Ep 83 SPECIAL Seismic Design
This week's engineering episode is about earthquakes (4:50). Seismic engineering has progressed over time to make sure we’re prepared (13:40) for earthquakes and the tsunamis (21:25) they cause.
Sources:
Seismic Design
Episode Summary
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
We’re back from summer and we have some news. We’ve made the very tough decision to extend our hiatus. But have no fear, like any good engineer, we have a plan. As of the release of this episode, we have 49 mini failures in our content library. Most of you have never heard those episodes before. So going forward from this episode, we will be releasing one mini failure a week, yes you heard that correctly, we are going to a weekly format for the mini failure releases. One mini failure + releases of a couple of our favourite regular episodes, will be released weekly for the next year.
If you are a patreon subscriber, and this is brand new information, go back and listen to mini failure 49 where we get into the details of how this change will impact our Patreon page.
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Also, important to mention, if you are listening to this on youtube, we will no longer be releasing episodes on that platform. So make sure to follow and subscribe to our show on one of the traditional apps, we will be over there.
Just as a final parting thought, thank you to all of you for the support over the last three years, and we hope you enjoy the mini failures over the next year.
Now on to this week’s engineering failure; seismic design or earthquake engineering.
● The overall goal of seismic design is to make structures like bridges or buildings more resistant to earthquakes. The main goal is for people to exit the building safely and for the building to not collapse and cause more damage. But beyond that there are many levels to seismic design. You could go as far as designing an earthquake proof building that still functions after an earthquake; and many evacuation centers or hospital trauma centers are built this way.
● There have been a number of major earthquakes over the years. Some common ones that come to mind are the Turkey earthquakes earlier this year, the 2011 Christchurch earthquake, and then about two weeks later the 2011 Tohoku earth off the coast of Japan that caused a huge tsunami and took out Fukushima Daiichi, the 2004 Indian Ocean earthquake which caused the Boxing Day tsunami off the coast of Indonesia, and the 1989 Loma Prieta earthquake in california, which we’ve talked about on a few previous episodes.
● Which brings me to my next point and a bit of an agenda for this episode. We’re going to talk about how earthquakes are measured and detected, how earthquake engineering has evolved over the years, and we’re also going to talk about the impact of earthquakes with respect to the generation of tsunamis. Because as interesting as earthquakes themselves are, I find I am drawn into the science of tsunamis just as much if not more.
Earthquake Detection
● Before we can dive into the science of earthquakes, tsunamis, and the engineering to protect us from them, let’s talk about what an earthquake is and how we know its coming.
● An earthquake is the shaking of the surface of the earth caused by a sudden release of energy in the earth’s lithosphere — which is the outermost rocky shell of the earth’s crust. There is likely a most sciency way to discuss this, but for the purpose of this podcast, rocky shell will do. We are not dirt people.
● This shaking of the earth causes seismic waves that vary in frequency, type and size. A big earthquake in a deserted area will likely have less impact than a smaller earthquake in a highly populated area that has not adopted seismic design regulations.
● The lithosphere, or rocky shell is made of tectonic plates that are continually moving and shifting. For the most part, as the plates shift, they slide past each other smoothly, but if there are any irregularities, the plates enter what’s called stick-slip behavior. Essentially, the energy to move the stuck plates builds up until it overcomes the sticking and the plates jerk forward past that irregularity. The bigger the irregularity and the higher the energy to get it past it, but bigger the earthquake.
● There are a number of types of faults depending on how the plates are sliding against each other. Sometimes they are moving horizontally and sometimes one plate is sliding over the other. We’re trying to keep this a bit more high level because we have lots to cover, but if you're interested, there is a lot of info on this online.
● As humans, we have a reasonably good understanding of where the fault lines are, or where the different plates touch, and can somewhat predict where earthquakes could occur. But we still have lots to learn about when they will occur and how big they’ll be.
Measuring earthquakes
● In 1935, Charles Francis Richter, an American seismologist and physicist presented a landmark paper where he developed a magnitude scale used to measure an earthquake size and intensity from amplitude of waves and their distance to the epicenter.
● This evolved into the surface wave magnitude in the 1950s to measure remote events and improve accuracy for future events.
● In the 1970s the moment magnitude scale was developed to measure the amplitude of the shock as well as the total rupture area, average slip of the fault, and rigidity of the rocks.
● There is also the Japan Meteorological Agency seismic intensity scale, the Medvedev–Sponheuer–Karnik scale, and the Mercalli intensity scale which are based on the observed effects and are related to the intensity of shaking.
● Today earthquake intensity is presented as magnitude on a scale from 1 to 10, with 1 being barely perceptible and 10 being completely devastating.
Preparedness
● The better we can detect and prepare for earthquakes, the more people will survive, which should be the goal for everyone.
● Seismic codes have been developed and adopted in earthquake prone areas to make buildings more resistant. Older buildings are often retrofitted to increase their resistance. Although this can be expensive and is not necessarily widely adopted. Which was one of the challenges we saw in Turkey, their infrastructure was wildly unprepared to seismic activity and many buildings collapsed or partially collapsed which greatly increased the death toll.
● In Canada, and I assume the US as well, the building codes dictate whether seismic design is required based on the location of the building and its usage. For example, in Calgary, we are not in a seismic region and therefore are not required to implement seismic design. However, there are some building types, like new hospitals where they are intended to be a gathering place for evacuees in an emergency that may require seismic design.
● In Vancouver, and a lot of BC, seismic design is required and is usually done by a separate engineering team who works with the consultant team to secure the major components of the building. In my experience, seismic design involves a lot of cross bracing and extra securing to make sure the building stays together and nothing large falls on anyone.
● Currently, there are several design philosophies in earthquake engineering, which use experimental results, computer simulations and observations from past earthquakes to determine the required performance for the seismic threat. These include appropriately sizing the structure to be strong enough to survive the shaking with an acceptable damage, to equipping it with base isolation or using structural vibration control technologies to minimize any forces and deformations.
● Based on studies from the Christchurch earthquake in New Zealand, precast concrete installed under new codes performed well. And another study noted precast panel buildings were durable during an earthquake compared to precast frame panels.
● Concurrent shake-table testing of two or more building models is a vivid, persuasive and effective way to validate earthquake engineering solutions experimentally.
● In 2008, the Municipal Services Building of the City of Glendale, California was seismically retrofitted using an innovative combined vibration control solution: the existing elevated building foundation of the building was put on high damping rubber bearings.
● The Ritz-Carlton/JW Marriott hotel building, a part of the LA Live development in Los Angeles, California, is the first building in Los Angeles that uses an advanced steel plate shear wall system to resist the lateral loads of strong earthquakes and winds.
● The Kashiwazaki–Kariwa Nuclear Power Plant, the largest nuclear generating station in the world by net electrical power rating, happened to be near the epicenter of the strongest Magnitude 6.6 July 2007 Chūetsu offshore earthquake. This initiated an extended shutdown for structural inspection which indicated that a greater earthquake-proofing was needed before operation could be resumed.On May 9, 2009, one unit (Unit 7) was restarted, after the seismic upgrades. The test run had to continue for 50 days. The plant had been completely shut down for almost 22 months following the earthquake.
● The Superframe earthquake proof structure is a proposed system composed of core walls, hat beams incorporated into the top-level, outer columns, and viscous dampers vertically installed between the tips of the hat beams and the outer columns. During an earthquake, the hat beams and outer columns act as outriggers and reduce the overturning moment in the core, and the installed dampers also reduce the moment and the lateral deflection of the structure. This innovative system can eliminate inner beams and inner columns on each floor, and thereby provide buildings with column-free floor space even in highly seismic regions
Tsunamis
● Now that we’ve talked about what happens on land, let’s talk about undersea earthquakes and more importantly tsunamis.
● I’ll start by mentioning that tsunamis are not exclusively caused by earthquakes, they can also be caused by volcanic eruptions or underwater explosions. In any case, a tsunami is caused when there is a substantial volume of water displaced out at sea.
● Remember before we were talking about the tectonic plates shifting and releasing energy if they get stuck? So where those plates meet underwater, that energy that’s released is transferred to the water and creates waves. The wave height offshore is small and very long (often hundreds of kilometers). This often causes them to go unnoticed at first. As the waves move closer to the shore and each shallower water, they change in height from an effect called wave shoaling because the wave energy transport velocity changes with water depth. Essentially, the wave frequency remains constant, but the waves slow down and the wave length is reduced, transferring that energy into taller waves. This is a whole rabbit hole if you want to go down it.
● If the earthquake epicenter is far away from land, the tsunami is referred to as a teletsunami. The 2004 Boxing Day tsunami was one such tsunami that occurred from a magnitude 9 earthquake 160 km off the western coast of northern Sumatra in the Indian Ocean, at a depth of 30 km. By the time it made landfall just before 8am on December 26th the waves were 30m high. There is a really good movie called The Impossible based on the true story of Maria Belon and her family who were in Khao Lak, Thailand when the tsunami struck. I highly recommend this movie, even though it's fictional I thought it did a good job of giving me an understanding of what it would be like to experience this tsunami.
● In some tsunami-prone countries, earthquake engineering measures have been taken to reduce the damage caused onshore. Japan, where tsunami science and response measures first began following a disaster in 1896, has produced ever-more elaborate countermeasures and response plans. The country has built many tsunami walls of up to 12 m high to protect populated coastal areas. Other localities have built floodgates of up to 15.5 m high and channels to redirect the water from an incoming tsunami. However, their effectiveness has been questioned, as tsunamis often overtop the barriers. The Fukushima Daiichi nuclear disaster was directly triggered by the 2011 Tōhoku earthquake and tsunami, when waves exceeded the height of the plant's sea wall.
● Iwate Prefecture, which is an area at high risk from tsunami, had tsunami barriers walls (Taro sea wall) totalling 25 km long at coastal towns. The 2011 tsunami toppled more than 50% of the walls and caused catastrophic damage.
● The Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake on July 12, 1993, created waves as much as 30 m tall—as high as a 10-storey building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it did not prevent major destruction and loss of life.
So there you have it, earthquake engineering and how we’re designing the built environment to protect us from seismic activity.
For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find us. If you want to chat with us, our Twitter handle is @failurology, you can email us thefailurologypodcast@gmail.com, or you can connect with us on Linked In. Check out the show notes for links to all of these. Thanks, everyone for listening. And tune in to the next episode for mini failure 1 which is about Lake Peigneur which drained into a salt mine in a matter of minutes.
Bye everyone, talk soon!