IL192598A - System and method for seismic isolation - Google Patents

Description

23813/08 1T D'ID ΠΪΠΙΠ ΏΏΊΏΏ SYSTEM AND METHOD FOR SEISMIC ISOLATION SYSTEM AND METHOD FOR SEISMIC ISOLATION Field of the Invention The present invention relates to the field of seismic isolation energy dissipation systems aimed at reducing the influence of externally induced motion on structures. More particularly, the invention relates to earthquake-proof structures owing their seismic resistance to special seismic isolation systems installed therein. The invention may also be used for insulating the vibrations of various objects, such as equipment pieces, resulting from dynamic loads.

Background of the invention The earthquake phenomenon presents kinetic energy in a wide spectrum of frequencies. Kinetic energy resulting from an earthquake travels outward from the epicenter, effecting movement of the ground in its path. For purposes of analysis, the kinetic energy may be considered as being comprised of horizontal and vertical components. In practice, existing multistory buildings have high ability to resist vertical forces but their ability to withstand horizontal forces is relatively low. Hence the horizontal component is more important for the design of seismically resistant multistory buildings.

Building constructions or structures have the foundations that are fixed to the ground. Consequently, energy presented during an earthquake is transmitted to the building foundation, and, hence, to the building itself, resulting in structural failure or significant damage caused to structural and non- structural elements and systems.

Many attempts in structural design and building construction have been made in order to improve the seismic response of structures. Seismic isolation is one of the known methods for allowing enhanced durability of structures during earthquakes. Generally, natural frequencies of most buildings having up to ten floors correspond to the most active frequencies of the earthquake. It yields resonance vibrations in the structure and significantly increases the forces acting in the structural elements. The main purpose of seismic isolation systems is to reduce the building's dominant natural frequency and to avoid resonance vibrations. In order to provide seismic resistance of structures, the following main requirements are put forth to seismic isolation systems: - the ability to reduce the structure's natural vibration frequency; - the ability to damp the vibrations that may result from seismic disturbances.

When both requirements are fulfilled, the response of the structure to the disturbing influence of the soil during an earthquake is considerably improved.

In addition, other important features should also be provided to seismic isolation systems, such as they should be durable, economical to manufacture, convenient to install in the structure, etc.

It is known that any earthquake-resistant structure with seismic isolation comprises a foundation, a superstructure, a load-bearing unit (for example, a load-bearing pillar) connected to the superstructure, and means providing horizontal displacement of the load-bearing unit relative to the foundation under seismic disturbances.

The consideration whether a seismic isolation system meets the above requirements depends, primarily, on the functioning principle and the design of the means provided for horizontal displacement of the load-bearing unit relative to the foundation.

According to US Patent 4,633,628, the means provided for horizontal displacement of the load-bearing unit relative to the foundation under seismic disturbances is a multi-layer metal-and-rubber support consisting of alternating raw rubber layers and thin steel plates. This support withstands considerable vertical loads simultaneously making possible horizontal displacement of the load-bearing unit and its return to the initial position due to the elasticity of raw rubber. However, this arrangement is not durable enough, lasting not more than 40 years, as the elasticity of raw rubber deteriorates in the course of time. Moreover, this arrangement is relatively expansive.

According to US Patent 4,974,378, the means provided for horizontal displacement of the load-bearing unit relative to the foundation under seismic disturbances comprises a bottom concave support rigidly coupled to the foundation, a ball or a cylinder resting upon this support, and the top concave member resting upon the said ball or cylinder and rigidly coupled to the load-bearing unit. In seismic disturbances the ball or the cylinder rolls between the concave support and the top concave member displacing the load-bearing unit against the foundation. This arrangement, like the previously mentioned one, provides for horizontal displacement of the load-bearing unit. However, this arrangement is more durable than the previous one but it is still expansive. Moreover, this arrangement requires high accuracy centering and testing of each isolator before installation in structures. An additional drawback of this arrangement is its low damping ability.

According to US Patent 6,115,972, the load-bearing unit is a load- bearing pillar, and the foundation bears the load-bearing unit. The means providing horizontal displacement of the load-bearing pillar relative to the foundation is a hinge unit in the form of a rod having spherical friction knuckle joints on its ends connecting the support unit to the load- bearing pillar and serving a hinged support for the load-bearing pillar. However, this arrangement is free of the aforementioned drawbacks but the spherical friction knuckle joints are complicated from the manufacturing viewpoint and have high cost. Energy dissipation in such arrangement is still low. Additionally, using spherical friction knuckle joints is problematic under relatively high vertical loads generally acting in columns of multistory buildings.

It is therefore an object of the present invention to provide an earthquake-resistant structure with a low natural vibration frequency and with enhanced ability of damping the vibrations in an earthquake.

It is another object of the present invention to provide an earthquake-resistant structure having a high load bearing capacity in vertical direction.

It is still another object of the present invention to provide a seismic isolation system having an enhanced durability.

It is a further object of the present invention to provide a simple seismic isolation system that may be manufactured directly on the building site with no need for testing before installation in a structure.

It is yet another object of the present invention to provide a seismic isolation system which is more economical and convenient to install within new buildings and for replacing columns with insufficient load bearing capacity required for providing desired seismic resistance in existing structures.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention The present invention relates to a seismic isolation system for reducing of structural vibrations of a structure under external dynamic loadings, including earthquakes, which comprises: a) an upper base affixed to a column at the underside of said structure; b) a lower base affixed to said column at the foundation of said structure; c) a first and a second "V" shaped elements positioned one into another with a turning of plane of said first "V" element by 90 degrees concerning plane of said second "V" shaped element around vertical axis, thus the narrow end of said first "V" shaped element is connected to the underside of said upper base and the narrow end of said second "V" shaped element is connected to upper surface of said lower base; and d) a chain for suspending said second "V" shaped element on said first "V" shaped element thereby forming a pendulum system which allows horizontal displacements of said structure relative to its foundation and energy dissipation effects. Preferably, the chain is a anchor chain.

According to an embodiment of the invention, the system further comprises a filling material for covering the free space between the upper and the lower bases.

The present invention further relates to a seismic isolation method for reducing of structural vibrations of a structure under external dynamic loadings, including earthquakes, comprising: a) revealing the original load bearing element of an existing column of said structure; b) adding reinforced concrete jacketing around the underside and upper side of the exposed area of said column, thereby creating an upper base and a lower base; c) adding one or more temporary reinforced columns between said upper and lower bases, wherein the total load carrying capacity of said temporary reinforced columns should not be less than the load carrying capability of said original load bearing element; d) cutting and eliminating said original load bearing element; e) installing a first "V" shaped element, in such a way that the narrow end of said first "V" shaped element is positioned adjacent to the underside surface of said upper base, while fixing the wide end of said first "V" shaped element to the upper side surface of said lower base, wherein said first "V" shaped element is provided with a chain hanged from the inner side of said narrow end, thus the first link of said chain is fixed to said inner side; f) temporary fixing the narrow end of said first "V" shaped element to the underside surface of said upper base; g) cutting and eliminating said temporary reinforced columns; h) installing a second "V" shaped element, in such a way that the narrow end of said second "V" shaped element is positioned adjacent to the upper side surface of said lower base while fixing the wide end of said second "V" shaped to the underside surface of said upper base and so that the first and a second "V" shaped elements positioned one into another with a turning of plane of said first "V" element by 90 degrees concerning plane of said second "V" shaped element around vertical axis; i) if required, adding links to said hanged chain, and connecting the lower most link of said hanged chain to the inner side of the narrow end of said second "V" shaped element; and j) lowering said first "V" shaped element by eliminating said temporary fixed elements, thereby generating a tension on said chain and forming a pendulum system which allows horizontal displacements and energy dissipation effects of said structure relative to its foundation, thus whenever a tension will be generated on said chain, a relatively small gap will remain between the narrow end of said first and second "v" shaped elements and said upper and lower bases respectively.

Brief Description of the Drawings In the drawings: - Fig. 1 schematically illustrates an existing column before replacement; Fig. 2 schematically illustrates the column after the replacement of the original load bearing element of the column with a seismic isolation system, according to one preferred embodiment of the invention; Fig. 3 schematically illustrates the appearance of the replaced column after filling; Fig. 4 schematically illustrates a closed view of the seismic isolation system inserted within a column, according to an embodiment of the present invention; Fig. 5 schematically illustrates connection of two toruses as a simulation model for chain links connection, according to an embodiment of the invention; Fig. 6a schematically illustrates a building with its original columns; Fig. 6b schematically illustrates the building of Fig. 6a after installation of the proposed seismic isolation units according to an embodiment of the invention; Figs 7-12 schematically illustrate the order of works for the replacement of existing column with the seismic isolation system, according to one embodiment of the invention; and Fig. 13 schematically illustrates the column provided with the seismic isolation system in a rest position, according to an embodiment of the invention.

Detailed Description of Preferred Embodiments The present invention proposes a system, which is interposed between a building structure and its foundation for supporting the building structure, decreasing the structures dominant frequency and providing supplemental damping for improving structural seismic response. While reference herein is made to a building structure, it is apparent that other forms of structures are equally applicable, such as bridges, tanks, machines, equipment or other objects subjected to seismic vibrations or other dynamic loads.

The present invention further discloses a method for replacing the existing columns in buildings, preferably with a soft story having low seismic resistance, by columns with the proposed seismic isolation system having sufficient load capacity for transmission of vertical static forces from the isolated structure to the foundation and the ability to withstand the horizontal seismic loading. The proposed columns may be used also in new structures.

According to one embodiment of the present invention, the replacement of the existing columns by columns with a seismic isolation system is carried out without using load lifting means, such as jacking system. Fig. 1 schematically illustrates an existing column 10 (i.e., the original column) with a load bearing element 14 before replacement. The isolated structure is schematically represented by structure 11 in Figs. 1, 2, 3, 7, 8, 9, 10, 11 and 12.

Fig. 2 schematically illustrates the column after the replacement of the original load bearing element 14 of the column with a seismic isolation system. According to one embodiment of the invention, the seismic isolation system comprises a rigid upper base 15, connected to the isolated structure and a rigid lower base 17 connected to the foundation. In this embodiment, the upper and lower bases 15 and 17 are made of concrete.

Load bearing elements, such as the two "V" shaped elements 20 and 21 are disposed between the upper base 15 and the lower base 17. Preferably, the two "V" shaped elements 20 and 21 are positioned with an angular shift of 90° with respect to each other, around the vertical axis.

According to an embodiment of the invention, the seismic isolation system represents a pendulum-like mechanism including a chain 13 (e.g., a ship anchor chain) that is secured by suitable means, such as anchor bolts, to the lower and upper "V" shaped elements 20 and 21. The loads bearing elements have the capability of transmitting vertical and horizontal loads from the structure 11 to its foundation 12. In addition thereto, the load bearing elements permit mutual horizontal displacement between the upper base 15 and the lower base 17. Hence, the load bearing elements allow mutual horizontal displacement between the structure 11 and its foundation 12.

The two "V" shaped elements 20 and 21 may be manufactured from standard steel beams. The anchor chain 13 is attached to each of the "V-shaped elements at their narrow ends. The two "V" shaped elements 20 and 21 are positioned with an angular shift of 90° with respect to each other, around the vertical axis. "V" shaped elements 20 and 21 are of equal height, which is shorter than the total height of the seismic isolation system (preferably 10 to 15 mm shorter). The seismic isolation system of the present invention allows mutual horizontal displacements between the two 'Ύ' shaped elements in a certain range that is obtained according to the design requirements.

As an example for a column with vertical load (W) equal to 100 tons, seismic zone factor (Z) equal to Z=0.2g, soil type coefficient S=1.2 and the length of the anchor chain 13 about 90 cm the maximum allowable horizontal displacement is about 14 cm and the design horizontal displacement is about 9.5 cm.

According to an embodiment of the present invention, the seismic isolation system can be covered with a relatively low strength material (e.g., low weight concrete) in such a way that such a material fills all the free space between the top and bottom surfaces 15 and 17 of the seismic isolation system, as shown by filling 31 in Fig. 3. Such filling material is used for: making the external view of the column with the proposed system homogenous like; - protecting steel structural elements of the seismic isolation system from influence of environment; - preventing foreign matter from entering into the load bearing elements, as foreign matter entering the load bearing elements tends to reduce the effectiveness or useful duration of the load bearing elements; and providing some critical value of design horizontal force, sufficient to prevent horizontal displacements due to low magnitude dynamic loadings like low magnitude earthquakes, slight wind, low magnitude sonic boom, etc.

As soon as the horizontal force will exceed a critical value, the filling material will collapse and the seismic isolation systems become free from the restraining elements and start to act, as will be described hereinafter. Of course, filling material should be restored within some time after earthquake.

The load bearing elements carry the vertical and horizontal forces. Due to the properties of the pendulum-like mechanism, there is a restoring force applied to the structure 11 when there is a horizontal displacement between the foundation 12 and the structure 11.

The upper link of the chain 13 is connected to the upper side of the lower "V" shaped element 20. The lower "V" shaped element 20 is rigidly connected to the upper side of the lower base 17. Similarly, the lower link of chain 13 is connected to the under side of the upper tcV" shaped element 21. The upper "V" shaped element 21 is rigidly connected to the upper base 15.

During an earthquake, the foundation 12 and the lower "V" shaped element will move horizontally in an arched path and the chain 13 will allow relative horizontal displacement between the isolated structure 11 and its foundation 12.

The anchor chain 13 connected between the "V" shaped elements 20 and 21 allows simultaneous movements of all the foundation 12 in any horizontal direction along an arched path. The anchor chain 13 connected between the "V" shaped elements 20 and 21 also yields effective damping of structural vibrations. The damping is obtained while adjacent links of chain 13 slide one against the other during horizontal displacement and friction forces are generated on the contact surfaces between adjacent links of chain 13. This sliding friction results in effective damping. Further details on links of the chain are described hereinafter with respect to Fig. 5.

Referring now to Fig. 4, according to an embodiment of the invention, the chain 13 connection to the "V" shaped elements 20 and 21 should satisfy the following requirements: - to carry all vertical and horizontal loads; - to have 3 degrees of freedom - i.e., rotation around any horizontal axis, passing joint; and - to represent a zone, where dissipation of energy is taking place (i.e., damping).

The following is a description of a three-dimensional joint created by connecting adjacent links of a loaded chain, such as the links of chain 13. The proposed joint is able to carry out very high loads and it is made using standard chains manufactured by the industry. The joint has certain damping properties illustrated by Fig. 5. Preferably, most effective damping can be achieved using ship anchor chains.

Fig. 5 illustrates ideal connection scheme of two chain links from the damping viewpoint. This scheme represents connection of two equal links each of which having a torus form (one torus is located into another and its section radius is equal to that of its hole), as shown. Theoretically, this connection has two rotational degrees of freedom that are implemented by overcoming the sliding friction. Initial rotation due to the tolerance is possible by overcoming the rolling friction and in order to increase the damping it is logical to have more strong tolerance requirements.

Actually, the chain links are deformed under tensile vertical loads. It changes the geometry of the links making it more close to ideal and as a result, damping is also closer to the idealized model.

Because the real connection has a certain tolerance and due to a combined action of all chain links, a third rotational degree of freedom of the system of the present invention is achieved.

Using anchor chains allows obtaining a three-dimensional joint connection. An additional important advantage of anchor chains is their high load bearing capacity (up to 1000 ton). Anchor chains are widely used in the industry and have high reliability. Each chain is tested by the manufacturer, and hence there is no need to test each column, (see ISO 1704:2008 - Ships and marine technology - Stud-link anchor chains).

Due to the above mentioned circumstances, the proposed solution has the following positive features, compared to the prototype: high load bearing capacity, significant damping characteristics, high reliability and the fact that it can be easily manufactured using regular elements used in the industry.

Fig. 6a illustrates a building 61 with the original columns 62, 63, 64 and 65 (i.e., before replacement). Building 61 is embedded with a system for providing a horizontal displacement between the structure 67 of building 61 and its foundation 66 (i.e., columns 62a, 63a, 64a and 65a after the replacement).

Replacement of the existing column with a seismic isolation system, according to an embodiment of the present invention, is described with reference to Figs. 7-12, as follows: - At the first step, uncovering of existing column 10 until revealing or exposing the load bearing elements 14 of that column (Fig. 7); - At the next step, executing reinforced concrete jacketing around the top and bottom surfaces of the exposed area of column 10, thereby creating an upper base 15 and a lower base 17. The top and bottom bases 15 and 17 are connected with one or more temporary reinforced columns, such as columns 74, 75 (Fig. 8). Of course, the total load carrying capacity of the temporary reinforced columns 74, 75 should not be less than the load carrying capability of the original load bearing element 14; - At the next step, cutting and elimination of the middle zone (i.e., the original load bearing element 14). Fig. 9 schematically illustrates the column 10 after cutting (i.e., disassembling) of the middle zone; - At the next step, installing the first "V" shaped element 20, preferably, in such a way that the narrow end of "V" shaped element 20 is positioned adjacent to the underside surface of the upper base 15, while the wide end of element 20 is fixed, by any suitable means, to the upper side surface of the lower base 17 (Fig. 10). The "V" shaped element includes one or more links of chain 13, wherein the first link of chain 13 is fixed to the inner side of the narrow end of element 20 (Fig. 10). At this step, the chain 13 is only fixed to element 20; - At the next step, temporary fixing the narrow end of the "V" shaped element 20 to the underside surface of upper base 15 by suitable means, such as anchor bolts; - At the next step, cutting and eliminating the temporary reinforced columns 74 and 75. Fig. 11 schematically illustrates column 10 after the elimination of the temporary reinforced columns 74 and 75; - At the next step, installing the second "V" shaped element 21 in such a way that the narrow end of "V" shaped element 21 is positioned adjacent to the upper side surface of the lower base 17 while the wide end of element 21 is fixed, by any suitable means, to the underside surface of the upper base 15 (Fig. 12). Preferably, the "V" shaped elements 20 and 21 are placed so that the angle between them would be 90 degrees; - At the next step, if required, adding additional links to the chain 13, and connecting the chain 13 between the first and second "V" shaped elements 20 and 21, thus the lower link of chain 13 is fixed to the inner side of the narrow end of second "V" shaped element 21; - At the next step, lowering the first "V" shaped element 20 with respect to the underside surface of the upper base 15 by removing the temporary fixed elements, thereby generating tension on chain 13. The tension is generated due to the load of the structure on this column. During external dynamic loading, such as earthquake, the foundations move, thereby causing chain 13 to move like a pendulum with an upper anchoring point. The chain 13 is connected between the two "V" shaped element 20 and 21 in such a way that a relatively small gap (preferably, about 20-30 mm) will remain between the narrow end of the second "V" shaped element 21 and the lower base 17 and between the narrow end of the first "V" shaped element 20 and the upper base 15 (see GAP in Fig. 13). Preferably, the total length of chain 13 is completed by a connecting shackle (not shown). According to this, the gap between element 20 and the upper base 15 will be formed just after lowering the first "V" shaped element 20 with respect to the underside surface of the upper base 15 by removing the temporary fixed elements; and - Optionally, covering by a light material 31, such as light weight concrete the exposed elements of the seismic isolation system as shown in Fig. 3.

Fig. 13 schematically illustrates the column provided with the seismic isolation system in a rest position, according to an embodiment of the invention. Chain 13 fully carries the load during rest state as well as during external dynamic loading, such as earthquake. Chain 13 is used for suspending the second "V" shaped element 21 on the first "V" shaped element 20, thereby forming a pendulum-like structure which allows horizontal displacements of the structure relative to its foundation and energy dissipation. Chain 13 connects between the two "V" shaped element 20 and 21 in such a way that a relatively small gap (preferably, about 20-30 mm) exists between the narrow ends of the first and second "V" shaped elements 20 and 21 and the upper and lower bases 15 and 17, respectively. Therefore, the total length of chain 13 during tension should allow these gaps to remain.

While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.

Claims (8)

1. # 23813/08 - 17 - V CLAIMS A seismic isolation system for reducing of structural vibrations of a structure under external dynamic loadings, including earthquakes, comprising: (a) an upper base affixed to a column at the underside of said structure; (b) a lower base affixed to said column at the foundation of said structure; (c) a first and a second "V" shaped elements positioned one into another with a turning of plane of said first "V" element by 90 degrees concerning plane of said second "V" shaped element around vertical axis, such that the narrow end of said first "V" shaped element is connected to the underside of said upper base and the narrow end of said second "V" shaped element is connected to upper surface of said lower base; and (d) a chain connected between the narrow ends of said "V" shaped elements for continuously carrying the load of said structure by suspending said second "V" shaped element on said first "V" shaped element thereby forming a pendulum-like hanged system with joints being the connection points, which allow horizontal displacements of said structure relative to its foundation and energy dissipation, in response to applied horizontal forces.

2. A system according to claim 1, in which the chain is an anchor chain.

3. A system according to claim 1, further comprising a filling material for covering the free space between the upper and the lower bases.

4. A seismic isolation method for reducing of structural vibrations of a structure under external dynamic loadings, including earthquakes, comprising: a) revealing the original load bearing element of an existing column of said structure; b) adding reinforced concrete jacketing around the underside and upper side of the exposed area of said column, thereby creating an upper base and a lower base; c) adding one or more temporary reinforced columns between said upper and lower bases, wherein the total load carrying capacity of said temporary reinforced columns should not be less than the load carrying capability of said original load bearing element; d) cutting and eliminating said original load bearing element; e) installing a first "V" shaped element, in such a way that the narrow end of said first "V" shaped element is positioned adjacent to the underside surface of said upper base, while fixing the wide end of said first "V" shaped element to the upper side surface of said lower base, wherein said first "V" shaped element is provided with a chain hanged from the inner side of said narrow end, thus the first link of said chain is fixed to said inner side; f) temporary fixing the narrow end of said first "V" shaped element to the underside surface of said upper base; g) cutting and eliminating said temporary reinforced columns; h) installing a second "V" shaped element, in such a way that the narrow end of said second "V" shaped element is positioned adjacent to the upper side surface of said lower base while fixing the wide end of said second "V" shaped to the underside surface of said upper base and so that the first and a second "V" shaped elements positioned one into another with a turning of plane of said first "V" element by 90 degrees concerning plane of said second "V" shaped element around vertical axis; 23813/08 - 19 - i) whenever required, adding links to said hanged chain, and connecting the lower most link of said hanged chain to the inner side of the narrow end of said second "V" shaped element; and j) lowering said first "V" shaped element by eliminating said temporary fixed elements, thereby generating a tension on said chain and forming a pendulum system which allows horizontal displacements and energy dissipation effects of said structure relative to its foundation, thus whenever a tension will be generated on said chain, a relatively small gap will remain between the narrow end of said first and second "v" shaped elements and said upper and lower bases respectively.

5. A method according to claim 4, wherein the chain is an anchor chain.

6. A method according to claim 4, further comprising covering, by a filling material, the free space between the upper and the lower bases.

7. A method according to claim 4, wherein the narrow end of the first "V" shaped element is temporary fixed to the upper base by anchor bolts.

8. A method according to claim 4, wherein the total length of the chain is obtained by a shackle.