0:00:07.080,0:00:10.586 Earthquakes have always been [br]a terrifying phenomenon, 0:00:10.586,0:00:14.051 and they've become more deadly[br]as our cities have grown, 0:00:14.051,0:00:17.720 with collapsing buildings posing[br]one of the largest risks. 0:00:17.720,0:00:20.279 Why do buildings collapse [br]in an earthquake, 0:00:20.279,0:00:22.756 and how can it be prevented? 0:00:22.756,0:00:24.994 If you've watched a lot of disaster films, 0:00:24.994,0:00:26.101 you might have the idea 0:00:26.101,0:00:29.573 that building collapse is caused directly[br]by the ground beneath them 0:00:29.573,0:00:32.977 shaking violently,[br]or even splitting apart. 0:00:32.977,0:00:35.298 But that's not really how it works. 0:00:35.298,0:00:39.374 For one thing, most buildings[br]are not located right on a fault line, 0:00:39.374,0:00:43.966 and the shifting tectonic plates [br]go much deeper than building foundations. 0:00:43.966,0:00:46.316 So what's actually going on? 0:00:46.316,0:00:50.077 In fact, the reality of earthquakes[br]and their effect on buildings 0:00:50.077,0:00:52.065 is a bit more complicated. 0:00:52.065,0:00:55.282 To make sense of it, architects [br]and engineers use models, 0:00:55.282,0:00:59.778 like a two-dimensional array of lines[br]representing columns and beams, 0:00:59.778,0:01:05.432 or a single line lollipop with circles [br]representing the building's mass. 0:01:05.432,0:01:09.280 Even when simplified to this degree,[br]these models can be quite useful, 0:01:09.280,0:01:12.009 as predicting a building's response[br]to an earthquake 0:01:12.009,0:01:14.553 is primarily a matter of physics. 0:01:14.553,0:01:16.868 Most collapses that occur [br]during earthquakes 0:01:16.868,0:01:20.332 aren't actually caused [br]by the earthquake itself. 0:01:20.332,0:01:23.259 Instead, when the ground moves[br]beneath a building, 0:01:23.259,0:01:26.284 it displaces the foundation [br]and lower levels, 0:01:26.284,0:01:28.975 sending shockwaves through [br]the rest of the structure 0:01:28.975,0:01:31.834 and causing it to vibrate back and forth. 0:01:31.834,0:01:36.136 The strength of this oscillation[br]depends on two main factors: 0:01:36.136,0:01:39.196 the buildings mass, [br]which is concentrated at the bottom, 0:01:39.196,0:01:40.568 and its stiffness, 0:01:40.568,0:01:44.595 which is the force required [br]to cause a certain amount of displacement. 0:01:44.595,0:01:48.155 Along with the building's material type[br]and the shape of its columns, 0:01:48.155,0:01:51.207 stiffness is largely a matter of height. 0:01:51.207,0:01:54.105 Shorter buildings tend to be stiffer[br]and shift less, 0:01:54.105,0:01:57.347 while taller buildings are more flexible. 0:01:57.347,0:02:00.557 You might think that the solution[br]is to build shorter buildlings 0:02:00.557,0:02:02.843 so that they shift as little as possible.[br] 0:02:02.843,0:02:08.700 But the 1985 Mexico City earthquake is[br]a good example of why that's not the case. 0:02:08.700,0:02:09.812 Durng the quake, 0:02:09.812,0:02:14.022 many buildings between six [br]and fifteen stories tall collapsed. 0:02:14.022,0:02:17.999 What's strange is that while shorter[br]buildings nearby did keep standing, 0:02:17.999,0:02:22.405 buildings taller than fifteen stories[br]were also less damaged, 0:02:22.405,0:02:24.555 and the midsized buildings that collapsed 0:02:24.555,0:02:28.730 were observed shaking far more violently[br]than the earthquake itself. 0:02:28.730,0:02:30.590 How is that possible? 0:02:30.590,0:02:34.322 The answer has to do with something[br]known as natural frequency. 0:02:34.322,0:02:35.988 In an oscillating system, 0:02:35.988,0:02:41.581 the frequency is how many back and forth[br]movement cycles occur within a second. 0:02:41.581,0:02:43.731 This is the inverse of the period, 0:02:43.731,0:02:47.520 which is how many seconds it takes[br]to complete one cycle. 0:02:47.520,0:02:51.763 And a building's natural frequency,[br]determined by its mass and stiffness, 0:02:51.763,0:02:55.330 is the frequency that its vibrations [br]will tend to cluster around. 0:02:55.330,0:03:00.277 Increasing a building's mass slows down[br]the rate at which it naturally vibrates, 0:03:00.277,0:03:03.835 while increasing stiffness[br]makes it vibrate faster. 0:03:03.835,0:03:06.192 So in the equation representing[br]their relationship, 0:03:06.192,0:03:09.911 stiffness and natural frequency[br]are proportional to one another, 0:03:09.911,0:03:14.184 while mass and natural frequency[br]are inversely proportional. 0:03:14.184,0:03:17.658 What happened in Mexico City[br]was an effect called resonance, 0:03:17.658,0:03:20.198 where the frequency [br]of the earthquake's seismic waves 0:03:20.198,0:03:24.535 happen to match the natural frequency[br]of the midsized buildings. 0:03:24.535,0:03:27.456 Like a well-timed push on a swingset, 0:03:27.456,0:03:31.211 each additional seismic wave[br]amplified the building's vibration 0:03:31.211,0:03:33.052 in its current direction, 0:03:33.052,0:03:36.616 causing it to swing even further back,[br]and so on, 0:03:36.616,0:03:41.303 eventually reaching a far greater extent[br]than the initial displacement. 0:03:41.303,0:03:44.685 Today, engineers work [br]with geologists and seismologists 0:03:44.685,0:03:48.702 to predict the frequency [br]of earthquake motions at building sites 0:03:48.702,0:03:51.626 in order to prevent [br]resonance-induced collapses, 0:03:51.626,0:03:55.023 taking into account factors[br]such as soil type and fault type, 0:03:55.023,0:03:57.947 as well as data from previous quakes. 0:03:57.947,0:04:00.997 Low frequencies of motion [br]will cause more damage to taller 0:04:00.997,0:04:02.865 and more flexible buildings, 0:04:02.865,0:04:05.789 while high frequencies of motion[br]pose more threat 0:04:05.789,0:04:08.553 to structures that [br]are shorter and stiffer. 0:04:08.553,0:04:11.316 Engineers have also devised ways[br]to abosrb shocks 0:04:11.316,0:04:14.947 and limit deformation[br]using innovative systems. 0:04:14.947,0:04:17.423 Base isolation uses flexible layers 0:04:17.423,0:04:21.346 to isolate the foundation's displacement[br]from the rest of the building, 0:04:21.346,0:04:25.225 while tuned mass damper systems[br]cancel out resonance 0:04:25.225,0:04:28.536 by oscillating out of phase[br]with the natural frequency 0:04:28.536,0:04:30.303 to reduce vibrations. 0:04:30.303,0:04:33.835 In the end, it's not the sturdiest [br]buildings that will remain standing, 0:04:33.835,0:04:35.477 but the smartest ones.