Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 6

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Building in its modern development owes as much to improvement in form as to the use of stronger materials, brick instead of clay, iron and steel instead of wood. A stick as cut from a tree makes a capital tent-pole, and will serve just as well to sustain the roof of a cabin. For structures so low and light it is not worth while to change the shape of a stick. By way of contrast let us glance at an office building of twenty-five stories, or the main piers of the new Quebec Bridge rising 330 feet above their copings. To compass such heights stout steel is necessary, and it must be disposed in shapes more efficient than that of a cylinder, as we shall presently see.

In most cases strength depends upon form, in some cases strength has nothing whatever to do with form; if we cut an iron bar in two its cross-section of say one square inch may be round, oblong, or of other contour, while the effort required to work the dividing shears will in any case be the same. But shearing stresses, such as those here in play, are not so common or important as the tension which tugs the wires of Brooklyn Bridge, or the compression which comes upon a pillar beneath the dome of the national capitol. When we place a lintel over a door or a window, we are concerned that it shall not sag and let down the wall above it in ruin: we ensure safety from disaster by giving the lintel a suitable shape. When we build a bridge we wish its roadway to remain as level as possible while a load passes, so that no hills and hollows may waste tractive power: levelness is secured by a design which is rigid as well as strong. If a railroad has weak, yielding rails, a great deal of energy is uselessly exerted in bending the metal as the wheels pass by. A stiff rail, giving way but little, avoids this waste. To create forms which in use will firmly keep their shape is accordingly one of the chief tasks of the engineer and the architect.

Rubber strip suspended plank-wise, and joist-wise.

Board doubled breadthwise through small semi-circle AB, then edgewise through large semi-circle CD.