Static structures (e.g. – buildings) – Means compensating earth-transmitted force
Reexamination Certificate
1998-06-11
2001-01-09
Kent, Christopher T. (Department: 3635)
Static structures (e.g., buildings)
Means compensating earth-transmitted force
C052S167300, C052S167800, C052S573100
Reexamination Certificate
active
06170202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of construction. More particularly, the invention relates to building structures having improved structural integrity by incorporating an adaptive control system using materials whose properties can be dynamically controlled.
2. Description of the Related Art
It is well known that an earthquake can cause serious damage to a building or other building structure. The extent of the damage depends on the vibration frequency and ground displacement or shaking. An earthquake can easily destroy a building if the earthquake frequencies coincide with one of the building's natural frequencies causing the building to resonate.
Past research shows that the number of natural frequencies in a building increase with the number of floors. When a new floor is added to the building, a new natural frequency, higher than the building's previous natural frequencies is created. This is not the only effect the new floor will have on the structure. This new frequency will shift the previous frequencies to lower values, thus making the first frequency of the structure lower. This effect makes the buildings susceptible to earthquakes of low frequencies. An example is the earthquake that occurred in Mexico City in the 1980's. This earthquake had a frequency around one to two hertz. All buildings of eight floors completely fell. Another one of this earthquake's characteristics is that even though it only registered 6 on the Richter Scale, it was accompanied by great earth displacement. Of course, multi-storied buildings are not the only building structures susceptible to earthquake damage. Recent earthquakes have destroyed one-story structures, bridges, and whole sections of elevated highways. The 1989 Loma Prieta earthquake in Northern California even threatened a sold-out baseball stadium during the World Series.
A classical tuned mass absorber was one of the first methods of vibration control applied to structures to prevent earthquake damage. When properly designed, a mass absorber will introduce an additional degree of freedom to the vibration system. This permanent change to the building will effectively cancel out the displacement of the other floors by shifting the natural frequencies of the structure. Although the system is usually used only in high rise buildings, the theory behind the operation of the absorber may be adapted to any structure.
The theory governing the operation of the mass absorber is relatively simple. The mass absorber introduces another degree of freedom to an existing system to reduce displacement at one of the structure's natural frequencies. It is a proven means of resonance prevention and displacement reduction, yet there are numerous shortcomings to this absorber system. Drawbacks of the mass absorber system include its permanent modification of a system's dynamic behavior upon its installation, its creation of another natural frequency, and its inability to adapt to changing environmental conditions.
Active Variable Stiffness (AVS) mechanisms are also used to control the vibration characteristics of a building in order to prevent resonance due to an earthquake motion, and to suppress the response of the building. The Kajima Corporation sells a type of AVS. An AVS system, as its name suggests, works by changing the stiffness of a structure. An increase in the stiffness of a building will result in an increment of the natural frequencies of the same. Thus, if the natural frequencies of a building increase, the tendency to get up to resonance in the building would be diminished.
Shape Memory Alloys (SMAs) are metallic materials that exhibit the ability to return to a previously defined shape or size when subjected to certain temperatures. An SMA has the ability to be trained for one, two, or several “memories.” Materials with One Way Memory, if plastically deformed at low temperature, will experience the recovery effect of its trained memory upon heating, or activation of the alloy. The material then will maintain this recovered shape as it cools to room temperature (25° C.). This recovery effect is also known as the Shape Memory Effect (SME). In materials trained for Two Way Memory, an SMA is trained for a specific memory at both high and low temperatures. After deformation at a low temperature and upon heating of the alloy, the material will recover the form it was trained for at high temperature. Then, as the SMA cools to room temperature (25° C.), it will switch from the high temperature memory to the shape given to it during low temperature training.
Shape Memory Effect (SME) was discovered in 1932 when Read and Chang made the first recorded observation of the shape memory transformation in a gold and cadmium (AuCd) alloy. Three decades later, Buehler, Wang, and co-workers discovered this memory behavior in a nickel-titanium (NiTi) alloy. It was later named Nitinol because of the alloy components, nickel and titanium, and for the place of its discovery, the U.S. Naval Ordinance Laboratory. Buehler, Wang, and co-workers were designing a material for the use in the nose cone of torpedoes. By accident, Buehler discovered the damping characteristic of Nitinol while he was transporting two bars of Nitinol, one cooled to room temperature, and the other one still warm from the casting furnace. He accidentally dropped the cooled bar on the concrete floor, and noticed that it landed with a dull thud, like a bar of lead. Curiosity caused him drop the warm bar, and it “rang with a bell-like quality.” Amazed, he cooled the warm bar and let it fall again onto the floor, and surprisingly obtained the same response as when the cooled bar was dropped before. Likewise, an accident led to the discovery of the SME, when at a conference, a member of Buehler's crew heated a piece of the Nitinol with a lighter and observed a change in shape.
When Nitinol was first discovered in the 1960's, SMA suddenly became an area of interest for vibrations applications, and other branches of science. Several types of SMAs such as CuZnAl, CuZnSn, CuZnSi, and FeMnSi have been discovered and studied since then. The research done on this material has made possible the increasing entrance of new SMA commercialized products into the markets everyday. Today, the scope of SMA application involves astronomy, energy resources, car industry, electronics, mechanics, medical equipment, and others.
Though several types of SMA have been studied, Nitinol is the most widely used of the alloys. Its approximate composition ranges from 49% to 51% of nickel (atomic weight). As with all SMA, Nitinol's SME occurs with changes in the material's temperature. At a low temperature, the Nitinol microstructure is based on a martensitic phase, while at high temperature an austenitic phase dominates the microstructure. Normally, the temperature at which the martensitic changes start, known as the transition temperature, ranges from −50° C. to 166° C. depending on the alloy composition. So, as the material is being heated at or above this transition temperature, Nitinol will start undergoing changes within its crystalline structure “reverting from the martensite to the austenite phase, and then to its parent phase.” These changes occurring in its microstructure are what allow the material to recover its original shape by showing the SME.
Nitinol can be trained for more than one memory. To train a specific memory to Nitinol, it should be fixed in the desired shape (parent shape), and then heated for 5 minutes at a temperature of 500° C. This high temperature causes the atoms to arrange themselves into the most compact and regular pattern possible, resulting in a rigid cubic arrangement known as the austenite phase. Once the austenite phase is reached in all the material, and with Nitinol still fixed in the desired shape, Nitinol is allowed then to cool to room temperature (25° C.). This procedure will program the material for a one way memory. A similar procedure must be followed to progra
Davoodi Hamid
Just Frederick A.
Noori Mohammad N.
Saffar Ali
Greenblum & Bernstein P.L.C.
Kent Christopher T.
University of Puerto Rico
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