Review Of Movie Engineering: The Palm Beach Story (1942)

Joel McCrea, on the left, is playing Tom Jeffers, a self-employed engineer unable to find investors to make his dream a reality. His dream, as shown in the model, is an airport in the middle of a city – Jeffers lives in Manhattan – made of steel cables, suspended above the streets and buildings. (The white stripes in the model represent runways, not the cables.) This strikes me as insane, but then again people around that time were seriously discussing having airships dock in the middle of cities, so maybe it didn’t sound quite so insane eighty years ago.

Was it feasible? (We never find out in the movie. Jeffers’s wife Gerry, played by Claudette Colbert, plans to divorce him and marry a rich man so that she can fund it, but that plan – amazingly – does not work. The airport remains unbuilt when the movie ends.) Commercial airplanes in 1942 were all propeller planes. I’ll use two example: the Boeing 307 (the first commercial plane with a pressurized interior) entered use in 1938, and was 74 feet long with a 107 foot wingspan, and weighed 45,000 pounds loaded; the DC-3 entered use in 1936, and was 65 feet long with a 95 foot wingspan, and weighed 25,000 pounds loaded. The minimum suggested runway for a DC-3 was 2500 feet (not quite half a mile) or about ten New York blocks with the intervening streets. The model looks to be about three blocks on a side, but let’s use the more realistic larger number. (Note that the recommended runway was also a hard surface, which a cable net would not be.) A logical layout would be boarding on one end and a single runway projecting out; the boarding area is where planes would be stored and so represents the heaviest loading.

If the planes were stored side by side, as if each was a rectangle in plan rather than a cross shape, the DC-3s would weigh 4 psf and the 307s would weigh 6 psf. That is quite light: code requirements for a car parking garage are 50 psf. If the cables are 1 inch in diameter and spaced at 12 inches on center, the cable net would weigh about 3.5 psf. A cable net would not collect much wind or much snow – unless it iced over, so let’s assume that the airport maintenance prevents icing. Since the runway has to be long, it would make sense for the overall airport to be a long narrow rectangle, which means that the cable net would be effectively spanning in one direction (the short way) with the long cables acting to even out loads among the short cables. If the airport is 600 feet wide – wide enough for a reasonable runway, and the equivalent of the model’s three blocks – the cable spans are the same. Using a basic, slightly simplified formula for a single vertically loaded catenary, with 10 psf loading and cables at 12 inches on center, we get a cable tension of 450,040 pounds if the cable sags one foot. That’s an L/600 sag, which is less than most ordinary structures. Planes back then were taking off and landing at slower speeds than modern jets, so I’d guess they were less sensitive to out-of-flatness of the runway. If we allow the cables to sag two feet, the tension is reduced to 225,020 pounds. (Yes, the simplified formula is linear with respect to the sag.) The 225,020 pound pull is a diagonal with a horizontal component of 225,000 pounds. So the buildings or towers supporting the cable net would be subjected to a lateral pull of 112 tons per foot at whatever height the airport is. A twenty-story building analyzed under the 1938 New York Building Code would be subject to a lateral wind load of 4000 pounds (2 tons) per foot of width. (The load would be somewhat higher under modern codes.) So the airport cable supports would subject the buildings to fifty times the lateral force of wind. If we allow the cables to sag ten feet, the force is down to 45,000 pounds, but the 10 foot difference in height in the 300 feet from the center of the span to the support is a noticeable slope and the force on the buildings is still five times that of wind.

I haven’t even gotten to the questions of how people walk on the cable net, the size of cables required to carry those forces, exactly how the magic de-icing works, and the zoning requirements for building heights around the cable net area. In short, the investors were right to ignore Tom Jeffers, and Gerry Jeffers should have divorced him and married J. D. Hackensacker (a John D. Rockefeller, Jr., parody played by Rudy Vallee) because Tom was completely out of his mind.

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