0 00:00:08,684 --> 00:00:11,404 - In the early 1900s, there were two theories 1 00:00:11,404 --> 00:00:13,697 for longspan suspension bridge design. 2 00:00:13,697 --> 00:00:15,787 One was called the Elastic Theory, and the other 3 00:00:15,787 --> 00:00:18,177 was called the Deflection Theory. 4 00:00:18,177 --> 00:00:20,427 The Elastic Theory relied on deep decks, 5 00:00:20,427 --> 00:00:24,053 so this deep deck, the stiffness of those decks 6 00:00:24,053 --> 00:00:27,744 was relied upon for the stiffness of the bridge. 7 00:00:27,744 --> 00:00:31,882 Typically, for Elastic Theory, the span-to-depth ratio 8 00:00:31,882 --> 00:00:33,226 was less than 40. 9 00:00:33,226 --> 00:00:35,594 Again, span being the difference between 10 00:00:35,594 --> 00:00:38,197 the two towers and depth being the 11 00:00:38,197 --> 00:00:40,824 depth of the deck, not the side D, 12 00:00:40,844 --> 00:00:41,955 don't get confused with D. 13 00:00:41,955 --> 00:00:45,155 I'm talking about span-to-depth, the depth of the deck. 14 00:00:45,155 --> 00:00:48,898 So span-to-depth, less than 40 for Elastic Theory. 15 00:00:48,898 --> 00:00:53,613 Deflection Theory led to these thin decks being designed. 16 00:00:53,613 --> 00:00:56,652 And it relied not on the stiffness of the deck, 17 00:00:56,652 --> 00:01:00,226 but it relied on the stiffness of the cables. 18 00:01:00,226 --> 00:01:01,783 The Williamsburg Bridge, for example, 19 00:01:01,783 --> 00:01:05,175 completed in 1903, was based on the Elastic Theory. 20 00:01:05,175 --> 00:01:08,289 So this had a span-to-depth ratio of 40, 21 00:01:08,289 --> 00:01:11,393 and you could see by an image of the Williamsburg Bridge 22 00:01:11,393 --> 00:01:15,787 the very deep deck, that deep truss that was used. 23 00:01:15,787 --> 00:01:18,432 The Manhattan Bridge, in 1909, 24 00:01:18,432 --> 00:01:20,128 was based on the Deflection Theory. 25 00:01:20,128 --> 00:01:23,403 So here, we have a span-to-depth ratio of 61. 26 00:01:23,403 --> 00:01:27,189 So we went from 40 to 61, whereas that ratio, 27 00:01:27,189 --> 00:01:29,013 the larger that ratio gets, 28 00:01:29,013 --> 00:01:32,149 that means the thinner the deck gets. 29 00:01:32,149 --> 00:01:35,199 The Bronx-Whitestone Bridge, completed in 1939, 30 00:01:35,199 --> 00:01:37,215 this was a design by Othmar Ammann, 31 00:01:37,215 --> 00:01:39,796 was also designed by the Deflection Theory, 32 00:01:39,796 --> 00:01:43,551 and now we see a span-to-depth ratio of 210. 33 00:01:43,551 --> 00:01:46,569 So we went from 40, from the Deflection Theory, 34 00:01:46,569 --> 00:01:49,918 now to 210 in the Bronx-Whitestone Bridge. 35 00:01:49,918 --> 00:01:52,169 Now, the Bronx-Whitestone Bridge, when completed, 36 00:01:52,169 --> 00:01:55,049 it was seen to, let's say, dance a little bit. 37 00:01:55,049 --> 00:01:56,297 It would go up and down, 38 00:01:56,297 --> 00:01:57,459 fluctuate a little bit, 39 00:01:57,459 --> 00:01:59,689 and there was a little bit of nervousness 40 00:01:59,689 --> 00:02:01,662 by the people using this bridge. 41 00:02:01,662 --> 00:02:03,518 Ammann kept saying the bridge was safe 42 00:02:03,518 --> 00:02:04,936 and the bridge was safe. 43 00:02:04,936 --> 00:02:06,280 Indeed, it was safe. 44 00:02:06,280 --> 00:02:10,365 But this was an indication of things that were to come. 45 00:02:10,365 --> 00:02:12,434 Let me tell you a little bit about Deflection Theory. 46 00:02:12,434 --> 00:02:16,317 Deflection Theory is a complex mathematical theory. 47 00:02:16,317 --> 00:02:18,546 I am not going to get into the mathematics of it, 48 00:02:18,546 --> 00:02:22,140 but I will just tell you the basic philosophy behind it. 49 00:02:22,140 --> 00:02:25,831 So the philosophy is that the heavier the span weight, 50 00:02:25,831 --> 00:02:28,807 the span weight is related to Q x L. 51 00:02:28,807 --> 00:02:30,705 Q is the load, the line load. 52 00:02:30,705 --> 00:02:32,039 L is the span. 53 00:02:32,039 --> 00:02:34,310 So the heavier that span weight, 54 00:02:34,310 --> 00:02:36,774 the greater the cable tension. 55 00:02:36,774 --> 00:02:38,235 Again, we could go back to the equation 56 00:02:38,235 --> 00:02:40,881 H = QL squared over 8D. 57 00:02:40,881 --> 00:02:44,204 So the larger QL, the larger the H is, 58 00:02:44,204 --> 00:02:47,094 which means the larger the cable tension. 59 00:02:47,724 --> 00:02:51,044 The larger the cable tension, the stiffer the span, 60 00:02:51,044 --> 00:02:53,369 because now you have a larger cable. 61 00:02:53,369 --> 00:02:56,568 So this is the stiffness of the top cable. 62 00:02:56,568 --> 00:02:58,286 So the stiffer the span, 63 00:02:58,286 --> 00:03:02,051 the less you have a need for a deck truss. 64 00:03:02,051 --> 00:03:05,784 And then that results in the elegance of the thin deck. 65 00:03:05,784 --> 00:03:08,440 So this is the philosophy, the thinking, 66 00:03:08,440 --> 00:03:11,853 behind Deflection Theory, which is followed up by, 67 00:03:11,853 --> 00:03:15,063 as I said, really complex mathematical formulations 68 00:03:15,063 --> 00:03:17,570 that were being used in the 1930s 69 00:03:17,570 --> 00:03:20,012 to design longspan bridges. 70 00:03:20,012 --> 00:03:22,935 So now let's look at the effects of the wind, 71 00:03:22,935 --> 00:03:25,356 the dynamic effects of the wind in particular, 72 00:03:25,356 --> 00:03:28,929 and what they had on these longspan suspension bridges. 73 00:03:28,929 --> 00:03:31,276 And in particular, the danger of this 74 00:03:31,276 --> 00:03:35,243 dynamic effect of wind on these very thin decks 75 00:03:35,243 --> 00:03:37,313 designed by Deflection Theory. 76 00:03:37,313 --> 00:03:39,328 So I mentioned that the Bronx-Whitestone Bridge 77 00:03:39,328 --> 00:03:41,718 was moving quite a bit when it was completed 78 00:03:41,718 --> 00:03:43,510 in its design. 79 00:03:43,510 --> 00:03:45,184 And that had to do with the effects 80 00:03:45,184 --> 00:03:46,827 that the wind had on this bridge. 81 00:03:46,827 --> 00:03:48,405 So from the scientific point of view, 82 00:03:48,405 --> 00:03:49,899 let's look at the wind. 83 00:03:49,899 --> 00:03:51,936 From the social point of view, 84 00:03:51,936 --> 00:03:54,421 I'm going to talk a little bit about science 85 00:03:54,421 --> 00:03:55,541 trumping history. 86 00:03:55,541 --> 00:03:57,482 There was no much reliance on science, 87 00:03:57,482 --> 00:03:59,605 and science had such prestige - 88 00:03:59,605 --> 00:04:02,484 the Deflection Theory had great prestige - 89 00:04:02,484 --> 00:04:06,004 that engineers and others forgot to go back 90 00:04:06,004 --> 00:04:07,136 and look at history, 91 00:04:07,136 --> 00:04:09,460 look at empirical data and evidence 92 00:04:09,460 --> 00:04:12,724 of how longspan suspension bridges perfomed in the wind 93 00:04:12,724 --> 00:04:14,302 in the past. 94 00:04:14,302 --> 00:04:16,286 And from the symbolic point of view, 95 00:04:16,286 --> 00:04:19,326 we're gonna look at the deck 96 00:04:19,326 --> 00:04:22,131 and different forms for bridges as they came about, 97 00:04:22,131 --> 00:04:24,755 as engineers and others learned about 98 00:04:24,755 --> 00:04:28,893 these dynamic effects that the wind had on bridges. 99 00:04:28,893 --> 00:04:31,165 So let's go back to the Bronx-Whitestone Bridge. 100 00:04:31,165 --> 00:04:33,970 This had a span-to-depth ratio of 210. 101 00:04:33,970 --> 00:04:36,392 And as I mentioned earlier, when it was completed, 102 00:04:36,392 --> 00:04:38,130 it started to move a bit in the wind 103 00:04:38,130 --> 00:04:40,658 and make people a little bit uncomfortable. 104 00:04:40,658 --> 00:04:45,212 After 1940, some retrofits were made to this bridge. 105 00:04:45,212 --> 00:04:47,623 For example, some cable stays were added, 106 00:04:47,623 --> 00:04:50,897 and a very deep deck truss 107 00:04:50,897 --> 00:04:52,828 was added to this bridge. 108 00:04:52,828 --> 00:04:56,006 So, why this change? What happened? 109 00:04:56,006 --> 00:04:58,353 What happened was the Tacoma Narrows. 110 00:04:58,353 --> 00:05:01,288 So the Tacoma Narrows was completed in 1940, 111 00:05:01,288 --> 00:05:03,976 and it spanned 2,800 feet. 112 00:05:03,976 --> 00:05:06,856 The Bronx-Whitestone, completed only a year earlier, 113 00:05:06,856 --> 00:05:10,386 had a similar span of 2,300 feet. 114 00:05:10,386 --> 00:05:11,944 There're some differences, not only of span 115 00:05:11,944 --> 00:05:13,341 between the bridges. 116 00:05:13,341 --> 00:05:16,050 The Tacoma Narrows was a very narrow bridge, 117 00:05:16,050 --> 00:05:19,577 so proportionately, it was different as well. 118 00:05:19,577 --> 00:05:22,706 This also had a very thin deck. 119 00:05:22,706 --> 00:05:26,354 It had a span-to-depth ratio of 350. 120 00:05:26,354 --> 00:05:28,487 Remember, we started at span-to-depth ratios 121 00:05:28,487 --> 00:05:30,642 of 40 with the Elastic Theory. 122 00:05:30,642 --> 00:05:32,786 Deflection Theory was moving to thin decks, 123 00:05:32,786 --> 00:05:34,194 and we see the extremity of that 124 00:05:34,194 --> 00:05:36,690 with the Tacoma Narrows. 125 00:05:36,690 --> 00:05:38,482 Now the Tacoma Narrows was also, 126 00:05:38,482 --> 00:05:39,900 we could say, a dancing bridge 127 00:05:39,900 --> 00:05:41,202 like the Bronx-Whitestone, 128 00:05:41,202 --> 00:05:44,519 but it was far more dramatic than the Bronx-Whitestone. 129 00:05:44,519 --> 00:05:46,076 It was said that actually, high school kids 130 00:05:46,076 --> 00:05:47,559 would love to drive over this bridge 131 00:05:47,559 --> 00:05:49,628 because it was like driving over a rollercoaster, 132 00:05:49,628 --> 00:05:51,441 and they often would have to close it 133 00:05:51,441 --> 00:05:54,385 because it was just moving too much. 134 00:05:54,385 --> 00:05:58,235 Well, unfortunately, on one November day, 135 00:05:58,235 --> 00:06:02,256 this bridge essentially twisted itself to death. 136 00:06:02,256 --> 00:06:03,675 And there's famous footage, 137 00:06:03,675 --> 00:06:05,456 it's all over the Internet. 138 00:06:05,456 --> 00:06:09,456 I'll show you here in this course, 139 00:06:09,456 --> 00:06:11,984 one of those videos that shows you 140 00:06:11,984 --> 00:06:13,328 the Tacoma Narrows Bridge, 141 00:06:13,328 --> 00:06:14,960 and how it collapsed. 142 00:06:14,960 --> 00:06:17,349 This was on, I wouldn't call it a windy day. 143 00:06:17,349 --> 00:06:19,855 The wind was on the order of 40 miles per hour, 144 00:06:19,855 --> 00:06:23,130 so it wasn't a significant amount of wind. 145 00:06:23,130 --> 00:06:26,703 We can't blame huge wind gusts or hurricane wind loads 146 00:06:26,703 --> 00:06:28,527 to bring down this collapse. 147 00:06:28,527 --> 00:06:31,630 I'll explain after you see the video footage 148 00:06:31,630 --> 00:06:33,166 what happened. 149 00:06:33,166 --> 00:06:38,035 (dramatic music) 150 00:06:40,131 --> 00:06:41,411 - Dawn of a fatal day. 151 00:06:41,411 --> 00:06:43,192 And the wind begins to speak with a roar 152 00:06:43,192 --> 00:06:45,560 that no man can fail to hear. 153 00:06:45,560 --> 00:06:50,040 (dramatic music) 154 00:06:50,040 --> 00:06:51,437 In a 40-mile-an-hour gale, 155 00:06:51,437 --> 00:06:53,176 the center span weaves like a ribbon 156 00:06:53,176 --> 00:06:55,757 in the swinging cliffs that you wouldn't believe possible 157 00:06:55,757 --> 00:06:58,566 unless you could see it, as you do now. 158 00:06:58,566 --> 00:07:02,747 (dramatic music) 159 00:07:02,747 --> 00:07:06,182 There's an automobile caught on the heaving roadway. 160 00:07:06,182 --> 00:07:08,902 The 11,000-ton centerspan twists and springs 161 00:07:08,902 --> 00:07:10,779 the giant cables that support it, 162 00:07:10,779 --> 00:07:14,800 cables of 6300 wireframes, each 17 inches thick. 163 00:07:14,800 --> 00:07:19,231 (dramatic music) 164 00:07:36,303 --> 00:07:37,668 Back out of the danger zone, 165 00:07:37,668 --> 00:07:39,993 all stricken spectators are driven to safety 166 00:07:39,993 --> 00:07:41,870 as the bridge gyrates like a nightmare 167 00:07:41,870 --> 00:07:43,150 high above the river, 168 00:07:43,150 --> 00:07:46,286 twisting, turning, curling! 169 00:07:46,286 --> 00:07:50,531 (dramatic music) 170 00:07:50,531 --> 00:07:52,856 The lone motorist is forced to abandon the car. 171 00:07:52,856 --> 00:07:57,240 He has but a few minutes in which to save himself. 172 00:07:57,240 --> 00:07:58,285 Face to face with fate, 173 00:07:58,285 --> 00:08:01,752 his destiny hanging in the balance. 174 00:08:01,752 --> 00:08:03,277 Will he heed the last warning, 175 00:08:03,277 --> 00:08:05,389 or perish with the doomed structure? 176 00:08:05,389 --> 00:08:10,033 (dramatic music) 177 00:08:13,613 --> 00:08:16,705 But he saved himself by seconds. 178 00:08:16,705 --> 00:08:21,705 (dramatic music) 179 00:08:28,193 --> 00:08:29,793 No structure of steel and concrete 180 00:08:29,793 --> 00:08:31,467 can stand such a strain. 181 00:08:31,467 --> 00:08:33,622 Steel girders buckle and giant cables snap 182 00:08:33,622 --> 00:08:34,976 like puny flint. 183 00:08:34,976 --> 00:08:36,736 There it goes! 184 00:08:36,736 --> 00:08:41,306 (wind blowing) 185 00:08:48,469 --> 00:08:51,413 Engineers are divided as to the cause of the disaster. 186 00:08:51,413 --> 00:08:53,620 Some claim it was the use of solid girders. 187 00:08:53,620 --> 00:08:54,644 Others differ. 188 00:08:54,644 --> 00:08:57,428 But whatever the reason, Tacoma will rebuild, 189 00:08:57,428 --> 00:08:59,465 this time, a bridge that will not provide 190 00:08:59,465 --> 00:09:02,121 a super thrill in the news. 191 00:09:02,121 --> 00:09:06,403 (dramatic music) 192 00:09:08,222 --> 00:09:10,377 Many textbooks will tell you that 193 00:09:10,377 --> 00:09:13,513 the Tacoma Narrows collapsed due to resonance, 194 00:09:13,513 --> 00:09:16,531 resonance of the wind with the bridge, 195 00:09:16,531 --> 00:09:17,747 but this is not true. 196 00:09:17,747 --> 00:09:19,550 It wasn't resonance that brought down 197 00:09:19,550 --> 00:09:21,085 the Tacoma Narrows Bridge, 198 00:09:21,085 --> 00:09:25,075 it was something called self-excitation, or flutter. 199 00:09:25,075 --> 00:09:27,101 So flutter is something you might see 200 00:09:27,101 --> 00:09:30,557 if you see a flag on a flagpole 201 00:09:30,557 --> 00:09:31,837 on a very windy day. 202 00:09:31,837 --> 00:09:35,229 If the day is windy, you don't see the flag 203 00:09:35,229 --> 00:09:36,647 flying straight. 204 00:09:36,647 --> 00:09:37,756 The flag will flutter. 205 00:09:37,756 --> 00:09:41,224 It will oscillate in a direction perpendicular to the wind, 206 00:09:41,224 --> 00:09:42,577 back and forth. 207 00:09:42,577 --> 00:09:44,689 So basically, there's a critical wind speed 208 00:09:44,689 --> 00:09:48,540 at which that flag, and this bridge as well, 209 00:09:48,540 --> 00:09:50,492 will start to oscillate in a direction 210 00:09:50,492 --> 00:09:53,393 perpendicular to the wind in the direction 211 00:09:53,393 --> 00:09:54,758 that it is blowing. 212 00:09:54,758 --> 00:09:56,337 And that is indeed what happened 213 00:09:56,337 --> 00:09:58,481 with the Tacoma Narrows Bridge. 214 00:09:58,481 --> 00:10:00,432 So you see, this twisting motion, 215 00:10:00,432 --> 00:10:03,451 it activated this torsional mode, or twisting mode, 216 00:10:03,451 --> 00:10:07,003 of the bridge, and eventually, as you saw, 217 00:10:07,003 --> 00:10:09,253 the bridge came down. 218 00:10:09,253 --> 00:10:12,016 Now by this time in 1940, there were several bridges 219 00:10:12,016 --> 00:10:13,946 designed with very thin decks. 220 00:10:13,946 --> 00:10:15,290 Not only the Bronx-Whitestone, 221 00:10:15,290 --> 00:10:17,423 but the Deer Isle Bridge in Maine, 222 00:10:17,423 --> 00:10:19,140 a very thin deck. 223 00:10:19,140 --> 00:10:21,722 The Golden Gate Bridge, that I will talk about soon, 224 00:10:21,722 --> 00:10:23,951 also, very thin deck, 225 00:10:23,951 --> 00:10:26,404 and was starting to experience some motions 226 00:10:26,404 --> 00:10:28,153 due to wind. 227 00:10:28,153 --> 00:10:28,847 So the question is, 228 00:10:28,847 --> 00:10:32,078 how could this have happened with so much prestige? 229 00:10:32,078 --> 00:10:35,022 This new science, Deflection Theory, 230 00:10:35,022 --> 00:10:37,550 was so mathematically beautiful, 231 00:10:37,550 --> 00:10:40,131 and soundproof, it semed. 232 00:10:40,131 --> 00:10:41,688 How did this happen? 233 00:10:41,688 --> 00:10:45,261 Well, basically, there was 234 00:10:45,261 --> 00:10:47,651 no study of going to the past. 235 00:10:47,651 --> 00:10:49,400 They had forgotten about the past. 236 00:10:49,400 --> 00:10:50,978 And if you go to the past, 237 00:10:50,978 --> 00:10:53,645 you would have seen empirical evidence of wind 238 00:10:53,645 --> 00:10:55,554 producing vertical oscillations 239 00:10:55,554 --> 00:10:57,015 on many suspension bridges, 240 00:10:57,015 --> 00:11:00,546 and some of them leading to their collapse. 241 00:11:00,546 --> 00:11:02,007 It didn't happen to the Brooklyn Bridge. 242 00:11:02,007 --> 00:11:05,079 Remember that Roebling remembered the Wheeling Bridge, 243 00:11:05,079 --> 00:11:06,657 how the Wheeling Bridge collapsed, 244 00:11:06,657 --> 00:11:08,930 and he added stays to his bridges. 245 00:11:08,930 --> 00:11:11,415 So the Brooklyn Bridge has stays, 246 00:11:11,415 --> 00:11:15,020 and those stays, those diagonal steel wires 247 00:11:15,020 --> 00:11:17,227 that connect the tower to the deck, 248 00:11:17,227 --> 00:11:19,873 they are there to prevent those oscillations 249 00:11:19,873 --> 00:11:21,857 of the deck due to wind. 250 00:11:21,857 --> 00:11:24,107 But again, if you go back in history, 251 00:11:24,107 --> 00:11:25,750 we can see several, 252 00:11:25,750 --> 00:11:29,397 I'm listing eight of them, in history, 253 00:11:29,397 --> 00:11:32,864 where we saw either collapse or damage 254 00:11:32,864 --> 00:11:34,538 due to the effects of wind. 255 00:11:34,538 --> 00:11:38,794 And these examples span from many years, 256 00:11:38,794 --> 00:11:42,410 and also different lengths of bridges. 257 00:11:42,410 --> 00:11:44,266 In the archival literature, 258 00:11:44,266 --> 00:11:47,103 you will see sketches of bridges 259 00:11:47,103 --> 00:11:50,356 that have been drawn in this collapsed mode. 260 00:11:50,356 --> 00:11:51,519 So this has been observed, 261 00:11:51,519 --> 00:11:54,761 this kind of collapse has been observed in the past. 262 00:11:54,761 --> 00:11:56,393 We've studied the Menai Straits Bridge 263 00:11:56,393 --> 00:11:57,365 with Telford. 264 00:11:57,365 --> 00:11:59,710 Now, Menai did not collapse in the wind, 265 00:11:59,710 --> 00:12:02,473 but it did suffer extensive damage due to wind. 266 00:12:02,473 --> 00:12:05,171 So Telford knew about the effects of wind 267 00:12:05,171 --> 00:12:07,400 on his longspan suspension bridges. 268 00:12:07,400 --> 00:12:09,854 And there's other examples, like the Brighton Chain, 269 00:12:09,854 --> 00:12:12,872 another example of a collapsed suspension bridge, 270 00:12:12,872 --> 00:12:15,197 a historical study of that. 271 00:12:15,197 --> 00:12:18,685 So now we have new designs for suspension bridges. 272 00:12:18,685 --> 00:12:22,066 Engineers learned and studied these dynamics effects 273 00:12:22,066 --> 00:12:25,159 that the wind has on these longspan bridges. 274 00:12:25,159 --> 00:12:29,874 So the new Tacoma Narrows now has a very deep truss. 275 00:12:29,874 --> 00:12:33,425 The Deer Isle Bridge now has stays to, again, 276 00:12:33,425 --> 00:12:36,423 control those effects of the wind on the deck. 277 00:12:36,423 --> 00:12:38,236 And the George Washington Bridge was always 278 00:12:38,236 --> 00:12:39,345 safe against the wind. 279 00:12:39,345 --> 00:12:41,713 It had four very heavy cables, 280 00:12:41,713 --> 00:12:44,379 and it had a very wide deck. 281 00:12:44,379 --> 00:12:47,611 But in 1962, a lower deck was added, 282 00:12:47,611 --> 00:12:50,726 and then the form, the cross-section of the deck, 283 00:12:50,726 --> 00:12:54,789 became a shape that would now be very 284 00:12:54,789 --> 00:12:58,363 rigid from a twisting or torsional perspective. 285 00:12:58,363 --> 00:13:00,154 Thin decks are still possible. 286 00:13:00,154 --> 00:13:02,087 This Severn River Bridge, for example, 287 00:13:02,087 --> 00:13:03,761 has air foils in it. 288 00:13:03,761 --> 00:13:06,097 So these air foils essentially, you could say, 289 00:13:06,097 --> 00:13:07,569 slice the wind. 290 00:13:07,569 --> 00:13:09,724 So it interacts with the wind in such a way 291 00:13:09,724 --> 00:13:12,487 that it doesn't pick up this fluttering reaction 292 00:13:12,487 --> 00:13:13,233 of the wind. 293 00:13:13,233 --> 00:13:15,409 So thin decks are still very possible 294 00:13:15,409 --> 00:13:18,246 if you have air foils, or if you have the cross-section 295 00:13:18,246 --> 00:13:19,942 that's in a closed loop, 296 00:13:19,942 --> 00:13:21,979 like a rectangle shape. 297 00:13:21,979 --> 00:13:25,072 Thse are very strong against torsional 298 00:13:25,072 --> 00:13:28,507 movements due to wind-induced excitation. 299 00:13:28,507 --> 00:13:31,408 And as I said earlier, the Brooklyn Bridge is safe. 300 00:13:31,408 --> 00:13:32,709 It has those stays. 301 00:13:32,709 --> 00:13:35,525 Stays are another way of preventing these movements 302 00:13:35,525 --> 00:13:36,815 from the wind. 303 00:13:36,815 --> 00:13:40,719 So Roebling's bridge will not be destroyed by wind, 304 00:13:40,719 --> 00:13:44,996 but Hollywood loves to destroy the Brooklyn Bridge 305 00:13:44,996 --> 00:13:47,033 and other great bridges. 306 00:13:47,033 --> 00:13:49,145 It's just all over Hollywood, which makes me cringe, 307 00:13:49,145 --> 00:13:51,076 not only because I'm a bridge lover, 308 00:13:51,076 --> 00:13:52,238 but also because usually, 309 00:13:52,238 --> 00:13:56,601 they get the anatomy of the collapse wrong. 310 00:13:56,601 --> 00:13:59,299 So let's take a look at what Hollywood is doing 311 00:13:59,299 --> 00:14:00,825 in one example. 312 00:14:00,825 --> 00:14:04,035 So if we take a screenshot of the Batman movie, 313 00:14:04,035 --> 00:14:07,075 Dark Night Rises, we see that the villain 314 00:14:07,075 --> 00:14:09,443 has destroyed the Brooklyn Bridge 315 00:14:09,443 --> 00:14:10,584 and Manhattan Bridge. 316 00:14:10,584 --> 00:14:12,088 They have essentially sliced those bridges 317 00:14:12,088 --> 00:14:14,040 right down the center. 318 00:14:14,040 --> 00:14:16,173 So let's take a closer look at that idea 319 00:14:16,173 --> 00:14:20,333 of slicing a suspension bridge right down the center. 320 00:14:20,333 --> 00:14:21,751 So let's assume that a supervillain 321 00:14:21,751 --> 00:14:24,023 has destroyed a bridge, 322 00:14:24,023 --> 00:14:27,319 very similar to the Batman movie. 323 00:14:27,319 --> 00:14:29,228 And he blows up the bridge 324 00:14:29,228 --> 00:14:31,617 so that the deck and the cable, 325 00:14:31,617 --> 00:14:34,774 the whole center section of that bridge is gone. 326 00:14:34,774 --> 00:14:37,281 My question to you is, if this supervillain indeed 327 00:14:37,281 --> 00:14:40,342 essentially chops this bridge in half, 328 00:14:40,342 --> 00:14:42,603 what would happen to that bridge deck? 329 00:14:42,603 --> 00:14:44,183 Would it be unaffected because it's suspended 330 00:14:44,183 --> 00:14:45,312 from the cable? 331 00:14:45,312 --> 00:14:47,702 Would it be unaffected because it's cantilevering 332 00:14:47,702 --> 00:14:48,566 from the tower? 333 00:14:48,566 --> 00:14:50,720 It's supported by the tower? 334 00:14:50,720 --> 00:14:52,768 Will the bridge collapse, or do you 335 00:14:52,768 --> 00:14:56,708 have no idea what will happen to this bridge? 336 00:15:01,589 --> 00:15:04,373 So the answer is that the bridge will collapse. 337 00:15:04,373 --> 00:15:05,908 And why is that? 338 00:15:05,908 --> 00:15:08,276 It's because the deck is suspended 339 00:15:08,276 --> 00:15:10,879 from the cables, essentially. 340 00:15:10,879 --> 00:15:12,777 If the cable is no longer there, 341 00:15:12,777 --> 00:15:15,103 there is nothing supporting that deck. 342 00:15:15,103 --> 00:15:17,940 The deck is not supported by the towers. 343 00:15:17,940 --> 00:15:21,555 So it goes deck to cable to tower. 344 00:15:21,555 --> 00:15:24,115 So the bridge in this scenario would collapse, 345 00:15:24,115 --> 00:15:26,270 in which case, in that still image 346 00:15:26,270 --> 00:15:28,126 of the Batman movie I showed you, 347 00:15:28,126 --> 00:15:29,640 Hollywood got it wrong. 348 00:15:29,640 --> 00:15:32,445 There is no way that that still image would be real 349 00:15:32,445 --> 00:15:35,181 in a real case.