How Earthquake Bearings Work

0:01 In 1994, the Northridge earthquake struck

0:04 the Greater Los Angeles area in California.

0:07 The shaking was so intense in the west San

0:11 Fernando Valley that buildings collapsed and bridges were ruined.

0:14 Billions of dollars of damages were inflicted on parking garages,

0:19 office towers, apartments, and warehouses,

0:21 thousands were injured, and 57 people died.

0:25 One factor that compounded the earthquake's devastation was

0:28 the extent of the damage it caused to hospitals.

0:31 Just when they were needed most,

0:33 eleven of the area's hospitals were completely or partially closed,

0:37 with patients evacuated to outside medical centers.

0:40 But one hospital stayed open.

0:42 At USC University Hospital, now USC Keck, patients felt the rumble,

0:48 but it was more of a gentle sway than

0:51 the violent tremors felt in other parts of Los Angeles.

0:54 The facility was one of the few in the area

0:56 that was able to accept new patients after the earthquake.

0:59 The reason it could stay open amid the chaos all comes down to the foundation.

1:05 The hospital was built in 1991, only three years prior to the quake,

1:09 and the design included what was a pretty innovative

1:12 idea for its time (at least in the US).

1:15 It is dead simple in theory:

1:16 just physically decouple the superstructure from the ground.

1:20 Of course, in practice, there are a lot of engineering details to get right.

1:25 So I built a little model to show you how this works.

1:28 I’m Grady, and this is Practical Engineering.

1:41 Earthquakes can put enormous demands on buildings.

1:44 In seismically active areas for low- and mid-rise buildings,

1:47 there really is no worst-case loading condition for structural engineers.

1:51 It’s a big deal.

1:53 Earthquakes are a lot like floods:

1:55 you can’t predict the timing, and their intensity varies tremendously.

1:58 In most situations, you just can’t afford to design for the worst case.

2:04 If you can imagine a destructive earthquake,

2:06 there’s an even stronger one possible.

2:08 Of course, they’re difficult to design against,

2:11 and we’ll get to that, but there’s the additional challenge

2:14 of just drawing a line in the sand that says:

2:16 “This is the level of risk we’re willing to accept.” With floods,

2:20 we often choose a somewhat arbitrary limit, like the 1-percent storm,

2:24 to size drainage infrastructure and delineate

2:27 the floodplain where development is restricted.

2:30 Same idea with earthquakes.

2:32 The code outlines site-specific seismic risks across the US and says,

2:37 essentially, your building has to “survive” this amount of shaking.

2:41 But that word “survive” might not mean what you expect.

2:45 Earthquake accelerations and the resulting deflections can be so strong that it

2:50 usually isn’t feasible to design a building to completely resist the forces.

2:55 Instead, we design buildings to absorb earthquake energy

2:58 by deforming and yielding in a controlled and predictable manner.

3:03 Seismic loads aren’t like other structural demands, like gravity and wind,

3:07 where we add strength as needed to resist them entirely.

3:11 If the “design earthquake” required by the code ever comes,

3:15 the expectation is that ordinary buildings are going to be damaged,

3:20 maybe even beyond repair.

3:21 The intent is simply that they don’t collapse.

3:24 Let’s go to the garage and I’ll show you what I mean.

3:27 I designed this little shake table so

3:29 we can simulate an earthquake on the bench.

3:33 It just uses a cordless drill for motion.

3:35 I’m doing this in one dimension for simplicity,

3:38 but most earthquake engineering has

3:40 to consider shaking in any horizontal direction.

3:43 Vertical motions usually aren’t as intense in earthquakes,

3:46 and buildings are a lot stiffer in the vertical direction

3:49 since they sustain a full G of vertical acceleration constantly,

3:53 so it’s not as big a factor in design.

3:56 I built a little building using these magnetic pieces,

3:59 and let’s give it a little shake.

4:04 This is exaggerated for effect, but I wouldn’t want to be in there!

4:09 It’s a little worse for wear after the quake,

4:11 but as a whole, it’s still standing.

4:13 This is what the code allows for many types of buildings.

4:17 It’s all about life safety: protect the people inside.

4:20 And it’s generally a reasonable tradeoff.

4:22 For rare events like earthquakes,

4:24 it’s often just not feasible to invest in a building

4:27 that’s strong enough to come away from the shaking unscathed.

4:30 But there are situations where it makes sense to have additional protection.

4:35 Hospitals are the perfect example.

4:37 Even if the building doesn’t collapse, it might need to be evacuated and closed

4:41 while the structure is repaired after an earthquake.

4:44 Lives are still endangered by interruptions in procedures and lack of capacity.

4:49 And think about all the expensive equipment and contents inside.

4:52 Even if the building itself survives a major earthquake,

4:56 a lot of that stuff is going to be damaged beyond repair.

4:59 When you factor in the impact of ruined contents and the time

5:02 it would take to get things back up and running,

5:05 it tilts the calculus of savings versus

5:08 safety when it comes to earthquake design.

5:10 This concept is called resilience.

5:12 For offices and homes,

5:13 we might be willing to accept the risk of having to rebuild

5:17 after an earthquake to avoid the cost of building an extremely strong structure.

5:23 For hospitals, fire stations, emergency shelters,

5:26 and other critical buildings, it’s just not acceptable.

5:29 They need to be more resilient.

5:31 But there’s a challenge there.

5:33 Let me stiffen up my building on the shake table, and I’ll show you what I mean.

5:38 Now this structure is resilient against earthquakes.

5:40 Give it a good shake and all the pieces

5:43 are still connected just the way they were before.

5:45 But watch what happens when I put some stuff inside.

5:51 Almost all the accelerations from the ground

5:53 are transmitted through the building, where contents feel the shaking.

5:57 You can see how stiffness plays two parts:

6:00 it keeps the structural members from bending and deflecting so much,

6:04 but it also means that more of the accelerations

6:07 are “felt” by the building and what’s inside.

6:09 So sometimes we need an alternative.

6:12 For that, we need to look at an earthquake.

6:16 This is an accelerogram of an earthquake.

6:18 It’s a simple plot to understand:

6:21 time on the x-axis; acceleration on the y-axis.

6:24 It looks almost like a sound wave you might record through a microphone.

6:28 By itself, it isn’t that useful at first glance, except, I guess,

6:32 to let you know that earthquakes

6:34 are “noisy.” The ground movements are broadband;

6:37 they contain a lot of different frequencies.

6:40 And frequency matters a lot in seismic engineering.

6:43 Let’s go back to the garage to see why.

6:46 I have a few rods set up on the table now.

6:50 These are very simple oscillators, and I can excite them with my drill.

6:58 If I shake it slowly,

6:59 the longest rod has an extreme response while the others are mostly unbothered.

7:03 A little faster, and now it’s the medium rod responding.

7:08 Faster still, and the smallest rod is

7:10 responding while the other two are barely moving.

7:13 The displacement is the same for all three oscillators,

7:17 but the response is completely frequency-dependent.

7:23 I think this is pretty intuitive.

7:25 Instead of frequency, we usually talk in terms of period in seismic engineering.

7:29 That’s the time it takes to complete one cycle of an oscillation.

7:34 Every structure has a fundamental period at which it naturally sways.

7:38 For short buildings it’s usually less than a second.

7:42 For skyscrapers, it can be multiple seconds.

7:45 And the closer an excitation gets to a building’s fundamental period,

7:50 the more its response to that excitation grows.

7:53 Imagine if I could build a shake table with a lot of these oscillators,

7:58 ranging in fundamental period,

7:59 and then take our accelerogram from earlier and feed it into the demo.

8:03 Some of the oscillators would shake a lot.

8:06 Some barely at all.

8:07 If we plot the response of the whole group,

8:10 we get this graph called a response spectrum.

8:13 This is the plot for a single event.

8:15 For many earthquakes, acceleration tends to peak for periods less than a second.

8:21 Plus, oscillators with a longer period naturally smooth out rapid ground motion.

8:26 So it’s pretty typical that you have a stronger response at the lower periods.

8:30 The building codes look at a wide range

8:33 of earthquakes to create site-specific hazard curves that look

8:36 a little like this: a high plateau

8:39 at the shorter periods that decays as periods get longer.

8:42 If you’re the designer of a low or mid-rise building,

8:46 this curve is an issue for you.

8:49 With a shorter fundamental period,

8:51 your building is at the worst part of this curve;

8:54 it’s the most susceptible to seismic ground movement.

8:57 This is why it’s common to see

9:00 more damage in shorter buildings after an earthquake,

9:02 when the skyscrapers survive unscathed.

9:05 Lower buildings have shorter fundamental periods,

9:08 so they have to be designed against much higher accelerations.

9:11 But what if you could just adjust a building’s natural

9:15 period so that it “feels” less power from the earthquake?

9:18 That’s the idea with base isolation.

9:21 Let me set up another example.

9:24 Now my building is on rollers, and I’m using rubber bands to hold it in place.

9:29 I’ll put the two examples side-by-side so you can see the difference.

9:34 Pretty remarkable.

9:36 And here’s what happens to the stuff inside.

9:38 What I love about this solution is how intuitive it is.

9:42 You know, all that exposition about

9:45 acceleration spectra and oscillators and ground

9:48 motion response… you don’t need any of that to understand how this works.

9:52 And because of that, it’s not really a new idea.

9:55 The concept of using a loose connection between

9:59 foundation and underlying soil has been used for centuries,

10:02 and many historic monuments and buildings

10:04 have probably benefitted from an isolated base,

10:07 whether it was an intentional seismic protection or not.

10:10 Lots of older patents on the idea make use of rollers or ball bearings,

10:16 kind of like my demo.

10:17 The general idea is the same in all cases:

10:20 make the structure’s fundamental period longer

10:23 to get the peak accelerations down.

10:25 Modern designs typically use one of two solutions.

10:29 The first is rubber bearings.

10:31 Instead of resting directly on the pile foundation,

10:34 isolation devices separate the superstructure from the substructure.

10:38 When an earthquake comes, the building acts more like a skyscraper,

10:43 smoothing out the high-frequency ground motions so the floors feel less shaking.

10:47 Early isolators were plain rubber, but it didn’t work that well.

10:51 It would bulge out under the weight of the building,

10:54 making the bearings less effective over time.

10:56 Modern isolation bearings use a composite

10:58 of steel plates with rubber in between them.

11:02 This makes them highly elastic in the horizontal direction,

11:05 but stiff in the axial direction so they

11:08 can withstand a lot more load without bulging.

11:10 And this system has a lot of advantages.

11:13 Of course you get the lengthening of the building’s fundamental period,

11:17 which lowers its response to most earthquakes.

11:19 That makes a huge difference in resilience

11:22 for both the structure and the stuff inside it,

11:25 shortening or entirely eliminating the time required to put

11:28 it back into service after a seismic event.

11:31 It also means that the structural members themselves don’t have to be as strong.

11:35 I put the original, more flexible structure

11:37 on the shake table with the isolation system, and it held up just fine.

11:42 Even the stuff inside it was mostly unbothered by the shaking.

11:46 So you sometimes see an offset in the cost

11:48 of the isolation system from the rest of the building,

11:51 and there are situations where they pay for themselves entirely.

11:54 They’re also also big for retrofits.

11:56 Strengthening older buildings to meet modern seismic codes is often

12:01 disruptive to architectural elements or even to the structure's function.

12:06 New beams and braces require removal

12:08 of decorative features or changing the floor plan.

12:11 Instead, you can just temporarily support

12:13 the building and insert isolators below.

12:15 That’s exactly what they’re doing on the Salt Lake Temple in Utah,

12:19 and the same idea has been used

12:22 in a lot of seismic retrofit projects across the world.

12:25 That’s a lot of advantages, but we can do even better.

12:29 Isolation lengthens the fundamental period,

12:31 reducing the building’s response to an earthquake,

12:33 but it doesn’t inherently absorb the energy.

12:36 We don’t want our buildings to vibrate like a guitar string ringing out,

12:40 so damping is also important.

12:41 There are lots of ways to improve the damping of buildings.

12:44 I’ve done a couple of videos on unique solutions in tall buildings.

12:49 But base isolation systems make it easy.

12:51 You can just use a special blend of rubber in the bearings that absorbs some

12:55 of the energy instead of transmitting it

12:58 into the superstructure (so called high damping rubber).

13:01 Another option is to use a lead plug in the center of the bearing.

13:06 As the plug plastically deforms, it absorbs the energy of the shaking.

13:10 This reduces the accelerations of the building even further

13:13 and helps kill the oscillations faster after an earthquake,

13:17 so the building doesn’t ring like a bell.

13:19 And not just buildings, but other structures too.

13:22 Lots of bridges use similar isolation systems to manage earthquake loads.

13:27 Rubber bearings are a great solution and probably

13:30 the most widely used base isolation system.

13:33 But their properties can change based on temperature, they’re kind of bulky,

13:37 they’re susceptible to damage from exposure to oils and ozone,

13:40 and they can degrade with age.

13:42 They’re not always the perfect fit for every application.

13:46 An alternative is curved surface sliding bearings,

13:49 often known by one of the most common trademark names,

13:54 friction pendulum isolators.

13:55 Instead of rubber, these use a sliding element

13:58 on a curved surface to separate the superstructure and substructure.

14:02 If an earthquake comes, the building rides on the bearings.

14:05 Damping comes from the sliding friction,

14:08 and the restoring force comes from the curved surface,

14:10 keeping the building in place when the shaking is over.

14:14 These get pretty creative with two or three separate elements that offer

14:18 more control over the displacement and period

14:21 under a wide range of accelerations.

14:24 The amount of displacement a building

14:26 experiences on the isolators is pretty important,

14:29 because buildings don’t just float on their foundations.

14:33 They’re connected to stuff!

14:34 People have to get in and out of buildings, of course.

14:38 Usually, that’s not so complicated,

14:39 since most structures don’t move relative to the ground,

14:43 but it’s not true for base-isolated buildings.

14:45 For the system to work,

14:47 a building’s only connection to the ground has to be through the bearings.

14:51 So buildings equipped with a seismic isolation system

14:54 often have some kind of “moat” around them, providing a space to move.

14:59 It’s usually pretty easy to spot an isolated building

15:02 because it has a big gap all the way around,

15:05 although often this joint is bridged with some kind of expandable cover,

15:08 similar to the way expansion joints work on bridges.

15:12 Utilities are also a challenge.

15:14 You can’t have a solid water or sewer

15:16 connection to a building that wiggles around.

15:19 So, engineers have to design sometimes elaborate,

15:22 flexible connections for water, sewer, gas, and electricity.

15:25 Finally, you have to be thoughtful about long-period earthquakes.

15:29 There are situations where a base isolation can

15:32 actually amplify the shaking if it’s not carefully tuned.

15:36 Earthquakes are such a challenge in engineering:

15:39 unpredictable both in time and intensity.

15:42 There’s only so much you can do

15:44 when the ground underneath your structure shakes uncontrollably.

15:47 But I love this solution of base isolation because it’s so intuitive.

15:52 It’s something my five-year-old would come up

15:54 with: just put a suspension system on a building.

15:57 But it works!

15:58 It works really well, actually, to the point where it's been installed

16:01 on thousands of buildings across the world.

16:04 It’s not a catch-all, but I suspect that, as the technology improves and as we

16:08 get more data about how these buildings perform in real-world situations,

16:13 it’s only going to be more common to see

16:16 little moats around important buildings in seismically active regions.

16:19 Most people will probably never notice,

16:22 but you and I will know what’s underneath.

16:26 I love talking about the engineering of the built world that often

16:30 goes unseen like the isolation bearings below lots of famous buildings.

16:34 It can be hard to appreciate how this stuff works,

16:37 so a lot of my videos feature these demonstrations I build in my garage.

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17:37 Thank you for watching, and let me know what you think!