Stupid Engineering Mistakes

lee1 writes "Wired has bestowed on us a list of the ten worst engineering mistakes of all time. We have the St. Francis Dam designed by 'self-taught' engineer William Mulholland, which burst and wiped out several towns near LA; the Kansas City Hyatt walkway collapse; the DC-10, and more, but my favorite is the one I'd never heard of: a giant tank of molasses that ruptured in 1919 and sent 'waves of molasses up to 15 feet high' through Boston, killing 21."

Comments

Re:15 foot high waves of molasses

[ckswift]

Actually according to Wikipedia the molasses flowed at 35mph exerting a pressure of 200 kPa.

At 529 Commercial Street, a huge molasses tank (50 ft (15 m) tall, 240 ft (70 m) around and containing as much as 2.5 million US gallons (9,500 m or 9,500,000 litres)) collapsed. The collapse unleashed an immense wave of molasses between 8 and 15 ft (2.5 to 4.5 m) high, moving at 35 mph (60 km/h) and exerting a pressure of 2 ton/ft (200 kPa). The molasses wave was of sufficient force to break the girders of the adjacent Boston Elevated Railway's Atlantic Avenue Elevated structure and lift a train off the tracks. Several nearby buildings were also destroyed, and several blocks were flooded to a depth of 2 to 3 feet. Twenty-one people were killed and 150 injured as the molasses crushed and asphyxiated many of the victims. Rescuers found it difficult to make their way through the syrup to help the victims.

Correct...

[Gadgetfreak]

I have a MechEng/ Materials dual degree, and one of my later courses was actually a "Metal Failures" course, dedicated to this kind of stuff. Most of it was more complicated. My professor was actually a retired PhD who worked on investigative teams that evaluated accidents like these, and acted as the 'expert witness' for technical information in many cour cases.

We studied this case, as well as many on the list above, in detail. In particular, the box beams in question ran horizontally to support the walkway, while the vertical rod was the support for the end of the box beams. The beams could have been made better, but they were good enough for their design loads.

The problem was that the original design called for one continuous vertical rod, with several levels of walkway hanging from it at different heights. However, due to construction issues, the installation was changed (for the worse) so that separate vertical rods were used. This unfortunately got written approval, and shouldn't have. Instead of the successive loads being applied to the rod, the box beam was then holding the weight of all the floors below it, which it was not designed to do.

Imagine one rope hanging from a ceiling, with 3 people hanging at various heights on the rope. The rope can hold the total weight of the 3 people easily, but each climber needs only enough grip to hold up his own weight. Now imagine due to "construction issues" you can't get one long rope, so you get 2 shorter lengths. Ideally, you'd tie the ropes together to create a nearly identical scenario, but in this case, it's like they tied the bottom rope to the middle guy's ankle, and expected him to hold on with the added weight of the guy below him.

Unfortunately, it was just strong enough to hold a few people, but let go when it was fully loaded.

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