1 00:00:02,770 --> 00:00:06,572 Narrator: Manhattan, new york city. 2 00:00:06,574 --> 00:00:08,841 Once it is home to two of america's 3 00:00:08,843 --> 00:00:11,844 most iconic buildings -- 4 00:00:11,846 --> 00:00:16,348 the world trade center twin towers. 5 00:00:16,350 --> 00:00:19,585 They were giants of their time. 6 00:00:19,587 --> 00:00:22,921 When constructed, they are an engineering triumph -- 7 00:00:22,923 --> 00:00:25,758 the tallest buildings on earth. 8 00:00:25,760 --> 00:00:29,161 The skyscraper has always been an engineering marvel, 9 00:00:29,163 --> 00:00:32,664 but it was the twin towers that really set the precedent. 10 00:00:34,268 --> 00:00:38,737 Narrator: Remembered across the globe for their tragic final moments, 11 00:00:38,739 --> 00:00:42,941 the story of their origins is rarely told. 12 00:00:42,943 --> 00:00:46,245 Today, investigators use pioneering technology 13 00:00:46,247 --> 00:00:49,048 to examine the twin towers' surviving footprint 14 00:00:49,050 --> 00:00:50,949 and secret history... 15 00:00:50,951 --> 00:00:54,486 We can see details you can't see from the human eye. 16 00:00:54,488 --> 00:00:57,990 Narrator: ...And put the materials they are built with to the test. 17 00:00:59,860 --> 00:01:03,962 They really broke records in structural design innovation. 18 00:01:03,964 --> 00:01:08,100 Narrator: How do engineers push the boundaries of building tall 19 00:01:08,102 --> 00:01:13,405 to erect these towering giants in the heart of new york city? 20 00:01:13,407 --> 00:01:18,610 To solve this mystery, we'll digitally reconstruct the towers 21 00:01:18,612 --> 00:01:22,481 and reveal the hidden secrets behind their iconic design 22 00:01:22,483 --> 00:01:25,517 that once defined new york's skyline. 23 00:01:28,756 --> 00:01:31,723 -- Captions by vitac -- www.Vitac.Com 24 00:01:31,725 --> 00:01:34,726 captions paid for by discovery communications 25 00:01:34,728 --> 00:01:36,428 narrator: New york city... 26 00:01:36,430 --> 00:01:41,300 Today over 6,000 high rises dominate the horizon, 27 00:01:41,302 --> 00:01:44,736 but back in the 1960s, only a handful of buildings 28 00:01:44,738 --> 00:01:47,606 stand higher than 40 stories. 29 00:01:47,608 --> 00:01:51,477 Darton: Something radical needed to be done to bring new york, 30 00:01:51,479 --> 00:01:55,047 really, into the 21st century, and in our culture, 31 00:01:55,049 --> 00:01:59,251 bold gestures are often made architecturally. 32 00:01:59,253 --> 00:02:03,188 Narrator: So new york's leaders invest hundreds of millions of dollars 33 00:02:03,190 --> 00:02:07,559 in a spectacular building complex. 34 00:02:07,561 --> 00:02:10,062 The architect, minoru yamasaki, 35 00:02:10,064 --> 00:02:12,731 went through literally hundreds of plans, 36 00:02:12,733 --> 00:02:14,533 and eventually he succeeded 37 00:02:14,535 --> 00:02:17,936 by coming up with the idea for the twin towers. 38 00:02:20,141 --> 00:02:22,274 Narrator: The world trade center design features 39 00:02:22,276 --> 00:02:27,312 five lowrise buildings surrounding a central plaza. 40 00:02:27,314 --> 00:02:28,981 Rising from the center, 41 00:02:28,983 --> 00:02:33,418 the most ambitious skyscrapers of all time, 42 00:02:33,420 --> 00:02:35,854 each soaring more than 1,300 feet 43 00:02:35,856 --> 00:02:38,757 above lower manhattan. 44 00:02:38,759 --> 00:02:42,628 Over four times taller than the statue of liberty. 45 00:02:42,630 --> 00:02:45,797 Together they will deliver 10 million square feet 46 00:02:45,799 --> 00:02:48,133 of real estate. 47 00:02:48,135 --> 00:02:53,639 Every day, 50,000 workers will flock to this city in the sky, 48 00:02:53,641 --> 00:02:57,075 but no one has ever tried erecting a building this tall, 49 00:02:57,077 --> 00:02:58,710 so how do they do it? 50 00:03:02,082 --> 00:03:06,118 Eric darton is an expert on the history of new york city. 51 00:03:06,120 --> 00:03:09,621 He investigates the challenges the engineers face. 52 00:03:09,623 --> 00:03:14,526 The idea was to acquire land and to build on a 16-acre site, 53 00:03:14,528 --> 00:03:15,961 which is not a lot, 54 00:03:15,963 --> 00:03:20,265 enough office space to accommodate 50,000 people. 55 00:03:20,267 --> 00:03:25,604 That's why the towers had to be as tall as they were. 56 00:03:25,606 --> 00:03:27,539 Narrator: The architects look to what is then 57 00:03:27,541 --> 00:03:30,609 the world's tallest skyscraper for inspiration -- 58 00:03:30,611 --> 00:03:34,813 the iconic empire state building. 59 00:03:34,815 --> 00:03:39,618 No skyscraper had come near to surpassing it in 40 years. 60 00:03:42,856 --> 00:03:45,090 The empire state building is the high point 61 00:03:45,092 --> 00:03:47,626 of skyscraper design -- 62 00:03:47,628 --> 00:03:50,162 a steel grid reinforced with concrete 63 00:03:50,164 --> 00:03:53,098 and encased in strong limestone. 64 00:03:53,100 --> 00:03:56,835 It weighs 365,000 tons, 65 00:03:56,837 --> 00:03:59,871 but using this building model for the twin towers 66 00:03:59,873 --> 00:04:02,174 would limit the internal space. 67 00:04:04,945 --> 00:04:08,547 Darton: The empire state building has 2 million square feet 68 00:04:08,549 --> 00:04:11,316 of floor space, office space. 69 00:04:11,318 --> 00:04:13,518 The program for the world trade center 70 00:04:13,520 --> 00:04:17,456 was 8 million square feet of office space. 71 00:04:17,458 --> 00:04:20,826 You can't just scale up the empire state building. 72 00:04:20,828 --> 00:04:22,027 There'd be so many columns, 73 00:04:22,029 --> 00:04:24,830 you couldn't put anybody in the building. 74 00:04:24,832 --> 00:04:28,033 Narrator: So experts have to figure out how to build skyscrapers 75 00:04:28,035 --> 00:04:29,301 that each have double 76 00:04:29,303 --> 00:04:32,070 the floor space of the empire state building. 77 00:04:35,743 --> 00:04:37,943 Professor adrian brugger investigates 78 00:04:37,945 --> 00:04:41,313 the empire state building and twin towers structures. 79 00:04:42,850 --> 00:04:44,583 Brugger: Here I have a 3d print 80 00:04:44,585 --> 00:04:47,686 of one of the floors of the empire state building, 81 00:04:47,688 --> 00:04:50,956 and you can see that we have this very thick core 82 00:04:50,958 --> 00:04:52,624 in the middle here. 83 00:04:52,626 --> 00:04:55,494 Narrator: The model shows how the weight of the empire state building 84 00:04:55,496 --> 00:04:57,796 is spread across a thick central core 85 00:04:57,798 --> 00:05:00,565 and hundreds of columns, 86 00:05:00,567 --> 00:05:02,200 but in order to beat the height of 87 00:05:02,202 --> 00:05:04,469 the empire state building and accommodate 88 00:05:04,471 --> 00:05:07,739 the 50,000 office workers, 89 00:05:07,741 --> 00:05:09,875 japanese american architect 90 00:05:09,877 --> 00:05:12,978 minoru yamasaki and american engineer 91 00:05:12,980 --> 00:05:15,514 leslie robertson will have to come up 92 00:05:15,516 --> 00:05:18,483 with a revolutionary solution. 93 00:05:18,485 --> 00:05:21,853 Minoru yamasaki had the idea of this stress skin 94 00:05:21,855 --> 00:05:23,622 architecture from nature, 95 00:05:23,624 --> 00:05:25,057 which is generally a very good place 96 00:05:25,059 --> 00:05:27,092 to get your motivation from. 97 00:05:27,094 --> 00:05:29,628 And he thought of bamboo. 98 00:05:29,630 --> 00:05:32,531 Narrator: Bamboo is hollow and its entire weight 99 00:05:32,533 --> 00:05:35,667 is supported by its outer skin. 100 00:05:35,669 --> 00:05:37,336 It's a very strong plant. 101 00:05:37,338 --> 00:05:41,239 In japan it's also a symbol of prosperity. 102 00:05:41,241 --> 00:05:45,711 The idea is the perfect solution for the twin towers. 103 00:05:45,713 --> 00:05:48,580 Instead of spreading the load across a grid, 104 00:05:48,582 --> 00:05:52,517 yamasaki creates a super strong hollow structure. 105 00:05:52,519 --> 00:05:55,754 All the weight is supported around the edge. 106 00:05:55,756 --> 00:05:58,590 It's a breakthrough in skyscraper design. 107 00:06:01,228 --> 00:06:05,931 Each tower is made up of 236 steel columns. 108 00:06:05,933 --> 00:06:11,203 They stand just 26 inches apart around the perimeter. 109 00:06:11,205 --> 00:06:15,607 More than 43,000 windows complete the outer skin. 110 00:06:15,609 --> 00:06:19,745 Inside, steel plates stretch across the void, 111 00:06:19,747 --> 00:06:22,714 topped with nearly a million tons of concrete 112 00:06:22,716 --> 00:06:26,184 to create 110 floors. 113 00:06:26,186 --> 00:06:29,721 The north tower is crowned with a final feature -- 114 00:06:29,723 --> 00:06:34,459 a 360-foot communications mast. 115 00:06:34,461 --> 00:06:37,929 Now they have a new task -- to make this outer skin 116 00:06:37,931 --> 00:06:40,799 strong enough to support the weight of the tower. 117 00:06:43,871 --> 00:06:45,604 Adrian has rare access 118 00:06:45,606 --> 00:06:49,141 to the surviving structures of the twin towers, 119 00:06:49,143 --> 00:06:51,209 carefully conserved in the basement of 120 00:06:51,211 --> 00:06:54,112 the 9/11 memorial museum. 121 00:06:54,114 --> 00:06:56,114 He examines the surviving columns 122 00:06:56,116 --> 00:06:58,583 from the outer skin of the building. 123 00:06:58,585 --> 00:07:01,153 So here you can see one of those mega columns 124 00:07:01,155 --> 00:07:02,754 that is landing the forces 125 00:07:02,756 --> 00:07:05,257 from 110 stories into the ground. 126 00:07:05,259 --> 00:07:08,093 And if we look up at the column, we'll see that one of these 127 00:07:08,095 --> 00:07:11,029 large columns goes through this beautiful trident structure 128 00:07:11,031 --> 00:07:14,166 that then picks up three smaller columns. 129 00:07:14,168 --> 00:07:17,969 Narrator: The hollow building structure is a breakthrough in engineering, 130 00:07:17,971 --> 00:07:21,239 but this revolutionary concept goes even further, 131 00:07:21,241 --> 00:07:23,642 to the skeleton itself. 132 00:07:23,644 --> 00:07:25,877 This column is actually hollow on the inside, 133 00:07:25,879 --> 00:07:27,379 and that's purposely done 134 00:07:27,381 --> 00:07:29,047 so that we can increase its stiffness. 135 00:07:29,049 --> 00:07:32,784 So the material is all along the perimeter. 136 00:07:32,786 --> 00:07:36,755 Narrator: Not only is the building hollow, but the columns are hollow, too. 137 00:07:39,293 --> 00:07:41,893 Finally the designers use an extra trick 138 00:07:41,895 --> 00:07:45,864 to reduce the overall weight of the twin towers. 139 00:07:45,866 --> 00:07:48,867 The metal is 4 inches thick at the base, 140 00:07:48,869 --> 00:07:53,538 but tapers to just 1/4-inch at the highest point. 141 00:07:53,540 --> 00:07:55,974 Brugger: They get lighter and lighter as you go up to the top. 142 00:07:55,976 --> 00:07:58,477 And that is essentially to make the building 143 00:07:58,479 --> 00:08:00,445 as efficient as humanly possible. 144 00:08:00,447 --> 00:08:01,913 So this is how structural engineers 145 00:08:01,915 --> 00:08:05,050 are able to build this high. 146 00:08:05,052 --> 00:08:08,153 Narrator: Structurally the design is perfect. 147 00:08:08,155 --> 00:08:09,821 It breaks through the constraints 148 00:08:09,823 --> 00:08:13,425 of contemporary skyscraper building. 149 00:08:13,427 --> 00:08:18,597 With this method, the height seems almost limitless. 150 00:08:18,599 --> 00:08:21,266 It will allow the twin towers to rise taller 151 00:08:21,268 --> 00:08:24,636 than any other building on the planet, 152 00:08:24,638 --> 00:08:30,475 reaching an astonishing 1,368 feet, 153 00:08:30,477 --> 00:08:33,745 but engineers face an additional challenge... 154 00:08:33,747 --> 00:08:35,580 Darton: How do you bring people in? 155 00:08:35,582 --> 00:08:38,016 The vast majority of people going into 156 00:08:38,018 --> 00:08:42,587 the world trade center came in underground. 157 00:08:42,589 --> 00:08:45,690 Narrator: Office workers arrive at the towers via the subway 158 00:08:45,692 --> 00:08:49,094 and then go straight to the elevators, 159 00:08:49,096 --> 00:08:51,863 but in the mornings, there are 50,000 people 160 00:08:51,865 --> 00:08:54,499 coming in at the same time. 161 00:08:54,501 --> 00:08:57,135 How can they all travel through the tower? 162 00:08:59,573 --> 00:09:01,439 In the empire state building, 163 00:09:01,441 --> 00:09:07,279 elevators occupy almost 3/10s of the buildings floor area. 164 00:09:07,281 --> 00:09:09,714 If engineers follow a similar design, 165 00:09:09,716 --> 00:09:13,184 the twin towers would lose vital floor space. 166 00:09:16,657 --> 00:09:19,658 The taller a building is, the more elevators it needs 167 00:09:19,660 --> 00:09:21,159 to service it, 168 00:09:21,161 --> 00:09:24,496 but the shafts take up valuable floor space. 169 00:09:26,900 --> 00:09:30,502 The subway system in new york provides inspiration 170 00:09:30,504 --> 00:09:33,204 where local trains stop at every station 171 00:09:33,206 --> 00:09:37,642 and express ones fast track to key hubs. 172 00:09:37,644 --> 00:09:41,446 So engineers add two sky lobbies to the twin towers. 173 00:09:41,448 --> 00:09:43,848 These are special floors where people can switch 174 00:09:43,850 --> 00:09:48,920 from a large capacity express elevator to a local one. 175 00:09:48,922 --> 00:09:52,223 So multiple elevators can share the same shaft, 176 00:09:52,225 --> 00:09:54,793 increasing the usable floor space. 177 00:09:57,197 --> 00:10:00,799 Engineering masterminds designed these iconic towers 178 00:10:00,801 --> 00:10:05,437 to break the record for the tallest building in the world, 179 00:10:05,439 --> 00:10:10,976 ready to steal the crown from the empire state building. 180 00:10:10,978 --> 00:10:12,811 But when construction begins, 181 00:10:12,813 --> 00:10:17,916 engineers face an unexpected challenge beneath the ground. 182 00:10:17,918 --> 00:10:22,253 Could it become a serious danger for the building's foundations? 183 00:10:31,031 --> 00:10:33,698 Narrator: The twin towers are once 184 00:10:33,700 --> 00:10:37,702 the most recognizable skyscrapers in the world 185 00:10:37,704 --> 00:10:40,338 and the tallest buildings of their time. 186 00:10:42,809 --> 00:10:45,644 The engineers who build them face huge challenges 187 00:10:45,646 --> 00:10:48,647 from the very start of their construction. 188 00:10:48,649 --> 00:10:50,482 That's because much of lower manhattan 189 00:10:50,484 --> 00:10:53,385 is built on reclaimed land. 190 00:10:53,387 --> 00:10:56,021 When engineers start to dig their foundations, 191 00:10:56,023 --> 00:10:58,089 they make a surprising discovery. 192 00:11:00,594 --> 00:11:02,994 Hidden below the pavement, engineers 193 00:11:02,996 --> 00:11:06,464 encounter water-logged mud and rubble, 194 00:11:06,466 --> 00:11:10,535 including a 400-year-old ship preserved in mud. 195 00:11:12,906 --> 00:11:17,142 As they dig deeper, a modern obstacle emerges -- 196 00:11:17,144 --> 00:11:18,910 an active railway tunnel 197 00:11:18,912 --> 00:11:23,648 that carries 100,000 passengers per day. 198 00:11:23,650 --> 00:11:27,352 And even further down, 70 feet below street level, 199 00:11:27,354 --> 00:11:29,954 they finally reach bedrock. 200 00:11:29,956 --> 00:11:35,093 This exceptionally hard material is the perfect base to build on, 201 00:11:35,095 --> 00:11:37,862 but in order to secure the twin towers' foundations 202 00:11:37,864 --> 00:11:39,197 into the bedrock, 203 00:11:39,199 --> 00:11:41,599 the engineers first have to reach it. 204 00:11:44,104 --> 00:11:48,106 Structural engineer rick ellman investigates how it's possible 205 00:11:48,108 --> 00:11:51,309 to build a skyscraper on reclaimed land. 206 00:11:53,346 --> 00:11:56,014 These tall towers that are all around us, 207 00:11:56,016 --> 00:11:58,516 if they were not constructed properly, 208 00:11:58,518 --> 00:12:02,353 they would lean and very likely topple over. 209 00:12:02,355 --> 00:12:06,291 Narrator: The most famous example of the dangers of unsteady foundations 210 00:12:06,293 --> 00:12:09,394 is the tower of pisa in Italy. 211 00:12:09,396 --> 00:12:11,029 It's build on marshy land, 212 00:12:11,031 --> 00:12:14,699 similar to the reclaimed land of manhattan. 213 00:12:14,701 --> 00:12:17,202 Over time, the soft ground causes the tower 214 00:12:17,204 --> 00:12:19,070 to tilt dangerously. 215 00:12:21,742 --> 00:12:25,944 But unlike pisa, manhattan has an incredibly tough bedrock 216 00:12:25,946 --> 00:12:28,379 just 70-feet underground. 217 00:12:28,381 --> 00:12:31,416 The twin towers, foundations can be firmly anchored 218 00:12:31,418 --> 00:12:36,087 into this solid rock, but there's a huge problem -- 219 00:12:36,089 --> 00:12:37,589 reaching it. 220 00:12:37,591 --> 00:12:41,126 Ellman: Excavating in reclaimed land can be a real challenge. 221 00:12:41,128 --> 00:12:45,096 Because of the weak soils and the high ground water, 222 00:12:45,098 --> 00:12:48,733 you're unable to just simply excavate. 223 00:12:48,735 --> 00:12:53,772 Narrator: If they excavate, water and mud just slide back into the hole, 224 00:12:53,774 --> 00:12:57,609 so engineers come up with an ingenious solution -- 225 00:12:57,611 --> 00:13:01,446 they create a wall to surround the foundations. 226 00:13:01,448 --> 00:13:04,149 The wall acts as a protective barrier. 227 00:13:07,521 --> 00:13:11,089 First, they dig long trenches around the edge of the site 228 00:13:11,091 --> 00:13:12,957 and fill them with a liquid slurry 229 00:13:12,959 --> 00:13:15,326 to stop them collapsing inwards. 230 00:13:18,165 --> 00:13:22,500 Then they drop reinforced steel cages into the trenches and pump 231 00:13:22,502 --> 00:13:25,069 in concrete to displace the slurry. 232 00:13:29,643 --> 00:13:32,477 Once these concrete walls set solid, 233 00:13:32,479 --> 00:13:35,547 engineers can excavate everything inside them, 234 00:13:35,549 --> 00:13:40,351 creating a giant underground bathtub. 235 00:13:40,353 --> 00:13:44,189 Instead of holding water in, the bathtub keeps it out 236 00:13:44,191 --> 00:13:46,624 so the foundations can now be built. 237 00:13:50,230 --> 00:13:54,599 At the 9/11 memorial site, rick ellman has rare access 238 00:13:54,601 --> 00:13:58,169 to the remains of the bathtub. 239 00:13:58,171 --> 00:14:01,906 Ellman: Here we are, 70-feet below the ground surface 240 00:14:01,908 --> 00:14:04,342 in the area that was once occupied 241 00:14:04,344 --> 00:14:07,645 by the original world trade center foundations. 242 00:14:07,647 --> 00:14:12,784 The space was created using the slurry wall construction method. 243 00:14:12,786 --> 00:14:15,987 Narrator: The wall holds back the surrounding mud and ground water 244 00:14:15,989 --> 00:14:20,325 to protect the towers' foundations. 245 00:14:20,327 --> 00:14:23,795 The wall itself is probably 3 to 5-feet thick, 246 00:14:23,797 --> 00:14:25,496 and this system, amazingly, 247 00:14:25,498 --> 00:14:29,701 is holdings thousands of pounds of force. 248 00:14:29,703 --> 00:14:32,604 Narrator: The twin towers engineers are some of the first 249 00:14:32,606 --> 00:14:35,573 to use this pioneering technique in new york. 250 00:14:39,379 --> 00:14:43,681 In 1968, the bathtub is complete 251 00:14:43,683 --> 00:14:47,285 and the main building work can begin. 252 00:14:47,287 --> 00:14:51,789 But engineers face additional challenges. 253 00:14:51,791 --> 00:14:54,726 The twin towers are the world's first skyscrapers 254 00:14:54,728 --> 00:14:57,562 built without stone or brick. 255 00:14:57,564 --> 00:14:59,931 Their flexible metal structure causes them 256 00:14:59,933 --> 00:15:03,368 to sway in high winds. 257 00:15:03,370 --> 00:15:05,470 How can engineers stop new yorkers 258 00:15:05,472 --> 00:15:07,939 from getting sea sick inside? 259 00:15:17,250 --> 00:15:20,084 Narrator: April 1973... 260 00:15:20,086 --> 00:15:22,921 In the heart of manhattan, two skyscrapers are set 261 00:15:22,923 --> 00:15:27,058 to emerge on the horizon -- the twin towers. 262 00:15:27,060 --> 00:15:31,562 Their hollow metal structure is a revolutionary design. 263 00:15:31,564 --> 00:15:34,132 They are the first skyscrapers ever built 264 00:15:34,134 --> 00:15:36,467 without stone or brick. 265 00:15:36,469 --> 00:15:38,569 The engineers use a new technique 266 00:15:38,571 --> 00:15:41,005 to construct these steel giants. 267 00:15:41,007 --> 00:15:44,542 They click the pieces together like lego. 268 00:15:44,544 --> 00:15:46,044 Engineer sawteen see 269 00:15:46,046 --> 00:15:47,845 is the managing partner of the company 270 00:15:47,847 --> 00:15:51,282 that built the world trade center. 271 00:15:51,284 --> 00:15:53,451 All the design of the twin towers 272 00:15:53,453 --> 00:15:58,923 allowed for faster and safer erection and construction, 273 00:15:58,925 --> 00:16:02,827 so it went up really fast. 274 00:16:02,829 --> 00:16:05,663 Narrator: Building the pieces off site allows engineers 275 00:16:05,665 --> 00:16:08,700 to complete them to the highest standard, 276 00:16:08,702 --> 00:16:13,304 and it means the structure can be assembled quickly on site. 277 00:16:13,306 --> 00:16:15,873 See: In a sense it was like an assembly plant. 278 00:16:19,713 --> 00:16:23,414 Narrator: The twin towers' external skin goes up first. 279 00:16:23,416 --> 00:16:29,087 59 steel columns per side, each just 14 inches wide. 280 00:16:29,089 --> 00:16:32,724 They bear most of the towers' weight. 281 00:16:32,726 --> 00:16:37,962 In the center of each tower sits a core of 47 steel columns 282 00:16:37,964 --> 00:16:40,965 to give the tower extra rigidity. 283 00:16:40,967 --> 00:16:44,535 Thin steel plates 60-feet wide bridge the gap, 284 00:16:44,537 --> 00:16:47,171 connecting the inner and outer columns, 285 00:16:47,173 --> 00:16:49,741 anchoring the entire structure together, 286 00:16:49,743 --> 00:16:53,378 and creating the vast open floor plans. 287 00:16:53,380 --> 00:16:56,147 It's a futuristic lightweight structure, 288 00:16:56,149 --> 00:16:58,383 but will this flexible design be a match 289 00:16:58,385 --> 00:17:01,452 for new york's high winds? 290 00:17:01,454 --> 00:17:03,421 At the top of the buildings, wind speeds 291 00:17:03,423 --> 00:17:06,424 can reach 100 miles per hour. 292 00:17:06,426 --> 00:17:08,626 To make it very tall, the building 293 00:17:08,628 --> 00:17:13,564 had to be very stiff to resist the wind loads. 294 00:17:13,566 --> 00:17:17,335 Narrator: If the building sways too much, it could sheer the steal, 295 00:17:17,337 --> 00:17:21,072 or the elevators could even get stuck in the shafts. 296 00:17:21,074 --> 00:17:24,776 So the twin towers become the first skyscrapers in the world 297 00:17:24,778 --> 00:17:27,645 to be evaluated for wind dynamics. 298 00:17:27,647 --> 00:17:31,115 A new field of research is born. 299 00:17:31,117 --> 00:17:34,252 Scale models on the twin towers 300 00:17:34,254 --> 00:17:36,421 and the complex, the surrounding areas 301 00:17:36,423 --> 00:17:41,392 were built and tested in colorado. 302 00:17:41,394 --> 00:17:44,429 Narrator: In the 1960s, the twin towers' engineers 303 00:17:44,431 --> 00:17:46,564 create the fist ever wind tunnel 304 00:17:46,566 --> 00:17:49,801 and put their models to the test. 305 00:17:49,803 --> 00:17:54,038 They shoot 150 mile per hour winds across it. 306 00:17:54,040 --> 00:17:57,141 The tests show they will hold strong structurally -- 307 00:17:57,143 --> 00:18:01,312 a success, but there's a problem... 308 00:18:01,314 --> 00:18:04,315 Even though the models can withstand the wind loads, 309 00:18:04,317 --> 00:18:07,452 they sway 1 inch from side to side, 310 00:18:07,454 --> 00:18:12,223 equivalent to 40 feet on the full size buildings. 311 00:18:12,225 --> 00:18:15,460 Workers at the top will sway with the buildings. 312 00:18:15,462 --> 00:18:17,662 Gazing out of the window at the second tower 313 00:18:17,664 --> 00:18:22,166 swaying will make the sense of movement worse. 314 00:18:22,168 --> 00:18:24,535 On the 110th story, a gust of wind 315 00:18:24,537 --> 00:18:26,971 will trigger instant nausea. 316 00:18:26,973 --> 00:18:29,807 It would be impossible to work there. 317 00:18:29,809 --> 00:18:32,677 So the wind tunnel test provided wind loads, 318 00:18:32,679 --> 00:18:37,148 but we didn't know whether those accelerations were acceptable 319 00:18:37,150 --> 00:18:40,251 to people who are in the building, the occupants. 320 00:18:40,253 --> 00:18:42,720 Would they feel sea sick? 321 00:18:42,722 --> 00:18:43,821 We don't know. 322 00:18:43,823 --> 00:18:45,723 There were no guidelines around. 323 00:18:49,696 --> 00:18:52,130 Narrator: How much sway humans can endure 324 00:18:52,132 --> 00:18:55,099 is a critical question for skyscraper engineers. 325 00:18:56,536 --> 00:18:59,670 To investigate, structural engineer antony darby 326 00:18:59,672 --> 00:19:03,074 launches the "v" simulator, 327 00:19:03,076 --> 00:19:05,776 a state of the art virtual reality skyscraper 328 00:19:05,778 --> 00:19:09,714 at the university of bath, England. 329 00:19:09,716 --> 00:19:13,251 Heavy duty hydraulics suspend a custom made office, 330 00:19:13,253 --> 00:19:18,789 simulating the sway motion of a skyscraper in gusts of wind. 331 00:19:18,791 --> 00:19:21,826 Inside, a volunteer wears 3d glasses 332 00:19:21,828 --> 00:19:24,362 to experience a virtual reality view 333 00:19:24,364 --> 00:19:27,765 out of the skyscraper windows. 334 00:19:27,767 --> 00:19:31,302 Antony's team records the volunteer's physical reactions 335 00:19:31,304 --> 00:19:35,406 as the skyscraper's sway intensifies. 336 00:19:35,408 --> 00:19:38,776 We can measure their response, their heart rate, 337 00:19:38,778 --> 00:19:40,978 their eye blink rate, 338 00:19:40,980 --> 00:19:43,848 things that measure stress or tiredness. 339 00:19:43,850 --> 00:19:46,584 We can also assess their work performance, 340 00:19:46,586 --> 00:19:49,921 how that degrades over time as well. 341 00:19:49,923 --> 00:19:53,524 Narrator: Antony's team collects data from hundreds of volunteers 342 00:19:53,526 --> 00:19:57,695 to determine the breaking point of humans. 343 00:19:57,697 --> 00:20:00,398 He measures the movement in g-force. 344 00:20:00,400 --> 00:20:04,502 The typical g-force felt on a roller-coaster is 4g 345 00:20:04,504 --> 00:20:08,739 compared to just 0.01g for walking. 346 00:20:08,741 --> 00:20:11,108 What we've shown here is that below 347 00:20:11,110 --> 00:20:16,347 about 0.005g, people don't feel the motion. 348 00:20:16,349 --> 00:20:19,750 Above 0.005g, you can perceive the motion, 349 00:20:19,752 --> 00:20:21,652 but it might not affect you that much. 350 00:20:21,654 --> 00:20:26,290 But as we steadily get higher up around 0.04g, 351 00:20:26,292 --> 00:20:29,327 then it does become intolerable. 352 00:20:29,329 --> 00:20:34,332 Narrator: The simulator tests reveal that 0.04g is the upper limit. 353 00:20:34,334 --> 00:20:37,134 Anything above this force is intolerable. 354 00:20:40,006 --> 00:20:43,608 Today, this research is used for high-tech skyscraper design 355 00:20:43,610 --> 00:20:47,011 and rigorous testing. 356 00:20:47,013 --> 00:20:50,648 But in the 1960s, tests like these don't exist. 357 00:20:50,650 --> 00:20:51,882 The twin towers team 358 00:20:51,884 --> 00:20:55,186 has to invent the very first test themselves. 359 00:20:58,157 --> 00:21:03,127 The twin towers engineers build their own test rig 360 00:21:03,129 --> 00:21:08,666 and hang it inside a ventilation shaft in the lincoln tunnel. 361 00:21:08,668 --> 00:21:11,802 Volunteers sit inside the fake office 362 00:21:11,804 --> 00:21:15,072 while two engineers push it from side to side 363 00:21:15,074 --> 00:21:19,176 and ask them to report when they feel sick. 364 00:21:19,178 --> 00:21:21,946 Even these crude tests are enough to confirm 365 00:21:21,948 --> 00:21:25,483 what the twin towers engineers feared -- 366 00:21:25,485 --> 00:21:30,421 people aren't able to work in these conditions. 367 00:21:30,423 --> 00:21:33,824 In order to reduce the 40-foot sway, the architects 368 00:21:33,826 --> 00:21:36,861 create special shock absorbers to reduce 369 00:21:36,863 --> 00:21:41,132 the movement between the wall and the floor. 370 00:21:41,134 --> 00:21:45,202 They install 10,000 of these dampers across the two towers, 371 00:21:45,204 --> 00:21:47,571 absorbing huge amounts of energy, 372 00:21:47,573 --> 00:21:50,308 reducing the sway to just 3 feet. 373 00:21:52,779 --> 00:21:54,779 They are a success. 374 00:21:54,781 --> 00:21:58,482 They are the first skyscraper dampers in the world. 375 00:21:58,484 --> 00:22:01,052 See: Everything in the original twin towers 376 00:22:01,054 --> 00:22:04,021 was innovative and ground breaking -- 377 00:22:04,023 --> 00:22:07,358 in the design, in the materials, 378 00:22:07,360 --> 00:22:10,594 in the construction, and erection. 379 00:22:10,596 --> 00:22:13,831 Narrator: The twin towers are a modern masterpiece 380 00:22:13,833 --> 00:22:17,501 created with exquisite attention to detail. 381 00:22:17,503 --> 00:22:21,272 By creating these technological solutions, the engineers 382 00:22:21,274 --> 00:22:24,175 keep the 50,000 people inside safe 383 00:22:24,177 --> 00:22:28,112 from these foreseeable dangers. 384 00:22:28,114 --> 00:22:30,948 But the threat that lies ahead is unimaginable 385 00:22:30,950 --> 00:22:32,616 to its creators -- 386 00:22:32,618 --> 00:22:35,886 the deadly impact of the terrorist attack on 9/11 387 00:22:35,888 --> 00:22:39,790 surpasses any force engineers could ever predict. 388 00:22:39,792 --> 00:22:46,230 ♪ 389 00:22:53,239 --> 00:22:56,240 narrator: In 1973, the twin towers 390 00:22:56,242 --> 00:22:59,777 become the tallest buildings on the planet. 391 00:22:59,779 --> 00:23:06,484 These engineering marvels rise 1,368 feet into the sky, 392 00:23:06,486 --> 00:23:08,686 but no one could foresee the challenges 393 00:23:08,688 --> 00:23:12,056 they would face over their lifetime. 394 00:23:12,058 --> 00:23:15,693 In 1975, an electric short circuit failure 395 00:23:15,695 --> 00:23:18,929 set six stories of the north tower on fire. 396 00:23:18,931 --> 00:23:21,499 Firefighters manage to control the blaze. 397 00:23:21,501 --> 00:23:23,534 Fortunately the building was unoccupied 398 00:23:23,536 --> 00:23:26,036 or there would have been, potentially, 399 00:23:26,038 --> 00:23:28,339 some serious consequences. 400 00:23:28,341 --> 00:23:32,009 Narrator: 20 years later, a bigger impact strikes -- 401 00:23:32,011 --> 00:23:34,812 a terrorist plants a 1,200-pound bomb 402 00:23:34,814 --> 00:23:37,648 in the north tower garage. 403 00:23:37,650 --> 00:23:40,751 It explodes with devastating force, 404 00:23:40,753 --> 00:23:44,722 yet the tower stands strong. 405 00:23:44,724 --> 00:23:48,726 But on September 11, 2001, a tragedy bigger 406 00:23:48,728 --> 00:23:52,029 than anything imaginable strikes the towers. 407 00:23:55,435 --> 00:23:59,737 The world watches in disbelief. 408 00:23:59,739 --> 00:24:05,876 Over 17,000 people are working inside the twin towers 409 00:24:05,878 --> 00:24:09,713 when suddenly a terrorist-hijacked boeing 767 410 00:24:09,715 --> 00:24:14,618 with 92 people on board crashes into the north tower. 411 00:24:14,620 --> 00:24:16,487 The impact creates a burning hole 412 00:24:16,489 --> 00:24:23,727 between the 94th and 99th floors of the 110-story skyscraper. 413 00:24:23,729 --> 00:24:28,199 The 10,000 dampers absorb the energy from the impact, 414 00:24:28,201 --> 00:24:30,901 and the hollow skeleton design supports the weight 415 00:24:30,903 --> 00:24:33,304 of the building despite the damage. 416 00:24:36,509 --> 00:24:39,210 17 minutes after the first plane hits, 417 00:24:39,212 --> 00:24:42,279 a second hijacked boeing 767 418 00:24:42,281 --> 00:24:44,882 with 65 passengers and crew on board... 419 00:24:47,787 --> 00:24:51,822 ...Crashes into the 77th to 85th floor of the south tower. 420 00:24:53,159 --> 00:24:56,994 There was a tremendous outburst of smoke, debris. 421 00:24:56,996 --> 00:25:01,165 Debris from both impacts, particularly the second one, 422 00:25:01,167 --> 00:25:03,434 rained down on the whole area. 423 00:25:06,606 --> 00:25:10,941 Narrator: Over 1,000 people perish in these sudden crashes. 424 00:25:10,943 --> 00:25:12,710 While the towers are still standing, 425 00:25:12,712 --> 00:25:16,447 over 15,000 people manage to evacuate. 426 00:25:19,719 --> 00:25:21,652 But the tragedy isn't over. 427 00:25:23,656 --> 00:25:26,190 The flammable jet fuel from the aircraft 428 00:25:26,192 --> 00:25:30,160 makes the crash much more deadly than anyone could have foreseen. 429 00:25:32,698 --> 00:25:35,766 When engineers designed the twin towers, they protect 430 00:25:35,768 --> 00:25:39,169 against a crash from the largest aircraft at the time -- 431 00:25:39,171 --> 00:25:42,039 a boeing 707. 432 00:25:42,041 --> 00:25:46,377 But the hijacked aircraft are much large boeing 767s 433 00:25:46,379 --> 00:25:50,114 carrying 20,000 gallons of kerosene. 434 00:25:50,116 --> 00:25:54,084 This jet fuel pours down through the 12 miles of elevator shafts 435 00:25:54,086 --> 00:25:57,421 onto other floors, spreading like wildfire. 436 00:25:59,525 --> 00:26:02,693 Inside the towers, there are thousands of tons of office 437 00:26:02,695 --> 00:26:04,662 furnishings, 438 00:26:04,664 --> 00:26:06,830 many of them highly flammable 439 00:26:06,832 --> 00:26:09,867 so the fire spreads unusually quickly. 440 00:26:09,869 --> 00:26:13,504 The firefighters soon realize that there was no controlling 441 00:26:13,506 --> 00:26:15,072 this blaze, and from then on, 442 00:26:15,074 --> 00:26:17,708 it was concentrating on not saving the building 443 00:26:17,710 --> 00:26:20,411 but on rescuing people inside. 444 00:26:20,413 --> 00:26:24,682 Narrator: At exactly 9:59 a.M., almost two hours 445 00:26:24,684 --> 00:26:27,685 after the first collision, 446 00:26:27,687 --> 00:26:29,853 the south tower collapses... 447 00:26:31,424 --> 00:26:33,223 ...In just 10 seconds. 448 00:26:37,797 --> 00:26:41,498 Half an hour later, the north tower goes down, too. 449 00:26:44,337 --> 00:26:47,871 More than 2,000 people are still inside the buildings. 450 00:26:49,875 --> 00:26:53,177 Darton: It was impossible, I think, for anyone -- 451 00:26:53,179 --> 00:26:55,479 engineer, tourist, new yorker, 452 00:26:55,481 --> 00:27:00,117 to imagine that these towers could possibly come down. 453 00:27:00,119 --> 00:27:03,487 Narrator: After standing strong for nearly two hours, 454 00:27:03,489 --> 00:27:06,657 what ultimately causes the towers to collapse? 455 00:27:09,428 --> 00:27:11,695 The twin towers' all-steel structure 456 00:27:11,697 --> 00:27:13,464 is exceptionally strong, 457 00:27:13,466 --> 00:27:17,635 but its innovative design hides potential weaknesses. 458 00:27:17,637 --> 00:27:19,236 At the base of each tower, 459 00:27:19,238 --> 00:27:22,039 the steel columns are 4 inches thick 460 00:27:22,041 --> 00:27:24,375 to bear the weight of the building above. 461 00:27:26,879 --> 00:27:32,816 But near the top, these columns are only 1/4-inch thick. 462 00:27:32,818 --> 00:27:36,153 To protect the steel from fire, engineers coat it 463 00:27:36,155 --> 00:27:38,055 in a fireproofing spray... 464 00:27:41,027 --> 00:27:46,263 ...But this protective layer is thin and easily dislodged. 465 00:27:46,265 --> 00:27:49,800 Could this be the downfall of these iconic buildings? 466 00:27:49,802 --> 00:27:55,706 ♪ 467 00:27:55,708 --> 00:27:58,642 structural engineer, adrian brugger investigates 468 00:27:58,644 --> 00:28:01,645 if the fire could be the cause of the collapse. 469 00:28:03,983 --> 00:28:06,617 Brugger: The ensuing fire raged inside the building. 470 00:28:06,619 --> 00:28:11,055 The impact caused massive swaths of windows to be torn out 471 00:28:11,057 --> 00:28:13,223 and this extra damage created winds 472 00:28:13,225 --> 00:28:16,126 that were actually fanning the flames. 473 00:28:16,128 --> 00:28:20,097 Narrator: Although the fireproof coating can withstand the flames, 474 00:28:20,099 --> 00:28:24,168 the impact of the aircraft leaves it compromised. 475 00:28:24,170 --> 00:28:26,403 The aircraft entered the building 476 00:28:26,405 --> 00:28:29,106 between 400 and 550 miles per hour 477 00:28:29,108 --> 00:28:31,775 and the impact lasted only 0.7 seconds. 478 00:28:31,777 --> 00:28:33,644 And of course, during that impact, 479 00:28:33,646 --> 00:28:37,347 we actually had all of that fireproofing stripped. 480 00:28:37,349 --> 00:28:40,617 Narrator: Adrian's team investigates how fire affects steel 481 00:28:40,619 --> 00:28:44,154 that has been stripped of its fireproofing. 482 00:28:44,156 --> 00:28:45,823 His team uses floor beams 483 00:28:45,825 --> 00:28:49,026 identical to those from the twin towers. 484 00:28:49,028 --> 00:28:51,795 They load them with 1.4 tons of weight 485 00:28:51,797 --> 00:28:56,033 to simulate the weight of each floor of the tower. 486 00:28:56,035 --> 00:28:58,502 They prepare to heat up the steel beam. 487 00:28:58,504 --> 00:29:02,272 We will apply heat by simulating a fire. 488 00:29:02,274 --> 00:29:03,907 Narrator: The temperature of the fire climbs 489 00:29:03,909 --> 00:29:06,610 to 1,800 degrees fahrenheit -- 490 00:29:06,612 --> 00:29:09,213 equivalent to that inside the towers. 491 00:29:09,215 --> 00:29:11,815 Now we're heating up the beam with flame throwers, 492 00:29:11,817 --> 00:29:13,784 and you can see the temperature is going up 493 00:29:13,786 --> 00:29:15,285 on our infrared camera. 494 00:29:15,287 --> 00:29:17,121 We're reaching about 500 degrees celsius, 495 00:29:17,123 --> 00:29:19,456 so that's 800 degrees fahrenheit. 496 00:29:22,394 --> 00:29:26,196 Narrator: At 800 degrees fahrenheit, the trusses hold strong, 497 00:29:26,198 --> 00:29:29,833 but then they start to show signs of weakness. 498 00:29:29,835 --> 00:29:34,104 Now we see a cherry red spot on the beam that's indicative 499 00:29:34,106 --> 00:29:36,974 of about a temperature of 750 celsius, 500 00:29:36,976 --> 00:29:39,409 which is equivalent to 1,300 fahrenheit. 501 00:29:39,411 --> 00:29:43,046 The beam is actually deflecting significantly. 502 00:29:43,048 --> 00:29:46,416 Narrator: The steel beam gives way after just 11 minutes 503 00:29:46,418 --> 00:29:48,519 at 1,300 degrees. 504 00:29:52,658 --> 00:29:56,193 Adrian's investigation shows that without fireproofing, 505 00:29:56,195 --> 00:29:59,163 the floor beams cannot withstand the high heat 506 00:29:59,165 --> 00:30:00,798 and lose their strength. 507 00:30:03,102 --> 00:30:06,937 The extreme heat weakens the steel floor plates 508 00:30:06,939 --> 00:30:09,273 causing them to sag, 509 00:30:09,275 --> 00:30:13,076 pulling the walls in on each other. 510 00:30:13,078 --> 00:30:15,279 The heat also weakens the steel bolts 511 00:30:15,281 --> 00:30:18,515 that hold the floors in place causing each floor 512 00:30:18,517 --> 00:30:21,418 to collapse down onto the one below -- 513 00:30:21,420 --> 00:30:25,622 a process known as pancaking. 514 00:30:25,624 --> 00:30:27,691 This catastrophe goes down in history 515 00:30:27,693 --> 00:30:29,793 as america's darkest day. 516 00:30:32,364 --> 00:30:35,365 The wreckage burns for 100 days. 517 00:30:35,367 --> 00:30:40,470 ♪ 518 00:30:40,472 --> 00:30:43,507 rescue operations salvage the structural remains 519 00:30:43,509 --> 00:30:45,976 of the twin towers. 520 00:30:45,978 --> 00:30:49,012 Senior vice president of the 9/11 memorial museum, 521 00:30:49,014 --> 00:30:51,014 anthony gardner, 522 00:30:51,016 --> 00:30:55,986 leads the mission to preserve the twin towers structures. 523 00:30:55,988 --> 00:30:58,822 1.8 million tons of debris fill 524 00:30:58,824 --> 00:31:02,759 ground zero in the aftermath of the attacks. 525 00:31:02,761 --> 00:31:07,231 Essentially it was 7-story smoldering pile of material 526 00:31:07,233 --> 00:31:09,199 that stretched up from street level 527 00:31:09,201 --> 00:31:13,070 and then also down seven stories to the bedrock level of the site 528 00:31:13,072 --> 00:31:15,839 where I'm standing today. 529 00:31:15,841 --> 00:31:20,477 Narrator: The scale of destruction is overwhelming. 530 00:31:20,479 --> 00:31:21,979 But despite the force, 531 00:31:21,981 --> 00:31:25,916 some structures survive the collapse. 532 00:31:25,918 --> 00:31:28,418 These elements are conserved on the 9/11 533 00:31:28,420 --> 00:31:31,855 memorial site to this day -- 534 00:31:31,857 --> 00:31:34,625 a display of respect and a place to remember 535 00:31:34,627 --> 00:31:37,761 those that lost their lives. 536 00:31:37,763 --> 00:31:39,696 One of the columns of the twin towers 537 00:31:39,698 --> 00:31:43,200 was found intact in its original position. 538 00:31:43,202 --> 00:31:48,005 So this is the last column that stood, despite the destruction. 539 00:31:48,007 --> 00:31:49,439 Rescue and recovery workers adorned it 540 00:31:49,441 --> 00:31:51,008 with all these inscriptions 541 00:31:51,010 --> 00:31:53,377 and tributes to their fallen colleagues. 542 00:31:58,317 --> 00:32:01,585 Narrator: As the clean up operation evolves, the twin towers 543 00:32:01,587 --> 00:32:04,554 deep foundations come into view. 544 00:32:04,556 --> 00:32:07,424 Buried by rubble, can the damaged bathtub wall 545 00:32:07,426 --> 00:32:10,627 continue to hold back the hudson river? 546 00:32:10,629 --> 00:32:14,731 What will it take to keep this area of lower manhattan safe? 547 00:32:14,733 --> 00:32:19,736 ♪ 548 00:32:24,910 --> 00:32:27,611 narrator: September 11, 2001... 549 00:32:30,149 --> 00:32:32,449 After months of wreckage clean up, 550 00:32:32,451 --> 00:32:36,887 just a handful of structures from the towers remain -- 551 00:32:36,889 --> 00:32:38,956 a reminder of the fight the skyscrapers 552 00:32:38,958 --> 00:32:41,692 put up against the attacks. 553 00:32:41,694 --> 00:32:43,260 Can any part of these buildings 554 00:32:43,262 --> 00:32:47,631 possibly stay standing to this day? 555 00:32:47,633 --> 00:32:51,969 Above ground, very little survives -- 556 00:32:51,971 --> 00:32:54,104 a single staircase. 557 00:32:54,106 --> 00:32:58,508 A steel column from the core of the south tower. 558 00:32:58,510 --> 00:32:59,776 And steel tridents that 559 00:32:59,778 --> 00:33:03,547 once formed the lower facade of the towers. 560 00:33:03,549 --> 00:33:08,618 But below ground, something far bigger survives -- 561 00:33:08,620 --> 00:33:10,887 the vast concrete bathtub 562 00:33:10,889 --> 00:33:14,391 built to protect the site's foundations. 563 00:33:14,393 --> 00:33:16,526 But deep cracks span the wall 564 00:33:16,528 --> 00:33:20,330 and many anchoring bolts begin to leak ground water. 565 00:33:20,332 --> 00:33:22,666 Without repair, the wall could fail, 566 00:33:22,668 --> 00:33:25,936 jeopardizing nearby buildings. 567 00:33:25,938 --> 00:33:29,840 What will it take to keep this area of lower manhattan safe? 568 00:33:31,944 --> 00:33:34,478 Head of conservation, lisa conte, 569 00:33:34,480 --> 00:33:38,682 leads the preservation mission at the 9/11 memorial. 570 00:33:38,684 --> 00:33:41,284 Her four-person team works all year round 571 00:33:41,286 --> 00:33:44,554 to keep this historic structure standing. 572 00:33:44,556 --> 00:33:49,159 Here we are looking at a portion of the original bathtub. 573 00:33:49,161 --> 00:33:53,530 Just beyond it is the hudson river. 574 00:33:53,532 --> 00:33:57,267 Narrator: Following the events of 9/11, the damage to the bathtub wall 575 00:33:57,269 --> 00:34:00,704 threatens new york city with a potential catastrophe. 576 00:34:03,709 --> 00:34:06,209 If the bathtub wall fails, 577 00:34:06,211 --> 00:34:08,645 the area will rapidly flood with groundwater 578 00:34:08,647 --> 00:34:11,314 and water from the nearby hudson river. 579 00:34:13,385 --> 00:34:15,919 This water would then inundate the subway tunnel 580 00:34:15,921 --> 00:34:17,854 that runs through the bathtub... 581 00:34:20,225 --> 00:34:22,125 ...Threatening widespread flooding of 582 00:34:22,127 --> 00:34:24,027 the underground rail network. 583 00:34:26,331 --> 00:34:27,931 With the bathtub breached, 584 00:34:27,933 --> 00:34:32,002 the surrounding ground would weaken, risking subsidence 585 00:34:32,004 --> 00:34:36,807 or even collapse of surrounding buildings. 586 00:34:36,809 --> 00:34:39,409 To detect any weak spots in the wall, 587 00:34:39,411 --> 00:34:42,479 lisa's team turns to cutting-edge technology. 588 00:34:44,750 --> 00:34:47,684 Every year scanning specialist scott lee 589 00:34:47,686 --> 00:34:53,256 laser scans the entire wall to millimeter accuracy. 590 00:34:53,258 --> 00:34:56,660 His lidar scanning works like echo location, 591 00:34:56,662 --> 00:34:58,495 bouncing beams off the surface 592 00:34:58,497 --> 00:35:02,799 and recording the distance with incredible accuracy. 593 00:35:02,801 --> 00:35:05,202 By doing the lidar or laser scanning, we create 594 00:35:05,204 --> 00:35:10,740 a very accurate millimetric model of the entire slurry wall. 595 00:35:10,742 --> 00:35:15,579 Narrator: The team also takes super high resolution photos of the wall 596 00:35:15,581 --> 00:35:19,449 to record the surface detail. 597 00:35:19,451 --> 00:35:22,018 A platform raises the team into the air 598 00:35:22,020 --> 00:35:24,888 to capture the wall from every angle. 599 00:35:24,890 --> 00:35:28,091 Every detail matters. 600 00:35:28,093 --> 00:35:30,393 Scott then combines the lidar scan 601 00:35:30,395 --> 00:35:31,895 and the thousands of photos 602 00:35:31,897 --> 00:35:37,134 into a super high resolution 3d model of the wall. 603 00:35:37,136 --> 00:35:39,970 We project those really high resolution photos 604 00:35:39,972 --> 00:35:41,805 onto that solid surface model 605 00:35:41,807 --> 00:35:45,742 to create a very detailed photo realistic model. 606 00:35:45,744 --> 00:35:47,410 We can see every nook and cranny. 607 00:35:47,412 --> 00:35:52,949 Often we can see details you can't see from the human eye. 608 00:35:52,951 --> 00:35:56,987 Narrator: Armed with this 3d model, lisa's team can hone in 609 00:35:56,989 --> 00:36:00,190 on the weak parts of the wall that need attention. 610 00:36:00,192 --> 00:36:04,227 The brown that you see here is corrosion staining related 611 00:36:04,229 --> 00:36:07,197 to the interaction of the water and the steel elements. 612 00:36:07,199 --> 00:36:11,001 Also present, but less easy to see, are salts. 613 00:36:11,003 --> 00:36:16,473 And those are the areas of white here 614 00:36:16,475 --> 00:36:21,278 and coming down the face here. 615 00:36:21,280 --> 00:36:23,580 Narrator: The 3d scan pinpoints areas 616 00:36:23,582 --> 00:36:26,917 where new water starts to seep through. 617 00:36:26,919 --> 00:36:30,453 It's really important that we're working to preserve the wall 618 00:36:30,455 --> 00:36:32,889 because everything needs to be cared for 619 00:36:32,891 --> 00:36:36,059 in order to preserve its integrity over time. 620 00:36:36,061 --> 00:36:39,896 Narrator: The wall shows great strength and resilience. 621 00:36:39,898 --> 00:36:43,033 With the help of lisa's team, it will remain standing for many 622 00:36:43,035 --> 00:36:48,271 more decades to keep the surrounding city safe. 623 00:36:48,273 --> 00:36:50,373 This colossal underground structure 624 00:36:50,375 --> 00:36:52,676 is a reminder of the innovative engineering 625 00:36:52,678 --> 00:36:57,981 that made these radical skyscrapers a reality. 626 00:36:57,983 --> 00:37:00,483 How have the twin towers' innovations 627 00:37:00,485 --> 00:37:02,252 and their tragic end 628 00:37:02,254 --> 00:37:05,355 changed the design of skyscrapers forever? 629 00:37:15,767 --> 00:37:19,469 Narrator: The twin towers, revolutionary skyscrapers 630 00:37:19,471 --> 00:37:22,539 that have changed the world forever. 631 00:37:22,541 --> 00:37:24,341 What can we learn from the towers 632 00:37:24,343 --> 00:37:28,778 and the events that lead to their collapse? 633 00:37:28,780 --> 00:37:31,114 The impact of two hijacked airplanes 634 00:37:31,116 --> 00:37:33,817 creates massive gashes in the towers 635 00:37:33,819 --> 00:37:38,955 and an unprecedented fire that causes them to collapse. 636 00:37:38,957 --> 00:37:43,493 2,753 people die, 637 00:37:43,495 --> 00:37:45,695 but the building's put up a remarkable fight 638 00:37:45,697 --> 00:37:47,964 in the face of the attack. 639 00:37:47,966 --> 00:37:49,766 On the north face of the north tower 640 00:37:49,768 --> 00:37:52,102 2/3 of the columns were taken out, 641 00:37:52,104 --> 00:37:55,705 but yet, the towers stood for more than an hour, 642 00:37:55,707 --> 00:37:57,440 which allowed occupants 643 00:37:57,442 --> 00:38:01,311 who were below the force of impact to escape. 644 00:38:02,414 --> 00:38:04,948 Darton: The death toll was horrific, 645 00:38:04,950 --> 00:38:08,485 but had they not been as well built or engineered 646 00:38:08,487 --> 00:38:10,420 in the wonderful way that they were, 647 00:38:10,422 --> 00:38:12,255 they probably would have collapsed much earlier 648 00:38:12,257 --> 00:38:16,493 and the death toll would have been exponentially higher. 649 00:38:16,495 --> 00:38:19,262 Narrator: The twin towers are revolutionary structures, 650 00:38:19,264 --> 00:38:22,432 revered by engineers all over the world. 651 00:38:22,434 --> 00:38:24,434 They are the first to break through the ceiling 652 00:38:24,436 --> 00:38:26,703 of skyscraper height, 653 00:38:26,705 --> 00:38:28,471 passing this legacy of innovative 654 00:38:28,473 --> 00:38:32,075 engineering onto their successors. 655 00:38:32,077 --> 00:38:35,512 The skyscraper has always been an engineering marvel, 656 00:38:35,514 --> 00:38:39,149 but it was the twin towers that really set the precedent 657 00:38:39,151 --> 00:38:41,985 with their revolutionary technology, 658 00:38:41,987 --> 00:38:44,954 with all the different extraordinary inventions 659 00:38:44,956 --> 00:38:47,590 that had to be created to even make it happen 660 00:38:47,592 --> 00:38:51,094 that set the precedent for the design and engineering 661 00:38:51,096 --> 00:38:54,331 of the even taller buildings around the world today. 662 00:38:57,269 --> 00:39:02,005 Narrator: Two of the most recognizable skyscrapers of all time. 663 00:39:02,007 --> 00:39:05,542 Two of the first buildings to use a bathtub design. 664 00:39:05,544 --> 00:39:07,610 They paved the way for skyscrapers 665 00:39:07,612 --> 00:39:10,580 on reclaimed land all over the world. 666 00:39:10,582 --> 00:39:12,782 The hollow tube design is now adopted 667 00:39:12,784 --> 00:39:15,985 by thousands of buildings worldwide. 668 00:39:15,987 --> 00:39:19,189 With subway inspired express elevators paving the way 669 00:39:19,191 --> 00:39:22,792 for people to travel through cities in the sky. 670 00:39:22,794 --> 00:39:25,328 And their lightweight flexible structure triggers 671 00:39:25,330 --> 00:39:30,333 a whole new field of research into skyscraper wind dynamics, 672 00:39:30,335 --> 00:39:36,005 setting the stage for cutting edge research for years to come. 673 00:39:36,007 --> 00:39:38,975 13 years after the 9/11 tragedy, 674 00:39:38,977 --> 00:39:42,846 a new building stands at the world trade center site -- 675 00:39:42,848 --> 00:39:46,316 the new one world trade center. 676 00:39:46,318 --> 00:39:52,455 Rising 1,772 feet tall, it is new york's tallest skyscraper. 677 00:39:52,457 --> 00:39:55,024 It is designed with features to remember the people 678 00:39:55,026 --> 00:39:57,961 and the original twin towers with protection 679 00:39:57,963 --> 00:39:59,629 as a top priority. 680 00:39:59,631 --> 00:40:02,866 See: Like world trade center one and the other one's here 681 00:40:02,868 --> 00:40:05,368 are designed with every safety feature 682 00:40:05,370 --> 00:40:09,272 that the engineers could come up with. 683 00:40:09,274 --> 00:40:12,108 Since 9/11, more public buildings 684 00:40:12,110 --> 00:40:15,645 like the world trade center and other office buildings 685 00:40:15,647 --> 00:40:19,215 have adopted bomb blast criteria. 686 00:40:19,217 --> 00:40:25,054 ♪ 687 00:40:25,056 --> 00:40:27,724 narrator: Today at ground level, massive voids 688 00:40:27,726 --> 00:40:29,993 show the footprints of the towers. 689 00:40:29,995 --> 00:40:35,198 ♪ 690 00:40:35,200 --> 00:40:38,001 so this is the north tower memorial pool, 691 00:40:38,003 --> 00:40:40,703 and the memorial pools mark the location 692 00:40:40,705 --> 00:40:44,140 of where the towers stood at the street level of the site. 693 00:40:44,142 --> 00:40:48,077 And here, etched in the bronze parapets 694 00:40:48,079 --> 00:40:50,713 are the names of the nearly 3,000 people, 695 00:40:50,715 --> 00:40:53,783 including my brother, who perished on 9/11. 696 00:40:57,989 --> 00:41:01,858 Narrator: The twin towers and those who lost their lives in 9/11 697 00:41:01,860 --> 00:41:03,593 are never to be forgotten. 698 00:41:03,595 --> 00:41:11,200 ♪ 699 00:41:11,202 --> 00:41:13,970 the twin towers rose on new york's skyline 700 00:41:13,972 --> 00:41:17,307 as a powerful message of american innovation, 701 00:41:17,309 --> 00:41:19,375 breaking boundaries and reaching higher 702 00:41:19,377 --> 00:41:22,312 than anyone had ever gone before. 703 00:41:22,314 --> 00:41:26,182 Their fall was the darkest day in america's history, 704 00:41:26,184 --> 00:41:29,752 but their legacy lives on through other skyscrapers. 705 00:41:29,754 --> 00:41:32,121 A legacy never to be forgotten. 706 00:41:32,123 --> 00:41:40,230 ♪ 707 00:41:40,232 --> 00:41:48,371 ♪ 708 00:41:48,373 --> 00:41:56,546 ♪ 709 00:41:56,548 --> 00:42:04,721 ♪