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July 16, 2005 Everyone knows that materials, particularly metals, expand when heated, but how many understand how much they expand? Until the 1970’s or so, most highway and railroad bridges in the US were made of steel girders, and in places like Michigan, where the temperature may vary as much as 120 degrees Fahrenheit in the course of a year, thermal expansion becomes an important factor in the design. Suppose that a continuous run of girders expanded 4 or 5 inches from winter to summer, then surely if both ends of the run were bolted down fast, something would have to give. Either the bearings would be torn out of the concrete or the girders would buckle, unless of course the fittings were designed to withstand the added strain, which would be very costly. According to Wikipedia, the temperature rate of change of the length of carbon steel elements, (1/L) x (dL/dT), is 10.8 / 1,000,000 per degree Kelvin. This comes out to .000006 per degree Fahrenheit, and I believe the accepted figure in the US steel industry is .0000057. Now suppose we have a continuous run of girders 1000 feet long. Of course the individual component girders are probably no more than around 150 feet long, because of shipping limitations, but if they are connected end-to-end with fully bolted splices, they behave like a single girder. So if we estimate the winter-to-summer temperature difference as 100 degrees, the change of length of the girders will equal .0000057 x 1000 x 100 feet, or .57 feet, which is almost seven inches. If the run of girders is supported on three or four concrete piers, as over a river, there will be a fixed bearing only at one end. This is a heavy-duty stool that is erect and immobile. At the other piers, there will be rockers or rollers. A rocker is like a fixed bearing, except the bottom is machined to a radius so that it can tilt one way or the other as the superjacent girder expands or contracts. Usually it has a press-fitted pintle sticking out of its lower surface that keys into a blind hole in the base plate on which it rests and the pintle holds the rocker in place. If the amount of contraction or expansion is too great, then rollers are used. These are like steel rolling pins 12 or 15 inches in diameter that have welded rims to keep them from slipping off the base plates. But the girders themselves are not bolted down at all. The upshot is that a giant could put his forearms under the whole bridge and lift it off its bearings. Only its own weight keeps it in place. Some states, notably Missouri, prescribe hold-down devices, sliding clamps that overlap the girder flanges, but in most states this is not the case. Anyway, some bridges are much longer than 1000 feet. Some may be 5000 or 10,000 feet long. For such bridges even the above method is insufficient, for expansion could reach nearly six feet. What is done in these cases is that the bridge is divided into several runs of 800 or 1000 feet, each treated as I’ve described. But now the beginning of one run hangs from the end of the preceding run. The two sets of girders are cut out to fit each other, one underslung below the other, but with a longitudinal gap of six or eight inches. The underslung girder is supported by two straps between large cylindrical bearings mounted in the girders. The straps behave like pendulums, and the lower girder is free to swing back and forth under the upper. As for the surface of the road itself, expansion is handled by using fingerplates. You’ve probably seen them at the ends of bridges, like two giant combs with interlocking fingers. Such an expansion joint can be designed to be displaced as much as a foot without causing any discontinuity in the driving surface. The expansion joints are mounted on the ends of the girders and travel with them as they expand and contract. On a long bridge, there may be fingerplates at several locations along the length, in fact wherever there are pin-type girder joints below. ------------ About the author Thomas Keyes: I have written two books: A SOJOURN IN ASIA (non-fiction) and A TALE OF UNG (fiction), neither published so far. I have studied languages for years and traveled extensively on five continents. Email: udikeyes@yahoo.com Comment on this article here! ------------ All articles are EXCLUSIVE to Useless-Knowledge.com and are not allowed to be posted on other websites. ARTICLE THIEVES WILL BE PROSECUTED! |
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