1 00:00:02,100 --> 00:00:05,567 [narrator] Every day, around the world, millions of motorists take 2 00:00:05,734 --> 00:00:07,467 to the roads and bridges. 3 00:00:09,367 --> 00:00:11,867 These marvels of design have pushed the boundaries 4 00:00:12,033 --> 00:00:14,266 of physics and engineering since their creation. 5 00:00:16,667 --> 00:00:17,934 [Ian] We're engineers. 6 00:00:18,100 --> 00:00:21,100 We design things to survive. We don't design things to fail. 7 00:00:21,266 --> 00:00:24,000 [Rob] But when things go wrong, their collapses are 8 00:00:24,166 --> 00:00:26,667 spectacular and unforgiving. 9 00:00:27,600 --> 00:00:31,767 In 2018, 43 people lost their lives when torrential 10 00:00:31,767 --> 00:00:35,367 rain caused the Morandi Bridge in Genoa, Italy, to collapse. 11 00:00:36,767 --> 00:00:38,967 Their journey to the peace and tranquility 12 00:00:39,133 --> 00:00:42,567 of the Italian coast will become a nightmare. 13 00:00:45,000 --> 00:00:47,266 In 1940, in the state of Washington, 14 00:00:47,266 --> 00:00:51,667 in the USA, the Tacoma Narrows Bridge, just months after 15 00:00:51,834 --> 00:00:55,367 its inauguration, broke apart due to an oscillating motion 16 00:00:55,533 --> 00:00:57,367 never before observed on a bridge. 17 00:00:57,867 --> 00:00:59,867 You don't have to be an expert to see that it's only going 18 00:00:59,867 --> 00:01:01,333 to end in one way. 19 00:01:03,767 --> 00:01:08,867 In 2007, a single structural element of the I-35W highway 20 00:01:08,867 --> 00:01:12,600 bridge in Minneapolis failed, causing the entire structure 21 00:01:12,767 --> 00:01:15,967 to collapse, killing 13 people. 22 00:01:16,133 --> 00:01:17,567 [Ian] It's just like a house of cards. 23 00:01:17,734 --> 00:01:20,867 You pull one card out, the whole thing falls down. 24 00:01:21,033 --> 00:01:24,867 What lessons did we learn from these catastrophic tragedies? 25 00:01:25,033 --> 00:01:27,867 How did the engineering community and the institutions 26 00:01:27,867 --> 00:01:30,266 responsible for maintaining these impressive 27 00:01:30,266 --> 00:01:31,967 structures respond? 28 00:01:32,133 --> 00:01:34,700 And what has been done to ensure that bridges designed 29 00:01:34,867 --> 00:01:37,667 today avoid fatal engineering? 30 00:01:37,667 --> 00:01:44,000 [electronic theme music playing] 31 00:01:47,767 --> 00:01:56,500 [suspenseful music playing] 32 00:01:56,667 --> 00:02:00,400 When it was inaugurated in Genoa, Italy, in 1967, 33 00:02:00,567 --> 00:02:03,500 the Polcevera Viaduct, more commonly known as 34 00:02:03,667 --> 00:02:07,166 the Morandi Bridge was widely celebrated as a masterpiece 35 00:02:07,333 --> 00:02:09,100 of engineering. 36 00:02:10,200 --> 00:02:12,567 To the north, it is supported by a series of fairly 37 00:02:12,567 --> 00:02:15,000 standard piers. 38 00:02:15,000 --> 00:02:18,467 In the south, the pillars are very different. 39 00:02:18,634 --> 00:02:22,567 It has these three Pylon structures, which are very, 40 00:02:22,734 --> 00:02:25,467 very visually dominant in the city. 41 00:02:27,266 --> 00:02:29,967 Each tower has four connection points, with two stays 42 00:02:30,133 --> 00:02:32,900 stretching in either direction of each Pylon. 43 00:02:34,667 --> 00:02:37,500 This unique triangular design is the trademark 44 00:02:37,667 --> 00:02:40,166 of its designer, the Italian engineer 45 00:02:40,333 --> 00:02:41,667 Riccardo Morandi. 46 00:02:42,367 --> 00:02:45,500 He had already experimented with it on the Maracaibo Bridge 47 00:02:45,667 --> 00:02:49,500 in Venezuela a few years earlier, a bridge eight times 48 00:02:49,667 --> 00:02:52,100 larger than the one in Genoa, which has already 49 00:02:52,266 --> 00:02:53,266 proved its worth. 50 00:02:54,367 --> 00:02:55,467 He's a brilliant man. 51 00:02:55,467 --> 00:02:58,266 He's already well known for doing, you know, 52 00:02:58,266 --> 00:02:59,667 pretty unusual designs. 53 00:03:00,767 --> 00:03:02,667 It's not just an ordinary bridge. 54 00:03:08,467 --> 00:03:12,467 But on the 14th of August, 2018, it's one of these, 55 00:03:12,467 --> 00:03:15,400 the vital component to the collapse of the bridge. 56 00:03:15,567 --> 00:03:18,400 [cracking, rumbling] 57 00:03:18,567 --> 00:03:20,667 [suspenseful music playing] 58 00:03:20,834 --> 00:03:22,767 It's 11:35 a.m. 59 00:03:22,767 --> 00:03:25,467 In the middle of August, many motorists traverse 60 00:03:25,467 --> 00:03:28,967 the Morandi Bridge, part of one of the busiest routes in Italy. 61 00:03:30,000 --> 00:03:33,266 [Shini] Whether on holiday or simply going about their daily lives, 62 00:03:33,433 --> 00:03:35,600 disaster is imminent. 63 00:03:35,767 --> 00:03:38,000 The Morandi Bridge is destined to fall. 64 00:03:38,000 --> 00:03:40,433 -[thunder rumbling] -[suspenseful music playing] 65 00:03:42,867 --> 00:03:46,867 No one, not even the engineers and inspectors responsible 66 00:03:46,867 --> 00:03:50,467 for ensuring the safety of the bridge, foresaw the tragedy 67 00:03:50,467 --> 00:03:51,867 that was about to unfold. 68 00:03:55,700 --> 00:03:59,900 People inside their cars, as they're driving along, 69 00:04:00,133 --> 00:04:02,166 minding their own business, 70 00:04:02,333 --> 00:04:07,066 suddenly, seemingly out of nowhere, dropping 40-odd meters 71 00:04:07,233 --> 00:04:08,867 into the valley. 72 00:04:08,867 --> 00:04:10,700 11:36 a.m. 73 00:04:10,867 --> 00:04:15,767 A stay cable on Pylon 9 breaks, causing 250 meters of deck 74 00:04:15,767 --> 00:04:17,667 to collapse just under a quarter 75 00:04:17,667 --> 00:04:19,767 of the total length of the bridge. 76 00:04:19,767 --> 00:04:21,467 [cracking, rumbling] 77 00:04:23,266 --> 00:04:25,967 Tons of concrete, and the vehicles on the bridge 78 00:04:26,133 --> 00:04:30,000 plunge into the Polcevera River, also falling 79 00:04:30,000 --> 00:04:32,533 into the railway tracks and buildings below. 80 00:04:35,000 --> 00:04:38,200 [shouting in Italian] 81 00:04:38,367 --> 00:04:43,400 [sirens wailing] 82 00:04:43,567 --> 00:04:45,567 [helicopter whirring] 83 00:04:45,734 --> 00:04:48,867 The collapse killed 43 people and injured many others. 84 00:04:50,600 --> 00:04:52,700 For the people of Genoa and those close to them, 85 00:04:52,867 --> 00:04:55,266 it has become clear that this visionary bridge, 86 00:04:55,266 --> 00:04:56,800 emblematic of the city, 87 00:04:56,967 --> 00:04:59,867 has become a symbol of a fatal engineering flaw. 88 00:05:03,100 --> 00:05:05,667 [speaking Italian] 89 00:05:11,900 --> 00:05:13,367 [in Italian] 90 00:05:15,367 --> 00:05:16,767 [Ian] This is another one of those cases 91 00:05:16,767 --> 00:05:19,567 where the engineering community around the world looks 92 00:05:19,567 --> 00:05:22,500 in horror -- and in this case, 93 00:05:22,667 --> 00:05:24,100 sadly, many people died -- 94 00:05:24,100 --> 00:05:25,500 and asks themselves, 95 00:05:26,667 --> 00:05:28,800 "Why? Why has it happened?" 96 00:05:33,000 --> 00:05:36,300 To understand why the Morandi Bridge collapsed, 97 00:05:36,467 --> 00:05:39,100 it is essential to consider the materials used 98 00:05:39,266 --> 00:05:40,266 in its construction. 99 00:05:42,467 --> 00:05:43,800 [airplane humming] 100 00:05:43,967 --> 00:05:45,066 [explosions blasting] 101 00:05:45,066 --> 00:05:47,767 After the second world war, with the production 102 00:05:47,767 --> 00:05:50,800 and availability of steel considerably reduced, 103 00:05:50,967 --> 00:05:53,667 architects and engineers turned to other materials 104 00:05:53,834 --> 00:05:54,800 for their projects. 105 00:05:56,266 --> 00:05:59,166 For Riccardo Morandi, it was concrete. 106 00:05:59,333 --> 00:06:02,767 [Rob] Riccardo Morandi was fascinated by the material properties 107 00:06:02,934 --> 00:06:05,800 of concrete. 108 00:06:05,967 --> 00:06:09,166 Prestressed concrete is subjected to compressive forces 109 00:06:09,166 --> 00:06:12,567 by means of cables tensioned before any loads are applied. 110 00:06:16,767 --> 00:06:18,600 This unique construction technique, 111 00:06:18,767 --> 00:06:20,767 invented in the 1920s by French 112 00:06:20,767 --> 00:06:22,667 engineer, Eugène Freyssinet, 113 00:06:22,834 --> 00:06:25,700 requires real craftsmanship to work perfectly. 114 00:06:30,867 --> 00:06:32,467 [Ian] But the thing that is particularly important 115 00:06:32,467 --> 00:06:35,300 in the story we're going to be talking about is 116 00:06:35,467 --> 00:06:36,467 the cable stays. 117 00:06:39,867 --> 00:06:42,166 With a length of 86 meters, between the top 118 00:06:42,166 --> 00:06:45,700 of each Pylon and the deck, these prestressed concrete 119 00:06:45,867 --> 00:06:48,400 stays, hailed for their elegance and unfailing 120 00:06:48,567 --> 00:06:52,767 strength, are responsible for keeping the deck in the air. 121 00:06:55,200 --> 00:06:58,800 However, it does not have the same properties as steel. 122 00:07:02,767 --> 00:07:06,767 Concrete as a material is very good under compression. 123 00:07:06,934 --> 00:07:10,200 It can resist being squished very, very well. 124 00:07:10,367 --> 00:07:12,767 [Ian] But if you put tension in concrete, it's gonna crack, 125 00:07:12,934 --> 00:07:14,433 and that's no good. 126 00:07:16,867 --> 00:07:20,266 To prevent the concrete from cracking, smaller cables are 127 00:07:20,433 --> 00:07:23,000 placed in a sheath around the other cables 128 00:07:23,166 --> 00:07:24,333 inside the concrete. 129 00:07:25,567 --> 00:07:28,667 But as motorists drive over the Morandi Bridge, 130 00:07:28,834 --> 00:07:31,367 they have no idea that one of these stays is 131 00:07:31,533 --> 00:07:32,867 severely compromised. 132 00:07:36,667 --> 00:07:40,567 [Shini] The pre-stressed concrete would be tested beyond its limits. 133 00:07:40,567 --> 00:07:43,500 -[cracking, rumbling] -[shouting in Italian] 134 00:07:50,767 --> 00:07:53,800 In its first year of existence, the Morandi Bridge was used 135 00:07:53,967 --> 00:07:56,400 by around 5 million vehicles. 136 00:07:56,567 --> 00:08:00,467 50 years later, a year before its dramatic collapse, 137 00:08:00,634 --> 00:08:05,000 traffic had increased five-fold to 25 million vehicles a year. 138 00:08:06,467 --> 00:08:08,900 Not only the volume, the weight of traffic 139 00:08:09,066 --> 00:08:12,467 on the bridge has also increased dramatically. 140 00:08:12,467 --> 00:08:14,467 Cars are bigger, more people are moving, 141 00:08:14,634 --> 00:08:16,667 but also the increase in the amount of freight 142 00:08:16,667 --> 00:08:18,200 from the nearby port. 143 00:08:19,100 --> 00:08:22,000 The port of Genoa is the largest port in Italy 144 00:08:22,000 --> 00:08:24,567 and one of the most important in Continental Europe. 145 00:08:26,100 --> 00:08:28,467 Heavy goods vehicles have to cross the Morandi Bridge 146 00:08:28,634 --> 00:08:30,266 in order to leave the port. 147 00:08:32,700 --> 00:08:35,467 Every time a vehicle crosses the Morandi Bridge, 148 00:08:35,634 --> 00:08:37,300 the structure and its concrete stays 149 00:08:37,467 --> 00:08:40,166 are subjected to varying loads. 150 00:08:40,166 --> 00:08:42,000 These changes in weight have the effect 151 00:08:42,166 --> 00:08:43,367 of deforming the structure. 152 00:08:44,967 --> 00:08:46,667 As the Morandi bridge continued to strain 153 00:08:46,667 --> 00:08:49,500 against the ever-increasing demand of traffic, 154 00:08:49,667 --> 00:08:51,734 cracks began to form. 155 00:08:54,166 --> 00:08:57,000 The cracks allow a silent and relentless threat 156 00:08:58,867 --> 00:09:02,000 capable of destroying even the strongest structures made 157 00:09:02,000 --> 00:09:03,367 of concrete and steel. 158 00:09:09,266 --> 00:09:12,100 Genoa is a coastal city where salty sea air, 159 00:09:12,100 --> 00:09:15,066 precipitation, and heat are all part of the climate. 160 00:09:20,166 --> 00:09:22,900 Any metal exposed to these conditions would rust 161 00:09:23,066 --> 00:09:25,567 and if left untreated, could fail. 162 00:09:25,567 --> 00:09:28,100 The problem with these cables inside that concrete sheath -- 163 00:09:29,200 --> 00:09:32,667 you can't inspect them, apart from just the outside surface, 164 00:09:32,834 --> 00:09:34,667 and you just don't know what's going on inside. 165 00:09:37,000 --> 00:09:41,500 The first to express concern was Riccardo Morandi himself, 166 00:09:41,667 --> 00:09:44,467 12 years after the inauguration of the bridge, 167 00:09:45,500 --> 00:09:48,767 he is publicly declaring that he is concerned about the level 168 00:09:48,934 --> 00:09:50,767 of deterioration he is observing 169 00:09:50,767 --> 00:09:53,433 on his prestressed concrete stays. 170 00:09:55,900 --> 00:09:59,300 The Italian engineer proposed solutions to prevent any 171 00:09:59,467 --> 00:10:02,100 further corrosion on or inside the structure. 172 00:10:03,600 --> 00:10:05,667 The problem is nobody did anything. 173 00:10:05,667 --> 00:10:09,000 It was not until the 1990s that repairs were carried out 174 00:10:09,166 --> 00:10:11,600 on Pylon number 11. 175 00:10:11,767 --> 00:10:15,166 Aware of the deterioration of the materials over time, 176 00:10:15,333 --> 00:10:18,467 authorities commissioned a detailed report and analysis 177 00:10:18,467 --> 00:10:21,767 of the condition of the stays. 178 00:10:21,767 --> 00:10:25,600 [Rob] It was discovered that 30% of the cables inside 179 00:10:25,767 --> 00:10:30,166 the concrete casing on Pylon 11 had corroded, 180 00:10:30,166 --> 00:10:33,767 resulting in a 50% loss of load capacity. 181 00:10:35,767 --> 00:10:37,166 But it's not the only one. 182 00:10:38,667 --> 00:10:42,367 Pylons 9 and 10 are in an equally damaged 183 00:10:42,533 --> 00:10:44,100 and worrying condition. 184 00:10:44,266 --> 00:10:46,066 Restoration work is being carried out 185 00:10:46,233 --> 00:10:48,467 to reinforce the Pylons. 186 00:10:48,467 --> 00:10:52,800 [Rob] Steel reinforcements were added to the cables on Pylon 11 187 00:10:52,967 --> 00:10:56,266 to bring back that load capacity. 188 00:10:56,266 --> 00:10:59,767 But in doing so, it ruins the clean aesthetic 189 00:10:59,767 --> 00:11:02,166 that Morandi was after. 190 00:11:02,333 --> 00:11:05,000 A few years later, more modifications were made 191 00:11:05,000 --> 00:11:09,567 to the cable stays on Pylon 10, this time steel 192 00:11:09,567 --> 00:11:12,667 cradles installed at the top of those cables. 193 00:11:12,667 --> 00:11:16,667 But nothing was ever done to Pylon 9. 194 00:11:17,867 --> 00:11:19,567 [Ian] The plan is... 195 00:11:19,734 --> 00:11:21,667 we're gonna do those in a few years' time. 196 00:11:23,900 --> 00:11:25,767 Unfortunately, we never got there. 197 00:11:27,100 --> 00:11:30,767 [Shini] Pylon 9 was left as originally designed. 198 00:11:30,934 --> 00:11:32,600 This was a fatal error. 199 00:11:33,567 --> 00:11:36,867 A fatal error, the gravity of which the whole world 200 00:11:36,867 --> 00:11:39,767 will realize on August 14th, 2018. 201 00:11:41,300 --> 00:11:42,967 The time is 11:35 a.m. 202 00:11:45,000 --> 00:11:48,900 It's lashing with rain, and there's water on the roads, 203 00:11:49,066 --> 00:11:50,300 adding even more weight. 204 00:11:52,767 --> 00:11:55,266 [suspenseful music playing] 205 00:11:56,266 --> 00:11:59,000 The torrential rain on the Morandi Bridge pummels 206 00:11:59,000 --> 00:12:00,867 the prestressed concrete stays. 207 00:12:01,800 --> 00:12:05,567 The water is spreading inside, relentlessly damaging the steel 208 00:12:05,567 --> 00:12:09,066 cables, which have no waterproof protection. 209 00:12:09,233 --> 00:12:12,967 The cables have corroded to such an extent, they just could 210 00:12:13,133 --> 00:12:14,200 not cope any longer. 211 00:12:15,800 --> 00:12:18,867 This is when the next event comes into play. 212 00:12:21,100 --> 00:12:27,467 One vehicle is on the approach to Pylon 9, a 44-ton truck. 213 00:12:30,867 --> 00:12:33,100 11:36 a.m. 214 00:12:33,100 --> 00:12:35,800 The heavy truck arrives at the Pylon and triggers 215 00:12:35,967 --> 00:12:38,200 the start of a chain reaction that will be fatal 216 00:12:38,367 --> 00:12:39,767 for the Morandi bridge. 217 00:12:45,967 --> 00:12:47,900 When the wheels of such a truck hit a step, 218 00:12:48,066 --> 00:12:49,667 it creates a big impact. 219 00:12:49,834 --> 00:12:54,767 So that truck is most likely to be the cause of what 220 00:12:54,767 --> 00:12:56,867 started the collapse. 221 00:12:57,033 --> 00:12:59,367 But ultimately, it didn't need very much. 222 00:12:59,367 --> 00:13:02,900 At that moment, one of the cables snaps. 223 00:13:03,066 --> 00:13:05,166 They break one after the other, 224 00:13:05,333 --> 00:13:06,467 leading to the total failure 225 00:13:06,467 --> 00:13:07,767 of the shroud. 226 00:13:07,934 --> 00:13:09,867 But it's creating a new phenomenon. 227 00:13:09,867 --> 00:13:12,767 If one of the cables snaps on one side of the tower, 228 00:13:12,934 --> 00:13:15,767 the tower itself becomes unstable and imbalanced. 229 00:13:15,934 --> 00:13:17,567 It's pulled the other way. 230 00:13:17,734 --> 00:13:20,367 So almost immediately after the snapping of the cable, 231 00:13:20,367 --> 00:13:24,967 the tower collapses and the bridge deck breaks. 232 00:13:27,166 --> 00:13:32,767 A section just under 250 meters in length, centered on Pylon 9, 233 00:13:32,934 --> 00:13:34,200 gives out. 234 00:13:34,367 --> 00:13:38,100 Any car on that section at that time was going down with it. 235 00:13:39,967 --> 00:13:42,200 [sirens wailing] 236 00:13:46,000 --> 00:13:48,800 The vehicles plummeted 45 meters, the height 237 00:13:48,967 --> 00:13:50,800 of a 13-story building. 238 00:13:50,967 --> 00:13:53,600 [somber music playing] 239 00:13:57,367 --> 00:13:59,800 Some died from the impact of the collapse. 240 00:13:59,967 --> 00:14:03,467 Others survived the drop, but were then crushed 241 00:14:03,467 --> 00:14:06,066 by falling pieces of concrete from the tower. 242 00:14:07,367 --> 00:14:09,700 A total of 43 people were killed 243 00:14:09,867 --> 00:14:11,667 when the Morandi Bridge collapsed. 244 00:14:13,500 --> 00:14:17,000 This led to a one-year state of emergency in the Genoa region. 245 00:14:19,266 --> 00:14:21,100 It highlighted the catastrophic state 246 00:14:21,100 --> 00:14:22,800 of Italy's infrastructure. 247 00:14:23,900 --> 00:14:26,066 The company in charge of the bridge, 248 00:14:26,233 --> 00:14:30,100 Autostrade, underwent a partial nationalization, 249 00:14:30,100 --> 00:14:32,867 offering to settle the legal case against the company 250 00:14:33,033 --> 00:14:34,800 for 1 million euros in damages 251 00:14:35,900 --> 00:14:38,700 and a further 26 million euros in compensation. 252 00:14:41,166 --> 00:14:45,667 [cracking, booming] 253 00:14:45,667 --> 00:14:48,767 Although Pylons 10 and 11 remained intact 254 00:14:48,934 --> 00:14:51,000 after the collapse of the Morandi Bridge, 255 00:14:51,000 --> 00:14:54,467 they were demolished on June 28th, 2019. 256 00:14:56,200 --> 00:14:59,467 A replacement bridge, the Genoa St. George Bridge, 257 00:14:59,634 --> 00:15:04,467 was inaugurated 13 months later on August 4th, 2020. 258 00:15:04,467 --> 00:15:07,266 A single moment brought about by years of negligence 259 00:15:07,433 --> 00:15:11,867 and a refusal to fully address evident issues can be 260 00:15:11,867 --> 00:15:13,967 catastrophically fatal. 261 00:15:22,467 --> 00:15:25,967 Wednesday, August 1st, 2007, in Minneapolis, Minnesota. 262 00:15:27,500 --> 00:15:29,000 It's 6:00 p.m. 263 00:15:29,000 --> 00:15:34,400 rush hour on Interstate 35 West, commonly known as I-35W. 264 00:15:34,567 --> 00:15:37,100 As part of that road, there's a bridge. 265 00:15:37,100 --> 00:15:40,166 [suspenseful music playing] 266 00:15:43,567 --> 00:15:46,100 A very, very busy bridge crossing the river, 267 00:15:46,100 --> 00:15:49,600 carrying thousands of vehicles every day. 268 00:15:49,767 --> 00:15:53,166 The I-35W Highway Bridge has three main spans 269 00:15:53,333 --> 00:15:56,166 made of steel and concrete. 270 00:15:56,333 --> 00:15:59,367 And a truss is a bridge where the elements are usually 271 00:15:59,367 --> 00:16:02,066 diagonals, and all the pieces are working 272 00:16:02,233 --> 00:16:04,266 in tension or compression. 273 00:16:04,433 --> 00:16:08,066 And this truss carries all the load, enables the thing 274 00:16:08,233 --> 00:16:09,967 to support all the traffic. 275 00:16:10,133 --> 00:16:12,166 It's a very efficient kind of structure. 276 00:16:12,333 --> 00:16:15,000 [Rob] But this bridge will become one of the most famous, 277 00:16:15,166 --> 00:16:17,867 or should I say infamous, in American history. 278 00:16:22,166 --> 00:16:25,800 It's 6:00 p.m. Rush hour on Interstate 35 West 279 00:16:25,967 --> 00:16:28,200 commonly known as I-35W. 280 00:16:31,600 --> 00:16:35,166 What they don't know is that this routine drive 281 00:16:35,166 --> 00:16:38,266 will turn into a major catastrophe. 282 00:16:38,266 --> 00:16:41,800 6:05 p.m., 40 years after its inauguration, 283 00:16:41,967 --> 00:16:44,867 the I-35W highway bridge collapses. 284 00:16:44,867 --> 00:16:46,634 Happens in the blink of an eye. 285 00:16:49,100 --> 00:16:51,000 [Shini] The bridge gives way, throwing all 286 00:16:51,000 --> 00:16:53,667 vehicles, passengers, construction workers, 287 00:16:53,834 --> 00:16:56,000 and equipment down into the cold waters 288 00:16:56,166 --> 00:16:57,767 of the Mississippi River. 289 00:17:00,066 --> 00:17:05,367 The accident kills 13 people and injures 145. 290 00:17:06,066 --> 00:17:09,667 Though authorities initially investigated a terror attack, 291 00:17:09,834 --> 00:17:13,467 this was a disaster that had been 40 years in the making. 292 00:17:15,500 --> 00:17:19,667 At first glance, the I-35W Highway Bridge is a solid 293 00:17:19,834 --> 00:17:22,500 structure, perfectly adapted to its environment. 294 00:17:22,667 --> 00:17:25,767 It's made up of interconnected steel beams, ensuring that 295 00:17:25,767 --> 00:17:27,467 loads are distributed evenly. 296 00:17:27,634 --> 00:17:30,567 But all of those members have to be joined together to create 297 00:17:30,567 --> 00:17:32,367 the overall truss. 298 00:17:32,367 --> 00:17:35,000 And usually, when we're doing this in a system where 299 00:17:35,000 --> 00:17:38,367 everything is bolted or riveted together, we use a thing called 300 00:17:38,367 --> 00:17:39,600 a gusset plate. 301 00:17:39,767 --> 00:17:41,567 The gusset plate, this small steel sheet 302 00:17:41,567 --> 00:17:43,166 will be one of the key elements 303 00:17:43,166 --> 00:17:46,066 that will make the Minneapolis Bridge one of the most tragic 304 00:17:46,233 --> 00:17:47,400 bridge collapses. 305 00:17:49,000 --> 00:17:52,467 More than 200 of these are riveted on both sides 306 00:17:52,467 --> 00:17:54,600 of the lattice junction points. 307 00:17:54,767 --> 00:17:58,400 Although small, they create essential links in the bridge. 308 00:17:58,567 --> 00:18:00,100 Each one must transfer the weight 309 00:18:00,100 --> 00:18:02,000 from one element to another. 310 00:18:02,166 --> 00:18:06,266 [Ian] The load transfers out of the member into the rivet, 311 00:18:06,433 --> 00:18:10,000 from the rivet, into the gusset plate, from the gusset plate 312 00:18:10,166 --> 00:18:12,767 into the rivet of the other member, and then from the rivet 313 00:18:12,934 --> 00:18:14,300 of the other member into the other member. 314 00:18:14,467 --> 00:18:18,667 It's the gusset plate that will take the stress of all of those 315 00:18:18,834 --> 00:18:20,066 forces together. 316 00:18:20,066 --> 00:18:23,567 So getting the right thickness of gusset plate is absolutely 317 00:18:23,567 --> 00:18:26,567 crucial to the strength of not only that joint, 318 00:18:26,734 --> 00:18:28,367 but to the strength of the structure as a whole. 319 00:18:30,166 --> 00:18:33,000 Investigators discover that the builders of the bridge 320 00:18:33,000 --> 00:18:36,300 neglected the thickness of these plates. 321 00:18:36,467 --> 00:18:40,600 At just 13 millimeters thick, the gusset plates on the I-35W 322 00:18:40,767 --> 00:18:43,700 bridge were not strong enough to support the bridge's weight 323 00:18:43,867 --> 00:18:45,000 and that of the traffic. 324 00:18:46,967 --> 00:18:48,700 It was designed to cope with the heavy traffic 325 00:18:48,867 --> 00:18:51,266 of a region like Minneapolis with a population 326 00:18:51,433 --> 00:18:55,000 of 3 million, but in 2007, 327 00:18:55,000 --> 00:18:58,100 100,000 vehicles cross it every day. 328 00:19:00,500 --> 00:19:03,767 As commuters head home on August 1st, the safety 329 00:19:03,767 --> 00:19:07,800 of the I-35W does not enter their mind. 330 00:19:07,967 --> 00:19:10,900 Having been a key route through the heart of Minneapolis since 331 00:19:11,066 --> 00:19:15,700 it was reopened in 1967, there's no reason for fear. 332 00:19:15,867 --> 00:19:18,500 But some warning signs were already apparent, 333 00:19:18,667 --> 00:19:22,967 because on that day, traffic on the bridge is disrupted. 334 00:19:23,133 --> 00:19:26,200 On the day in question, there are new modifications 335 00:19:26,367 --> 00:19:28,266 being made to the bridge. 336 00:19:28,433 --> 00:19:30,567 [Shini] Heavy machinery and building materials 337 00:19:30,567 --> 00:19:33,000 ready to resurface the bridge's deck. 338 00:19:34,700 --> 00:19:38,166 Four of the eight lanes of the 35-meter-wide deck were 339 00:19:38,166 --> 00:19:41,300 once again closed for renovations, renovations 340 00:19:41,467 --> 00:19:43,567 that don't worry drivers. 341 00:19:43,567 --> 00:19:44,867 [Rob] They've seen it for months, though. 342 00:19:44,867 --> 00:19:46,467 It's nothing out of the ordinary. 343 00:19:46,467 --> 00:19:48,867 What is out of the ordinary is what's happening beneath 344 00:19:49,033 --> 00:19:50,400 the bridge deck. 345 00:19:51,600 --> 00:19:55,767 The renovations are underway to combat an almost hidden threat, 346 00:19:55,934 --> 00:19:59,500 a dangerous process that is slowly but surely eating away 347 00:19:59,667 --> 00:20:02,700 at the structure, another bridge falling victim 348 00:20:02,867 --> 00:20:04,834 to corrosion. 349 00:20:07,266 --> 00:20:09,867 This relentless enemy is fed not only by the river 350 00:20:09,867 --> 00:20:12,900 that the bridge crosses, but also by the only natural 351 00:20:13,066 --> 00:20:18,667 waterfall on the Mississippi, 875 meters downstream. 352 00:20:18,834 --> 00:20:20,200 Saint Anthony Falls. 353 00:20:21,400 --> 00:20:24,100 The spray from the falls directly attacks the concrete 354 00:20:24,100 --> 00:20:26,000 slab on which the cars drive. 355 00:20:27,300 --> 00:20:30,133 But it's not the only factor damaging the bridge. 356 00:20:32,600 --> 00:20:35,166 If you've ever spent a winter in Minnesota, you know 357 00:20:35,166 --> 00:20:38,800 that the winters get brutal. The temperatures plummet. 358 00:20:38,967 --> 00:20:41,800 There's ice. There's lots of rain. 359 00:20:41,967 --> 00:20:44,767 And so the authorities, as we do all over the world, 360 00:20:44,934 --> 00:20:49,266 use salt on the roads to melt the ice and make it safer 361 00:20:49,266 --> 00:20:50,467 for drivers. 362 00:20:50,634 --> 00:20:53,867 While cold and water are real threats, salt adds 363 00:20:54,033 --> 00:20:56,500 its corrosive power to the concrete slab, 364 00:20:56,667 --> 00:20:59,100 and, more importantly, to the steel bars 365 00:20:59,266 --> 00:21:00,433 that reinforce it. 366 00:21:01,367 --> 00:21:04,467 To avoid this, they add concrete to the deck. 367 00:21:04,634 --> 00:21:07,266 Several tons will be added to the initial weight. 368 00:21:07,433 --> 00:21:10,467 The bridge is designed to support this increased weight, 369 00:21:10,467 --> 00:21:14,166 but this isn't the first time weight has been added. 370 00:21:14,333 --> 00:21:16,867 Because when it started life, it was only six and a half 371 00:21:17,033 --> 00:21:20,867 inches thick, 165 millimeters, and that's super thin. 372 00:21:21,033 --> 00:21:23,700 [Shini] Moisture and salt used during the winter months to de-ice 373 00:21:23,867 --> 00:21:26,567 the road would slowly seep into the concrete and corrode 374 00:21:26,567 --> 00:21:27,667 the metal within. 375 00:21:27,667 --> 00:21:30,266 In the years following its inauguration, 376 00:21:30,266 --> 00:21:32,867 the concrete deck began to crack. 377 00:21:32,867 --> 00:21:36,000 To address this issue, engineers took a major decision 378 00:21:36,166 --> 00:21:40,066 in 1977, only 10 years after the bridge's construction. 379 00:21:40,233 --> 00:21:43,667 They poured more concrete on top to protect and renew 380 00:21:43,667 --> 00:21:48,000 that surface so that the bridge deck was structurally sound 381 00:21:48,166 --> 00:21:50,600 despite all the environmental factors that were trying 382 00:21:50,767 --> 00:21:52,767 to constantly attack it. 383 00:21:52,767 --> 00:21:56,867 However, this added 512 tons to the bridge's initial weight. 384 00:21:56,867 --> 00:21:58,867 They don't know it yet, but this is the beginning 385 00:21:59,033 --> 00:22:01,967 of a nightmare scenario that will slowly but surely 386 00:22:02,133 --> 00:22:05,567 doom the I-35W highway bridge. 387 00:22:09,000 --> 00:22:10,767 Drivers crossing the bridge couldn't know 388 00:22:10,934 --> 00:22:14,066 that the structure was now much heavier than when it was built. 389 00:22:15,100 --> 00:22:17,467 But the experts should have realized this during 390 00:22:17,467 --> 00:22:19,166 a previous inspection. 391 00:22:19,166 --> 00:22:23,100 It's really important that engineers are sent out to do 392 00:22:23,100 --> 00:22:26,667 detailed inspections and analyses of structures like 393 00:22:26,834 --> 00:22:29,800 the I-35 West Bridge. And that happened. 394 00:22:29,967 --> 00:22:33,567 In fact, it happened four years prior to this disaster. 395 00:22:33,567 --> 00:22:37,066 And there is evidence from those engineers' reports 396 00:22:37,233 --> 00:22:40,066 of the smoking gun in this disaster. 397 00:22:41,467 --> 00:22:44,467 This is the photograph that showed signs of the already 398 00:22:44,467 --> 00:22:46,166 impending catastrophe. 399 00:22:46,333 --> 00:22:49,967 This image shows gusset plates and shows signs of warping. 400 00:22:50,133 --> 00:22:52,567 As an engineer, when you see a component that's started 401 00:22:52,734 --> 00:22:57,600 to bow, like this gusset plate, that tells you that the forces 402 00:22:57,767 --> 00:23:00,867 within your structure are doing something different to how 403 00:23:01,033 --> 00:23:02,767 they were originally designed. 404 00:23:02,934 --> 00:23:06,000 The consequences would be dramatic because, although very 405 00:23:06,000 --> 00:23:10,166 solid, the I-35W highway bridge has an Achilles heel. 406 00:23:10,166 --> 00:23:12,800 This is known as a non-redundant bridge. 407 00:23:12,967 --> 00:23:15,867 [Rob] It means that there are no redundancies built 408 00:23:15,867 --> 00:23:17,467 within the structure. 409 00:23:17,467 --> 00:23:19,200 If one member in the truss fails, 410 00:23:20,467 --> 00:23:22,100 then the whole bridge can fail. 411 00:23:22,266 --> 00:23:25,667 To avoid that kind of disaster scenario, all parts of a bridge 412 00:23:25,834 --> 00:23:29,300 like this must be perfectly calibrated and inspected. 413 00:23:29,467 --> 00:23:32,767 However, the bending of this gusset plate was not considered 414 00:23:32,767 --> 00:23:34,767 a danger by inspectors. 415 00:23:34,934 --> 00:23:36,400 The rivets and the buckling plate 416 00:23:37,367 --> 00:23:40,900 experience fluctuating stresses all the time, and that causes 417 00:23:41,066 --> 00:23:43,800 something called fatigue, which eventually creates 418 00:23:43,967 --> 00:23:46,500 a crack, and then if the loading continues like that, 419 00:23:46,667 --> 00:23:49,100 that crack will grow and as the crack grows 420 00:23:49,266 --> 00:23:51,166 eventually there'll be a point when...bang. 421 00:23:57,100 --> 00:24:00,667 During the fateful day of August 1st, 2007, 422 00:24:00,834 --> 00:24:03,467 construction workers prepared the materials needed to add 423 00:24:03,467 --> 00:24:05,467 the additional layer that evening. 424 00:24:07,066 --> 00:24:10,967 571 tons of sand and gravel ready for concreting, 425 00:24:11,133 --> 00:24:14,867 and a 36-ton tanker are now on the bridge. 426 00:24:15,033 --> 00:24:18,266 In total, more than 600 tons of material were added 427 00:24:18,266 --> 00:24:21,066 to the structure above the bent gusset plate. 428 00:24:22,400 --> 00:24:24,500 After years of development, which increased 429 00:24:24,667 --> 00:24:28,300 the dead load of the I-35W, this added weight is placed 430 00:24:28,467 --> 00:24:31,166 directly above a weakened pressure point. 431 00:24:32,266 --> 00:24:35,300 At 6:00 pm, everything was in place for the bridge workers 432 00:24:35,467 --> 00:24:38,200 to continue with the deck renovation work. 433 00:24:40,667 --> 00:24:43,266 They have to start their shift at 7:00 pm. 434 00:24:43,433 --> 00:24:46,867 What they don't know is that by 7:00 pm, 435 00:24:47,033 --> 00:24:48,800 there'll be no bridge left to resurface. 436 00:24:50,600 --> 00:24:51,967 6:03 pm... 437 00:24:53,900 --> 00:24:55,200 It's rush hour. 438 00:24:55,367 --> 00:24:57,000 People are making their way home from work. 439 00:24:57,000 --> 00:25:01,100 They're crossing the I-35 bridge in huge numbers. 440 00:25:01,100 --> 00:25:05,567 But that gusset plate has now got to the point where 441 00:25:05,567 --> 00:25:07,367 it can't take it any longer. 442 00:25:11,767 --> 00:25:15,200 The micro cracks within that gusset plate have grown. 443 00:25:17,000 --> 00:25:21,467 The gusset plate fails, and it fails catastrophically. 444 00:25:21,467 --> 00:25:24,166 6:05 p.m., the beginning of the end 445 00:25:24,166 --> 00:25:28,500 for the I-35W Highway Bridge and tragedy for those on it. 446 00:25:28,667 --> 00:25:31,000 The members that are connected to it detach. 447 00:25:31,166 --> 00:25:32,767 They're no longer attached to each other, 448 00:25:32,767 --> 00:25:35,200 the force has got nowhere to go, 449 00:25:35,367 --> 00:25:38,667 so it just collapses and it collapses instantly. 450 00:25:38,667 --> 00:25:41,767 [cracking, rumbling] 451 00:25:46,000 --> 00:25:48,967 The central span collapses, sending vehicles, 452 00:25:49,133 --> 00:25:52,900 people, and building materials plummeting 35 meters, 453 00:25:53,066 --> 00:25:56,200 17 vehicles fall into the Mississippi River. 454 00:25:56,367 --> 00:25:59,100 Some are crushed by the deck's pieces falling on them, 455 00:25:59,266 --> 00:26:00,600 but there's more to come. 456 00:26:03,767 --> 00:26:05,667 After the central span collapses, 457 00:26:05,667 --> 00:26:08,100 the two adjoining spans follow. 458 00:26:10,567 --> 00:26:12,567 Everything falls into a railway yard, 459 00:26:12,734 --> 00:26:14,667 and the Mississippi River. 460 00:26:14,834 --> 00:26:18,667 111 vehicles, including 25 construction machines 461 00:26:18,834 --> 00:26:22,667 and a school bus loaded with 63 students, were on the bridge 462 00:26:22,667 --> 00:26:24,800 when it fell. 463 00:26:24,967 --> 00:26:28,367 While many miraculously come out of the disaster alive, 464 00:26:28,533 --> 00:26:32,166 for others, a new nightmare begins. 465 00:26:33,400 --> 00:26:36,200 Emergency services take six minutes to reach 466 00:26:36,367 --> 00:26:37,700 the accident scene. 467 00:26:38,266 --> 00:26:41,767 In the meantime, solidarity is building among the survivors 468 00:26:41,767 --> 00:26:44,500 who are trying to rescue people trapped in their cars, 469 00:26:44,667 --> 00:26:46,367 which are filling up with water. 470 00:26:46,533 --> 00:26:49,800 The collapse of the I-35W Highway Bridge injures 471 00:26:49,967 --> 00:26:53,166 145 and leaves 13 people dead. 472 00:26:53,166 --> 00:26:57,166 The I-35W bridge became fatal for three main reasons. 473 00:26:58,266 --> 00:27:00,700 Firstly, the design of the gusset plate has to be 474 00:27:00,867 --> 00:27:03,400 up there as being a major problem. 475 00:27:03,567 --> 00:27:06,166 Number two is the question of maintenance. 476 00:27:06,166 --> 00:27:08,367 Maintenance is vital. 477 00:27:08,367 --> 00:27:10,700 And number three, actually, is that the bridge is carrying 478 00:27:10,867 --> 00:27:13,900 more load than it was designed for. 479 00:27:14,066 --> 00:27:19,000 As it turns out, that problem was not located purely 480 00:27:19,000 --> 00:27:20,333 within Minnesota. 481 00:27:21,567 --> 00:27:27,567 In 2007, the Federal Highway Administration counted 19,273 482 00:27:27,734 --> 00:27:30,200 non-redundant bridges in the United States alone, 483 00:27:30,367 --> 00:27:35,700 465 of which had a steel truss similar to that of the I-35W. 484 00:27:35,867 --> 00:27:40,767 Of those, 31%, or 145 bridges, were classified 485 00:27:40,767 --> 00:27:42,567 as structurally deficient. 486 00:27:42,734 --> 00:27:45,600 On all these bridges, if a key structural element 487 00:27:45,767 --> 00:27:48,200 were to fail, the bridge would present a high risk 488 00:27:48,367 --> 00:27:50,166 of partial or total collapse. 489 00:27:51,867 --> 00:27:55,100 In memory of this tragic accident, the defective gusset 490 00:27:55,100 --> 00:27:58,166 plate is now exhibited in a museum in Minneapolis. 491 00:27:59,100 --> 00:28:01,100 It's a haunting reminder that many people that failures 492 00:28:01,266 --> 00:28:04,900 like this could become all too common. 493 00:28:07,367 --> 00:28:09,567 [Rob] What is unique is what happened on the other side 494 00:28:09,734 --> 00:28:13,667 of the country, in Washington state, when, in 1940, 495 00:28:13,667 --> 00:28:17,567 a bridge ripped itself apart. 496 00:28:17,567 --> 00:28:21,567 The famous, iconic Tacoma Narrows bridge. 497 00:28:34,266 --> 00:28:37,667 [Ian] Tacoma Narrows Bridge represents one of those moments 498 00:28:37,667 --> 00:28:39,800 in history where everything changed. 499 00:28:41,400 --> 00:28:44,300 There's a before Tacoma and there's an after Tacoma. 500 00:28:44,467 --> 00:28:48,567 11:09 a.m. on November 4th, 1940. 501 00:28:49,567 --> 00:28:52,500 A little more than four months after its inauguration, 502 00:28:52,667 --> 00:28:56,100 the cameras of the reporters on site capture these exceptional 503 00:28:56,100 --> 00:28:59,266 and unprecedented images that leave people speechless. 504 00:29:00,400 --> 00:29:03,166 It's overcast and the wind is already causing the bridge 505 00:29:03,166 --> 00:29:06,100 to showcase its wave-like motion. 506 00:29:06,667 --> 00:29:08,166 11:10 a.m. 507 00:29:08,333 --> 00:29:11,967 The brand-new Tacoma Narrows Bridge collapses spectacularly 508 00:29:12,133 --> 00:29:14,867 in this inlet of the Pacific Ocean. 509 00:29:15,033 --> 00:29:18,066 The deck, now twisting in unpredictable motions, 510 00:29:18,233 --> 00:29:21,367 continued to shed concrete and steel, with successive 511 00:29:21,533 --> 00:29:23,800 portions of the bridge falling into the water 512 00:29:23,967 --> 00:29:25,400 of the narrows below. 513 00:29:30,200 --> 00:29:32,767 Hailed as a triumph of human perseverance when it was 514 00:29:32,934 --> 00:29:35,500 inaugurated, the Tacoma Narrows Bridge would become a symbol 515 00:29:35,667 --> 00:29:38,500 of contemporary engineering failure. 516 00:29:40,967 --> 00:29:44,066 Why was such an important bridge destined for disaster? 517 00:29:49,066 --> 00:29:52,867 In 1937, the local authorities decided to build a bridge 518 00:29:52,867 --> 00:29:56,100 to link the cities of Tacoma and Gig Harbor. 519 00:29:57,367 --> 00:30:00,200 [Shini] It's intended to help commuters traverse the cold waters 520 00:30:00,367 --> 00:30:03,066 of the Puget Sound, providing a vital economic 521 00:30:03,233 --> 00:30:05,066 and military route. 522 00:30:05,233 --> 00:30:07,200 [Ian] The way to get from one side to the other was 523 00:30:07,367 --> 00:30:08,667 a very long way around. 524 00:30:08,667 --> 00:30:11,700 So this is a really good example of where a bridge is 525 00:30:11,867 --> 00:30:14,367 gonna make a massive difference to people's lives. 526 00:30:14,533 --> 00:30:17,667 An initial design was proposed by a local engineer, 527 00:30:17,834 --> 00:30:19,100 Clark Eldridge. 528 00:30:20,166 --> 00:30:24,367 Clark Eldridge is proposing a steel truss bridge, of which 529 00:30:24,533 --> 00:30:28,600 there are thousands at this time in the United States. 530 00:30:28,767 --> 00:30:32,200 Its design incorporates 7.6 meter-deep trusses 531 00:30:32,367 --> 00:30:34,767 to reinforce the rigidity of the bridge. 532 00:30:34,934 --> 00:30:38,767 But at a cost of 11 million US dollars, or at today's value, 533 00:30:38,934 --> 00:30:42,000 $250 million, the Eldridge Project is 534 00:30:42,000 --> 00:30:43,367 considered too expensive. 535 00:30:43,533 --> 00:30:46,100 [suspenseful music playing] 536 00:30:50,266 --> 00:30:52,467 [Rob] On the other hand, you have Leon Moisseiff, 537 00:30:54,000 --> 00:30:58,500 who is a steel rope expert, and he is proposing a sleek 538 00:30:58,667 --> 00:31:00,634 suspension bridge. 539 00:31:03,100 --> 00:31:05,667 But crucially, the suspension bridge will do 540 00:31:05,834 --> 00:31:07,200 that job cheaper. 541 00:31:09,100 --> 00:31:11,967 At $8 million US dollars, $3 million less 542 00:31:12,133 --> 00:31:13,700 than the Eldridge project, 543 00:31:13,867 --> 00:31:16,000 the proposal by this experienced engineer, 544 00:31:16,166 --> 00:31:17,867 who has worked on the Manhattan Bridge 545 00:31:17,867 --> 00:31:20,700 in New York and the Golden Gate Bridge in San Francisco, 546 00:31:20,867 --> 00:31:22,867 also has other advantages. 547 00:31:25,000 --> 00:31:27,000 [Ian] It was using less material. 548 00:31:27,000 --> 00:31:28,567 It was gonna be easier to build. 549 00:31:28,734 --> 00:31:31,600 So that design was selected. 550 00:31:34,066 --> 00:31:36,367 The Léon Moisseiff Bridge is supported by 551 00:31:36,367 --> 00:31:38,667 two 130-meter-high towers, 552 00:31:38,834 --> 00:31:42,000 linked by cables that suspend the deck 60 meters 553 00:31:42,000 --> 00:31:43,800 above the waters of the Tacoma Narrows. 554 00:31:45,667 --> 00:31:50,000 The longest span is 853 meters, making it the longest 555 00:31:50,000 --> 00:31:53,367 suspension bridge in the United States and the third longest 556 00:31:53,367 --> 00:31:55,266 in the world. 557 00:31:55,266 --> 00:31:58,300 The engineer did away with the usual steel trusses 558 00:31:58,467 --> 00:32:01,700 and replaced them with two 2.4-meter-high girders 559 00:32:01,867 --> 00:32:02,867 along the deck. 560 00:32:04,000 --> 00:32:07,266 It's very, very thin, like a ribbon, 561 00:32:07,266 --> 00:32:09,000 but also because it's very narrow. 562 00:32:13,300 --> 00:32:17,767 With a deck only 12 meters wide, the length of three cars, 563 00:32:17,767 --> 00:32:21,000 the bridge's visionary elegance was to become the weak point 564 00:32:21,166 --> 00:32:23,634 of this revolutionary work of art. 565 00:32:26,100 --> 00:32:28,100 July 1st, 1940. 566 00:32:28,100 --> 00:32:30,767 The cameras and the public flocked to the inauguration 567 00:32:30,767 --> 00:32:33,967 of what is regarded as a masterpiece of engineering. 568 00:32:34,867 --> 00:32:37,400 [Shini] What locals don't know is that the bridge is 569 00:32:37,567 --> 00:32:40,667 already doomed in his efforts to create a visually pleasing 570 00:32:40,667 --> 00:32:44,100 cheaper bridge, Moisseiff made fatal miscalculations. 571 00:32:45,467 --> 00:32:48,867 [Rob] Very early on the workers are regularly subjected 572 00:32:49,033 --> 00:32:52,567 to high winds that are funneled down this valley. 573 00:32:53,667 --> 00:32:54,967 The wind... 574 00:32:56,166 --> 00:32:59,300 one of the main enemies of a bridge, that absolutely must 575 00:32:59,467 --> 00:33:01,867 be taken into account when designing it. 576 00:33:03,867 --> 00:33:06,667 If it was a truss bridge, then obviously the wind would 577 00:33:06,667 --> 00:33:08,767 pass through it to an extent. 578 00:33:08,934 --> 00:33:10,900 Unlike trusses, these girders don't allow wind 579 00:33:11,066 --> 00:33:12,467 to pass through. 580 00:33:13,367 --> 00:33:15,767 Although visionary, Leon Moisseiff's proposal 581 00:33:15,934 --> 00:33:18,667 to replace the trusses with steel girders on the sides 582 00:33:18,834 --> 00:33:20,000 of the deck... 583 00:33:21,367 --> 00:33:23,667 would be the first mistake that would prove fatal 584 00:33:23,834 --> 00:33:26,200 to the Tacoma Narrows Bridge. 585 00:33:28,667 --> 00:33:31,467 [Ian] The wind hits the side of the bridge, 586 00:33:31,634 --> 00:33:35,367 and it's presented with an eight-foot-deep girder, 587 00:33:35,367 --> 00:33:37,667 which is a solid object. 588 00:33:37,667 --> 00:33:39,200 And the wind, of course, has to go somewhere. 589 00:33:41,166 --> 00:33:45,066 As the wind hits the truss, it creates small vortices. 590 00:33:45,233 --> 00:33:48,266 These vortices move along the truss and find a completely 591 00:33:48,266 --> 00:33:50,300 new direction when they reach the edges. 592 00:33:51,467 --> 00:33:53,567 [Ian] Well, it goes over the top and underneath. 593 00:33:53,567 --> 00:33:58,567 The wind separates from the structure, and that creates 594 00:33:58,567 --> 00:34:03,066 a little vacuum, a little area of low pressure every time one 595 00:34:03,233 --> 00:34:04,367 of those vortices forms. 596 00:34:06,000 --> 00:34:08,567 That pressure creates a sort of suction which draws 597 00:34:08,734 --> 00:34:10,867 the bridge down if it's on the bottom, 598 00:34:10,867 --> 00:34:13,200 and if it's on the top, it'll draw the bridge up. 599 00:34:20,066 --> 00:34:24,567 [Rob] You may think of bridges as rigid, stationary structures. 600 00:34:24,567 --> 00:34:28,567 In fact, all bridges are designed to have some movement, 601 00:34:28,567 --> 00:34:30,533 some flex in them. 602 00:34:33,000 --> 00:34:37,000 Despite these worrying signs, the bridge was open to traffic. 603 00:34:37,967 --> 00:34:40,967 The bridge's strange behavior earned it the nickname, 604 00:34:41,133 --> 00:34:42,367 "Galloping Gertie." 605 00:34:43,600 --> 00:34:49,200 From day one, the bridge deck sways, it moves, it dances 606 00:34:49,367 --> 00:34:50,700 in the wind. 607 00:34:50,867 --> 00:34:54,300 Some people enjoy the process of driving across it, 608 00:34:54,467 --> 00:34:57,100 almost like being on a roller coaster. 609 00:34:59,567 --> 00:35:04,000 Engineers are concerned because this amount of movement was not 610 00:35:04,166 --> 00:35:05,266 part of the design process. 611 00:35:10,066 --> 00:35:12,767 They are attempting to stabilize the deck by securing 612 00:35:12,767 --> 00:35:16,467 it at both ends using cable connected to 50 ton concrete 613 00:35:16,467 --> 00:35:17,467 blocks on the bank. 614 00:35:17,634 --> 00:35:19,967 [suspenseful music playing] 615 00:35:22,300 --> 00:35:23,700 But they break. 616 00:35:25,867 --> 00:35:28,600 Two months after the inauguration of the bridge, 617 00:35:28,767 --> 00:35:31,600 hopes were pinned on a team of engineers and scientists led 618 00:35:31,767 --> 00:35:35,266 by Professor Farquaharson from the University of Washington. 619 00:35:37,400 --> 00:35:40,266 They are building this miniature replica of the bridge 620 00:35:40,266 --> 00:35:43,867 and carrying out tests to reproduce its exact behavior 621 00:35:43,867 --> 00:35:45,000 when the wind blows. 622 00:35:46,100 --> 00:35:47,900 They have come up with two proposals 623 00:35:48,066 --> 00:35:49,700 to stop its undulations. 624 00:35:55,467 --> 00:35:59,767 [Rob] The first proposal is to drill holes in the steel girders 625 00:35:59,934 --> 00:36:03,867 along the side of the bridge so that the wind has a passage 626 00:36:03,867 --> 00:36:07,567 through and doesn't have to go up and under the bridge. 627 00:36:09,300 --> 00:36:12,767 [Shini] The second is to add deflector veins and fairings along 628 00:36:12,767 --> 00:36:14,767 the deck to increase the bridge's 629 00:36:14,767 --> 00:36:16,367 aerodynamic profile. 630 00:36:16,533 --> 00:36:19,467 The first option is rejected because of the irreversible 631 00:36:19,467 --> 00:36:21,000 nature of the solution, 632 00:36:21,000 --> 00:36:23,400 but the second must be implemented. 633 00:36:23,567 --> 00:36:26,867 Unfortunately, the Tacoma Narrows Bridge would not hold 634 00:36:26,867 --> 00:36:29,767 out long enough for work to begin. 635 00:36:32,200 --> 00:36:37,000 November 7th, 1940. The time is 10:30 a.m. 636 00:36:37,000 --> 00:36:39,266 Winds of 68 kilometers per hour 637 00:36:39,266 --> 00:36:41,667 blow across the Tacoma Narrows Bridge. 638 00:36:41,834 --> 00:36:45,166 It's waving up and down, oscillating up and down 639 00:36:45,333 --> 00:36:48,867 and side to side, as it has done for months before, 640 00:36:48,867 --> 00:36:52,066 but the bridge starts to move in a slightly different way. 641 00:36:54,166 --> 00:36:57,266 A phenomenon never before seen on a superstructure. 642 00:36:59,266 --> 00:37:03,266 The very thin deck, praised for its elegant design, is proving 643 00:37:03,433 --> 00:37:06,600 to be the second weak link of the Tacoma Narrows Bridge. 644 00:37:08,600 --> 00:37:12,166 [Ian] This bridge, which had longitudinal girders 645 00:37:12,166 --> 00:37:18,367 down each side, had very little transverse stiffness. 646 00:37:18,533 --> 00:37:23,266 The bridge deck has started to twist, and that twisting 647 00:37:23,266 --> 00:37:26,667 movement was the fatal difference on this day. 648 00:37:27,867 --> 00:37:30,967 Faced with the terrifying sight of this deck twisting along 649 00:37:31,133 --> 00:37:33,166 its entire length, the engineers are 650 00:37:33,166 --> 00:37:35,400 increasingly worried. 651 00:37:35,567 --> 00:37:38,066 The bridge is immediately closed to traffic. 652 00:37:41,467 --> 00:37:45,000 The time is 10:31 a.m. 653 00:37:45,166 --> 00:37:48,367 [Shini] Unfortunately, one driver has already been allowed past 654 00:37:48,367 --> 00:37:51,900 and is on a deadly route towards the bridge. 655 00:37:52,066 --> 00:37:53,200 Leonard Coatsworth. 656 00:37:53,367 --> 00:37:56,166 It's just him and one passenger in his car. 657 00:37:56,166 --> 00:37:57,800 His dog, named Tubby. 658 00:37:57,967 --> 00:37:59,600 [dog barking] 659 00:38:01,266 --> 00:38:03,367 Tubby and his master find themselves 660 00:38:03,367 --> 00:38:05,800 stranded in the middle of the raging bridge. 661 00:38:08,266 --> 00:38:12,166 [Shini] As the bridge sways almost uncontrollably, a major failure 662 00:38:12,166 --> 00:38:13,667 goes unnoticed. 663 00:38:13,667 --> 00:38:16,867 A cable band at the bridge's mid-span snaps. 664 00:38:18,200 --> 00:38:19,867 [dog barking] 665 00:38:19,867 --> 00:38:22,900 Leonard can't go any further and he can't go back. 666 00:38:23,066 --> 00:38:26,266 He decides to abandon his car. 667 00:38:26,433 --> 00:38:29,600 [Shini] He climbs out, desperate to make it to safety with Tubby, 668 00:38:29,767 --> 00:38:32,400 but is knocked from his feet by the powerful twisting motion 669 00:38:32,567 --> 00:38:36,300 of the concrete, throwing him first to one side of the bridge 670 00:38:36,467 --> 00:38:38,266 and then back to the other. 671 00:38:38,433 --> 00:38:41,100 Tubby, still sitting in Leonard's car and terrified 672 00:38:41,100 --> 00:38:44,300 by the movement, refuses to leave the vehicle. 673 00:38:44,467 --> 00:38:46,967 Leonard has to accept the inevitable if he wants 674 00:38:47,133 --> 00:38:48,500 to save himself. 675 00:38:50,700 --> 00:38:51,867 11 a.m. 676 00:38:51,867 --> 00:38:54,667 Professor Farquharson came to observe the new torsion 677 00:38:54,834 --> 00:38:56,066 behavior of the bridge. 678 00:38:57,667 --> 00:39:00,667 He's about to witness something completely new, and even 679 00:39:00,667 --> 00:39:02,033 more dangerous. 680 00:39:04,500 --> 00:39:07,667 As the bridge continues to sway, it draws its power 681 00:39:07,834 --> 00:39:11,200 from the winds that continue to batter the deck. 682 00:39:13,667 --> 00:39:16,867 Not only is Galloping Gertie twisting out of control, 683 00:39:17,033 --> 00:39:19,100 it's also building energy. 684 00:39:19,100 --> 00:39:23,700 There is a critical point where the speed of the eddies being 685 00:39:23,867 --> 00:39:27,500 shed coincides with the natural frequency of the bridge. 686 00:39:29,600 --> 00:39:32,667 It is no longer just the wind that creates this twisting 687 00:39:32,667 --> 00:39:35,667 movement along the bridge, but the transfer of energy 688 00:39:35,834 --> 00:39:39,400 within the structure itself, known as resonance. 689 00:39:41,867 --> 00:39:45,166 It's a runaway train bound for the end of the track. 690 00:39:46,500 --> 00:39:49,567 To make matters worse, the two ends of the bridge are 691 00:39:49,567 --> 00:39:52,200 twisting in opposite directions. 692 00:39:52,367 --> 00:39:56,667 The bridge twists violently up to 45 degrees in one direction, 693 00:39:56,834 --> 00:39:58,033 and then the other. 694 00:39:58,800 --> 00:40:01,467 This creates a propeller effect, with only the middle 695 00:40:01,467 --> 00:40:04,367 of the bridge locked in place while the two sides 696 00:40:04,533 --> 00:40:05,500 twist violently. 697 00:40:07,967 --> 00:40:09,600 11:02 a.m. 698 00:40:09,767 --> 00:40:13,367 A girder bulges out on the gig harbor side of the bridge, 699 00:40:13,533 --> 00:40:16,767 unable to endure this violent twisting motion. 700 00:40:16,767 --> 00:40:19,567 The weight that is now put on the hangers next to it 701 00:40:19,567 --> 00:40:21,467 immediately is too much. 702 00:40:21,467 --> 00:40:24,200 They fail and the whole thing falls like a domino rally. 703 00:40:27,200 --> 00:40:32,767 [cracking, rumbling] 704 00:40:40,166 --> 00:40:43,100 [Shini] Professor Farquharson and Leonard can do nothing more 705 00:40:43,266 --> 00:40:46,300 than watch the car and poor Tubby fall with the rest 706 00:40:46,467 --> 00:40:50,100 of the bridge's main span deck. 707 00:40:52,567 --> 00:40:56,166 Tacoma is one of those moments in history when everybody just 708 00:40:56,333 --> 00:40:59,500 stops and says, "Wow, now we've learned something 709 00:40:59,667 --> 00:41:00,867 we didn't know." 710 00:41:00,867 --> 00:41:03,467 It was a phenomenon that had just not been seen before. 711 00:41:04,767 --> 00:41:07,467 Following the collapse of the Tacoma Narrows Bridge, 712 00:41:07,634 --> 00:41:10,567 many other suspension bridges have been built and have become 713 00:41:10,734 --> 00:41:12,667 benchmarks of structural engineering. 714 00:41:18,166 --> 00:41:22,000 Bridges are icons within our cities and cityscapes 715 00:41:22,000 --> 00:41:23,767 around the world. 716 00:41:23,767 --> 00:41:26,600 But when things go wrong, their collapses are 717 00:41:26,767 --> 00:41:29,266 spectacular and unforgiving. 718 00:41:30,700 --> 00:41:33,367 We're engineers. We don't design things to fail. 719 00:41:33,367 --> 00:41:36,967 We design things to survive. Sometimes, mistakes happen. 720 00:41:37,133 --> 00:41:40,100 Sometimes humanity has made the same mistake twice. 721 00:41:41,266 --> 00:41:44,266 Whether it's a design error, an unknown phenomenon, 722 00:41:44,266 --> 00:41:48,000 or negligence, when a bridge collapses, the whole world is 723 00:41:48,166 --> 00:41:49,367 in shock. 724 00:41:49,367 --> 00:41:52,266 When these errors are not caught, then a collapse can 725 00:41:52,433 --> 00:41:55,567 become a tragic case of fatal engineering.