Earthquake engineering is a subdivision of structural engineering that focuses on designing structures capable of withstanding the immense levels of stress caused by seismic forces. Civil engineers specializing in this field usually work in geographic areas that frequently experience earthquakes, allowing them to test new technologies in real-world earthquake scenarios. As the science behind earthquake engineering is constantly advancing, current engineering professionals can benefit from examining several of the innovations introduced to the industry.
The seismic waves caused by earthquakes weaken the stability of buildings. To withstand an earthquake, buildings need to be designed with seismic control—especially taller buildings, as their collapse could cause significant damage. One inexpensive method of achieving seismic control is base isolation. This passive method isolates the base of a structure from its foundation using a set of lead-rubber bearings within the structure’s foundation that can effectively deflect or absorb the vibrations caused by seismic waves.
Lead-rubber bearings are comprised of a lead core set within a rubber housing, which is then encased between two thick steel plates and fixed at the base of a building’s foundation. The flexibility of this design aids in deflecting seismic waves, while the plasticity of the rubber components absorbs energy from vibrations that would otherwise cause significant damage. Finally, the solid lead core dissipates residual energy that has not already been absorbed or deflected by the outer layers. Studies on the effectiveness of lead-rubber bearing systems have shown that the presence of this form of isolator effectively reduces a frame’s response to vibrations, compared with a structural frame that does not utilize a lead-rubber bearing. As the science supporting lead-rubber bearings continues to advance, civil engineers are tasked with discovering new materials with which to enhance the performance of these isolators.
Steel Plate Shear Walls
Steel plate shear wall systems have been used to reinforce buildings since the 1970s, particularly in Japan and North America, and are considered a promising alternative to conventional earthquake-resistant systems that are being used in many high-risk seismic regions. These walls are designed to limit lateral force in buildings by using steel shear walls that absorb stress and bend but do not entirely buckle under pressure. The walls are also significantly thinner than concrete shear walls, offering similar levels of resistance and stability, reducing construction costs and lowering total building weight—all without compromising public safety. In addition, steel walls don’t need to be cured, allowing a much faster and fluid erection process.
Controlled rocking systems prevent damage by minimizing the drifts that occur in a structure during an earthquake. These high-performance systems utilize braced steel frames that have elastic properties; this allows the steel frames to rock upon their foundation. The elastic element creates a self-centering, restoring force that dissipates seismic vibrations throughout the structure and allows the frame of a structure to rock in a controlled fashion within a gap that has been intentionally placed in the foundation.
Another major component of engineering effective controlled rocking systems is the implementation of replaceable energy-dissipating devices that produce high initial system stiffness. These devices function similarly to an electric fuse, yielding under the stress of sudden significant loads and quickly being replaced once they have failed.
Tuned Mass Dampers
Traditional mass dampers are designed to control the movement of high-rise buildings. To create a mass damper, civil engineers suspend large metal pendulums attached to cables at the top of a tall building; these pendulums act as an inertial counterweight that keeps the building as centered as possible. These dampers effectively lower the speed at which a building is allowed to oscillate, as well as the total distance of each oscillation.
In circumstances where the use of a traditional mass damper has been deemed unsafe or unreasonable because of excessive amounts of sway, tuned mass dampers may be utilized instead. Tuned mass dampers work similarly to traditional mass dampers but include the use of an additional control system, such as an electromagnet, to limit the motion of the damper’s pendulum element.
An example of tuned mass dampers can be found in China’s Shanghai Tower, the world’s second-tallest building, standing at 2,073 feet. Because of the building’s massive size, traditional dampers were not a realistic option. Therefore, the engineers paired the weighted pendulums with a magnetic system that would safely limit their range of motion. As the building sways, the 1,000-ton iron weights swing above the magnets, inducing an electrical current in the copper plate mounted beneath the damper and immediately creating an opposing magnetic field capable of counteracting the motion of the weight. This maximizes the damping effect of the system while hydraulic shock absorbers keep the weight from swinging too quickly during seismic events or other occurrences that have the potential to cause increased structural sway.
Described as a completely new approach to earthquake resistance, seismic cloaks are currently being tested as a means of creating protective barriers capable of rerouting seismic energy away from aboveground structures. Seismic cloaking involves the modification of soil and other ground materials surrounding a building to deflect or redirect the force created by an earthquake. This innovation revolves around the theory that seismic waves pass energy between the potential energy stored in the planet’s crust and the kinetic energy within the seismic wave itself. Armed with this theoretical knowledge, earthquake engineers are tasked with creating a cloaking structure that can control these destructive seismic waves.
Tests have shown that under the correct circumstances, the oscillations of seismic vibrations can be stopped using modified soil. The application of seismic cloaking is far-reaching, and public and private firms are currently exploring the use the technology to defend high-priority structures, such as nuclear reactors. The downside to this process is the significant space required for a seismic cloak: roughly equal to the size of the structure being protected. There’s also the potential damage to neighboring structures when seismic vibrations are reflected away from the cloaked structure and into these surrounding areas. Researchers are currently developing seismic cloaks that can control the flow of seismic waves while also leaving nearby structures unaffected.
Earthquakes can result in devastating loss of life and property damage if not properly prepared for, and earthquake engineers are vital to the effort of designing buildings and innovative solutions that can defend the force of a potentially deadly earthquake. A keen understanding of engineering principles coupled with an in-depth knowledge of earthquake engineering theory and practice is necessary in order to safely plan and implement these crucial safeguards.
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Steel Plate Shear Walls: Practical Design and Construction, American Institute of Steel Construction
Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames, American Society of Civil Engineers
The World's Second-Tallest Building Sways, But Here's Why You Can't Feel It, Popular Mechanics
Seismic cloak could minimize earthquake damage, Physics World
Effect of Steel Plate Shear Wall on Behavior of Structure, Research India Publications
Controlled Rocking System with Replaceable Fuses –Minimizing Earthquake Damage in Buildings, Masterbuilder