The seismic mitigation of buildings has been a challenge for engineers since the construction of the first two-story structure. Today, buildings are designed to be taller in order to accommodate population growth. Consequently, the topic of seismic mitigation is becoming increasingly more important.
The National Earthquake Information Centre (NEIC) estimates that there are approximately 12,000 to 14,000 earthquakes per year around the world. The majority of these earthquakes are minor, with a magnitude of 2 or less on the Richter scale. However, if an earthquake of magnitude 5 or more occurs, it can cause significant structural damage and loss of life (IRIS, 2011). This can be seen in the infamous earthquake of 2010, in which over 316,000 people were killed in Haiti despite early warning systems. In addition to fatalities, large earthquakes cause significant economic damage due to mass damage to infrastructure.
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Why Earthquakes Occur?
Earthquakes occur because stress accumulates in rock as a result of the movement of tectonic plates. As the stress exceeds the strength of the rock, energy is released in the form of an earthquake. Seismic waves are emitted which cause ground vibration. These vibrations can travel far from the initial point of rupture and act as a horizontal oscillating force on the base of buildings, causing oscillation in the structure and ultimately failure.
In an earthquake, structures experience inelastic deformations. These deformations cause the stiffness and strength of the buildings’ frames to weaken. To reduce these inelastic deformations, seismic protection systems can be used. These are becoming increasingly more important in overpopulated cities where demand for space and housing has resulted in numerous tall, slender, and lightweight structures. While these types of structures are extremely efficient for static loading, their construction makes them more susceptible to resonance caused by dynamic loading, such as results from seismic activity.
All structures are designed to withstand vertical loading. Earthquakes present a challenge to standard structural design, however, because of the introduction of horizontal loads. Engineers must establish systems that can mitigate the dynamic loading, reduce lateral displacements and accelerations, and abate the damage caused as a product of excessive movement.
Large displacements cause damage to the external façade and inelastic deformations in the frame. These are also known as plastic hinges, which cause a reduction in the element’s strength. These deformations also increase the displacement and inter-story drift in multi-story buildings. This can cause other problems, such as seismic pounding.
The majority of conventional seismic resistant structures rely on dampening techniques for dynamic response mitigation. However, damping is only one part of the equation. The dynamic response of a structure is directly related to the characteristics of the load and the natural frequency of the structure. Single degree of freedom systems can be modeled using the equation of motion of a mass-spring system:
f(x) = M(x ̈)+K(x)+C(x ̇)
M is the mass matrix as a function of acceleration, K is the stiffness matrix as a function of displacement, and C is the damping matrix of the system as a function of velocity. F is the dynamic loading on the system. By limiting one of these three terms, the seismic response of a structure is also limited.
Seismic Mitigation Methods
Seismic mitigation methods, or seismic control systems, are techniques by which structures are designed to prevent or reduce the damage caused by earthquakes. There are numerous ways this can be done, but the objective of all the systems is to divert seismic energy away from the main structural system in order to limit the seismic response.
One of the earliest examples of earthquake-resistant buildings is the Japanese pagoda. In the 6th century, the multi-story pagoda originally arrived from China, where they were traditionally built from stone. However, the structural design was adopted in Japan due to the frequency of seismic activity in the country, and the engineers developed a multi-faceted approach to earthquake-proof engineering. Horyuji Temple is a 5-story wooden pagoda that was built in 711 A.D. Since it was built Japan has sustained over 40 7.0 magnitude earthquakes and yet it still stands today.
This is because, like modern-day homes, the structure incorporates several different vibration control devices including base isolation, slip joints, friction dampers, and a tuned mass damper. (Nakahara, Hisatoku, Nagase, & Takahashi).
Tuned Mass Dampers and Their Types
There is extensive research on this topic and on benefits of tuned mass dampers (TMDs), as they are the most frequently vibration control devices. There are four main types of tuned mass damper: conventional, pendulum (PTMDs), bi-directional (BTMDs), and tuned liquid column dampers (TLCDs) (Gutierrez Soto & Adeli, 2013).
Research and development into earthquake-resistant homes is an ever-present topic in the world of structural engineering. New technology, analysis methods, and construction procedures continue to make homes safe to live in, regardless of where in the world they are.
- Gutierrez Soto, M., & Adeli, H. J. A. o. C. M. i. E. (2013). Tuned Mass Dampers. 20(4), 419-431. doi:10.1007/s11831-013-9091-7
- IRIS. (2011). How often do earthquakes occur? Retrieved from www.iris.edu website: https://www.iris.edu/hq/files/publications/brochures_onepagers/doc/EN_OnePager3.pdf
- Nakahara, K., Hisatoku, T., Nagase, T., & Takahashi, Y. Earthquake Response of Ancient Five-Storey Pagoda Structure of Horyu-Ji Temple in Japan. 12WCEE, 6.