Throughout history man has attempted to manipulate and modify his environment to suit his needs and desires. He has bore through mountains and bridged waters. However, despite major technological advancements, man has yet to tame the unpredictable and devastating forces of Mother Nature. Instead, man has been reduced to adapting to these natural disasters. Many precautionary measures have been adopted in order to prevent and protect such occurrences from disrupting the essential elements of man’s established social infrastructure, such as energy, water, health, and government. Currently, the most effective structural control available comes in the form of semi-active controls.
Civil engineers have devised multiple methods aimed toward stabilizing and controlling buildings during times of stress. In earlier years, engineers practiced a philosophy that emphasized strengthening the overall structure thereby enabling it to withstand shaking. Unbeknown at the time, this design did not permit escape of the vibration and instead prompted greater resonance. (Shimz 2002)
Techniques have since been developed that absorb and dissipate seismic energy allowing most any structural design to endure shaking. Such a system was included in the construction of an early Japanese pagoda. The first documented use of structural control occurred in the 1800s. John Milne, a professor at the Imperial Engineering College in Tokyo, Japan built a structure on irregularly shaped ball bearings and in doing so both isolated the structure from the ground and provided minimal damping. (Chen 19-2)
Many advances have since been made in the area of structural control. Presently, there are four structural control concepts: passive, active, hybrid, and semiactive. This paper will discuss the new technology incorporated in these systems.
Passive control systems are systems of structural control whose “characteristic is determined once by an appropriate design method before the devices are in place” (Chen 19-7). In other words, after installation these devices require no sensors, actuators, or controllers to be effective. However, once in position, there is no way to adapt the system to meet new challenges other than replacing the devices. Therefore, these systems are relatively easy to maintain. The energy required for passive systems to create the control forces is produced, not by external influences, but rather “through the motion of the mechanism during dynamic excitement” (Chen 19-5). The oscillations of the mechanism directly affect the amplitude and direction of the controlling force. Although there are numerous types of passive control systems, four main systems are most popular: tuned mass damper (TMD) and tuned liquid damper (TLD), yielding metal dampers, base isolation, and viscoelastic and fluid viscous dampers.
Tuned mass dampers work to counterbalance the vibrations of a structure. Although there are many different setups, most TMDs involve a mass, spring, and viscous damper (Chen 19-7). Basically, as lateral movement is introduced, the TMD is provoked and filters the kinetic energy of the vibrations to the viscous damper where it is absorbed. The TLD is similarly set up and the same effect is achieved. TMDs and TLDs are commonly used in concrete structures. Although reliable, these systems require long periods of vibration before they can reach their full potential and effectively counteract the seismic vibrations. It is suggested that such systems be utilized in an area where long periods of vibrations are common, instead of quick bursts of seismic activity.
Another type of passive structure control system is yielding metal dampers. “Inelastic hysteretic behavior of steel and lead elements absorbs the [seismic] energy, reducing the structural vibration (Chen 19-8). Hysteresis is “the lagging of an effect behind its cause” (Dictionary.com). In other words, because steel and lead are slow to succumb to seismic convulsions, these materials are the metals that are most frequently used in yielding metal damper structural control systems. The arrangement of this system is comparable to that of a carjack. The building rests upon a flat layer of metal that is supported by multiple metal rods. The metal, through the process of plasticization, absorbs the vibrations. Often, rubber is added to the setup to create an even more absorbent system. Due to the stiffness of the materials, yielding metal damper systems are most effective when introduced to large earthquake motions (Chen 19-8).
Base isolation (BI) systems rest between the foundation and the superstructure of a building. The device allows for movement of the foundation with minimal transfer of seismic energy to the supported structure. One method of base isolation requires the installation of bearings with low horizontal stiffness, but high vertical stiffness between the structure and its foundation. Due to the low horizontal stiffness of the device, “the natural period of the structure will be significantly lengthened and shifted away form the dominant high frequency range of the earthquakes” (Chen 17-2). The perpendicular rigidity still allows for the support of a significant vertical load, namely the building. Elastomeric bearings are comprised of thin layers of rubber alternating with steel plates that move horizontally quite easily, thus absorbing earthquake vibration. [They] are rigid in the perpendicular direction because they support the weight of the building ( Shimizu). To gain additional dampening, a lead center is occasionally added to the setup. It is interesting to note that even if the lead plugs become seriously deformed during the seismic activity, they eventually revert back to their original shape, thereby sustaining their effectiveness ( Shimizu). In addition the central lead plug, hydraulic dampers, steel coils, and viscous dampers also have been introduced into the system as supplemental dampeners to further isolate the structure (Chen 17-2).
More recent technology in passive structural control has been established in the field of frictional dampers. Frictional dampers use the natural force of friction to absorb the seismic shaking. The frictional pendulum system (FPS) is the most popular frictional dampening system because it utilizes concaved surfaces to prompt the recentering of the structure. Installed between a structure and its foundation, these bearings isolate the structure from the more dramatic movements by lengthening the structure’s natural period. “ When activated by an earthquake, [a] articulated slider moves along [a] concave surface, causing the supported structure to move with small pendulum motions. The dynamic friction force generated provides the required damping to absorb the energy of the earthquake. Consequently, lateral loads and shaking movements transmitted to the structure are greatly reduced” (EPS). To ensure the supported structure returns to its original position high-tension springs and elastomeric bearings are occasionally used to stimulate the restoring force. These pendulum systems can be customized to support structures within close proximity to fault lines.
The final type of passive structural support system that will be discussed is viscoelastic and fluid viscous dampers. A common viscoelastic damper consists of layers of viscoelastic material alternatively bonded to steel plates. “Traditionally, viscoelastic dampers have been used exclusively in steel and reinforced concrete structures; however, there have been some recent studies demonstrating the feasibility of using viscoelastic dampers in wood frame structures” (Lewicki). Fluid viscous dampers work similarly to an automobile shock absorber. A piston transmits the seismic energy to the fluid in the damper, causing the fluid to move inside the damper. This movement absorbs the kinetic energy by converting it to heat (Constantinou). Such systems have proven to be an affordable and effective solution for both steel and reinforced concrete structures during seismic activity.
Active control systems rely on many of the same fundamental principles that make passive structural control effective; however, an outside algorithm can alter the characteristics of active control systems. This is advantageous in that systems may be updated and manipulated to meet new criteria without replacing the entire structural control device. These systems use tremendous power to constantly push on the structure, neutralizing any disturbances. Active control systems can be grouped into two categories: active mass dampers (AMD) and active tendon and bracing systems (Chen 19-8).
The AMD system uses sensors to relay the vibration intensity to a computer. The computer then calculates the optimal vibration control power. This information prompts the repositioning of an auxiliary mass with an actuator, which thereby reduces the seismic effect on the structure (Takenaka Corporation). Active mass damper systems are designed to counteract small to moderate earthquakes (Chen 19-8).
“The most direct way to control vibrations is through active tendons or active braces installed in the structure” (Chen 19-9). Because tendons and braces can directly connect to the main infrastructure, these systems provide immediate counteraction to seismic forces. To be constantly effective, a connected computer continually adjusts the stiffness and elasticity of the support systems to meet the demands of seismic activity.
Both passive and active structural control systems have their disadvantages. Passive systems have limited responsiveness and active systems require a great amount of power in order to be effective. The latest structural control systems combine aspects of both active and passive control devices, thereby accenting each system’s advantages and eliminating any weaknesses in order to maximize control. Although more intricate in design, these hybrid systems have become more reliable than most active systems.
One type of hybrid system, active tuned mass dampers (ATMD), unite the strengths of tuned mass dampers and active mass dampers. The vibration resistant characteristics of a passive tuned mass damper are actively controlled to provide a greater control during more intense vibrations. An ATMD that incorporates characteristics of a pendulum has relatively recently been developed. “When sensors detect sway vibration, the computer-controlled servomotors drive ball screws to position the damper mass” (Chen 19-10). Another company, Fujita has created an ATMD system that has the capability of operating in both active and passive modes. The active mode controls the efficiency actuator, however, when the displacement becomes larger than the actuator can accommodate, the system reverts to strictly passive mode (Chen 19-10).
The final type of structural control is semiactive control. Like the hybrid control systems, semiactive systems utilize the reliability of passive control systems and the versatility of active control systems. Contrastingly, semiactive systems require less power than both active and hybrid control systems. “External energy is used only for adjustment of the mechanical characteristics of the system” (Chen 19-6). Instead of pushing on the building, to reduce motion semiactive systems counteract the seismic motion with a determined resistive force. “Unlike active devices they do not have the potential to go out of control and destabilize the structure” (Bonsor).
Two most commonly used semiactive control systems are the active variable stiffness (AVS) and active variable damper (AVD) systems. In both systems, the stiffness and damping intensity is changed by small adjustments in oil circuits; thereby significantly decreasing the amount of energy required to maintain the systems. A company named Kurata released the first semiactive damper in 1999. Using only seventy watts of power the semiactive hydraulic damper (SHD) is capable of producing a dampening force of one thousand kilonewtons.
Another type of semiactive control device is MR fluid dampers. The MR fluid dampers are comparable to pistons and are attached to chevron braces that are in turn attached to a steel crossbeam. As the building oscillates, the MR fluid dampers move back and forth offseting the vibrations. Inside the piston, MR fluid fills the chamber of an electromagnetic coil. As the coil is charged the MR fluid changes from liquid to solid like. The seismic vibrations accelerate this process and cause the MR fluid to change states thousands of times per second. “When magnetized, the MR fluid increases the amount of force that the dampers can exert.” (Bonsor)
Semiactive control devices, while still in research stages, will eventually be the norm for structural control. Utilizing the most important attributes of active and passive systems, they are a happy medium which ends up being more effective and more cost efficient during an earthquake.
Although still unable to prevent such natural disasters as earthquakes from occurring, man has made enormous advancements in the area structural control. This paper has described four types of structural control systems: passive, active, hybrid, and semiactive. Each of these systems has specific characteristics that make it more desirable or more effective in certain circumstances. Passive systems are relatively inexpensive to maintain and require no power to be effective, but these systems cannot be altered once installed. Active systems rely on intricate algorithms to combat vibrations; however, these systems require an immense external power supply. Hybrid systems and semiactive systems combine passive and active systems to maximize the dampening effect, but are more expensive and are still being researched. Through major technological advances, man is now able to defend himself against the brute forces of Mother Nature. Eventually, future research and development may eliminate the need for these structural systems, but for now man can sleep soundly knowing that while the potential for earthquakes is inherent, the possibility of dire consequences has been dramatically reduced by implementation of structural control systems.
Works Cited
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Dinehard, David W. and Lewicki, David E. “Combining Wood and Viscoelastic Material.” Updated July 2000. Available: http://www90.homepage.villanova.edu/david.dinehart/Abstracts/WCTE%20Lewicki%20Dinehart%20Abstract.doc. [This is an article from a journal that is posted on line.]
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