What's on the News
The recent strong quake in Taitung, Taiwan, highlighted the importance of earthquake-resilient planning in building design.
Why It Matters?
Hazard Risk is not the sole profession's responsibility. Like all other hazards, earthquakes are one of the most frightening natural disasters. Traditionally the structural engineer has been regarded as the primary responsibility for the seismic performance of a building, while contractor responsible for quality of the construction. Yet, a closer look at the site, soil condition, and building geometry at the beginning of a project planning phase could help mitigate the risk.
Dive Deeper:
To understand the earthquake situation in Taitung, Wikipedia has more descriptively information as the following: "The first earthquake struck Taitung on September 17, 2022, occurring with a magnitude of 6.5 Mw..." It also read, "the largest foreshock occurred on September 17, measuring 6.4 ML ..."
"The earthquake reached a maximum intensity of VII and VIII respectively on the Modified Mercalli Intensity Scale." But it later reads, "it struck at a depth of 7.3 km (4.5 mi) and had a maximum CWB seismic intensity of 6+."
So what does that information mean to us? The intensity scales and the magnitude come from our standard ground motion measurement.
Earthquake fundamental: the ground motion
Earthquakes happen worldwide, especially at the edge of the tectonic plates. Two broad classes of waves constitute the ground motion: the Body waves and the Surface wave. Because of these waves, the ground generates a force that causes building damage. Six fundamental measurements below helped us determine the strength and related seismic hazards at different locations.
Magnitude is expressed based on the maximum amplitude of certain seismic waves. It is recorded at a distance of 100 kilometers from the epicenter.
The 6.4 ML in the article is the description from Richter magnitude, which Charles Francis Richter defines. Each unit of Richter magnitude indicates a ten-times increase in wave amplitude and approximately 31-32 times energy increase. Thus, the Jiji earthquake in 1999, with a magnitude of 7.3 ML, was thirty times more "impactful than the recent earthquake in Taitung.
The 6.5 Mw in the article is the moment magnitude scale. It is a measurement based on its seismic moment. It was defined by Thomas Hanks and Hiroo Kanamori and became the standard scale used by U.S. Geological Survey to report an earthquake's impact summary.
Acceleration is the measurement used to estimate the earthquake impact on buildings. We apply this measurement in the equation, such as F=MA, to estimate the force.
Duration brings a more accurate impact when combined with the acceleration measurement.
A combination of these two measurements (acceleration and duration) predicts the overall destructive force of an earthquake over time. Generally, when a moderate acceleration lasts longer, a building is more challenging to withstand than a shorter but more vital force.
Dynamic (Shaking) is the measurement that indicates a displacement with a direction. This factor is measured in inches. It refers to the distance of a particle removed from the rest of its position.
Intensity is the information that describes how the quake is felt and the potential damage that might occur. The most commonly used scale is the Modified Mercalli scale (MM). The MM scale is a 12-level system, where scale MM I is described as "no felt," and MM X is described as "extreme."
Period is another critical characteristic of seismic waves. All objects have their natural period, including the earth and the buildings.
While the ground period in the United States generally varies from about 0.4 seconds to 1.5 seconds, depending primarily on the hardness of the ground, a building's natural period is altered by its height. For example, a one-story building has about 0.1 seconds in the period; a four-story building will sway at about 0.5 seconds, and taller buildings of 10 to 20 stories in height will swing at about 1 to 2 seconds.
When the ground shaking is at the building's natural period, the structure tends to "resonate." The resonate phenomenon "amplifies" the vibration, and its acceleration may increase by four or five times, which brings more dramatic damage to the building. To avoid amplification, professionals developed a response spectrum tool to anticipate the maximum shaking a structure will respond to.
The building code recognized this resonating aspect; therefore, professionals use table 1613.2.3 to determine the suitable coefficient for the maximum spectral response acceleration.
I did not access CWB’s website right after the earthquake event, thus the earthquake report excerpt shown here is from 9/19/2022 which is a day after the event.
Back to the Original Story, What's The Takeaway?
"In Taoyuan city, a man was injured by a ceiling collapse on the 5th floor of a sports center..."
To summarize the earthquake characteristics in the context of the earthquake in Taitung, we might come up with this educational guess that the ceiling might still fall (if the project team follows the standard design and construction procedure.)
According to the news, the ceiling on the fifth floor of the sports center in Taoyuan fall off, but the rest of the one-to-four stories remain functional. That might be because the current code recognizes the benefit of making a more prolonged period structure by permitting a lower site coefficient.
That being said, the sports center's structure might have followed the standard and been designed with a safer coefficient. However, the non-structural component inside the building might still experience resonate due to the similar period as the ground shakes. Hence, these non-structural components, such as ceilings, partitions, or even cabinets, may suffer considerable damage during moderate shakes.