1 00:00:02,503 --> 00:00:05,137 ¶ ¶ 2 00:00:05,139 --> 00:00:07,106 NARRATOR: For more than 50 years, 3 00:00:07,108 --> 00:00:10,576 the Polcevera Bridge stood as a landmark 4 00:00:10,578 --> 00:00:12,478 in the city of Genoa. 5 00:00:12,480 --> 00:00:15,647 CAMILLO NUTI: It was a masterpiece, 6 00:00:15,649 --> 00:00:19,518 and everybody knows it was a masterpiece. 7 00:00:21,122 --> 00:00:24,556 NARRATOR: It was one of the most famous bridges in Italy. 8 00:00:24,558 --> 00:00:27,559 TULLIA IORI: It was the bridge of the future. 9 00:00:27,561 --> 00:00:30,396 The bridge was really beautiful. 10 00:00:31,866 --> 00:00:35,134 NARRATOR: But on August 14, 2018, 11 00:00:35,136 --> 00:00:37,903 disaster strikes. 12 00:00:39,740 --> 00:00:45,411 MAN (yelling in Italian, repeating): 13 00:00:49,917 --> 00:00:54,319 NARRATOR: A huge 800-foot section of the bridge, 14 00:00:54,321 --> 00:00:56,455 carrying four lanes of traffic, 15 00:00:56,457 --> 00:00:58,457 collapses. 16 00:00:58,459 --> 00:01:00,692 (people calling in background) 17 00:01:00,694 --> 00:01:04,830 NARRATOR: 27 vehicles plummet into the valley below. 18 00:01:07,101 --> 00:01:10,669 43 people die. 19 00:01:12,773 --> 00:01:15,874 It is one of the worst road bridge collapses in Europe 20 00:01:15,876 --> 00:01:18,811 for more than a century. 21 00:01:18,813 --> 00:01:23,482 And yet there was no warning. 22 00:01:23,484 --> 00:01:26,618 Why did this seemingly sound bridge 23 00:01:26,620 --> 00:01:29,555 suddenly fall down? 24 00:01:29,557 --> 00:01:32,524 And is anyone to blame? 25 00:01:34,895 --> 00:01:37,262 In the last ten years, 26 00:01:37,264 --> 00:01:39,898 more than 60 bridges have collapsed 27 00:01:39,900 --> 00:01:41,967 around the world. 28 00:01:41,969 --> 00:01:45,537 How many more are about to give way? 29 00:01:47,041 --> 00:01:51,009 In the United States, over 47,000 bridges 30 00:01:51,011 --> 00:01:53,779 are classified as structurally deficient. 31 00:01:53,781 --> 00:01:56,915 We have had over two dozen bridges fail 32 00:01:56,917 --> 00:01:58,584 in the United States since 2000. 33 00:01:58,586 --> 00:02:03,555 NARRATOR: Can engineers guarantee the safety of our aging bridges 34 00:02:03,557 --> 00:02:07,526 before it is too late? 35 00:02:07,528 --> 00:02:11,997 "Why Bridges Collapse," right now on "NOVA." 36 00:02:14,502 --> 00:02:17,369 ¶ ¶ 37 00:02:19,507 --> 00:02:23,408 ¶ ¶ 38 00:02:25,813 --> 00:02:28,380 NARRATOR: Genoa. 39 00:02:30,184 --> 00:02:34,486 One of Italy's busiest ports. 40 00:02:35,389 --> 00:02:38,257 Over 6,000 ships a year 41 00:02:38,259 --> 00:02:40,325 carry 54 million tons of cargo through here, 42 00:02:40,327 --> 00:02:43,829 nearly all of it placed on trucks. 43 00:02:43,831 --> 00:02:48,167 ¶ ¶ 44 00:02:48,169 --> 00:02:51,570 The trucks fan out in all directions, 45 00:02:51,572 --> 00:02:54,373 but many end up on this highway, 46 00:02:54,375 --> 00:02:56,875 the E80. 47 00:02:56,877 --> 00:02:59,278 ¶ ¶ 48 00:02:59,280 --> 00:03:00,345 It's a critical link 49 00:03:00,347 --> 00:03:03,315 between the South of France and Rome, 50 00:03:03,317 --> 00:03:06,652 shuttling more than 70,000 cars and trucks past Genoa 51 00:03:06,654 --> 00:03:08,887 every day. 52 00:03:08,889 --> 00:03:11,690 (car engine humming) 53 00:03:11,692 --> 00:03:14,293 August 14, 2018. 54 00:03:14,295 --> 00:03:18,797 Firefighter Davide Capello drives his S.U.V. 55 00:03:18,799 --> 00:03:20,799 along route E80, 56 00:03:20,801 --> 00:03:24,069 heading east towards Genoa. 57 00:03:24,071 --> 00:03:27,839 CAPELLO (speaking Italian): 58 00:03:27,841 --> 00:03:30,142 (Radio announcer speaking Italian) 59 00:03:30,144 --> 00:03:33,378 CAPELLO: 60 00:03:33,380 --> 00:03:38,817 (radio announcer continues) 61 00:03:39,820 --> 00:03:44,423 CAPELLO: 62 00:04:04,678 --> 00:04:06,178 NARRATOR: Gianluca Ardini, 63 00:04:06,180 --> 00:04:07,512 a foreman for a furniture company, 64 00:04:07,514 --> 00:04:12,985 heads in the opposite direction along the E80 highway. 65 00:04:13,721 --> 00:04:17,756 ARDINI (speaking Italian): 66 00:04:23,163 --> 00:04:25,230 ¶ ¶ 67 00:04:25,232 --> 00:04:26,999 NARRATOR: Both Gianluca and Davide 68 00:04:27,001 --> 00:04:30,035 are heading towards the Polcevera Bridge, 69 00:04:30,037 --> 00:04:33,171 locally known as the Morandi Bridge. 70 00:04:33,173 --> 00:04:40,178 This iconic landmark connects the two sides of Genoa. 71 00:04:40,180 --> 00:04:42,681 At 11:36 a.m., 72 00:04:42,683 --> 00:04:48,553 the world in front of them begins to come apart. 73 00:04:48,555 --> 00:04:52,624 (speaking Italian): 74 00:05:00,167 --> 00:05:03,201 MAN (yelling in Italian, repeating): 75 00:05:10,878 --> 00:05:13,378 (speaking Italian): 76 00:05:18,185 --> 00:05:22,788 CAPELLO (speaking Italian): 77 00:05:48,115 --> 00:05:52,184 NARRATOR: A massive 800-foot section of the bridge, 78 00:05:52,186 --> 00:05:54,319 carrying four lanes of traffic, 79 00:05:54,321 --> 00:05:55,754 collapses. 80 00:05:55,756 --> 00:06:01,159 Gianluca's truck plummets 100 feet. 81 00:06:01,161 --> 00:06:05,063 (speaking Italian): 82 00:06:07,801 --> 00:06:12,371 ¶ ¶ 83 00:06:35,496 --> 00:06:39,564 ¶ ¶ 84 00:06:39,566 --> 00:06:44,403 NARRATOR: 27 vehicles fall with the bridge. 85 00:06:44,405 --> 00:06:48,974 This is Davide Capello's S.U.V. 86 00:06:48,976 --> 00:06:50,375 It falls 130 feet 87 00:06:50,377 --> 00:06:56,181 and lands inside a cavity formed by a section of the bridge. 88 00:06:58,619 --> 00:07:03,488 (speaking Italian): 89 00:07:21,442 --> 00:07:25,677 NARRATOR: Across from Davide's car, 90 00:07:25,679 --> 00:07:30,382 trapped inside the remains of the collapsed bridge tower, 91 00:07:30,384 --> 00:07:33,351 rescuers make a shocking discovery. 92 00:07:33,353 --> 00:07:38,824 Hanging inside this truck is Gianluca Ardini. 93 00:07:40,828 --> 00:07:43,995 ARDINI (speaking Italian): 94 00:07:51,538 --> 00:07:56,541 NARRATOR: Firefighters begin the precarious job 95 00:07:56,543 --> 00:07:59,711 of rescuing him from the mangled cab. 96 00:07:59,713 --> 00:08:02,981 (people talking in background) 97 00:08:02,983 --> 00:08:06,251 NARRATOR: It takes 40 minutes to cut Gianluca free 98 00:08:06,253 --> 00:08:11,156 and carry him to safety. 99 00:08:11,158 --> 00:08:15,494 400 firefighters from all over Italy 100 00:08:15,496 --> 00:08:17,996 arrive with specialized equipment 101 00:08:17,998 --> 00:08:22,400 most often used to rescue people from earthquakes. 102 00:08:22,402 --> 00:08:27,138 Sniffer dogs search for survivors. 103 00:08:27,140 --> 00:08:30,509 ¶ ¶ 104 00:08:30,511 --> 00:08:36,281 After four days, firefighters finally recover the last body. 105 00:08:36,283 --> 00:08:39,217 ¶ ¶ 106 00:08:39,219 --> 00:08:42,287 The collapse of the Polcevera Bridge 107 00:08:42,289 --> 00:08:46,391 kills 43 people and injures 15. 108 00:08:46,393 --> 00:08:54,633 It's far and away Italy's worst road bridge disaster. 109 00:08:54,635 --> 00:08:56,868 ¶ ¶ 110 00:08:56,870 --> 00:09:01,406 The catastrophe sounds a chilling alarm. 111 00:09:01,408 --> 00:09:05,110 Engineers around the world want answers-- 112 00:09:05,112 --> 00:09:08,446 why was there no warning? 113 00:09:08,448 --> 00:09:11,516 What caused the sudden collapse? 114 00:09:11,518 --> 00:09:17,522 And most important, how many other bridges are at risk? 115 00:09:18,692 --> 00:09:20,792 The stakes are huge. 116 00:09:20,794 --> 00:09:22,994 If the bridge failed due to age, 117 00:09:22,996 --> 00:09:25,664 that could mean disaster for thousands of other bridges 118 00:09:25,666 --> 00:09:28,233 built at the same time. 119 00:09:28,235 --> 00:09:30,769 ¶ ¶ 120 00:09:30,771 --> 00:09:34,506 All products of a highway construction boom 121 00:09:34,508 --> 00:09:39,177 that began more than a half a century ago. 122 00:09:39,179 --> 00:09:42,447 Across many countries, the Second World War 123 00:09:42,449 --> 00:09:47,552 destroyed thousands of roads and bridges. 124 00:09:49,790 --> 00:09:51,990 In the years that followed, 125 00:09:51,992 --> 00:09:55,460 engineers set about rebuilding the bridges, 126 00:09:55,462 --> 00:09:59,331 while also constructing new, super-fast highways. 127 00:10:02,803 --> 00:10:07,105 This was the dawn of the golden age of the car. 128 00:10:07,107 --> 00:10:09,307 And nowhere more so than in the United States. 129 00:10:09,309 --> 00:10:11,543 ¶ ¶ 130 00:10:11,545 --> 00:10:14,346 In 1956, President Eisenhower 131 00:10:14,348 --> 00:10:15,981 signed the Federal Aid Highway Act 132 00:10:15,983 --> 00:10:20,619 to create 41,000 miles of interstate highways. 133 00:10:20,621 --> 00:10:24,522 FILM NARRATOR: As far as the eye can see, in almost every city, 134 00:10:24,524 --> 00:10:26,658 concrete and asphalt are changing the patterns 135 00:10:26,660 --> 00:10:28,627 of land and land use. 136 00:10:28,629 --> 00:10:31,463 Soon, traffic will flow smoothly 137 00:10:31,465 --> 00:10:36,267 in, around, and between every major city and town in America. 138 00:10:36,269 --> 00:10:38,570 NARRATOR: Today in America, 139 00:10:38,572 --> 00:10:39,738 just as in Europe, 140 00:10:39,740 --> 00:10:46,444 many of these concrete bridges are growing old. 141 00:10:46,446 --> 00:10:47,445 About two-thirds 142 00:10:47,447 --> 00:10:49,214 of our interstate highway system bridges 143 00:10:49,216 --> 00:10:53,051 were built in either the 1960s or 1970s, 144 00:10:53,053 --> 00:10:56,221 so a number of those structures are coming to the end 145 00:10:56,223 --> 00:10:58,223 of what's called their useful design life. 146 00:11:00,594 --> 00:11:05,463 NARRATOR: The United States has more than 600,000 road bridges. 147 00:11:05,465 --> 00:11:08,700 Over 47,000 of them are classified with the term 148 00:11:08,702 --> 00:11:11,136 "structurally deficient." 149 00:11:11,138 --> 00:11:15,006 BLACK: What that means is that when the bridge is inspected, 150 00:11:15,008 --> 00:11:16,841 one of the key structural elements 151 00:11:16,843 --> 00:11:20,645 is rated in poor or worse condition. 152 00:11:20,647 --> 00:11:23,214 So it means they're bridges that need to be repaired. 153 00:11:23,216 --> 00:11:25,917 ¶ ¶ 154 00:11:25,919 --> 00:11:27,152 NARRATOR: Are these bridges, 155 00:11:27,154 --> 00:11:30,388 located all across America and the world, 156 00:11:30,390 --> 00:11:34,459 ticking time bombs? 157 00:11:34,461 --> 00:11:38,596 And particularly, if they share the same faults 158 00:11:38,598 --> 00:11:41,066 as the Polcevera Bridge in Genoa, 159 00:11:41,068 --> 00:11:43,635 then lives could be at stake. 160 00:11:43,637 --> 00:11:47,839 ¶ ¶ 161 00:11:47,841 --> 00:11:49,741 Within hours of the disaster in Italy, 162 00:11:49,743 --> 00:11:53,945 the site becomes the focus of a major investigation. 163 00:11:53,947 --> 00:11:57,082 Engineers need to find out 164 00:11:57,084 --> 00:11:59,918 what caused the bridge to collapse. 165 00:12:01,722 --> 00:12:05,090 Camillo Nuti is an independent engineer 166 00:12:05,092 --> 00:12:06,624 who worked on the Italian government inquiry 167 00:12:06,626 --> 00:12:11,262 into the disaster. 168 00:12:11,264 --> 00:12:14,733 One should be very cautious before removing anything. 169 00:12:14,735 --> 00:12:18,136 Obviously, you have to remove the debris, 170 00:12:18,138 --> 00:12:20,705 but then be very careful, 171 00:12:20,707 --> 00:12:23,708 because you have to take the pieces, 172 00:12:23,710 --> 00:12:28,780 then try to understand if there was failure somewhere. 173 00:12:28,782 --> 00:12:32,784 If there was failure, you should find those pieces broken. 174 00:12:34,321 --> 00:12:39,290 NARRATOR: Finding those pieces is a daunting challenge. 175 00:12:39,292 --> 00:12:41,025 Sifting thousands of tons of debris, 176 00:12:41,027 --> 00:12:44,562 investigators need to identify the single part of the bridge 177 00:12:44,564 --> 00:12:46,798 that failed and triggered the disaster. 178 00:12:46,800 --> 00:12:53,104 They move any suspect pieces to this secure warehouse. 179 00:12:54,841 --> 00:12:58,676 The operation will take months, so at the same time, 180 00:12:58,678 --> 00:13:02,147 they pursue a parallel line of inquiry. 181 00:13:02,149 --> 00:13:06,217 ¶ ¶ 182 00:13:06,219 --> 00:13:09,687 They turn their attention to the way the bridge was built 183 00:13:09,689 --> 00:13:12,023 and its original design. 184 00:13:15,929 --> 00:13:17,428 ¶ ¶ 185 00:13:17,430 --> 00:13:20,231 The planning of the Polcevera Bridge 186 00:13:20,233 --> 00:13:24,335 began in the early 1960s. 187 00:13:24,337 --> 00:13:29,274 (speaking Italian): 188 00:13:29,276 --> 00:13:31,709 NARRATOR: Its designer, Riccardo Morandi, 189 00:13:31,711 --> 00:13:37,115 was a star engineer of Italy's postwar reconstruction boom. 190 00:13:38,351 --> 00:13:42,654 His work included astonishing modern structures, 191 00:13:42,656 --> 00:13:45,857 like this aircraft hangar in Rome, 192 00:13:45,859 --> 00:13:50,061 and dozens of bridges. 193 00:13:50,063 --> 00:13:53,798 IORI: He was the best bridge design engineer in Italy. 194 00:13:53,800 --> 00:14:00,038 There is no Italian engineer who built what Morandi built. 195 00:14:00,040 --> 00:14:05,376 NARRATOR: To construct the Polcevera Bridge in Genoa, 196 00:14:05,378 --> 00:14:09,047 Morandi faced a huge challenge. 197 00:14:09,049 --> 00:14:13,318 The bridge had to span the Polcevera River, 198 00:14:13,320 --> 00:14:18,122 multiple railway lines, and four apartment buildings. 199 00:14:19,926 --> 00:14:25,230 His plan was to build three enormous trestles, 200 00:14:25,232 --> 00:14:30,401 each with a tower nearly 300 feet high. 201 00:14:30,403 --> 00:14:32,237 Then, to hold up the road deck, 202 00:14:32,239 --> 00:14:36,474 he would use giant supports called cable stays, 203 00:14:36,476 --> 00:14:39,844 made of steel and concrete. 204 00:14:39,846 --> 00:14:45,250 When complete, the vast bridge would be over half a mile long. 205 00:14:45,252 --> 00:14:47,552 ¶ ¶ 206 00:14:47,554 --> 00:14:49,654 To build it, Morandi used 207 00:14:49,656 --> 00:14:52,223 a relatively new method of construction, 208 00:14:52,225 --> 00:14:56,227 one that he had helped develop. 209 00:14:56,229 --> 00:15:00,231 Before this time, most concrete bridges 210 00:15:00,233 --> 00:15:03,768 used simple steel reinforcement. 211 00:15:03,770 --> 00:15:06,871 But by stretching steel cables 212 00:15:06,873 --> 00:15:09,874 threaded through hollow guides in the concrete, 213 00:15:09,876 --> 00:15:13,912 engineers found they could build longer spans. 214 00:15:13,914 --> 00:15:18,449 The tension in the wires acted to compress the concrete, 215 00:15:18,451 --> 00:15:23,021 making it less likely to bend and crack. 216 00:15:23,023 --> 00:15:26,624 This was called prestressed concrete, 217 00:15:26,626 --> 00:15:33,197 and Morandi relied on it for the Polcevera Bridge. 218 00:15:34,868 --> 00:15:39,437 In his design, he does something radical. 219 00:15:39,439 --> 00:15:44,375 In most cable-stay bridges, the road deck is supported 220 00:15:44,377 --> 00:15:49,747 by many cables, spread out across the span. 221 00:15:49,749 --> 00:15:52,050 But to support his deck, 222 00:15:52,052 --> 00:15:55,453 Morandi bunches all the cables together 223 00:15:55,455 --> 00:15:59,857 to create massive cable stays, 224 00:15:59,859 --> 00:16:03,328 each one made of 52 steel cables 225 00:16:03,330 --> 00:16:07,265 wrapped in concrete to protect them from corrosion. 226 00:16:11,304 --> 00:16:14,639 NUTI: The technique was a novelty. 227 00:16:14,641 --> 00:16:18,943 He put just four cables for each pier, 228 00:16:18,945 --> 00:16:21,212 but they were incredibly strong cables. 229 00:16:21,214 --> 00:16:25,183 ¶ ¶ 230 00:16:25,185 --> 00:16:27,352 NARRATOR: After four years' construction, 231 00:16:27,354 --> 00:16:32,824 in 1967, Italy's president opened the bridge 232 00:16:32,826 --> 00:16:34,525 to an enthusiastic reception. 233 00:16:34,527 --> 00:16:38,963 IORI: The Morandi Bridge was the most important bridge in Italy. 234 00:16:38,965 --> 00:16:43,501 The biggest, the more beautiful, the most... 235 00:16:43,503 --> 00:16:44,902 The most famous. 236 00:16:44,904 --> 00:16:46,104 Absolutely. 237 00:16:46,106 --> 00:16:49,874 NUTI: It was a masterpiece, 238 00:16:49,876 --> 00:16:53,578 and everybody knows it was a masterpiece. 239 00:16:57,183 --> 00:16:58,282 ¶ ¶ 240 00:16:58,284 --> 00:17:00,985 NARRATOR: But 50 years later, 241 00:17:00,987 --> 00:17:04,489 the bridge fails catastrophically. 242 00:17:05,525 --> 00:17:09,027 Did Morandi's pioneering design play any role 243 00:17:09,029 --> 00:17:11,696 in causing it to collapse? 244 00:17:11,698 --> 00:17:15,867 ¶ ¶ 245 00:17:15,869 --> 00:17:18,036 To find out what caused the disaster, 246 00:17:18,038 --> 00:17:21,439 investigators need to discover which part of the bridge 247 00:17:21,441 --> 00:17:24,942 failed first. 248 00:17:24,944 --> 00:17:28,079 They look for CCTV cameras in the area 249 00:17:28,081 --> 00:17:31,349 that would have recorded the moment the bridge collapsed. 250 00:17:31,351 --> 00:17:34,819 ¶ ¶ 251 00:17:34,821 --> 00:17:38,956 Rain obscures the view in almost all the cameras. 252 00:17:38,958 --> 00:17:42,126 But in this scrapyard, 253 00:17:42,128 --> 00:17:45,196 they find a video that captures the disaster 254 00:17:45,198 --> 00:17:46,731 from beginning to end. 255 00:17:46,733 --> 00:17:49,634 The recording is part 256 00:17:49,636 --> 00:17:51,602 of an ongoing criminal investigation, 257 00:17:51,604 --> 00:17:54,672 and is impounded. 258 00:17:54,674 --> 00:17:59,277 Francesco Cozzi is the state's prosecutor 259 00:17:59,279 --> 00:18:03,081 in charge of the criminal investigation. 260 00:18:03,083 --> 00:18:08,052 COZZI (speaking Italian): 261 00:18:25,438 --> 00:18:27,572 NARRATOR: Sources familiar with what happened 262 00:18:27,574 --> 00:18:28,840 have provided clues 263 00:18:28,842 --> 00:18:31,476 that allow a reconstruction of the probable sequence 264 00:18:31,478 --> 00:18:34,011 of the bridge collapse. 265 00:18:34,013 --> 00:18:39,951 At just after 11:36 a.m., something happened 266 00:18:39,953 --> 00:18:42,220 that caused the southeast cable stay on pylon nine 267 00:18:42,222 --> 00:18:44,622 to break. 268 00:18:46,059 --> 00:18:51,195 Unsupported, that side of the road deck sags. 269 00:18:51,197 --> 00:18:53,998 This unbalances the whole structure, 270 00:18:54,000 --> 00:18:55,933 and increases the load on other components 271 00:18:55,935 --> 00:18:58,136 beyond their breaking points. 272 00:18:58,138 --> 00:19:02,940 This sends the road deck plummeting to the ground, 273 00:19:02,942 --> 00:19:09,447 and causes the giant tower to collapse. 274 00:19:13,786 --> 00:19:16,053 ¶ ¶ 275 00:19:16,055 --> 00:19:19,891 It looked like the unthinkable had happened. 276 00:19:19,893 --> 00:19:22,793 Some of the steel cables Morandi had embedded in concrete 277 00:19:22,795 --> 00:19:25,830 to protect them from corroding 278 00:19:25,832 --> 00:19:27,064 had snapped. 279 00:19:27,066 --> 00:19:30,568 But why? 280 00:19:30,570 --> 00:19:32,637 ¶ ¶ 281 00:19:32,639 --> 00:19:35,406 The bridge had stood firm for over half a century, 282 00:19:35,408 --> 00:19:38,176 carrying millions of travelers. 283 00:19:38,178 --> 00:19:43,548 So what caused it to fail the very moment it did? 284 00:19:43,550 --> 00:19:46,551 ¶ ¶ 285 00:19:46,553 --> 00:19:49,754 Like most road bridges built during the building boom 286 00:19:49,756 --> 00:19:51,789 of the 1960s and '70s, 287 00:19:51,791 --> 00:19:54,926 the Polcevera Bridge had been experiencing 288 00:19:54,928 --> 00:19:57,929 steadily rising traffic loads. 289 00:19:59,465 --> 00:20:02,833 By 2009, it carried four times as many vehicles 290 00:20:02,835 --> 00:20:05,836 as 30 years earlier. 291 00:20:08,508 --> 00:20:12,743 Over 25 million vehicles crossed it each year. 292 00:20:14,447 --> 00:20:16,714 Could the weight of traffic on the bridge 293 00:20:16,716 --> 00:20:20,384 have brought it down? 294 00:20:22,021 --> 00:20:24,589 Like all major bridges in Italy, 295 00:20:24,591 --> 00:20:26,557 the Polcevera Bridge was designed 296 00:20:26,559 --> 00:20:31,996 to carry the load of a convoy of heavy military vehicles. 297 00:20:31,998 --> 00:20:34,632 This meant it should have been able to carry the weight 298 00:20:34,634 --> 00:20:38,302 of modern traffic. 299 00:20:40,306 --> 00:20:43,374 But a small error in design or construction 300 00:20:43,376 --> 00:20:45,676 can have catastrophic consequences, 301 00:20:45,678 --> 00:20:51,616 as happened to this bridge in the United States. 302 00:20:51,618 --> 00:20:53,784 On August 1, 2007, 303 00:20:53,786 --> 00:20:58,956 Kelly Kahle was driving towards the I-35W bridge 304 00:20:58,958 --> 00:21:01,225 in Minneapolis. 305 00:21:01,227 --> 00:21:04,161 KAHLE: It's rush-hour traffic, 6:00. 306 00:21:04,163 --> 00:21:05,663 I'm going north, 307 00:21:05,665 --> 00:21:09,100 and we get right on the interstate. 308 00:21:09,102 --> 00:21:12,103 All of a sudden, there's a rumbling. 309 00:21:12,105 --> 00:21:14,171 And then all of a sudden, the bridge in front of me, 310 00:21:14,173 --> 00:21:15,573 far in front of me, 311 00:21:15,575 --> 00:21:17,174 looks like it rose up. 312 00:21:17,176 --> 00:21:22,146 And it almost looked like it was oscillating. 313 00:21:22,148 --> 00:21:23,748 Looking back on it, 314 00:21:23,750 --> 00:21:26,450 I can see that we were on a point that was failing, 315 00:21:26,452 --> 00:21:30,821 and we were starting to fall. 316 00:21:35,695 --> 00:21:38,829 NARRATOR: This is Kelly's car. 317 00:21:38,831 --> 00:21:43,634 It narrowly escaped plunging into the river. 318 00:21:43,636 --> 00:21:45,469 I climbed onto the hood of the car, 319 00:21:45,471 --> 00:21:48,572 and skimmied onto this concrete island, 320 00:21:48,574 --> 00:21:52,243 and just looking around and seeing other people 321 00:21:52,245 --> 00:21:56,514 kind of getting their wits about them. 322 00:21:58,017 --> 00:22:02,153 NARRATOR: 111 vehicles were on the bridge when it gave way. 323 00:22:02,155 --> 00:22:04,955 13 people died. 324 00:22:04,957 --> 00:22:08,759 145 were injured. 325 00:22:11,531 --> 00:22:14,565 When investigators reassemble the pieces 326 00:22:14,567 --> 00:22:16,801 of the collapsed bridge, 327 00:22:16,803 --> 00:22:18,703 their attention focuses on a part known 328 00:22:18,705 --> 00:22:22,239 as a gusset plate. 329 00:22:24,310 --> 00:22:26,677 Gusset plates are flat plates of steel 330 00:22:26,679 --> 00:22:31,115 that hold the girders of a truss bridge together. 331 00:22:31,117 --> 00:22:34,618 The key pieces of the I-35W bridge 332 00:22:34,620 --> 00:22:37,421 have been preserved. 333 00:22:39,158 --> 00:22:42,760 These pieces are all connected to the single gusset plate 334 00:22:42,762 --> 00:22:45,463 that failed when the bridge collapsed. 335 00:22:45,465 --> 00:22:47,298 During the collapse, 336 00:22:47,300 --> 00:22:49,233 this folded in on itself and was completely mangled 337 00:22:49,235 --> 00:22:52,436 by the weight coming down on top of it. 338 00:22:57,110 --> 00:22:58,542 NARRATOR: At the time of the collapse, 339 00:22:58,544 --> 00:23:01,579 workers were preparing to re-lay the road. 340 00:23:01,581 --> 00:23:05,282 Trucks had dumped piles of sand and gravel 341 00:23:05,284 --> 00:23:08,152 on the closed lanes. 342 00:23:08,154 --> 00:23:10,488 REIERSON: There were 800,000 pounds of construction equipment 343 00:23:10,490 --> 00:23:11,455 on the top of the bridge, 344 00:23:11,457 --> 00:23:14,692 as well as over 160 people in cars, 345 00:23:14,694 --> 00:23:17,261 because it was the very end of rush hour that day. 346 00:23:17,263 --> 00:23:21,065 NARRATOR: That combined load 347 00:23:21,067 --> 00:23:23,634 proves too much for one of gusset plates. 348 00:23:23,636 --> 00:23:26,604 REIERSON: The gusset plate split along a line of rivets. 349 00:23:26,606 --> 00:23:31,308 You can see that it's completely torn apart. 350 00:23:31,310 --> 00:23:34,545 NARRATOR: The bridge was overloaded. 351 00:23:34,547 --> 00:23:38,816 But its designers had also made a mistake. 352 00:23:38,818 --> 00:23:41,685 The gusset plate was only a half an inch thick, 353 00:23:41,687 --> 00:23:43,087 and it should have been one inch. 354 00:23:43,089 --> 00:23:45,089 So it should have been double the thickness 355 00:23:45,091 --> 00:23:46,457 that it, that it was, 356 00:23:46,459 --> 00:23:48,125 and that's what led to the failure. 357 00:23:48,127 --> 00:23:53,798 ¶ ¶ 358 00:23:55,334 --> 00:23:57,201 NARRATOR: Did Genoa's Polcevera Bridge 359 00:23:57,203 --> 00:23:59,470 collapse because of a similar fault-- 360 00:23:59,472 --> 00:24:02,473 a failure to make it strong enough 361 00:24:02,475 --> 00:24:04,942 to carry heavy loads? 362 00:24:04,944 --> 00:24:08,879 When Morandi planned the bridge, 363 00:24:08,881 --> 00:24:12,183 he designed it to support a lot of weight. 364 00:24:12,185 --> 00:24:14,285 NUTI: The ideas of Morandi was, 365 00:24:14,287 --> 00:24:20,691 let's build a bridge with very, very strong cables 366 00:24:20,693 --> 00:24:25,229 to where it's overdesigned, very much overdesigned. 367 00:24:25,231 --> 00:24:29,099 So the design was incredibly safe for the time. 368 00:24:30,736 --> 00:24:32,536 NARRATOR: When the bridge was built, 369 00:24:32,538 --> 00:24:35,206 the cable stays were capable of carrying 370 00:24:35,208 --> 00:24:38,476 over twice the weight of the vehicles on it. 371 00:24:40,279 --> 00:24:42,413 The bridge should have been easily strong enough 372 00:24:42,415 --> 00:24:45,916 to carry the light traffic on it when it fell. 373 00:24:45,918 --> 00:24:47,785 COZZI (speaking Italian): 374 00:24:57,497 --> 00:24:59,296 NUTI: What is well known 375 00:24:59,298 --> 00:25:02,600 is that the total load that the bridge could support 376 00:25:02,602 --> 00:25:07,872 was well beyond the loads that was on the bridge. 377 00:25:10,042 --> 00:25:12,610 NARRATOR: So if the weight of the vehicles on the bridge 378 00:25:12,612 --> 00:25:15,546 didn't cause it to collapse, 379 00:25:15,548 --> 00:25:16,547 what did? 380 00:25:16,549 --> 00:25:19,283 Expert sources suggest 381 00:25:19,285 --> 00:25:21,886 that the southeast cable stay snapped 382 00:25:21,888 --> 00:25:25,756 near the top of the pylon. 383 00:25:25,758 --> 00:25:28,259 If it was this cable stay that broke first, 384 00:25:28,261 --> 00:25:33,230 something had gone seriously wrong. 385 00:25:33,232 --> 00:25:35,032 ¶ ¶ 386 00:25:35,034 --> 00:25:39,970 This is the kind of fault that all engineers fear. 387 00:25:39,972 --> 00:25:43,774 It's known as a single-point failure, 388 00:25:43,776 --> 00:25:46,277 when one component breaks 389 00:25:46,279 --> 00:25:49,813 and brings down an entire bridge. 390 00:25:49,815 --> 00:25:55,619 It's a lesson driven home by an earlier disaster. 391 00:25:55,621 --> 00:25:59,490 ¶ ¶ 392 00:25:59,492 --> 00:26:01,625 In 1928, the people 393 00:26:01,627 --> 00:26:03,761 of Point Pleasant in West Virginia 394 00:26:03,763 --> 00:26:05,996 celebrated the opening of their new bridge 395 00:26:05,998 --> 00:26:07,998 over the Ohio River. 396 00:26:08,000 --> 00:26:11,702 ¶ ¶ 397 00:26:11,704 --> 00:26:13,304 JACK FOWLER: It was a really exciting day. 398 00:26:13,306 --> 00:26:14,605 There was 10,000 people in town. 399 00:26:14,607 --> 00:26:16,674 There's pictures that show the bridge was completely 400 00:26:16,676 --> 00:26:18,842 covered with people. 401 00:26:18,844 --> 00:26:20,210 They were very proud of it, 402 00:26:20,212 --> 00:26:23,047 you know, to have a nice, big, shiny bridge like that. 403 00:26:23,049 --> 00:26:26,083 Well, it was the talk of the, talk of the area, 404 00:26:26,085 --> 00:26:28,085 the East Coast, you know? 405 00:26:28,087 --> 00:26:29,353 The Silver Bridge. 406 00:26:30,990 --> 00:26:33,891 NARRATOR: It was nicknamed the Silver Bridge 407 00:26:33,893 --> 00:26:37,461 because of its shiny coat of aluminum paint. 408 00:26:37,463 --> 00:26:39,964 But unknown to the people that used it, 409 00:26:39,966 --> 00:26:44,268 its design contained a serious flaw. 410 00:26:44,270 --> 00:26:47,204 ¶ ¶ 411 00:26:47,206 --> 00:26:49,607 On December 15, 1967, 412 00:26:49,609 --> 00:26:54,745 Peggy Huber was driving towards the Silver Bridge. 413 00:26:54,747 --> 00:26:57,715 HUBER: Traffic was heavy. 414 00:26:57,717 --> 00:27:01,151 It was Friday and near Christmastime. 415 00:27:01,153 --> 00:27:03,787 I got behind a dump truck. 416 00:27:03,789 --> 00:27:05,889 And it was moving so slow, 417 00:27:05,891 --> 00:27:08,692 it stopped right at the light where the bridge was. 418 00:27:08,694 --> 00:27:10,361 And I was behind it. 419 00:27:10,363 --> 00:27:15,466 And I had reached down to turn the radio dial, 420 00:27:15,468 --> 00:27:18,769 and I heard this awful noise, and when I looked up, 421 00:27:18,771 --> 00:27:22,840 the bridge was just like a tinker toy, waving, 422 00:27:22,842 --> 00:27:23,974 and it just fell. 423 00:27:23,976 --> 00:27:26,110 ¶ ¶ 424 00:27:26,112 --> 00:27:28,879 It just sounded like a bunch of metal. 425 00:27:28,881 --> 00:27:31,548 It was very loud. 426 00:27:31,550 --> 00:27:34,918 FOWLER: It collapsed, I think, at 4:58, 427 00:27:34,920 --> 00:27:36,854 so it was a busy time. 428 00:27:36,856 --> 00:27:42,092 There was 31 vehicles that fell. 429 00:27:42,094 --> 00:27:46,230 NARRATOR: The collapse killed 46 people. 430 00:27:46,232 --> 00:27:50,501 It remains the worst road-bridge disaster in American history. 431 00:27:50,503 --> 00:27:54,672 ¶ ¶ 432 00:27:54,674 --> 00:27:58,008 To find out what had caused the bridge to fail, 433 00:27:58,010 --> 00:28:01,679 investigators searched the riverbed for wreckage. 434 00:28:01,681 --> 00:28:05,382 FOWLER: They brought in a whole bunch of divers, 435 00:28:05,384 --> 00:28:07,051 and they would bring pieces up, 436 00:28:07,053 --> 00:28:08,886 and of course, they would examine all of it, 437 00:28:08,888 --> 00:28:10,320 see if it would fit in any way. 438 00:28:10,322 --> 00:28:12,389 ¶ ¶ 439 00:28:12,391 --> 00:28:16,026 NARRATOR: The bridge was an eyebar suspension bridge. 440 00:28:18,297 --> 00:28:21,665 Giant bars, up to 55 feet long, 441 00:28:21,667 --> 00:28:24,301 with a hole called an eye at each end, 442 00:28:24,303 --> 00:28:28,005 were linked together with steel pins. 443 00:28:28,007 --> 00:28:32,409 ¶ ¶ 444 00:28:32,411 --> 00:28:38,115 Crucially, each link of the suspension chain 445 00:28:38,117 --> 00:28:41,085 contained only two eyebars. 446 00:28:41,087 --> 00:28:46,423 ¶ ¶ 447 00:28:46,425 --> 00:28:48,092 FOWLER: And they finally found 448 00:28:48,094 --> 00:28:51,628 one of the eyebars that was broken-- split. 449 00:28:51,630 --> 00:28:54,732 And when they examined it, 450 00:28:54,734 --> 00:29:00,604 they found that there had been a, a hairline fracture in it. 451 00:29:00,606 --> 00:29:02,940 ¶ ¶ 452 00:29:02,942 --> 00:29:06,376 NARRATOR: Nearly 40 years of corrosion in the steel 453 00:29:06,378 --> 00:29:07,978 had triggered a fracture 454 00:29:07,980 --> 00:29:12,015 that ripped the eyebar from its pin. 455 00:29:12,017 --> 00:29:14,485 The weight of the bridge proves too much 456 00:29:14,487 --> 00:29:16,587 for the remaining eyebar, 457 00:29:16,589 --> 00:29:21,692 pulling it with an extreme force that tears the link apart. 458 00:29:22,795 --> 00:29:24,862 The failure of this single point 459 00:29:24,864 --> 00:29:27,731 triggers an unexpected domino effect, 460 00:29:27,733 --> 00:29:31,702 causing the entire bridge to collapse. 461 00:29:34,073 --> 00:29:35,906 In the United States alone, 462 00:29:35,908 --> 00:29:38,742 around 18,000 bridges have been identified 463 00:29:38,744 --> 00:29:41,111 as "fracture-critical." 464 00:29:41,113 --> 00:29:43,714 This means if one component fails, 465 00:29:43,716 --> 00:29:48,852 part or all of the bridge would probably collapse. 466 00:29:48,854 --> 00:29:51,255 Many were built in the 1960s and '70s 467 00:29:51,257 --> 00:29:55,092 during the rush to complete the interstate highway network. 468 00:29:55,094 --> 00:29:58,562 Today, engineers aim to design bridges 469 00:29:58,564 --> 00:30:01,799 so that if a single component fails, 470 00:30:01,801 --> 00:30:06,203 it won't bring down the whole structure. 471 00:30:06,205 --> 00:30:09,606 Modern cable-stay bridges are built with a fan of cables 472 00:30:09,608 --> 00:30:12,676 to spread the load. 473 00:30:12,678 --> 00:30:15,579 This design is very different 474 00:30:15,581 --> 00:30:19,716 to that of Morandi's Polcevera Bridge in Italy. 475 00:30:22,888 --> 00:30:26,123 Morandi bundled all his cables together 476 00:30:26,125 --> 00:30:28,392 and covered them in concrete 477 00:30:28,394 --> 00:30:32,162 to create single, giant stays. 478 00:30:32,164 --> 00:30:37,000 His design was meant to protect the cables from corrosion. 479 00:30:38,337 --> 00:30:41,305 But he inadvertently made the bridge vulnerable 480 00:30:41,307 --> 00:30:43,473 to a single-point failure. 481 00:30:43,475 --> 00:30:48,011 If just one of the huge stays broke, 482 00:30:48,013 --> 00:30:50,948 the result would be disastrous. 483 00:30:50,950 --> 00:30:55,652 Sadly, Morandi's design was completed 484 00:30:55,654 --> 00:30:59,089 a few years before the Silver Bridge disaster. 485 00:30:59,091 --> 00:31:03,393 Luckily, it has not been widely copied. 486 00:31:03,395 --> 00:31:08,632 Other engineers realized the potential problems, 487 00:31:08,634 --> 00:31:14,104 and only a handful of similar bridges were ever built. 488 00:31:14,106 --> 00:31:17,007 ¶ ¶ 489 00:31:17,009 --> 00:31:18,408 The Polcevera Bridge 490 00:31:18,410 --> 00:31:23,447 was routinely inspected, maintained, and monitored. 491 00:31:23,449 --> 00:31:28,318 For it to collapse, something must have gone seriously wrong. 492 00:31:28,320 --> 00:31:31,788 But what? 493 00:31:31,790 --> 00:31:33,991 ¶ ¶ 494 00:31:33,993 --> 00:31:37,027 Four months after the disaster, 495 00:31:37,029 --> 00:31:39,997 workers remove the last of the debris. 496 00:31:41,267 --> 00:31:45,569 Forensic engineers now have several key pieces of evidence. 497 00:31:45,571 --> 00:31:47,504 ¶ ¶ 498 00:31:47,506 --> 00:31:52,042 Their initial inspections reveal that some of the steel cables, 499 00:31:52,044 --> 00:31:53,944 that were supposedly well protected 500 00:31:53,946 --> 00:31:56,914 inside the concrete-covered stays, 501 00:31:56,916 --> 00:31:59,449 have suffered from corrosion. 502 00:31:59,451 --> 00:32:03,487 NUTI: Some cables appeared broken. 503 00:32:03,489 --> 00:32:04,755 When there is corrosion, 504 00:32:04,757 --> 00:32:07,491 the corroded cable are brittle. 505 00:32:07,493 --> 00:32:11,028 So if you just bent them, you break them, 506 00:32:11,030 --> 00:32:12,930 which is not usual for steel. 507 00:32:12,932 --> 00:32:16,600 But when you have corrosion, this may happen very easily. 508 00:32:16,602 --> 00:32:18,368 ¶ ¶ 509 00:32:18,370 --> 00:32:22,539 NARRATOR: The investigators identify 17 key sections of the bridge 510 00:32:22,541 --> 00:32:26,143 for further investigation. 511 00:32:26,145 --> 00:32:31,315 Their prime suspect is part number 132. 512 00:32:32,651 --> 00:32:33,984 This is believed to be 513 00:32:33,986 --> 00:32:37,120 the part of the cable stay near the top of the bridge 514 00:32:37,122 --> 00:32:40,691 that broke. 515 00:32:40,693 --> 00:32:46,330 They ship the pieces to a laboratory in Switzerland, 516 00:32:46,332 --> 00:32:48,732 where investigators will attempt to find out 517 00:32:48,734 --> 00:32:53,236 what caused the steel cables to corrode. 518 00:32:53,238 --> 00:32:54,671 ¶ ¶ 519 00:32:54,673 --> 00:32:57,708 When Morandi covered the cables with concrete, 520 00:32:57,710 --> 00:33:00,510 they should have been protected. 521 00:33:00,512 --> 00:33:07,451 Concrete is very alkaline, the opposite of acidic. 522 00:33:07,453 --> 00:33:10,821 This produces a thin layer of stable iron oxide 523 00:33:10,823 --> 00:33:14,157 around the steel, 524 00:33:14,159 --> 00:33:17,995 and should almost completely prevent the steel from rusting. 525 00:33:17,997 --> 00:33:22,399 ¶ ¶ 526 00:33:22,401 --> 00:33:25,202 Thousands of bridges rely on this protection. 527 00:33:25,204 --> 00:33:29,006 In most cases, it works fine. 528 00:33:29,008 --> 00:33:34,511 But just occasionally, it goes badly wrong. 529 00:33:34,513 --> 00:33:36,580 ¶ ¶ 530 00:33:36,582 --> 00:33:38,715 London. 531 00:33:40,386 --> 00:33:43,120 The Hammersmith Flyover. 532 00:33:43,122 --> 00:33:47,124 It's one of the city's busiest bridges. 533 00:33:47,126 --> 00:33:51,695 SHARAN GILL: Hammersmith Flyover is critical to the London road network. 534 00:33:51,697 --> 00:33:53,330 It provides one of the main routes 535 00:33:53,332 --> 00:33:55,932 from Heathrow Airport into central London, 536 00:33:55,934 --> 00:33:58,602 and it's also one of the major routes out 537 00:33:58,604 --> 00:34:00,670 to west London in itself. 538 00:34:00,672 --> 00:34:02,305 ¶ ¶ 539 00:34:02,307 --> 00:34:04,274 NARRATOR: In December 2011, 540 00:34:04,276 --> 00:34:08,712 just months before the 2012 London Olympics, 541 00:34:08,714 --> 00:34:10,313 engineers discover 542 00:34:10,315 --> 00:34:13,917 this vital bridge is at risk of collapse. 543 00:34:13,919 --> 00:34:16,253 ¶ ¶ 544 00:34:16,255 --> 00:34:20,090 The overpass was built in the early 1960s 545 00:34:20,092 --> 00:34:23,193 from giant concrete sections. 546 00:34:23,195 --> 00:34:26,596 The problem lay in the way they were held together. 547 00:34:26,598 --> 00:34:30,867 To build the Hammersmith Flyover, 548 00:34:30,869 --> 00:34:34,037 construction crews use a form of prestressed concrete 549 00:34:34,039 --> 00:34:38,475 made using a technique called posttensioning. 550 00:34:38,477 --> 00:34:42,212 They hoist giant, prefabricated sections of the bridge 551 00:34:42,214 --> 00:34:44,081 into position, 552 00:34:44,083 --> 00:34:49,286 then thread 64 steel tendons through holes in the structure. 553 00:34:49,288 --> 00:34:51,555 ¶ ¶ 554 00:34:51,557 --> 00:34:53,657 They then stretch and tension the wires 555 00:34:53,659 --> 00:34:58,995 to hold the concrete sections of the bridge together. 556 00:34:58,997 --> 00:35:02,432 To prevent the steel cables from corroding, 557 00:35:02,434 --> 00:35:08,338 workers cover them with a concrete grout. 558 00:35:08,340 --> 00:35:13,510 Like Morandi, the engineers of the Hammersmith Flyover 559 00:35:13,512 --> 00:35:18,014 thought the concrete would prevent the cables ever rusting. 560 00:35:18,016 --> 00:35:20,717 But in the early 2000s, 561 00:35:20,719 --> 00:35:26,323 inspections reveal they were wrong. 562 00:35:26,325 --> 00:35:31,094 Water is seeping into gaps in the concrete grout, 563 00:35:31,096 --> 00:35:34,431 causing the steel tendons that held the bridge together 564 00:35:34,433 --> 00:35:36,933 to rust. 565 00:35:36,935 --> 00:35:39,936 GILL: As you can see here, they're all encased in grout. 566 00:35:39,938 --> 00:35:42,439 And that means it's very difficult 567 00:35:42,441 --> 00:35:45,242 for the water to escape. 568 00:35:45,244 --> 00:35:48,912 Um, and it's also very difficult for us to inspect the tendons 569 00:35:48,914 --> 00:35:50,914 and make sure that everything is okay. 570 00:35:52,351 --> 00:35:53,783 NARRATOR: Unfortunately, 571 00:35:53,785 --> 00:35:57,587 the concrete grout that was meant to prevent rust 572 00:35:57,589 --> 00:35:59,156 trapped water in cavities 573 00:35:59,158 --> 00:36:04,628 and hid any corrosion that did occur. 574 00:36:04,630 --> 00:36:07,430 Transport for London calls in 575 00:36:07,432 --> 00:36:10,534 structural-integrity engineer Jon Watson 576 00:36:10,536 --> 00:36:15,372 to try to identify where the corrosion is worst. 577 00:36:15,374 --> 00:36:16,907 ¶ ¶ 578 00:36:16,909 --> 00:36:21,611 He installs over 500 microphones inside the bridge. 579 00:36:21,613 --> 00:36:24,014 WATSON: We put a microphone about every three meters 580 00:36:24,016 --> 00:36:25,382 along the structure, 581 00:36:25,384 --> 00:36:27,284 to listen out for anything that's distinct, 582 00:36:27,286 --> 00:36:28,585 like a wire break. 583 00:36:29,788 --> 00:36:32,422 NARRATOR: Each of the cables buried in the concrete 584 00:36:32,424 --> 00:36:37,627 contains 19 separate strands of wire. 585 00:36:37,629 --> 00:36:40,297 The cables themselves are built up of individual wires. 586 00:36:40,299 --> 00:36:43,700 When these wires corrode, a small crack grows in the wire. 587 00:36:43,702 --> 00:36:44,834 Eventually, the wire snaps 588 00:36:44,836 --> 00:36:47,437 with a very distinct snapping sound. 589 00:36:47,439 --> 00:36:49,506 (wire snapping) 590 00:36:49,508 --> 00:36:51,808 And it's this energy, this sound wave, 591 00:36:51,810 --> 00:36:55,145 that's detected by the microphone on the structure. 592 00:36:55,147 --> 00:36:57,547 It enables us to determine the position of the wire break 593 00:36:57,549 --> 00:36:58,815 on the cable itself. 594 00:37:00,118 --> 00:37:03,920 Over the period of three- and-a-half years of monitoring, 595 00:37:03,922 --> 00:37:07,824 we detected in excess of 1,100 wire breaks. 596 00:37:07,826 --> 00:37:10,026 So typically one or two a day. 597 00:37:11,830 --> 00:37:13,196 Within a couple of months, 598 00:37:13,198 --> 00:37:16,266 we were able to determine exactly where the hotspots were, 599 00:37:16,268 --> 00:37:18,802 where most wire breaks were occurring. 600 00:37:18,804 --> 00:37:20,604 NARRATOR: This told engineers 601 00:37:20,606 --> 00:37:22,973 where the corrosion was most severe, 602 00:37:22,975 --> 00:37:26,443 allowing them to focus their inspection. 603 00:37:26,445 --> 00:37:28,378 GILL: We were able to break out the grout 604 00:37:28,380 --> 00:37:31,648 around full sections of those bundles 605 00:37:31,650 --> 00:37:33,750 of highly stressed steel cables 606 00:37:33,752 --> 00:37:36,219 to see just how deep 607 00:37:36,221 --> 00:37:38,722 the corrosion had penetrated those cables. 608 00:37:39,992 --> 00:37:42,192 NARRATOR: What they discovered was alarming. 609 00:37:42,194 --> 00:37:46,096 The bridge was at risk of collapse. 610 00:37:47,633 --> 00:37:49,733 Although they thought the risk small, 611 00:37:49,735 --> 00:37:53,036 to be safe, they decided to close it. 612 00:37:53,038 --> 00:37:56,740 ¶ ¶ 613 00:37:56,742 --> 00:37:58,642 To repair the bridge, 614 00:37:58,644 --> 00:38:05,315 they attached four miles of new cables to the outside 615 00:38:05,317 --> 00:38:08,251 and stretched them, 616 00:38:08,253 --> 00:38:13,189 just like a super-sized version of the original posttensioning. 617 00:38:13,191 --> 00:38:16,559 ¶ ¶ 618 00:38:16,561 --> 00:38:19,896 It's estimated that this rescue operation 619 00:38:19,898 --> 00:38:22,832 has extended the bridge's life by 60 years. 620 00:38:22,834 --> 00:38:26,202 ¶ ¶ 621 00:38:26,204 --> 00:38:29,439 The close shave with the Hammersmith Flyover 622 00:38:29,441 --> 00:38:32,275 shows how cables buried inside concrete 623 00:38:32,277 --> 00:38:34,377 can rust. 624 00:38:34,379 --> 00:38:38,381 So did a similar problem with corrosion 625 00:38:38,383 --> 00:38:44,154 cause the Polcevera Bridge in Italy to fail? 626 00:38:44,156 --> 00:38:46,556 ¶ ¶ 627 00:38:46,558 --> 00:38:48,725 In Switzerland, scientists examine 628 00:38:48,727 --> 00:38:52,395 the pieces of the bridge brought from Genoa. 629 00:38:52,397 --> 00:38:53,763 They need to find out 630 00:38:53,765 --> 00:38:56,333 what caused the steel cables that held up the bridge 631 00:38:56,335 --> 00:39:00,403 to corrode. 632 00:39:00,405 --> 00:39:03,640 Like the Hammersmith Flyover, 633 00:39:03,642 --> 00:39:09,212 the Polcevera Bridge relied on posttensioned steel cables. 634 00:39:09,214 --> 00:39:11,915 They held the road deck together 635 00:39:11,917 --> 00:39:16,886 and gave the cable stays their strength. 636 00:39:16,888 --> 00:39:21,191 Back in the 1960s, to construct each stay, 637 00:39:21,193 --> 00:39:26,229 workers cover its 52 cables with steel ducts, 638 00:39:26,231 --> 00:39:29,399 then cast the concrete around them. 639 00:39:32,471 --> 00:39:34,471 Once this sets, 640 00:39:34,473 --> 00:39:39,576 engineers stretch the cables to posttension the stay. 641 00:39:39,578 --> 00:39:43,813 Finally, they inject a concrete grout into the ducts 642 00:39:43,815 --> 00:39:49,486 to cover the cables and prevent the steel from rusting. 643 00:39:49,488 --> 00:39:51,321 ¶ ¶ 644 00:39:51,323 --> 00:39:53,022 When scientists examine 645 00:39:53,024 --> 00:39:55,859 the steel cables inside the stays, 646 00:39:55,861 --> 00:39:58,728 they also carry out a forensic examination 647 00:39:58,730 --> 00:40:01,631 of the ducts. 648 00:40:01,633 --> 00:40:06,002 In some, they find cavities-- 649 00:40:06,004 --> 00:40:08,605 places where the grout is missing, 650 00:40:08,607 --> 00:40:11,274 leaving the steel cables unprotected. 651 00:40:13,645 --> 00:40:16,780 Once moisture entered these cavities, 652 00:40:16,782 --> 00:40:23,319 the steel wires would corrode, become brittle, and break. 653 00:40:23,321 --> 00:40:25,555 ¶ ¶ 654 00:40:25,557 --> 00:40:27,957 Investigators have not yet announced the cause 655 00:40:27,959 --> 00:40:31,027 of the bridge's collapse. 656 00:40:31,029 --> 00:40:34,764 But one theory is that corrosion of the cable stay 657 00:40:34,766 --> 00:40:39,269 played a major role. 658 00:40:39,271 --> 00:40:42,405 COZZI (speaking Italian): 659 00:40:57,155 --> 00:40:59,155 ¶ ¶ 660 00:40:59,157 --> 00:41:02,125 NARRATOR: Autostrade, the company that managed the bridge, 661 00:41:02,127 --> 00:41:04,694 maintains that a failure of the cable stays 662 00:41:04,696 --> 00:41:08,398 was not the primary cause of the collapse. 663 00:41:08,400 --> 00:41:09,766 It says that, 664 00:41:09,768 --> 00:41:13,870 with the amount of corrosion the Zurich laboratory found, 665 00:41:13,872 --> 00:41:15,538 the cables would still have had enough strength 666 00:41:15,540 --> 00:41:18,641 and would not have broken. 667 00:41:18,643 --> 00:41:21,978 It also says the ducts were present 668 00:41:21,980 --> 00:41:27,250 and doing their job of containing the cement grout. 669 00:41:27,252 --> 00:41:32,522 If the bridge's collapse was, in fact, due to its poor condition, 670 00:41:32,524 --> 00:41:36,793 could Autostrade have detected that it was about to fail? 671 00:41:36,795 --> 00:41:41,397 ¶ ¶ 672 00:41:41,399 --> 00:41:45,702 Corroded steel is easy to see-- if it is exposed. 673 00:41:45,704 --> 00:41:49,138 But when steel is embedded in concrete, 674 00:41:49,140 --> 00:41:53,042 it's very hard to check that it isn't corroding. 675 00:41:53,044 --> 00:41:54,878 WATSON: Concrete is very difficult 676 00:41:54,880 --> 00:41:57,780 to inspect non-destructively. 677 00:41:57,782 --> 00:42:01,518 It's very difficult to detect actual wire breaks 678 00:42:01,520 --> 00:42:04,420 and really quantify the extent of corrosions. 679 00:42:06,157 --> 00:42:07,624 NARRATOR: On the Polcevera Bridge, 680 00:42:07,626 --> 00:42:09,926 engineers used different techniques 681 00:42:09,928 --> 00:42:12,795 to assess the level of corrosion of the cables 682 00:42:12,797 --> 00:42:15,965 inside the concrete stays. 683 00:42:15,967 --> 00:42:19,068 One they appear to have relied on heavily 684 00:42:19,070 --> 00:42:23,306 is called reflectometric impulse measurement technique, 685 00:42:23,308 --> 00:42:25,875 or RIMT for short. 686 00:42:25,877 --> 00:42:30,513 ¶ ¶ 687 00:42:30,515 --> 00:42:32,482 This method aims to detect corrosion 688 00:42:32,484 --> 00:42:36,953 by sending short electrical pulses up the steel tendons 689 00:42:36,955 --> 00:42:39,822 and recording the reflected signal. 690 00:42:39,824 --> 00:42:42,325 ¶ ¶ 691 00:42:44,162 --> 00:42:45,461 NUTI: At the beginning, 692 00:42:45,463 --> 00:42:48,331 everybody was very optimistic with this method. 693 00:42:48,333 --> 00:42:53,269 In 1994, '95, some technicians from Autostrade 694 00:42:53,271 --> 00:42:56,906 published some papers where they said, 695 00:42:56,908 --> 00:43:00,243 "Oh, the method is fantastic, you can find everything." 696 00:43:00,245 --> 00:43:02,245 ¶ ¶ 697 00:43:02,247 --> 00:43:04,447 NARRATOR: But in 1997, 698 00:43:04,449 --> 00:43:07,517 independent scientists conducted a study 699 00:43:07,519 --> 00:43:13,623 and found that RIMT could not reliably detect faults. 700 00:43:15,260 --> 00:43:16,926 Part of the prosecution case 701 00:43:16,928 --> 00:43:22,065 is likely to question the use of the RIMT method. 702 00:43:22,067 --> 00:43:25,802 Autostrade says that since 2000, 703 00:43:25,804 --> 00:43:29,105 engineers have used a new version of RIMT 704 00:43:29,107 --> 00:43:31,407 called RIMT2. 705 00:43:31,409 --> 00:43:34,510 It says this is completely different, 706 00:43:34,512 --> 00:43:39,182 and does enable experts to accurately assess corrosion, 707 00:43:39,184 --> 00:43:41,684 and that its reliability has been verified 708 00:43:41,686 --> 00:43:45,254 by multiple international scientific studies. 709 00:43:47,525 --> 00:43:51,861 RIMT2 is not widely used in the industry. 710 00:43:51,863 --> 00:43:55,431 ¶ ¶ 711 00:43:55,433 --> 00:44:00,370 In 2017, Autostrade submitted a detailed plan 712 00:44:00,372 --> 00:44:03,373 to replace the cable stays. 713 00:44:04,909 --> 00:44:06,876 But before the work could start, 714 00:44:06,878 --> 00:44:09,612 the bridge collapsed. 715 00:44:09,614 --> 00:44:13,950 ¶ ¶ 716 00:44:16,021 --> 00:44:17,754 The Polcevera Bridge 717 00:44:17,756 --> 00:44:20,857 is one of the worst road-bridge disasters to hit Europe 718 00:44:20,859 --> 00:44:23,259 in over a century. 719 00:44:23,261 --> 00:44:26,763 It raises troubling and pressing questions 720 00:44:26,765 --> 00:44:30,933 about how our aging bridges are inspected and maintained. 721 00:44:32,871 --> 00:44:36,572 ¶ ¶ 722 00:44:36,574 --> 00:44:40,710 The highway-building boom of the 1960s and '70s 723 00:44:40,712 --> 00:44:45,982 has left us with thousands of bridges that are growing old, 724 00:44:45,984 --> 00:44:50,386 and we don't know how many contain hidden flaws 725 00:44:50,388 --> 00:44:53,990 that have yet to reveal themselves. 726 00:44:55,260 --> 00:44:57,326 Prestressed concrete bridges, 727 00:44:57,328 --> 00:44:59,662 with their concealed steel wires, 728 00:44:59,664 --> 00:45:04,200 are now a particular concern. 729 00:45:04,202 --> 00:45:06,335 ¶ ¶ 730 00:45:06,337 --> 00:45:07,770 Berlin, Germany. 731 00:45:07,772 --> 00:45:11,107 This is the Elsen Bridge. 732 00:45:11,109 --> 00:45:15,011 It was built in the mid-1960s. 733 00:45:15,013 --> 00:45:17,547 In the summer of 2018, 734 00:45:17,549 --> 00:45:20,183 inspectors find a 90-foot-long crack 735 00:45:20,185 --> 00:45:23,519 in its concrete span. 736 00:45:23,521 --> 00:45:25,888 Fearing that some of the steel cables inside the bridge 737 00:45:25,890 --> 00:45:28,024 have corroded and snapped, 738 00:45:28,026 --> 00:45:29,192 they restrict traffic. 739 00:45:29,194 --> 00:45:32,795 ¶ ¶ 740 00:45:32,797 --> 00:45:36,365 Many aging prestressed-concrete bridges 741 00:45:36,367 --> 00:45:40,002 have similar problems. 742 00:45:40,004 --> 00:45:42,672 Some, like the Hammersmith Flyover, 743 00:45:42,674 --> 00:45:46,709 can be saved by retrofitting external cables. 744 00:45:48,880 --> 00:45:51,247 Others, like the Elsen Bridge, 745 00:45:51,249 --> 00:45:54,650 will have to be demolished and replaced. 746 00:45:58,623 --> 00:46:01,457 So how likely is another bridge collapse 747 00:46:01,459 --> 00:46:04,827 like the one in Genoa? 748 00:46:04,829 --> 00:46:06,629 BLACK: You know, it's hard to say 749 00:46:06,631 --> 00:46:08,164 how likely a collapse like that would be, 750 00:46:08,166 --> 00:46:11,601 but we have had over two dozen bridges fail 751 00:46:11,603 --> 00:46:13,202 in the United States since 2000, 752 00:46:13,204 --> 00:46:14,537 for various reasons. 753 00:46:14,539 --> 00:46:17,106 So there are concerns these are bridges that... 754 00:46:17,108 --> 00:46:18,307 We have a number of them that need 755 00:46:18,309 --> 00:46:21,010 to be repaired and replaced. 756 00:46:21,012 --> 00:46:23,079 I think we have a significant challenge. 757 00:46:23,081 --> 00:46:26,182 We have underinvested in our transportation system 758 00:46:26,184 --> 00:46:27,483 for decades. 759 00:46:27,485 --> 00:46:29,452 NARRATOR: But safety has a price. 760 00:46:29,454 --> 00:46:32,989 BLACK: We could probably double or triple what we're spending 761 00:46:32,991 --> 00:46:34,957 across all levels of government 762 00:46:34,959 --> 00:46:38,027 to really make those repairs that need to be made. 763 00:46:38,029 --> 00:46:40,396 ¶ ¶ 764 00:46:40,398 --> 00:46:42,965 NARRATOR: Maintaining this aging infrastructure 765 00:46:42,967 --> 00:46:45,501 is a huge challenge, 766 00:46:45,503 --> 00:46:47,804 but engineers are rising to it. 767 00:46:47,806 --> 00:46:52,108 Modern cable-stay bridges are designed 768 00:46:52,110 --> 00:46:56,946 so that the cables can be removed and replaced one by one. 769 00:46:56,948 --> 00:46:59,282 ¶ ¶ 770 00:46:59,284 --> 00:47:01,818 And engineers have even developed a solution 771 00:47:01,820 --> 00:47:04,620 to one of the most challenging problems 772 00:47:04,622 --> 00:47:06,823 in bridge maintenance. 773 00:47:06,825 --> 00:47:11,027 The main cables that run between the towers on suspension bridges 774 00:47:11,029 --> 00:47:15,097 are extremely difficult to replace. 775 00:47:15,099 --> 00:47:17,033 So what do engineers do 776 00:47:17,035 --> 00:47:22,238 if they discover that these cables are corroding? 777 00:47:22,240 --> 00:47:26,242 ¶ ¶ 778 00:47:26,244 --> 00:47:31,147 Pittsburgh, Pennsylvania. 779 00:47:31,149 --> 00:47:34,951 It has been called "the City of Bridges," 780 00:47:34,953 --> 00:47:39,288 as it has over 400. 781 00:47:39,290 --> 00:47:43,359 Richard Connors manages some of them, 782 00:47:43,361 --> 00:47:48,164 including the city's Philip Murray Bridge. 783 00:47:48,166 --> 00:47:53,569 CONNORS: Now, this bridge gets about 17,000 vehicles a day. 784 00:47:53,571 --> 00:47:54,770 It's very important. 785 00:47:54,772 --> 00:47:57,006 This is a major physical asset in the city. 786 00:47:57,008 --> 00:47:59,242 ¶ ¶ 787 00:47:59,244 --> 00:48:02,478 NARRATOR: The bridge depends on its main cables, 788 00:48:02,480 --> 00:48:05,581 which are over 85 years old. 789 00:48:05,583 --> 00:48:09,352 CONNORS: This cable's about 13 inches in total diameter. 790 00:48:09,354 --> 00:48:14,357 It's made up of 4,864 individual wires, steel wires, 791 00:48:14,359 --> 00:48:17,660 that were installed in 1933. 792 00:48:17,662 --> 00:48:22,098 NARRATOR: This film, of San Francisco's Golden Gate Bridge, 793 00:48:22,100 --> 00:48:24,500 shows how the Pittsburgh bridge's cables 794 00:48:24,502 --> 00:48:26,903 would have been made. 795 00:48:28,706 --> 00:48:32,308 Individual wires were stretched across the river, 796 00:48:32,310 --> 00:48:34,076 bundled together, 797 00:48:34,078 --> 00:48:36,545 then wrapped inside a waterproof skin 798 00:48:36,547 --> 00:48:41,250 to prevent moisture seeping inside. 799 00:48:42,787 --> 00:48:44,153 But when engineers opened up 800 00:48:44,155 --> 00:48:46,289 the cables of the Pittsburgh bridge 801 00:48:46,291 --> 00:48:47,356 in 2009, 802 00:48:47,358 --> 00:48:51,527 they found that water had gotten inside. 803 00:48:53,464 --> 00:48:56,132 CONNORS: We noticed, upon inspection of the main cable, 804 00:48:56,134 --> 00:48:58,200 there was significant corrosion, 805 00:48:58,202 --> 00:49:03,239 to as much as 21% to 23% loss in certain sections. 806 00:49:03,241 --> 00:49:05,841 Most of the corrosion was at the center of the bridge, 807 00:49:05,843 --> 00:49:07,076 the lowest point, 808 00:49:07,078 --> 00:49:08,811 where the water would just sag to the low point. 809 00:49:08,813 --> 00:49:10,546 ¶ ¶ 810 00:49:10,548 --> 00:49:12,381 NARRATOR: To stop the corrosion, 811 00:49:12,383 --> 00:49:16,819 engineers first seal the cable in a new waterproof skin. 812 00:49:16,821 --> 00:49:19,288 Then they install a dehumidifier 813 00:49:19,290 --> 00:49:23,893 to blow dry air through the cable. 814 00:49:23,895 --> 00:49:27,730 This should reduce the humidity inside the cables 815 00:49:27,732 --> 00:49:30,766 to less than 40%. 816 00:49:30,768 --> 00:49:34,603 Below this level, steel won't rust. 817 00:49:34,605 --> 00:49:39,809 CONNORS: The dry air travels up to the top of the main cables. 818 00:49:39,811 --> 00:49:42,011 And then it's blown through the cable at very low pressure, 819 00:49:42,013 --> 00:49:44,280 one to two pounds per square inch. 820 00:49:44,282 --> 00:49:47,583 And the dry air collects moisture along the way, 821 00:49:47,585 --> 00:49:49,051 inside the cable. 822 00:49:49,053 --> 00:49:50,786 And the moisture comes out right here. 823 00:49:50,788 --> 00:49:52,722 ¶ ¶ 824 00:49:52,724 --> 00:49:55,458 NARRATOR: Removing the moisture from the cables 825 00:49:55,460 --> 00:49:57,460 will stop them corroding 826 00:49:57,462 --> 00:49:59,729 and hopefully extend the life of the bridge 827 00:49:59,731 --> 00:50:03,265 for years to come. 828 00:50:03,267 --> 00:50:04,967 ¶ ¶ 829 00:50:04,969 --> 00:50:10,072 This ingenious system prolongs the life of suspension bridges. 830 00:50:10,074 --> 00:50:12,641 Engineers are using other techniques 831 00:50:12,643 --> 00:50:17,013 to help preserve structures built of prestressed concrete. 832 00:50:17,015 --> 00:50:19,315 Innovative technologies like these 833 00:50:19,317 --> 00:50:22,718 should enable our aging bridges to remain safe 834 00:50:22,720 --> 00:50:24,286 into the future-- 835 00:50:24,288 --> 00:50:28,157 if their faults are spotted in time. 836 00:50:29,160 --> 00:50:31,427 The sudden collapse of the bridge in Genoa 837 00:50:31,429 --> 00:50:33,596 is still felt in the city. 838 00:50:33,598 --> 00:50:36,198 The loss of this vital transport link 839 00:50:36,200 --> 00:50:41,404 continues to cause traffic jams and delays. 840 00:50:41,406 --> 00:50:42,972 To get the traffic moving again, 841 00:50:42,974 --> 00:50:48,177 planners are racing to build a new bridge. 842 00:50:48,179 --> 00:50:50,212 Its designers have a big advantage 843 00:50:50,214 --> 00:50:54,583 over previous generations of engineers. 844 00:50:54,585 --> 00:50:59,522 They have had the chance to learn from earlier mistakes. 845 00:50:59,524 --> 00:51:02,625 Bridges now are rarely built 846 00:51:02,627 --> 00:51:06,595 with components that are prone to single-point failure. 847 00:51:10,568 --> 00:51:13,803 Regulations on designing gusset plates 848 00:51:13,805 --> 00:51:16,572 now require engineers to calculate their strength. 849 00:51:16,574 --> 00:51:22,244 ¶ ¶ 850 00:51:22,246 --> 00:51:25,081 And when building posttensioned bridges, 851 00:51:25,083 --> 00:51:26,816 greater attention is paid 852 00:51:26,818 --> 00:51:30,986 to ensure cavities can't form in the concrete grout. 853 00:51:32,690 --> 00:51:35,991 We may never completely design all weaknesses 854 00:51:35,993 --> 00:51:40,196 out of the structures we build. 855 00:51:41,299 --> 00:51:44,433 But every disaster is a lesson we can learn from, 856 00:51:44,435 --> 00:51:49,939 to make bridges safer in the future. 857 00:51:49,941 --> 00:51:52,842 ¶ ¶ 858 00:51:57,882 --> 00:51:59,949 NARRATOR: Traffic, deadly crashes... 859 00:51:59,984 --> 00:52:01,884 Is there a better way? 860 00:52:01,919 --> 00:52:05,621 Self-driving cars will eliminate road accidents. 861 00:52:05,656 --> 00:52:07,456 NARRATOR: A mobility revolution... 862 00:52:07,492 --> 00:52:09,024 AMNON SHASHUA: We're venturing into science fiction. 863 00:52:09,060 --> 00:52:11,093 NARRATOR: Or are we moving too fast? 864 00:52:11,129 --> 00:52:12,194 PHILIP KOOPMAN: Right in front of my car, 865 00:52:12,230 --> 00:52:13,195 it doesn't see him. 866 00:52:13,231 --> 00:52:16,432 It's just simply not ready. 867 00:52:16,467 --> 00:52:19,135 NARRATOR: The stakes are high. 868 00:52:19,170 --> 00:52:20,936 "Look Who's Driving," 869 00:52:20,972 --> 00:52:23,205 next time on "NOVA." 870 00:52:23,241 --> 00:52:26,008 ¶ ¶ 871 00:52:28,613 --> 00:52:30,546 Major funding for NOVA is provided by the following: 872 00:52:33,050 --> 00:52:34,550 exploration continues on "NOVA's" website, 873 00:52:34,552 --> 00:52:36,919 where you can watch this and other "NOVA" programs, 874 00:52:36,921 --> 00:52:39,388 see expert interviews, interactives, 875 00:52:39,390 --> 00:52:41,223 video extras, and more. 876 00:52:41,225 --> 00:52:43,159 Follow "NOVA" on Facebook and Twitter, 877 00:52:43,161 --> 00:52:46,962 and find us online at pbs.org/nova. 878 00:52:46,964 --> 00:52:51,600 ¶ ¶ 879 00:53:04,582 --> 00:53:06,448 To order this "NOVA" program on DVD, 880 00:53:06,450 --> 00:53:11,187 visit ShopPBS or call 1-800-PLAY-PBS. 881 00:53:11,189 --> 00:53:15,958 This program is also available on Amazon Prime Video. 882 00:53:15,960 --> 00:53:21,764 ¶ ¶