INNOVATIVE TECHNIQUE TO CONSTRUCT A ROBUST DURABLE SEISMIC PROTECTIVE DEVICE

The invention is an innovative technique to construct a robust durable protection device for structures against dynamic loadings such as earthquakes, wind, or mechanical vibrations. The technique utilizes friction between a sliding material such as PTFE and Cementitious Material (CM) (not metal) to protect structures. The invention controls the surface roughness of the CM by employing a designed mix with small size to no aggregates and cast it in molds with different roughness to produce a friction coefficient ranging from 0.3% to 40%. The CM mix preferably has a ultimate strength higher than 3,000 psi. The CM includes, but is not limited to, polymer concrete, high performance concrete, geopolymer and ultra-high-performance concrete. The resulting sliding material and the specially molded CM can be used in several forms to protect structures against dynamic loading such as dampers or base isolators.

BACKGROUND OF THE INVENTION

The present invention relates to seismic protective devices and in particular to seismic isolation devices and damping devices for structures.

Dynamic forces, such as those result from earthquakes, winds, and mechanical vibrations, cause significant damage to unprotected structures. These structures include, but are not limited to, buildings, bridges, houses, hospitals, data centers, wharves, constructed facilities, nuclear plants, and critical infrastructure. The hazard is also extended to nonstructural components such as mechanical and electrical equipment, utilities, building contents, piping systems and architectural partitions. Among the most vulnerable structures are the substandard non-retrofitted ones. Such structures impose risks on human lives. Once structures are damaged, millions of dollars will be spent on either repairing, demolishing, or replacing them. Many lives will be affected or lost as a result. There is a human and economic cost to earthquakes and other natural disasters.

Seismic protective technologies such as base isolation, passive and semi-active damping devices, self-centering walls and frames, and other emerging technologies, help improve the damage-mitigation and post-earthquake functionality, predictably, resiliency and reliably. However, most of the prior-art patents for these technologies have been implemented on a limited scale in actual structures because of the prohibitive costs associated with their implementation. The implemented system in actual structures is often expensive and poses durability problems such as poor fire and corrosion resistance. Corrosion constitutes a major drawback since it significantly deteriorates the device's performance, and the structure becomes unprotected over time.

The current state of the art uses one of the following devices to protect the structures from earthquakes. These devices can be divided into two categories:

1) Base isolation:a) Elastomeric-based such as Lead Rubber Bearings (LRBs);b) Friction based such as friction pendulums, for example described in U.S. Pat. No. 8,371,075 and Published US Application No. 2006/0174555 for:I) Single friction pendulums;ii) Double friction pendulums; andiii) Triple friction pendulums.
2) Passive and semi active damping devices (for example in U.S. Pat. No. 7,774,966).

These devices use either friction or material damping to dissipate the energy exerted on a structure by seismic events, wind buffering or mechanical vibration. Frictional devices rely on the friction generated between sliding surfaces, one is made of metal such as stainless-steel, and one made of materials such as Polytetrafluoroethylene (PTFE) or Ultra-High Molecular Weight Polyethylene (UHMWPE) friction material. These devices are, however, expensive, complicated to manufacture and have durability issues as stated earlier. Devices which use material damping such as LRB, can only be used as base isolators and not necessarily as dampers. These also pose high fabrication cost while exhibiting the same durability issues. They are also vulnerable to fire and can be damaged quickly causing catastrophic failures of structures.

More than 90% of previous patents related to seismic protection isolators or dampers have not been put into practical use due to the substantial expense linked to their application. While many of these prior-art patents have demonstrated significant effectiveness in mitigating seismic forces on supported structures, their lack of cost-effectiveness and their durability problems have rendered them unused. A primary goal of the innovative method outlined below is to establish an economically viable, cost-effective, and durable earthquake protection system.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing a technique to construct a seismic protective device that addresses known problems. The present invention utilizes friction between a sliding material such as Polytetrafluoroethylene (PTFE) and non-metallic materials such as Cementitious Material (CM), not metal, to protect structures. The CM is a mix with small size to no aggregates and cast in special molds to produce a wide array of surface roughness that results in a wide array of friction coefficients ranging from 0.3% to 40%. The product is highly durable, fire resistant, low-cost and can be used in many configurations. The technique uses existing material in an innovative way to create a device, which utilizes friction to protect structures.

Experimental evaluations were conducted to determine and regulate/control the surface roughness of the CM. By manipulating the surface roughness of the CM a desired Coefficient of Friction (COF) between the CM and various sliding materials, such as PTFE, UHMWPE, among others is obtained. The COF between the CM and these materials is important as it determines the level of friction/damping generated by earthquake protection devices.

The CM mixture incorporated no aggregates or aggregates of small to medium sizes and is cast in specialized molds having specified roughness, facilitating the creation of CM surfaces with diverse roughness levels. This diversity in surface textures yielded a broad spectrum of friction coefficients, ranging from 0.3% to 40%. Experimental tests utilized molds fabricated from materials including plastic, wood, steel (with varying finishes), stainless steel (also with varying finishes), and plexiglass. The surface roughness of these molds was quantitatively assessed using a commercially available surface roughness tester, before pouring the CM. Subsequently, the CM is poured into these molds to create pads. After the CM material hardened, the surface roughness of the CM pad is measured employing the same device used for the molds.

The resultant surface roughness of the CM closely matches that of the molds, with a variance of +/−10%. Certain molds exhibited exceptionally smooth surfaces, and the CM successfully replicated this level of smoothness. Achieving such smooth surfaces on CM enables its application in earthquake protection devices as a replacement for steel or stainless-steel plates traditionally used with sliding materials to create damping effects.

The present invention provides a method for manufacturing concrete with different surface roughness. Since COF between different materials is dependent on the surface roughness of such materials, thus, through precise control over the CM surface roughness, it is possible to regulate the COF between the CM and sliding materials. Consequently, this allows for the design and adjustment of the damping effect generated by the device. Utilizing different molds for casting the CM can result in varied surface roughness, thereby enabling the customization of the damping effect during seismic events.

By facilitating variable damping through the interaction between the CM and sliding materials, these devices can be tailored for specific applications. For instance, they can serve as seismic base isolation devices, whereby a building or structure is mounted onto them, effectively isolating it from ground movements. Additionally, these devices may be employed as dampers along a building's height, providing supplementary damping through controlled friction.

A comparative study was conducted to evaluate the anticipated COF between the finalized CM pad and various sliding materials. This study employed COF data from established materials like stainless steel and polished stainless steel, tested against a spectrum of sliding materials including Verigin PTFE, woven PTFE, and UHMWPE. Surface roughness served as the parameter for gauging the expected COF between the measured CM pads and the aforementioned range of sliding materials. The findings from these assessments are provided in Table 1.

TABLE 1Expected COF based on the comparable studyCMMaterialSurfaceComparedSlidingExpectedMoldsRoughnessto CMMaterialCOF*Stainless0.4 μmStainlessVirgin PTFE5% to 20%Steel 2Bsteel 2BWoven PTFE3% to 12%Polished0.08 μmPolishedVirgin PTFE3% to 8%StainlessStainlessUHMWPE1% to 1.5%steel orsteelLubricated0.3% to 1.5%PlexiglassPTFE orLubricatedUHMWPE*A range is provided since COF depends on many factors such as axial stress applied and velocity.A COF of 40% is expected to be achieved if the mold used is wood with very high surface roughness.

DETAILED DESCRIPTION OF THE INVENTION

Where the terms “about” or “generally” are associated with an element of the invention, it is intended to describe a feature's appearance to the human eye or human perception, and not a precise measurement, or typically within 10 percent of a stated value.

The present invention is a seismic protective device and an innovative technique to construct a robust durable protective device. The technique uses existing material in an innovative way to create the protective device, which utilizes friction to protect structures. As described inFIG.1, the device consists of a sliding puck10supporting a protected structure11, a slider14, and a pad12made of a designed Cementitious Material (CM) mix. The sliding puck10and the slider14slides on the pad12. The slider14is preferably made up of material such as Polytetrafluoroethylene (PTFE) or Ultra-High Molecular Weight Polyethylene (UHMWPE). The pad12may, for example, be flat, spherical, cylindrical, or a concave surface made of CM instead of metal. Because minimizing the friction was a key performance factor for prior protective devices, the pad12was usually made of metal in the prior art. The present invention utilizes the pad12made from CM not metal.

This modification has two major advantages:1) significantly increases device durability and fire resistance; and2) reduces the cost of fabrication since it eliminates the cost of machining, welding, and painting the metallic surfaces. The technique controls the surface roughness of the CM, which is a fundamental characteristic of the device, by employing a designed CM mix with small size to no aggregates and cast in special molds to produce a wide array of surface roughness that results in a wide array of friction coefficients ranging from 0.3% to 40%. The CM mix preferably has ultimate strength higher than 3,000 psi to be able to withstand the weights of the structures. The CM includes, but is not limited to, polymer concrete, high performance concrete, geopolymer and ultra-high-performance concrete.

The resulting device can be used as either a damper or a base isolator to protect structures by dissipating the energy exerted on them by seismic events, wind buffering or mechanical vibrations, or by isolating the structure from the ground. It can be used in different configurations and in different locations in the structure as illustrated inFIGS.3A to6B.

FIG.3Ashows the sliding puck10and the slider14sliding over a flat surface13aof a CM pad12andFIG.3Bshows the sliding puck10and the slider14sliding over the flat surface13aof the CM pad12with stoppers16.

FIG.4Ashows the sliding puck10and the slider14sliding over a concave surface13bof the CM pad12bandFIG.4Bshows the sliding puck10and the slider14sliding over the concave surface13bthe CM pad12bwith stoppers16.

FIG.5Ashows the sliding puck10and slider14sliding over a double curved CM surfaces12aand12bhaving concave surfaces13aand13b. Depending on the mold type, material and cast technique, the surface13acould have the same roughness as the surface13bor the surface roughness can be different. Having higher roughness values for surface13bwill produce more damping and limit the deformations during strong earthquakes.

FIG.5Badds stoppers16to limit the movement of the sliding puck10and slider14and is otherwise similar toFIG.5A.

FIG.6Ashows the sliding puck10and a curved upper sliding puck10b, and sliders14,14b, and14c. The sliding puck10and slider14slide on the lower curved cementitious material surface13b. A curved upper sliding puck10band sliders14band14cslide between a curved top surface15of the sliding puck10and the upper concave surface13a. The concept is the same asFIG.5Abut with more rotational capacity by including more sliding surfaces for the sliding puck in between surface13aand surface13b.

FIG.6Badds the stoppers16to limit the movement of the sliding pucks10and10band sliders14,14band14cand is otherwise similar toFIG.6A.

FIG.7Ais an elevation view showing a diagonal configuration of the present invention showing damping devices40used within a structural system50comprised of beams30, diagonal braces32, columns34, and CM damping devices40of a building to produce damping and absorb the energy from dynamic loading like earthquakes,FIG.7Bis a top view of a damping device40, according to the present invention, andFIG.7Cis a cross-sectional view of the damping device40taken along line7C-7C ofFIG.7B, according to the present invention.

The CM damping device40is comprised of the CM pad12, the slider14, and the sliding puck10. The slider14is fixed to the sliding puck10and the CM pad12slides over the slider14and the sliding puck10. The slider14material may be PTFE or UHMWPE or other material.

FIG.8Ashows a horizontal configuration of the damping device40that may be used within the structural system50of a building to produce damping and absorb the energy from dynamic loading like earthquakes. The structural system50is comprised of beams30, braces32, columns34. The CM damping device40is shown inFIG.8Band a cross-sectional view of the damping device40taken along line8C-8C ofFIG.8Bis shown inFIG.8C. The CM/Concrete damping device40is comprised of CM pad12, the slider14, and the sliding puck10. The slider14is fixed to the sliding puck10, and CM pad12sliding over the slider14. The slider14material may be PTFE or UHMWPE or other material.

FIG.11shows actual test results for roughness coefficients for eight specimens using different molds (steel, stainless steel, plexiglass, plastic, and others).

The innovative technique produces a protective device. Below is a summary of the key characteristics and advantages over other solutions:

1) The slider14, the sliding puck, and the CM/Pad12can be used in different configurations:a) The slider10and pad12can be used as base isolators in a manner similar to metallic friction pendulums without the inherent problems of the metallic friction pendulums, as mentioned herein above (problems with durability, corrosion, fire, and high cost).b) The slider10and CM pad12can be used as dampers in a manner similar to metallic friction dampers without the inherent problems of the metallic friction dampers, as mentioned herein above (problems with durability, corrosion, fire, and high cost).

2) The CM pad12is cast using CM instead of metal thus reducing manufacturing and life cycle costs while improving its durability and fire resistance.

3) Surface roughness can be controlled to accommodate different levels of damping and lateral displacements. Thus, the slider10and CM pad12are applicable to resist a spectrum of dynamic loads such as weak to moderate to strong earthquakes. Surface roughness is achieved by changing the mold type and material and cast technique.

4) The slider10and CM pad12can be formed in different shapes, such as flat or curved to serve different functions and applications.

5) The slider10and CM pad12can have more than one friction surface to protect structures against different load intensities.

6) The slider10and CM pad12are easy to manufacture and do not require specialized equipment such as Computer Numeric Control (CNC) machines.

7) The slider10and CM pad12are inexpensive and can be mass-produced without significant investment. This will allow developing countries with significant seismic hazard to protect their buildings and lives at an affordable cost. The device claimed herein is also affordable for typical residential construction. This is particularly advantageous since many homeowners and developers elect not to seismically protect their structures due to the high cost of current protective devices.

8) The slider10and CM pad12offers high durability, low maintenance, extended service life, decreased life-cycle cost, not prone to corrosion, and high fire resistance.

The invention is tested and verified using different molds such as steel, stainless steel, plexiglass, plastic, and others. The final surface roughness of the CM varied significantly depending on the molds used (seeFIG.9). For example, the CM pads casted in smooth molds such as stainless steel or plexiglass showed low roughness coefficients (Ra=around 0.1 μm). While other molds such as steel or rough plastic gave higher roughness coefficients (Ra=0.5 to 2.7 μm). Therefore, by controlling the mold and the mix, different damping levels and displacements can be accommodated.