WEBVTT Kind: captions Language: en 1 00:00:00.070 --> 00:00:02.530 Today’s guest video is from Grady at Practical Engineering. 2 00:00:02.530 --> 00:00:04.540 He’s talking about a tragedy: 3 00:00:04.540 --> 00:00:08.250 and the lessons we learned and the reasons why it happened are fascinating. 4 00:00:08.250 --> 00:00:10.660 Also, he made his own props. 5 00:00:10.660 --> 00:00:11.950 Grady, take it away. 6 00:00:11.950 --> 00:00:17.710 In the summer of 1981, the Hyatt Regency Hotel in Kansas City, Missouri, 7 00:00:17.710 --> 00:00:21.450 hosted a large party in the multi-story atrium area. 8 00:00:21.450 --> 00:00:22.450 During the event, 9 00:00:22.450 --> 00:00:26.640 suspended walkways on the second and fourth floors collapsed, 10 00:00:26.640 --> 00:00:31.550 killing over 100 people and injuring over 200 more. 11 00:00:31.550 --> 00:00:36.280 At the time, the accident was the worst structural failure in US history. 12 00:00:36.280 --> 00:00:37.600 In the wake of the disaster, 13 00:00:37.600 --> 00:00:41.980 investigators discovered a change to the original design of the walkways 14 00:00:41.980 --> 00:00:47.480 that was proposed by the fabricator and mistakenly approved by the structural engineer. 15 00:00:47.480 --> 00:00:51.870 What, at first glance, seemed like a subtle adjustment to the design, 16 00:00:51.870 --> 00:00:55.379 turned out to be the root cause of the failure. 17 00:00:55.379 --> 00:00:58.879 Two suspended walkways spanned the atrium in the Hyatt Regency 18 00:00:58.879 --> 00:01:03.539 with the second-floor walkway directly below the fourth-floor walkway. 19 00:01:03.540 --> 00:01:06.260 Each was supported by a series of box girders 20 00:01:06.260 --> 00:01:10.580 suspended by hanger rods and retained by nuts and washers. 21 00:01:10.590 --> 00:01:13.950 The original design called for a single pair of hanger rods, 22 00:01:13.950 --> 00:01:18.830 which would pass through each fourth-floor girder to the second-floor girder below. 23 00:01:18.830 --> 00:01:22.810 The fabricator responsible for constructing the walkways objected to this plan 24 00:01:22.810 --> 00:01:27.369 because it would require screw threads for the entire length of the hanger rods, 25 00:01:27.369 --> 00:01:29.930 which could easily be damaged during construction. 26 00:01:29.930 --> 00:01:34.220 So, they proposed to split the hanger rods into two sets. 27 00:01:34.220 --> 00:01:36.979 One to connect the fourth-floor walkway to the roof 28 00:01:36.979 --> 00:01:42.110 and one to connect the second-floor walkway up to the fourth-floor girders. 29 00:01:42.110 --> 00:01:45.820 If you don’t notice the significance of this change, you’re not alone. 30 00:01:45.820 --> 00:01:50.380 It was approved by the engineer without a detailed review or calculations, 31 00:01:50.380 --> 00:01:53.189 which would have revealed its inherent flaw. 32 00:01:53.189 --> 00:01:56.110 In this setup, which represents the original design, 33 00:01:56.110 --> 00:02:01.110 the load of the two walkways is independently transferred to the hanger rods. 34 00:02:01.110 --> 00:02:04.490 Notice how I can lift each girder without affecting the other. 35 00:02:04.490 --> 00:02:07.680 Now, let’s look at the design change. 36 00:02:07.680 --> 00:02:09.510 In this configuration, 37 00:02:09.510 --> 00:02:13.520 notice that the entire weight of the second-floor walkway is being borne 38 00:02:13.520 --> 00:02:15.780 not directly by the hanger rod, 39 00:02:15.780 --> 00:02:18.640 but instead, by the girder above. 40 00:02:18.640 --> 00:02:23.860 If I lift the fourth-floor walkway, the second-floor walkway is lifted as well. 41 00:02:23.860 --> 00:02:27.270 The hanger rods are still carrying the same load at the top, 42 00:02:27.270 --> 00:02:32.830 but the two nuts on the upper girder are supporting the weight of both walkways. 43 00:02:32.830 --> 00:02:38.020 This simple change effectively doubled the load on those bolted connections. 44 00:02:38.020 --> 00:02:41.150 Imagine that you and a friend are both hanging on a rope. 45 00:02:41.150 --> 00:02:46.220 The original design is the equivalent of you both holding onto the rope independently, 46 00:02:46.220 --> 00:02:51.760 whereas the design change is the equivalent of your friend hanging onto your ankles. 47 00:02:51.760 --> 00:02:55.160 The total weight supported by the rope is the same in both cases, 48 00:02:55.160 --> 00:02:59.300 but your likelihood of maintaining a grip is not. 49 00:02:59.300 --> 00:03:04.480 This subtle change was identified by investigators as the primary cause of failure. 50 00:03:04.480 --> 00:03:06.970 With so many people on the walkways that evening, 51 00:03:06.970 --> 00:03:09.710 the load on the connections was too great. 52 00:03:09.710 --> 00:03:13.570 The box girders split open, slipping past the washers and nuts 53 00:03:13.570 --> 00:03:16.950 leading to the collapse of both walkways. 54 00:03:16.950 --> 00:03:22.020 There is an implicit handshake between a society and its engineers. 55 00:03:22.020 --> 00:03:26.150 We hardly have a choice but to trust that the constructed environment that we live in 56 00:03:26.150 --> 00:03:28.420 is safe and sound. 57 00:03:28.420 --> 00:03:30.080 When an engineer seals a design, 58 00:03:30.080 --> 00:03:36.120 he or she takes responsibility for its accuracy and safety to the general public. 59 00:03:36.120 --> 00:03:39.090 But the err is human and that includes engineers. 60 00:03:39.090 --> 00:03:42.320 So, we try to develop conventions and processes 61 00:03:42.320 --> 00:03:46.730 that can catch and correct from mistakes before they get too far. 62 00:03:46.730 --> 00:03:50.660 And that includes studying and learning from errors made in the past. 63 00:03:50.660 --> 00:03:54.820 The failure of the Hyatt Regency walkways is an important case study 64 00:03:54.820 --> 00:03:56.980 taught to nearly every engineer 65 00:03:56.980 --> 00:04:00.640 with the goal that such a tragedy will never occur again. 66 00:04:00.640 --> 00:04:03.760 Practical Engineering is filled with great videos, so go subscribe! 67 00:04:03.760 --> 00:04:07.340 I would recommend starting with Grady’s video on hydrostatics. 68 00:04:07.340 --> 00:04:08.340 Next time, 69 00:04:08.340 --> 00:04:10.780 a guest video with a prop that has been to the moon.