and that includes the
foundations, generally having negligible horizontal deformability and
directly subject to the movements imposed by the seismic movement
of the ground;
- "superstructure" or "second foundation", the part of the structure
situated above the isolation interface, and therefore isolated.
The polyurethane or polystyrene or other similar material (3) of the
prefabricated module (1) is used to support the concrete casting of the
superstructure in the first 28 days of curing of the said concrete.
The sintered alumina with which the spheres (2) of the prefabricated module
(1) are made is a ceramic material resulting from the sintering of alumina, a
substance present in bauxite, consisting of a thermal and mechanical process
through which the powdered materials are reduced to a compact mass of a
given shape; it combines the advantages of aluminium alloys and of powder
metallurgy.
The sintered alumina spheres are characterized by very high hardness and
compressive strength and therefore high resistance to axial loads, such that
laboratory tests show that a sintered alumina sphere, about 5 cm in diameter,
subjected to a vertical axial load of 9,000 kg, does not show any plastic
effect
on its contact surface.
The advantages of aluminium alloys are:
- low specific weight (about 2.7 g/cm3);
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- good corrosion resistance;
- remarkable mechanical properties;
- good wear resistance;
- good fatigue resistance;
the advantages of powder metallurgy are:
- low production costs;
- good control of tolerances without subsequent processing;
- possibility of obtaining complex shapes at limited cost.
The thickness of the prefabricated panel or module (1) is equal to the
diameter of the spheres (2), (example: 3-5-8 cm, etc.) with a variable surface
area that is suitable for transport, (example: 3.00 x 1.50 m, etc.) or below
standard sizes.
The installation of these prefabricated panels or modules (1) envisages that
they be placed touching each other on the horizontal plane, between the first
and second foundation.
The sintered alumina spheres (2) should be covered with a suitable additive
on the market, a silicone release agent, to ensure that the polyurethane or
polystyrene or other similar material (3) does not come into contact with the
spheres (2), since these must be allowed the possibility of rotating
independently from the structure made of binding material (3) that surrounds
them.
The possibility of multidirectional rotation of the spheres (2) in the event
of
an earthquake absorbs and isolates the horizontal oscillating motion of the
ground (5) without transmitting stresses to the superstructure of the building
(7), which, by inertia, will tend to maintain the position, so reducing the
well-
known disastrous effects.
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The binding material (3), of the sintered alumina spheres (2), at low and
variable density allows their controlled rotation. The prefabricated module
(1) does not undergo deformations during the earthquake, since the sintered
alumina spheres (2) have high compressive strength and are without plastic
consequences; consequently, the response, with dissipative-isolating effect,
will always be the same even during the next earthquake shock, without the
panel (1) components ever having to be replaced.
For the system of the foundations (6), both the intradosal ones of the
isolator
and the extradosal ones, normal strength concretes Rck 30 can be used, with
usual loads without plastic effects on the concrete due to the sintered
alumina
sphere (2) of the isolator (tests carried out in a laboratory). For particular
loads on the foundation (6), a foundation concrete with suitable strengths
will
be used.
The centre-to-centre distances between the sintered alumina spheres (2) may,
in particular cases, be adapted to the requirements of the weights above so as
to optimize the point load on the sphere (2).
According to a further embodiment of this prefabricated panel or module (1)
for seismic dissipation and isolation, the binding material (3) of the
sintered
alumina spheres (2) is shaped in such a way that each sphere (2) must move
inside a localized circular area (9) that delimits its possibility of movement
and, in the same way, allows its controlled rotation.
The materials and the dimensions of the above-described invention,
illustrated in the accompanying drawings and later claimed, may be varied
according to requirements. Moreover, all the details may be replaced by other
technically equivalent ones without for this reason straying from the
protective scope of the present invention patent application.
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