Today's topic is "seismic verification" of structures.
Numerous earthquakes occur due to global tectonic activity, volcanic eruptions, and collisions of geological plates.
In today's technologically advanced world, witnessing global seismic events has heightened public concern, and various approaches are being pursued to minimize damage alongside growing awareness of the risks.
Looking at damage cases, the collapse of buildings, bridges, and ground is typically what draws the most immediate attention.
In practice, however, secondary damage such as power outages, communication failures, and fires can cause exponential harm to society, significantly elevating risk levels making preparedness essential.
Earthquakes occur due to various causes, including global tectonic activity, volcanic eruptions, and collisions of geological plates. At first glance, seismic events are most memorably associated with large-scale structural collapses — such as the collapse of buildings, ground fissures, and bridge failures — making it easy to assume that all seismic verification analyses are solely aimed at preventing collapse.
In reality, earthquake damage includes primary damage such as the overturning or crushing of large structures due to collapse, but massive losses are also caused by secondary damage — including power supply interruption, communication failure, sprinkler malfunction, hospital equipment failure, gas leaks, and fire outbreaks. When the capacity to respond to these cascading problems is completely lost, damage increases exponentially.
To prevent such problems, critical equipment including power facilities, communication devices, and fire suppression systems is evaluated not only at the collapse prevention level but also at the functional performance level. The review standard has been elevated to a level equivalent to special-grade buildings, which is higher than previous standards. In South Korea, following the 2016 Pohang earthquake, seismic grades have been raised and redefined across all sectors, significantly increasing the importance of accurate seismic design.
For general structures, load conditions are specified at two levels — the Safe Shutdown Earthquake (collapse prevention) level and the Operating Basis Earthquake (functional performance) level — with corresponding damping ratios provided for each. For general mechanical equipment, a damping ratio of approximately 2% is specified for the collapse prevention level, while approximately 3% or 1/2 of the collapse prevention damping ratio is applied for the functional performance level when data are available.
Beyond the load levels, Response Spectrum Analysis itself combines the responses of single-degree-of-freedom systems, so it is necessary to carefully account for rigid response and residual mass that may otherwise be omitted. Regulatory bodies such as the U.S. Nuclear Regulatory Commission prescribe analysis procedures and mode combination methods by regulation.
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