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.