May 21, 2021 posted in Design and Engineering
About 50,000 earthquakes large enough to be noticed without seismic equipment occur annually across the globe. Of these, approximately 100 (0.2%) are significant enough to produce substantial damage.
Whether you experience a moderate to large earthquake depends on the seismicity of your location. The United States Geological Survey (USGS) compiles all known earthquake sources in the National Seismic Hazard Maps (NSHMs). These maps serve as the basis for determining site-specific seismic design forces for structures. The ground accelerations noted in the NSHMs are entered into algorithms that output project-specific design accelerations that account for characteristics such as building shaking frequency and site soil classification. The NSHMs (and underlying model) are updated every six years to provide the basis for earthquake provisions in building codes. Regular updates ensure that engineers can access the most accurate information about potentially damaging earthquakes throughout the United States.
The time between earthquakes is also essential in determining the magnitude of an earthquake. The longer the forces build up along a fault, the more energy is released when the fault ruptures, creating a more significant quake.
The West Coast is the most active area in the United States for earthquakes due to two tectonic plates, the Pacific and the North American Plate. However, this does not mean the other parts of the country do not experience earthquakes.
The largest earthquake recorded in North America took place on the New Madrid Fault in Missouri in 1811. The Charleston, South Carolina region has also experienced significant earthquakes in the past hundred years.
Practices like fracking are making low-risk areas more susceptible to disturbances. Structures in these areas are not built to the same stringent seismic codes as those in California. This increases the risk of significant structural damage if seismic forces occur.
The most significant risk from an earthquake is that of life safety. Structural building collapses, and failure of non-structural components such as heavy furniture and hanging elements can cause substantial loss of life during seismic events. Modern building code requirements are intended to protect people. While the building may be damaged beyond repair, building codes aim to ensure safe evacuation at a minimum.
A serious problem facing society today is that many buildings were designed and constructed in the infancy of seismic design methodologies. We gain tremendous knowledge with each major earthquake. Construction techniques and details that have not performed well under seismic force stress are identified and refined. This insight is incorporated into Building Codes, allowing for the construction of more resilient buildings. Some buildings built before the 1970s have an increased likelihood of suffering significant structural damage because they were constructed using obsolete detailing and methodologies. Such buildings pose a substantial risk to life safety in the event of a major earthquake. For example, older unreinforced masonry buildings are among the most vulnerable types of structures, and many occur in the high-seismic and densely populated region of Southern California.
Earthquakes are estimated to cost the nation $6.1 billion annually in building stock losses, according to an updated report published by Federal Emergency Management Agency (FEMA) in 2017. Assessment and repair can cause production to stop and create a financial strain on a company.
The path towards seismically resilient cities starts with informed owners. Owners should understand the seismicity of the regions where their properties are located and be aware of their buildings’ seismic vulnerabilities. Here are the critical questions for every owner:
If owners do not know the answers to these questions or need to understand the impact seismic disturbances have on their building, they should contact a licensed structural engineer proficient in seismic design. If appropriately designed, both existing and new buildings can survive severe earthquakes and significantly reduce risk to life-safety, property damage, and lost-time production.
Although earthquakes remain a deadly threat, today’s buildings are more resilient than ever, thanks to seismic research and technology advancements. Yet, there is still a great deal to learn. Until we can tame mother nature, designers and engineers will continue to explore ways to minimize damage from seismic disturbances.
The Austin Company has a great legacy of seismic design projects across the world. Our parent company, Kajima, is located in Japan and is a leader in advanced seismic design research.