SKYSCRAPERS OF NEW YORK VIBRATE LIKE TUNING-FORKS.

Star (Christchurch), Issue 19239, 28 November 1930, Page 6

SKYSCRAPERS OF NEW YORK VIBRATE LIKE TUNING-FORKS.

To-Day’s Signed Article.

Specially Written For The “ Star.” By James K. Foreland .

There are many stories told about skyscrapers and their peculiarities when weather conditions are abnormal. Engineers have been listening to these accounts and have investigated some of them and others are the subject of exhaustive reports. Some of the stories have to do with clocks that stop or run away and pictures that slide to and fro along the wall. There is said to be a hanging lamp in one great and lofty tower that swings six inches or so whenever there is a stiff north-west breeze.

JJOW MUCH TRUTH is there in these tales of life in America’s tall buildings? Construction engineers first scorned to listen to them, but now, with the constant trend to still higher buildings, they are proceeding to check the facts; they are doing their own detective work. What about this weird and disquieting behaviour of familiar objects in the upper stories of a skyscraper? How much damage to life and limb is indicated by the sight of pictures sliding back and forth along the wall ? Some of the questions cannot be answered positively, some can. No one knows how many of the stories are precisely true. So far as safety goes, the engineers that design the tallest of the tall buildings are certain that none of them can be blown down. The engineers believe that there are now enough tall buildings in use to make it possible to find out something about their general behaviour under wind strains. Leaning and Vibration. The distinction between leaning and vibration is an important one; it seems highly probable that all tall buildings, including lighthouses and church spires, bend or lean out of the vertical to a measureable degree. When the sun shines on one side of a building, and it is cool on the other side, the warm side will naturally expand, and the top will lean away from the sun. All these questions are now being exhaustively studied. Whether it is a fact that tall buildings lean as much as a foot is not actually known, but, if it is found true, the leaning motions are slow, occupying several minutes or several hours to take effect. They cannot be perceived by the sense, and they cannot produce any disconcerting effect like stopping clocks or making the electric fixtures sway rapidly to and fro. The engineer is interested because the facts have to do with stresses in the building and the relative effects of walls and steel frame. Vibration Causes Worry. To the occupants of a tall building it is the vibration that causes the sense of swaying. It is the same thing that happens in a tuning-fork when it is struck. After being struck the ends of the fork swing to and fro for several seconds with a harmonic motion; that is, it is a regular beat with a frequency or pitch which is always the same for any particular fork. Like the end of a tuning fork, the building is an elastic cantilever, held by the earth at one end and free to vibrate at the other. Like the fork it has a definite frequency of its own, in which it always vibrates, and it would give off a musical note if the vibrations were not so slow or the human ear could hear it. A tall building is set to swinging by a sudden gust of wind. The average pressure of the wind all over the building makes it lean over, but the gust, or the “holes” in the wind, that push and pull it suddenly, cause it to vibrate. After a hard blow the vibrations may continue for five or ten seconds before another gust . strikes the building going forward and increases the swing, or it may hit it coming back and stop the vibrations dead. m in si ® ® ® s is in ® m ® hi a ® ® in ® s is m m i

When a gust of wind strikes the building on the forward beat it may almost double the length of the next stroke, and it is probable at such moments, accompanied as they are by the sudden shriek of the blast, that the occupants feel the motion. Whether anyone has ever been able actually to feel the sway of a skyscraper is speculative and one hard to settle in a positive manner. Checking the Amount of Sway. As to the amount of motion that occurs, it is hard to say accurately, according to the construction engineers. However, readings have been taken in most of the tall buildings in New York during gales, and the indications were that they swayed as much as an inch at least, making two inches from vertical counting the swing back and forth. The frequency of the motions depends on many separate considerations. A slender tower will move more slowly than a stout one, a high tower moves more slowly than a short one, a heavily braced tower is faster than one of the same size and shape which is not so well braced. A tower with heavy floors or walls at the top is slower than one of light construction; one with heavy walls Jin the lower stories, especially of stone masonry, is faster than one of light brick or terra-cotta construction. This all comes down to the fact, that the stiffer it is for its size and weight, the faster it goes. The rates vary from fifteen complete vibrations per minute for the very tall, slender towers and for the medium-sized towers and of cheaper construction, to thirty per minute. Buildings May Be Braced. The frequency of vibration is of considerable interest because of its relation to the sort of happenings which cause gossip. _ It is a well recognised fact that if any object has a harmonic motion of its own, like a pendulum, and if it is given a series of slight pushes, timed to its own frequency, it will get to swinging violently. Hence the trouble. Nearly all the wild tales of skyscraper movements seem to deal not with sensations of motion, but with the queer behaviour of objects. In so far as they have any truth in them it is probably connected with this characteristic of swinging bodies to respond to anything that tunes in on their own wave length. The fact that a hanging lamp swings six inches does not mean that the building is swaying six inches. A motion of an eighth of an inch is enough to account for it, if properly timed. Professor Ferdinand Schuler suggests that in the tall building of the future when far greater heights are obtained, it will he found advisable to brace the towers with long girders which, in turn, will be used for communication purposes, trains or tracks for cars that travel at great speed being placed within the giant braces. In this way the tallest of the be braced with all the other skyscrapers and there will be no visible movement that will tend to create a seasick feeling when watching the pictures sway or the lighting fixtures swing. (Anglo-American N.S.—Copyright.)