Seismic and vibration isolation system

Abstract

The invention relates to A method for protecting a structure from seismic ground motion and forced vibration comprising the following steps:

Description

FIELD OF THE INVENTION

[0001] The invention relates to improvements in the design, layout and manufacturing process of seismic and vibration isolation systems protecting structures from seismic ground motion or induced forced vibration. These devices are located between foundation and structure in the case of seismic isolation of structures (buildings) or simply between the prepared natural grade (or foundation) and the bottom plate of a liquid filled structure when used in petrochemical facilities, e.g. an oil tank. When used as vibration isolation devices, these systems will be placed underneath the vibrating machine or underneath a proper support in the case of subways and trains.

BACKGROUND OF THE INVENTION

[0002] It is well known that structures can be protected against earthquakes by flexible bearings, decoupling the supported structure from the ground motion so that vibrations are prevented from propagating into the structure.

[0003] Typical earthquake accelerations have dominant periods of about 0.1-1 seconds (1-10 Hz) with maximum severity often in the range between 0.2-0.6 seconds. Structures whose natural periods of vibrations lie within the range of 0.1-1 seconds are therefore particularly vulnerable to seismic ground motions because they may be subjected to high acceleration. The main feature of seismic base isolation is to increase the flexibility of the structure. A higher flexibility translates into an increase in the natural period of the structure beyond that of earthquakes, resonance and near-resonance are avoided and the seismic induced forces are reduced. The increased flexibility also affects the horizontal displacements of structures during seismic events. Excessive displacements may be balanced by introducing better and higher damping properties to the isolation system. Conventional seismic design solutions based on structural strengthening provide sufficient stiffness, deformability and energy-dissipating locations throughout the structure to withstand the forces and dissipate the energy generated by earthquakes. The peak acceleration in the structure is often greater than the peak acceleration of the driving ground motion. On the other hand, seismic base isolation limits the effect of earthquakes, since a flexible base decouples the structure from the horizontal ground motion and the structural response accelerations are usually less than the ground accelerations. The addition of other energy dissipating devices (viscous dampers) may further reduce the energy transmitted to the isolated structure.

[0004] For effective protection of the structure it is therefore necessary to use isolation methods to shift the natural frequency of the system consisting of the structure and the isolation means to a value below 1 Hz.

[0005] The most widely used system for seismic isolation is to place several elastomeric bearings on a foundation and build the structure on these bearings. The seismic isolation bearings most commonly used today are a multilayered laminated composite with alternating layers of rubber and steel plates. The steel plates provide a lateral constraint of the rubber when the bearing is subjected to vertical load, but they do not influence the horizontal shearing behaviour of the rubber bearing. Therefore it is possible to produce bearings, which are very stiff to vertical applied loads, but very flexible in horizontal direction so that the natural frequency of the building can be shifted appropriately.

[0006] An alternative design of such bearings is disclosed in U.S. Pat. No. 5,904,010 (Javid et al.). Javid shows an elastomeric bearing comprising a stacked series of elastomeric laminae forming a unit cell having a stacked height corresponding to an overall horizontal and vertical stiffness of the unit, the laminae being in a vulcanized adherent connection with each other; and wherein at least one of the laminae includes a series of pretensioned continuous fibers extending across opposite side edges of at least one laminae. Structures supported by such elastomeric elements are efficiently protected from the danger of earthquakes. However, it is a rather expensive way to use such elements since the costs of producing these elements are high and the structures have to be adapted in order to be connected to these elements.

[0007] Especially for structures like oil tanks or other ground supported liquid filled structures, which have in general a very thin bottom surface and are placed directly on the prepared natural grade without proper foundation, the use of seismic bearings is problematic. In fact, the use of discrete, relatively small sized seismic bearings would require the construction of a stiff support structure in order to transmit the uniform distributed load (pressure) from the bottom plate to a finite number of seismic bearings. Furthermore, mechanical connectors (e.g. bolts) are needed to permanently connect individual bearings to the tank or to a supporting structure. Similar connections are needed to fix the bottom plate of the bearings with a supporting concrete like rigid foundation.

SUMMARY OF THE INVENTION

[0008] The objective of the present invention is to provide an improved method to manufacture and build seismic protection systems for structures subjected to seismic ground motion. The present invention also allows improvements in the design, manufacture and efficiency of energy dissipating devices in order to minimize the transfer of forced vibration from manufacturing equipment, trains and subways to the surroundings. Improvements are given by:

[0009] Reduce the weight of the isolation system;

[0010] Reduce manufacturing costs;

[0011] Reduce transportation costs;

[0012] Reduce installation costs;

[0013] Increase flexibility in the design concepts.

[0014] The method of the invention comprises the following steps:

[0015] preparing a horizontal surface, which may be the properly modified natural grade in the case of liquid filled structures or a continuous foundation in case of building like structure;

[0016] providing flexible damping systems each consisting essentially of a layered composite of unreinforced isotropic rubber sheets and fiber-reinforced elastomeric slabs. These layers are stacked on top of each other in order to satisfy proper vertical and horizontal stiffness requirements;

[0017] the layered damping elements will cover the same surface areas as the supported liquid storage tank or the supported continuous foundation;

[0018] The supported structure is not physically connected to the damping elements, thus no anchorage system is required;

[0019] The individual layers comprising the damping element are not vulcanised or permanently connected. The generated friction force between them is sufficient to eliminate any slipping.

[0020] It is a significant aspect of this invention that there is no need to provide a large number of single isolation bearings with a distance one to the other, but to cover the substantially entire bearing surface with several damping layers arranged next to each other, if needed by cutting prefabricated slabs. In the same way additional layers are stacked on top of each other to reach the vertical and horizontal design stiffness. The structure is supported by the upper surface of the uppermost layer of these damping elements. Therefore it is possible to build for example an oil tank on the damping elements without providing for a thick concrete base or any other anchorage system. Since the damping layers are flat it is not necessary to connect the single rubber sheets and fiber reinforced composites by vulcanizing or glueing. It is a simple way to roll the damping elements off from a roll or coil and cutting them into the required dimension in situ. The fibers constrain the rubber in horizontal direction so that vertical stiffness is attained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows schematically a layered isolation system of the invention installed directly on the prepared natural grade for protecting an oil tank in an axonometric view;

[0022] FIG. 2 is a detail of an isolation system showing the method of production;

[0023] FIG. 3 is a view similar to FIG. 1 showing a building supported by an isolation system of the invention on top of a continuous solid foundations;

[0024] FIG. 4 shows an isolation system for bearing a rail track;

[0025] FIG. 5 shows a machine based upon an isolation system of the invention;

[0026] FIG. 6 is an explanation of the method of the invention in an axonometric view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] FIG. 1 shows the use and assembly of a typical layered isolation system 2 for the protection of liquid filled structures such as a cylindrical tank 1. During an earthquake the ground 3 will be subjected to strong ground motion and the layered isolation system will decouple the superstructure 1 in order to minimize damage.

[0028] This isolation system may be located directly between the prepared natural grade 3 and the bottom plate 4 of the tank 1. The isolation system 2 consists of several layers of unreinforced rubber slabs 5 and fiber-reinforced elastomeric mats 6 stacked on top of each other.

[0029] A large contact area exists between the tank bottom 4 and the uppermost rubber laminae 5, 6, as well as between individual layers 5, 6. In this particular case sufficient friction develops to ensure that no slipping takes place. There is no need to use any form of mechanical connector between the natural grade 3, the isolation system 2 and the bottom surface 4 of the superstructure 1. In order to increase the frictional contact, the uppermost elastomeric laminae 5, 6 may be connected to the superstructure 1 using a strong epoxy bond.

[0030] FIG. 2 shows that fiber direction may change between one layer to the next in case an anisotropic design of the isolation system is asked for. The layers 7a, 7b to assemble the final isolation system 2 consist of fiber-reinforced elastomeric mats 8 wherein each mat 8 consisting of a reinforcing layer 10 sandwiched between rubber layers 9.

[0031] FIG. 3 shows the use of the proposed layered isolation system located between a rigid continuous or non-continuous foundation 11 and a superstructure e.g. a building 12. A large contact area is necessary to ensure sufficient frictional forces to eliminate or minimize slipping. The elastomeric layers are reinforced by using fibers in both in-plane directions.

[0032] FIG. 4 shows the layered isolation system to reduce energy transferred to the ground from traffic-induced vibrations. FIG. 4 shows sleepers 13 of a train truck 14 mounted on a rigid concrete slab 15. The isolation system 2 is located between the rigid slab 15 and the natural grade 3, or between rigid slab 15 and a concrete foundation. Fibers in the elastomer slabs may have a preferred direction or be installed in order to have transverse isotropy.

[0033] Another application of the isolation system is depicted in FIG. 5. It shows the isolation of vibrating equipment, e.g. a machine 16 or any other induced forced vibration by installing the isolation system 2 between the bottom surface 17 of the equipment and the rigid foundation, e.g. the floor of a manufacturing hall.

[0034] Further FIG. 6 shows a schematic representation of the in-situ assembly of the isolation system. Individual reinforced layers each consisting of an unreinforced rubber slab 5 or a fiber-reinforced elastomeric mat 6 will be unrolled at the construction site and used to cover the area underneath the superstructure. The first layer of fiber-reinforced material may lie directly on the prepared natural grade or on a concrete-like foundation. The fiber-reinforced slabs may change direction in each layer in order to increase the friction contact. No vulcanization is needed to permanently connect individual fiber-reinforced slabs, in special cases a strong epoxy bond may be used. In general, due to the large contact area and due to the permanent vertical load, sufficient friction will exist to ensure that no sliding takes place.

Claims

1. A method for protecting a structure from seismic ground motion and forced vibration comprising the following steps:

preparing a horizontal bearing surface or a solid foundation on top of which the isolation system will be placed;

providing flexible damping elements of a group consisting of unreinforced rubber slabs and fiber-reinforced elastomeric mats;

each mat consisting essentially of a piece of a rubber mat reinforced with fibers extending parallel to the surfaces of the mat;

covering the horizontal bearing surface or the solid foundation with several layers of damping elements, each layer consisting of a plurality of elongated pieces of the mat arranged side by side;

erecting the structure by putting the lower surface of the structure directly upon the upper surface of an uppermost layer of damping elements.

2. A method of claim 1, wherein the layers of unreinforced rubber slabs and fiber-reinforced elastomeric mats are not connected to each other.

3. A method of claim 1, wherein damping elements of one layer are not mechanically or chemically connected to damping elements of an adjacent layer.

4. A method of claim 1, wherein the damping elements are obtained by rolling off from a roll or coil and cutting them to the required dimension in situ.

5. A method of claim 1, wherein between two layers of unreinforced rubber slabs at least one fiber-reinforced elastomeric mat is provided.

6. A method of claim 1, wherein several fiber-reinforced elastomeric mats are stacked directly on top of each other.

7. An elastomeric seismic isolation system for bearing a structure supported by a foundation comprising:

a first layer of flexible damping elements each consisting essentially of a piece of a rubber slab reinforced with fibers extending parallel to the surfaces of the mat disposed on a horizontal bearing surface on the top of the foundation or a prepared natural grade;

at least one intermediate layer of flexible damping elements each consisting essentially of a piece of a rubber mat reinforced with fibers extending parallel to the surfaces of the mat disposed on an upper surface of a previous layer of flexible damping elements;

an uppermost layer of flexible damping elements each consisting essentially of a piece of a rubber mat reinforced with fibers extending parallel to the surfaces of the mat disposed on an upper surface of a previous layer of flexible damping elements and disposed for bearing a lower surface of the structure.

8. An elastomeric seismic isolation system of claim 7, wherein damping elements of one layer are not interconnected one to the other.

9. An elastomeric seismic isolation system of claim 7, wherein damping elements of one layer are not connected to damping elements of an adjacent layer.

10. An elastomeric seismic isolation system of claim 7, wherein the structure is an oil tank supported directly by the prepared natural grade without concrete foundations.

11. An elastomeric seismic isolation system of claim 7, wherein each layer covers an area, which is essentially equal to an area of the lower surface of the structure.

12. An elastomeric seismic isolation system of claim 7, wherein the damping elements have an essentially longitudinal form with a longitudinal axis and that the longitudinal axis of the damping elements of one layer are parallel one to the other.

13. An elastomeric seismic isolation system of claim 12, wherein the longitudinal axis of the damping elements of at least two adjacent layers are oriented in angle.

14. An elastomeric seismic isolation system of claim 13, wherein said angle is within a range between 45° and 90°.

15. An elastomeric seismic isolation system of claim 7, wherein between two layers of unreinforced rubber slabs at least one fiber-reinforced elastomeric mat is provided.

16. An elastomeric seismic isolation system of claim 7, wherein several fiber-reinforced elastomeric mats are stacked directly on top of each other.