US20070044395A1

US20070044395A1 - Seismic isolator with variable curvature

Info

Publication number: US20070044395A1
Application number: US11/209,632
Authority: US
Inventors: Lyan-Ywan Lu; Ming-Hsiang Shih
Original assignee: Individual
Current assignee: National Kaohsiung First University of Science and Technology
Priority date: 2005-08-24
Filing date: 2005-08-24
Publication date: 2007-03-01

Abstract

A seismic isolator includes a slider, and a path with non-fixed curvature; the path is positioned on a base; the slider is placed on the path, and bears the weight of a super-structure; the path with non-fixed curvature can be a smoothly curved surface; a layer of ductile material with high compressibility is mounted between the slider and the path with non-fixed curvature; therefore, when an earthquake occurs, the slider will slide on the path with non-fixed curvature to produce vibration isolation effect for the superstructure; loading capacity of the isolator increases, and resonance between the isolator and low-frequency earthquakes is prevented.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a seismic isolator with variable curvature, more particularly one, which doesn't easily resonate with low-frequency earthquakes, and has increased loading capacity, and can function more effectively.
2. Brief Description of the Prior Art
When an earthquake occurs, the violent ground motion is the major reason that damages a structure. The violent ground motion is passed on from the base of the structure to the super-structure. In seismic isolation technology, special isolation bearings are installed between the base and the super-structure to reduce the amount of movement passed on from the ground to the super-structure, thus reducing the seismic force, to which the structure is subjected. Common seismic isolation bearings are roughly grouped into two types, namely, sliding bearings and elastomeric bearings.
Friction pendulum system (FPS) is the most widely used type of sliding bearings. Referring to FIG. 12, a sliding bearing 3 includes a three-dimensional curved plate 31, and a slider 32. The curved plate 31 is positioned on and fixed to a base 33. The slider 32 bears the weight of a super-structure. When an earthquake occurs, the slider 32 will slide back and forth on the curved surface of the curved plate 32. When the slider 32 slides away from the center of the curved plate 31, the weight of the super-structure will make the bearing produce a restoring force that causes the slider return to the center (lowermost point) of the curved surface, thus reducing the post-earthquake residual displacement of the isolation bearing. If friction is neglected, the forward and backward sliding motion of the slider 32 on the curved plate 31 is the same as the motion of a pendulum. For a FPS isolator, the isolation frequency only relates to the radius R of curvature of the curved surface, and it doesn't relate to the weight of the super-structure. Therefore, the isolation frequency of this kind of isolation system solely depends on the radius of curvature of the curved surface. Furthermore, the curvature of the friction surface of the slider 32 has to be the same as the curvature of the curved plate 31 in order for the slider 32 to closely contact with the curved surface of the curve plate 31, thus preventing stress concentration.
The above sliding bearing (FPS) has the sliding curved surface to provide restoring force, thus reducing the post-earthquake residual displacement of the bearing as well as reducing permanent location change of the structure. However, when the above sliding bearing is put to practical use, it is found that because the radius of curvature of the curved surface is fixed, there will be a constant isolation frequency, and resonance is very likely to happen if the predominant frequency of the earthquake approximates to the constant isolation frequency. FIG. 13 shows the frequency response function, a graph which shows the relationship between the maximum structural acceleration and the excitation frequency of the ground motion, of a fixed-base structure and of a sliding isolated structure. The horizontal axis is the excitation frequency of the ground motion, the vertical axis the maximum structural acceleration, the natural frequency of the structure 1.67 Hz, the isolation frequency 0.4 Hz, and the friction coefficient is 0.1. From FIG. 13, it can be seen that the sliding isolated structure will be subjected to less earthquake effect than the fixed-base structure when the excitation frequency approximates to the natural frequency (1.67 Hz). However, it can also be found that the structure with the sliding isolation system will be subjected to even greater earthquake effect than the fixed base when the excitation frequency is lower than 0.6 Hz. And, significant resonance will occur, when the excitation frequency approximates to the isolation frequency (0.4 Hz). Therefore, we can conclude that a conventional sliding isolation system can effectively reduce vibration only when the excitation frequency is higher than the isolation frequency (0.4 Hz); a conventional sliding isolation system will cause an adverse effect, when the earthquake contains low-frequency components. In other words, the vibration-reducing effect of a conventional sliding isolation system depends on the frequency content of earthquakes, and low-frequency resonance may happen. Furthermore, recent theories and experiments on seismic isolation technology have revealed that sliding isolation systems can't effectively reduce vibration in a near-fault earthquake because it is affected by the low-frequency pulse waves characterizing a near-fault earthquake. Although pulse waves can only exert a transient response of the structure, seismic isolated structures with a fixed frequency still could cause resonance, and this fact agrees with the phenomenon shown in FIG. 13.
The followings are a review of some current applications of sliding isolation technology:
Referring to FIG. 14, a conventional sliding isolation includes a curved sliding surface in contact with a slider, which has a spherically curved shape with the same fixed radius of curvature of the sliding surface. Therefore, the isolation frequency is a fixed value, and resonance with a low-frequency earthquake is likely to happen.
Referring to FIG. 15, a conventional two-directional rolling isolation system includes upper and lower layers. Each layer consists of four rollers or wheels, which roll on curved grooves. Because the rollers are in point-contact with the curved grooves, there can be material damage caused by stress concentration. And, the system has to be equipped with additional damping devices in order to prevent excessive bearing displacement.

SUMMARY OF THE INVENTION

It is the main objective of the invention to provide a seismic isolator with variable curvature to overcome the above-mentioned problems. The seismic isolator of the present invention includes a slider, and a path with non-fixed curvature. The path is placed on a base. The slider touches the path, and bears the weight of a super-structure. The path with non-fixed curvature can be a smoothly curved surface. And, a layer of ductile material with high compressibility is attached to the slider and placed between the slider and the path of non-fixed curvature. Therefore, when an earthquake occurs, the slider will slide on the path of non-fixed curvature to produce a seismic isolation effect for the super-structure. At the same time, this invention increases loading capacity of the isolator, and prevents resonance with low-frequency earthquakes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by referring to the accompanying drawings, wherein:
FIG. 1 is a side view of the first preferred embodiment of a seismic isolator according to the present invention,
FIG. 2 is a side view of the second preferred embodiment,
FIG. 3 is a side view of the third preferred embodiment,
FIG. 4 is a perspective view of the third embodiment,
FIG. 5 is a view showing the first way to join the ductile material and the slider,
FIG. 6 is a view showing the second way to join the ductile material and the slider,
FIG. 7 is a view showing the third way to join the ductile material and the slider,
FIG. 8 is a view showing the fourth way to join the ductile material and the slider,
FIG. 9 compares the frequency response curves of the invention and the conventional bearing,
FIG. 10 compares acceleration responses over a period of time under the effect of low-frequency pulse waves, for the present invention and the conventional isolator,
FIG. 11 is a graph showing the relationship between the shear and isolator displacement of the present invention, based on theoretical and experimental results,
FIG. 12 is a side view of the first conventional sliding bearing,
FIG. 13 compares the frequency response curves of the first conventional sliding bearing with that of a fixed base structure,
FIG. 14 is a side view of the second conventional sliding bearing, and
FIG. 15 is a side view of the third conventional sliding bearing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a first preferred embodiment 1 of a seismic isolator in the present invention includes a path with non-fixed curvature, a slider 11 for bearing the weight of a super-structure, e.g. building, bridge, common civil structure, facility, storage tank, and pipeline, and a layer of ductile material 12 with high compressibility and low friction coefficient. In the present embodiment, the path with non-fixed curvature can be a smoothly curved surface 10, which can be made of stainless steel, metallic materials, metallic compound materials, synthetic materials or fibrous materials. The layer of ductile material 12 is placed between the smoothly curved surface 10 and the slider 11, and it can be made of high polymer materials such as plastic steel, and UPE, etc. When the slider 11 moves to a certain point on the curved surface 10, the layer of ductile material 12 will change shape accordingly, so the slider 11 can closely contact the curved surface 10; thus, the bearing stress will be uniformly distributed and stress concentration of the parts of the seismic isolator can be prevented. The radius of curvature of the curved surface 10 varies with position of the slider 11. The slider 11 will move on the curved surface 10 with non-fixed curvature when an earthquake occurs. Because the radius of curvature of the curved surface 10 isn't a fixed value, the present seismic isolator doesn't have a fixed isolation frequency, and in turn the seismic isolator will not be resonant with low-frequency components of an earthquake.
The curved surface 10 can be a continuous mathematical function with single variable or two variables, and the first derivative of the continuous mathematical function can be a continuous or non-continuous one. The curved surface 10 is placed on a base 13, which can be a three-dimensional curved plate or a two-dimensional curved bar with a certain thickness. Assuming that the curved surface 10 is axially symmetric about y-axis, a cross-sectional curve of the curved surface 10 can be represented by the following function:
y=y(x) (1)
where y′(x) represents the elevation function of the curved plate. If the displacement of the slider 11 is x, the restoring force provided to the slider 11 by the curved surface, u_r(x), can be obtained by the following formula: $\begin{matrix} u_{r} (x) = W \frac{ⅆ y (x)}{ⅆ x} = {Wy}^{'} (x) & (2) \end{matrix}$
wherein y′(x) represents the first derivative of y(x) with respect to x, and W represents the weight of the super-structure. The above formula shows that u_r(x), the restoring force, is proportional to the first derivative of y(x), the elevation function of the curved plate. At the same time, the instantaneous restoring stiffness of the bearing, k_r(x), can be represented by the following formula: $\begin{matrix} k_{r} (x) = \frac{ⅆ u_{r} (x)}{ⅆ x} = {Wy}^{″} (x) & (3) \end{matrix}$
wherein yⁿ(x) is the second derivative of y(x) with respect to x. The above formula shows that the instantaneous restoring stiffness k_r(x), is proportional to the second derivative of the elevation function of the curved plate, y(x). And, instantaneous isolation frequency of the bearing, ω_b(x), can be derived from formula (3): $\begin{matrix} ω_{b} (x) = \sqrt{\frac{k_{r} (x)}{m}} = \sqrt{\frac{{Wy}^{″} (x)}{m}} = \sqrt{\frac{{mgy}^{″} (x)}{m}} = \sqrt{g} \sqrt{y^{″} (x)} & (4) \end{matrix}$
wherein m represents the mass of the super-structure above the isolation system, and g represents gravitational acceleration. The last formula shows that the instantaneous isolation frequency ω_b(X) is proportional to the square root of the second derivative of y(x). According to formulas (3) and (4), the restoring stiffness and the isolation frequency of the present seismic isolator do vary with x, the bearing displacement. Because there is no fixed isolation frequency, the present isolator can't be resonant with a low-frequency earthquake.
Referring to FIG. 2, a second preferred embodiment of a seismic isolator is provided, which includes a lower bearing member 13, a semispherical slider 11 supported on the lower bearing member 13, an upper bearing member 14, and a ductile material 12 is filled in between the semispherical slider 11 and the curved surface 10 on the lower bearing member 13. The upper bearing member 14 is joined to a lower side of a super-structure 2 by means of bolts 141, and it is formed with an articulated ball-shaped recess 142 on a lower side thereof. The lower bearing member 13 is joined to a structure base 15. The semispherical slider 11 is rotatably held in the articulated ball-shaped recess 142 of the upper bearing member 14. The ductile material 12 has high compressibility and low friction coefficient, and it can slide on the curved surface 10, whose radius of curvature isn't a fixed value, in order to produce isolation effect. The lower bearing member 13 can be a three-dimensional curved plate or a two-dimensional curved bar with a certain thickness; if the lower bearing member 13 is a three-dimensional curved plate, the curved surface 10 can be a mathematical function of two variables or a curved surface axially symmetric about a vertical axis.
Referring to FIGS. 3 and 4, a third preferred embodiment of a seismic isolator is provided, which includes a lower bearing member 13, a semispherical slider 11 supported on the lower bearing member 13, an upper bearing member 14, a bearing member 16, and a ductile material 12 sandwiched between the semispherical slider 11 and the curved surface 10 on the lower bearing member 13. The upper bearing member 14 is joined to a lower side of a super-structure 2 by means of bolts 141, and it has a screw hole 143 on a lower side. The bearing member 16 is formed with screw threads 161, and an articulated ball-shaped recess 163 on a lower end thereof. The bearing member 16 is placed on the semispherical slider 11 at the lower end, and it is joined to the lower side of the upper bearing member 14 with the screw threads 161 being threadedly engaged with the screw hole 143. A nut 162 is used to secure the bearing member 16 in position. Therefore, the height of the upper bearing member 14 can be adjusted by means of turning the bearing member 16 relative to the upper bearing member 14. The lower-bearing member 13 is joined to a structure base 15. The semispherical slider 11 is rotatably held in the articulated ball-shaped recess 163 of the bearing member 16. The ductile material 12 has high compressibility and low friction coefficient, and it can slide on the curved surface 10, whose radius of curvature isn't fixed, to produce isolation effect. The lower bearing member 13 can be a three-dimensional curved plate or a two-dimensional curved bar with a certain thickness; if the lower bearing member 13 is a three-dimensional curved plate, the curved plate 10 can be a mathematical function with two variables or a curved surface axially symmetric about a vertical axis.
Furthermore, in order to increase the shear strength of the ductile material 12, the ductile material 12 can be directly fitted in the slider 11, as shown in FIG. 5, or the slider 11 can be equipped with a shear key 111 in a middle portion, as shown in FIG. 6. Or alternatively, the slider 11 can be equipped with several shear keys 112 as shown in FIG. 7. Or alternatively, a confinement ring 113 is positioned around the slider 11 in an up and down movable manner to confine the ductile material 12, as shown in FIG. 8, such that compressibility and shear strength of the ductile material 12 can be increased.
FIG. 9 is a frequency response function, a graph that shows the relationship between the maximum structural acceleration and the excitation frequency of the ground. In FIG. 9, a comparison is made between the present invention and the conventional sliding isolation bearing, wherein the horizontal axis is the ground excitation frequency, the vertical axis the maximum structural acceleration, the natural frequency of the structure 1.67 Hz, the isolation frequency of the conventional isolation bearing 0.4 Hz, and the friction coefficient is 0.1. From FIG. 9, it can be easily seen that unlike the conventional isolation bearing, the present seismic isolator does not have a resonance frequency under a particular excitation frequency. In other words, low-frequency resonance is avoided in the present invention; meanwhile, the present invention can reduce vibration as effectively as the prior art in high frequency area. In addition, referring to FIG. 10, another comparison is made between the structural acceleration responses of the present invention and the prior art adopting the conventional sliding isolation bearing structure subjected to a near-fault earthquake with low-frequency pulse waves. From FIG. 10, it can be seen that the present invention has a much better vibration-reducing effect than the conventional bearing.
Furthermore, the present invention has been proven feasible through experiments. Referring to FIG. 11, which includes two graphs showing the relationship between the shear and the displacement of the invention (i.e., hysteresis loop of energy dissipation), based on theoretical and experimental results, respectively. Therefore, comparison can be made between theoretical values and experimental values, which are obtained by a cyclic test; the horizontal axis is the displacement of the seismic isolator, and the vertical axis is the horizontal shear force, which the seismic isolator is subjected to. The theoretical values shown in part (b) of FIG. 11 are obtained through using formula (2) shown above. It can be seen that the experimental values closely match the theoretical values, proving the feasibility of the present invention.
From the above description, it can be understood that when compared with the conventional seismic isolation system, the technology of the present invention has advantages as followings:
1. When the present invention is adopted, the isolation frequency and restoring stiffness vary with the displacement of the bearing. Therefore, there is not a fixed isolation frequency, and the resonance between the seismic isolator and a low-frequency earthquake can be prevented. In other words, isolation frequency varies with the isolator displacement as well as varying with earthquake intensity, and the present invention can function effectively in earthquakes with different seismic frequency contents, and won't resonate with earthquakes.
2. The sliding surface is neither single-point contact nor multiple-points contact therefore there won't be excessive stress concentration that may cause material yielding and damage. And loading capacity of the bearing can be increased.

Claims

1. A seismic isolator with variable curvature, comprising

a smoothly curved surface with non-fixed curvature positioned between a base and a super-structure;

a slider positioned between the base and the super-structure, and touching the smoothly curved surface; and

a layer of ductile material with high compressibility positioned between the smoothly curved surface and the slider;

whereby the slider will slide on the smoothly curved surface with non-fixed curvature to produce vibration isolation effect for the super-structure when an earthquake occurs.

2. A seismic isolator with variable curvature, comprising

a path with non-fixed curvature positioned between a base and a super-structure;

a slider positioned between the base and the super-structure, and touching the path; and

a layer of ductile material with high compressibility positioned between the path and the slider;

whereby the slider will slide on the path with non-fixed curvature to produce vibration isolation effect for the super-structure when an earthquake occurs.

3. The seismic isolator with variable curvature as claimed in claim 1, wherein the curved surface is positioned on the base, and the slider bears weight of the super-structure.

4. The seismic isolator with variable curvature as claimed in claim 1, wherein the curved surface bears weight of the super-structure, and the slider is positioned on the base.

5. The seismic isolator with variable curvature as claimed in claim 1, wherein the base is a three-dimensional curved plate.

6. The seismic isolator with variable curvature as claimed in claim 1, wherein the base is a two-dimensional curved bar with a certain thickness.

7. The seismic isolator with variable curvature as claimed in claim 1; wherein the smoothly curved surface is a continuous mathematical function.

8. The seismic isolator with variable curvature as claimed in claim 1, wherein the smoothly curved surface is axially symmetric about a central axis.

9. The seismic isolator with variable curvature as claimed in claim 1, wherein the slider works together with a bearing member adjustable in height, and it is connected to the super-structure with help of the bearing member.

10. The seismic isolator with variable curvature as claimed in claim 1, wherein the slider is semispherical, and it is received in an articulated ball-shaped recess to be connected with the super-structure.

11. The seismic isolator with variable curvature as claimed in claim 10, wherein the articulated ball-shaped recess is formed on a bearing member adjustable in height.

12. The seismic isolator with variable curvature as claimed in claim 1, wherein the ductile material is high polymer.

13. The seismic isolator with variable curvature as claimed in claim 1, wherein the ductile material is a metallic one.

14. The seismic isolator with variable curvature as claimed in claim 1, wherein the ductile material is fitted in the slider.

15. The seismic isolator with variable curvature as claimed in claim 1, wherein shear keys are formed on a joint between the ductile material and the slider.

16. The seismic isolator with variable curvature as claimed in claim 1, wherein a confinement ring is positioned around an edge of a joint between the ductile material and the slider.

17. The seismic isolator with variable curvature as claimed in claim 1, wherein the super-structure is a civil engineering structure.

18. The seismic isolator with variable curvature as claimed in claim 1, wherein the super-structure is a facility.

19. The seismic isolator with variable curvature as claimed in claim 1, wherein the smoothly curved surface is a machined metallic material.

20. The seismic isolator with variable curvature as claimed in claim 1, wherein the smoothly curved surface is a synthetic material with high hardness and low friction coefficient.