The Third Little Pig built his house out of brick, so when the Big Bad Wolf huffed and puffed – it didn’t blow down.
But what if that childhood fable’s menace had been an earthquake instead?
The outcome of that story could be much different.
Brick, concrete, and steel were once considered indestructible wonders of civilization, but we’ve seen their rigidity can be extremely dangerous – even fatal – in an earthquake.
Catastrophic lessons of the past have shown earthquakes can cause these buildings to crumble – taking many lives, causing injuries and destroying livelihoods. Research into the field of earthquakes and engineering has made phenomenal advances in recent years. Structural engineers today better understand that when it comes to keeping buildings standing, flexibility is key.
Seismic engineers today consider events from past earthquakes, geophysical and ground-motion modeling (both predictive and random), the dynamics of physics, and mathematical calculations to identify structural designs and components able to withstand even the most severe consequences imaginable.
Why plan for the worst?
Because it will come.
Smarter, not stronger
Seismic engineering requires an understanding of structural behavior at a local and global level – examining the impacts of earthquakes of the past to better prepare for future events.
The essential premise behind seismic engineering is to determine the threats posed to a building and to pinpoint the weakest points where damage to a structure is most likely to occur.
These calculations include ground and building dynamics to determine a building’s best responses to a wide variety of possibilities – depending on the composition of the ground, the type of seismic wave occurring, its proximity and the direction from which is comes.
Engineers and scientists aim to think smarter rather than just stronger.
Dr Stefano Pampanin from the University of Canterbury believes seismic engineering relies on the merging of knowledge from many scientific areas, using an interdisciplinary approach and combining the best tools and know-how to obtain high level results.[i]
He explains that seismic engineers need a good understanding of physics, mathematics, and other sciences, as well as a broad general knowledge and a scientific approach.
“It is tempting to create buildings that are rigid and strong,” he told Science Hub. “But one day, an earthquake will come along that is stronger than the building, and it will break.”[ii]
When the Earth shakes, it does so with tremendous force – prompted by the intense energy of vibrations in the ground caused by two categories of waves: body waves deep underground, and surface waves.
Body waves are grouped into two areas: compressional or “P” waves that cause the ground to shake in a back-and-forth motion[iii], and shear or “S” waves, which shake the ground in a perpendicular motion that moves the ground up and down or from side to side.
Both cause high-frequency vibrations.
Surface waves are categorized as “Rayleigh” and “Love,” (named for John William Strutt (Lord Rayleigh) and Augustus Edward Hough Love, who mathematically proved their existence. These waves usually arrive later than the initial body waves. A Rayleigh wave causes the ground to shake in an elliptical motion, like the rolling pattern of an elliptical workout machine. A Love wave, shakes the surface in a motion perpendicular to the general direction of the body wave[iv].
Building resilience
Making a new building resilient so that it is not only safe, but repairable and occupiable after a major earthquake may result in perhaps a 1% to 2% increase over minimum code-based design costs, according to calculations by the United States Resiliency Council[v], a nonprofit organization dedicated to developing a rating system that – Like LEED certification for green buildings – indicates a structure’s ability to withstand seismic shaking.
The organization has established Platinum, Gold, and Silver rating criteria for both new construction and older buildings that have been retrofitted for seismic safety. They are used by:
- Building owners with properties that receive high USRC ratings may benefit from an increase in perceived value, potentially increasing leasing rates and transaction efficiency
- Lenders and insurers to inform real estate transactions associated with lending decisions and defining insurance products
- Tenants to assess both safety and recovery time following a major seismic event
Seismic engineering is a complex field with many approaches to making buildings more resilient.
Some new approaches include base isolation and dampers – where the building rests on flexible bearings or pads known as base isolators, which disperse an earthquake’s energy much like a car’s shock absorbers dissipate roadway vibration.
This approach has proven to be extremely effective in preventing damage in earthquakes. A retrofit of the William Clayton building in Wellington, New Zealand, with seismic dampers and isolators protected the structure during the 2016 quake – making the building a refuge for workers displaced from other buildings at the time.[vi]
Approaches such as these help a structure to withstand the pressures, vibrations, and lateral expansion forces that put a building under stress during an earthquake.[vii]
Other techniques used, depending on the structural design and composition of the building, include:
- The installation of external post-tensioning to help keep a building centered
- Reinforcement of concrete columns and connections within buildings
- Reinforcement of masonry to prevent it from falling loose
- Installation of shear walls or moment frames to help improve the integrity of the structure
- Beam-column joint connections
Seismic engineering
No two buildings are exactly alike. In fact, some that may seem at first appearance to be identical can display significant differences based on a number of factors, including structural composition, materials, the wear and tear on the building, even the composition of the soil beneath it.
So, when it comes to seismic safety, the top considerations in seismic engineering are: design, construction, and location.
When the ground beneath a building shakes, it makes the building sway as the energy of a quake’s waves moves through it.
Proximity to an earthquake fault, and the composition of the ground below a structure can be as important a safety consideration as its construction.
Bedrock absorbs more wave energy than sandy soils or landfill, so buildings on solid rock will be much less affected than those built on softer soils. And if softer soils have water in them, they can become a little like quicksand during an earthquake. When seismic waves pass through saturated soil, they give it a strong squeeze. The soil loses its strength and behaves like a liquid, a process called liquefaction.[viii]
Another factor is the materials a building is constructed from, which determine its strength and more importantly – its flexibility. Wood and steel have more give than unreinforced concrete, or masonry, and they are favored materials for building in fault zones.
Finally, the design of the building must be considered. Many structures constructed in the past were built – not intentionally – with vulnerabilities that only became apparent when a major quake caused them to topple.
The following is a list of the most commonly used building designs that may require seismic retrofitting to make them safe:
- Soft-Story: a design commonly found among apartment buildings, these structures are characterized by open parking on the ground floor and dwelling units built above. In some instances, the ground floor may be used as retail space and enclosed by windows that do not provide any structural support. When built prior to 1978, they can be extremely vulnerable to collapse in a major earthquake. The composition of these buildings lacks the ability to withstand lateral forces that push the building from side to side. The swaying can cause the first floor to collapse, and the upper stories to pancake on top of it.
- Non-ductile Concrete: Non-ductile concrete buildings built before 1978 are characterized as having concrete floors and/or roofs supported by concrete walls, columns and/or frames. Due to their rigid construction and limited capacity to absorb the energy of strong ground-shaking, these structures are at risk of collapse in an earthquake. In fact, non-ductile concrete buildings make up the majority of earthquake losses around the world. Because they are frequently used for office and retail uses that draw large numbers of people, the potential for death and injury with these structures is of grave concern.
- Tilt-up concrete: This type of building became popular in the post-World War II construction boom. This cost-effective technique of pouring a building’s walls directly at the jobsite and then raising or “tilting” the panels into position was and continues to be a popular way to meet California’s demand for new commercial buildings. The walls of a concrete tilt-up building can weigh between 100,000 and 300,000 pounds. Steel plates with headed studs are positioned into the forms prior to pouring the concrete to establish viable connection points that secure the walls to the foundation and the roof trusses to hold them in place. Many tilt-up structures built prior to the late 1970s were constructed with limited or weak connections proven to fail in an earthquake, causing severe damage and/or collapse. These building defects can be easily corrected with seismic retrofitting.
- Steel Moment Frame: Steel moment frame construction dates back to the 1880s with the very first skyscraper, the Home Insurance Building in Chicago. This building technique was most commonly used in the 1960s to 1990s. Steel moment frame construction uses a rigid steel frame of beams connected to columns to support the many floors of the structure. Those that were designed and built prior to the mid-1990s can sustain brittle fracturing of the steel frames at the welded joints between the beams and the columns. Moment frame buildings in Southern California, which have been through major seismic events, may today contain frame cracks and fissures and be susceptible to collapse in a major earthquake.
- Unreinforced Masonry: Unreinforced masonry buildings make up many of the older structures typical in downtown communities. They are characterized by walls (both load bearing and not) and other structures such as chimneys that are made of brick, cinderblock, or other masonry materials not braced with rebar or another reinforcing material. URM structures are vulnerable to collapse in an earthquake, due to a general failure of the mortar or when portions of the masonry such as parapets peel from the building façade and fall onto the sidewalk below. Most of these structures were identified as part of a California mandate for all cities within seismic Zone 4. During the late 1980s and 1990s, many cities enacted mandatory ordinances to require retrofits of these buildings. Thousands of these buildings are yet to be retrofitted.
Why does this matter?
The Earthquake Engineering Research Institute asserts that the safety and habitability of buildings after earthquakes is critical to a community’s ability to recover from a major temblor.
Residents should be able to ‘shelter-in-place,’ following a quake – but that is only possible when their homes are safe and habitable. According to EERI’s estimates, housing is the most vulnerable of all building types.
“ In fact, research shows that over half of the financial loss in earthquakes occurs in housing. But earthquake damage to housing has implications beyond cost. Residents are the foundation of any vibrant community and a key to social, environmental, and economic recovery following a natural hazard event. The loss of a home can lead to job loss, resulting from forced relocation, moving from one place to another, or searching for a new or temporary home. Serious damage to one’s home harms the entire neighborhood, while a damaged but usable home can benefit the resilience of a neighborhood. These housing losses in turn cause a sharp decrease in the available workforce. Housing loss can also lead to poor medical and mental health. Finally, rebuilt housing is likely to be more expensive, leading to gentrification, changes in neighborhood character, and loss of affordable housing, particularly since multi-family housing can take many years to replace.”[ix]
To protect the health of communities and their residents, EERI has recommended the following steps[x]:
- Enact policies to define under what conditions residents will be able to shelter-in-place and inform residential building owners and tenants of their risks including the level of expected damage and usability of their home after an earthquake
- Buyers and renters should have a right to reliable information, and developers, lenders, insurers, and sellers should have an obligation to provide it
- Promote and implement codes, standards, and guidelines to increase the number and quality of seismic retrofits, including standardized retrofit plans and training for contractors for retrofit of single-family homes, as well as improvements in existing building codes for single-family and multi-family housing
- Provide financial and other incentives for owners to upgrade housing, ensuring that these incentives are tied to consensus retrofit standards
- Require structural upgrades of seismically at-risk buildings when substantially altered or damaged, upon sale, or by a specific date
- Adoption of modern building codes should be universal, and developers and organizations should encourage faster incorporation of existing cost-benefit research into regulation and then use stronger codes where needed
Impacts of an earthquake can be widespread, which dramatically increases the urgency to make our cities safer.
If a 7.8-magnitude earthquake were to strike along the San Andreas Fault in Los Angeles , one in every 16 buildings in the region would be damaged, according to the U.S. Geological Survey.
That’s 300,000 structures.
Other USGS projections for this scenario include[xi]:
- 800 deaths
- 50,000 injuries
- $213 billion in property and infrastructure losses
- 121,339 displaced households (3.5 million individuals)
This USGS “ShakeOut” study anticipates that at least five pre-1994 steel moment-frame high-rise buildings would collapse under this scenario, with about 5,000 people inside them if the quake strikes during regular business hours. As many as 50 low- and mid-rise concrete moment-frame buildings would collapse, and 900 unreinforced masonry buildings would be irreparably damaged.
Our cities cannot afford to do nothing and wait for the next disaster to strike. If you own a building that you believe may be vulnerable to damage – or if you live or work in one – it’s important to educate yourself on cost-effective measures that can be taken to save lives, protect property and investments, and preserve the well-being of the community-at-large.
Visit optimumseismic.com for more information, or call us at 323-978-7664.
[i] Science Hub, https://www.sciencelearn.org.nz/resources/331-seismic-engineering#:~:text=Seismic%20engineering%20is%20a%20branch,bridges%2C%20resistant%20to%20earthquake%20damage.
[ii] Ibid.
[iii] U.S. Geological Survey, https://earthquake.usgs.gov/learn/glossary/?alpha=O
[iv] U.S. Geological survey, https://earthquake.usgs.gov/learn/glossary/?term=Love%20wave
[v] United States Resiliency Council, “The Resilience Advantage,” www.usrc.org
[vi] Science Learning, https://www.sciencelearn.org.nz/resources/331-seismic-engineering#:~:text=Seismic%20engineering%20is%20a%20branch,bridges%2C%20resistant%20to%20earthquake%20damage.
[vii] International Geophysics https://www.sciencedirect.com/topics/earth-and-planetary-sciences/seismic-engineering
[viii] Exploratorium, https://www.exploratorium.edu/faultline/damage/building.html
[ix] Earthquake Engineering Research Institute, Policy Paper, https://www.eeri.org/wp-content/uploads/2.01-2.02-PPA-Promote-Safe-and-Resilient-Housing-Position-Statement.pdf
[x] Ibid.
