Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 12

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Rails are girders used by themselves: girders are often combined in trusses; of these much the largest and most important are employed for bridges. There is now under construction near Quebec a cantilever bridge whose channel span of 1,800 feet will be the longest in the world. See page 29. It will take us a little while to understand how so bold a flight as this was ever dared. We will begin with a glance at a truss of the simplest sort, such as we may find beneath the roof of an old-fashioned barn. A pair of rafters, AB and AC, are inclined to each other at an obtuse angle, and are fastened to the horizontal beam, BC, at B and C. Their apex, A, is joined to BC by the king-post, AK, which binds the three strongly and firmly. This whole structure makes up a triangle, and so does each of its halves, ABK and AKC. No other shape built of straight pieces will keep its form under strain. Take in proof say four pieces of lath and unite them with a freely turning pin at each corner to make the frame, ABCD; it is easily distorted by a slight pull or push; but insert cross-pieces, AC and BD so as to divide the square into triangles, and at once the frame resists any strain not severe enough to break the wood or crush its fastenings. As the roof presses down the frame ABC, its sides, AB and AC, tend to slide away at their lower ends, B and C, but this is prevented by the horizontal beam, BC, which while it holds them in place is itself so stretched as to be held level and straight. This calling into play of tension constitutes the chief merit of the truss, and enables it in roofs and bridges to span breadths impossible to simple beams bending downward under compressive strains. Not only in houses, but in ships, the truss has great value; it was introduced in this field by Robert Seppings of Chatham, in England, about 1810. To resist the pressure of grinding ice, the “Roosevelt” is built with trusses of great strength. She sailed in 1905, under Commander Peary, for a voyage of Arctic discovery.

Frames of four sides. For rigidity diagonals are needed, AC, BD.

Were our barn roof flat instead of sloping to form a truss, its supporting timbers, under compression, would have a decided sag from which BC is free. When we fashion a small model of a king-post truss, its sides, AB and AC, must be of metal or wood because they will be in compression; the king-post, AK, and the base, BC, which will be under tension, may be of rubber or cord. Always as in this case the parts of a truss exposed to compression must be of rigid material. When a part may be of cord, rope or wire, we know that it is resisting tension.[2]

[2] A model easily put together illustrates the truss in its simplest form. Take a pair of wooden compasses, each half of which is 15 inches long, such as are sold for blackboard use by the Milton Bradley Co., Springfield, Mass., at 50 cents. At each tip fasten, by the ring provided with the compasses, a chair castor such as may be had at any hardware store. Join the tips of the castors by a rubber strip. Holding the compasses upright, and applying pressure from the hand, they will extend until the rubber will be so stretched as to become almost perfectly horizontal. Various weights may in succession be suspended from the compass-joint, replacing manual pressure, and serving to measure the exerted tensions.

Cross-section of the “Roosevelt,” Commodore Peary’s new Arctic ship. Reproduced by permission from the Scientific American, New York.

Pair of compasses stretch a rubber strip.

Wrought iron exerts about as much resistance to compression as to tension; so does steel. For this reason, and on account of their great strength, they have immense value in building. Cast iron can bear only about one sixth as much tension as compression, so that it is useful as foundations, for the bed-plates of engines and machinery and the like, but is unsuitable for girders. Wood is much stronger under tension than compression; in white pine this proportion is as eight to one. In designing timber bridges the strains are, therefore, as far as possible, arranged for tension.

Queen-post truss.

DE, HO, queen-posts.

Upper part of a roof truss.

Interborough Power House, New York.

Let us now enter another barn, about one half wider than the first, and look upward at its rafters. We see its roof sustained by timbers disposed as DCMH, to avoid the undue weight necessary for a design resembling that of our first roof, ABC. Instead of one upright post, AK, as in that case, we have now two, DE and HO, called queen-posts, sustaining the horizontal beam, CM. In large modern roofs the simple queen-post is modified and multiplied, as in the main power house of the Interborough Rapid Transit Company, West 59th St., New York. Returning to our simple queen-post design, let us imagine a creek flowing between walls spanned by DCMH; that truss and a mate to it, parallel at a distance of say ten feet, would easily carry a roadway and give us a bridge. A truss for a bridge must be much stronger than for a roof of equal span, because a bridge has to bear moving loads which may come upon it suddenly, giving rise not only to serious strains but to severe vibrations, all varying from moment to moment.

Two queen-post trusses form a bridge.

Palladio trusses.