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Saturday, November 09, 2013

Structural Failures

Why Buildings Fall Down: Why Structures Fail by Matthys Levy | LibraryThing:

Why did the pyramid at Meidum shed 250,000 tons of limestone outer casings when few of the others have? Why is the pyramid shape a logical structure for a country where the only available building material is stone? Those and many other questions are answered in a fascinating book by Matthys Levy. The bottom blocks of a pyramid must support the weight of all the blocks above it; those on top support only their own weight, much like a mountain. The classical 52o angle was adopted only after it was understood that the foundation had to be laid on limestone. At Meidum, the bottom layers and foundation were supported by sand, and the casing blocks lay in horizontal layers and were not inclined inward, unlike in all the pyramids that followed.

The light, airy dome has replaced the pyramid as man's alternative to monolithic monuments. The dome originated thirty-three hundred years ago in Assyria. By 200 A.D., the Roman Pantheon represented the peak of the builders' skill. In 1420, Filippo Brunelleschi completed the dome over the Santa Maria de Fiore without using a scaffold. What gives domes their stability and permanence is their curved continuous shape. Unlike arches that require enormous buttresses, the dome shares the load among all its members. They are exceptionally strong and support gravity loads well. Their rigidity makes them susceptible to earthquakes and soil settlement, however. The advent of Christian liturgical requirements, i.e., the cross shape of many buildings, posed great difficulties for medieval builders who wished to incorporate the dome into religious structures. Eggshells, which are difficult to squash when pressed from the ends toward the middle, are really just two domes glued together. The ratio of an eggshell's span to thickness is about 30. That of a conservatively designed dome is usually at least 300, or about ten times stronger.

The book analyzes assorted structural failures. The collapse of the dome at the C.W. Post College of Long Island University provides an interesting example of how a dome that met and exceeded code standards could still collapse because of a failure to anticipate natural conditions. The assumption behind the design was that snow loads on the roof would be uniform. During the storm that collapsed the roof, an east wind blew snow in huge drifts on one side of the dome, stressing it beyond design limits. That, coupled with the natural lifting effect caused by wind passing over the top of a dome (much like a sail) caused the structural members to fail. So even though the total snow load was one-quarter the maximum, it was concentrated on less than one-third of the dome's structure, bringing it down.

A particularly interesting section describes how tuned, dynamic dampers work in large buildings and why they are necessary. All tall buildings oscillate because of the pressure of the wind. This movement, while not necessarily structurally dangerous — although it can be — can cause airsickness in the occupants. A huge tuned (set to the same frequency as the oscillations of the building) concrete block is set on a thin layer of oil at the top of the building. It is connected to the outer walls by enormous steel springs and shock absorbers. When the building begins to oscillate, the damper tends to stay put because of its large inertia and allows the building to slide under it on the layer of oil. The springs on one side of the damper become longer and literally pull the building back into shape. Those on the other side push it to its original position.

Thermal stresses must also be considered in building and bridge design. As steel beams in a bridge expand from the heat in summer, the bridge must be permitted to expand by using rollers, or the compression caused by the prevention of expansion would reduce the carrying capacity. Air-conditioned buildings pose unique problems because the outside beams may be expanding while the inside beams are contracting because of the temperature differential. This can cause unwanted bending unless structural reinforcement is present.

The chapter on dams is instructive. Many earthen dams, built centuries ago, have survived thousands of years. The Romans built numerous masonry dams on a base three to four times the width of the dam's height. It remained for a Scottish engineer in the 19th century to show that the base width need be no more than the height. Of 1,764 dams built in the United States before 1959, one in fifty failed for a variety of reasons, a rather high failure rate. The most famous collapse is that of the Johnstown dam in 1889, which killed more than three thousand people. It had been completed in 1853 and was intended to provide a steady source of water for a Pennsylvania canal. By 1860, the canal was already obsolete; railroads were taking over the hauling of freight. Soon the dam was in a state of disrepair. When it was sold to new owners they made dangerous modifications that reduced the spillway to less than one-third of its original capacity. The five-inch rainfall that was blamed for the dam's failure would never have destroyed the dam in its original configuration.

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