. The Americana; a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc., of the world. pans from 120 to200 feet and over. Riveted trusses are usuallyof the Warren type, but for spans under 180feet the pin-connected Pratt truss with inclinedend posts is the most economical. As the spanincreases from 180 to 250 feet it is modified bycurving the upper chord and takes the form ofa single intersection truss. For longer spansranging from 250 to 450 feet the newer simpletruss bridges employ the Petit or the Balti-more trusses. span p
. The Americana; a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc., of the world. pans from 120 to200 feet and over. Riveted trusses are usuallyof the Warren type, but for spans under 180feet the pin-connected Pratt truss with inclinedend posts is the most economical. As the spanincreases from 180 to 250 feet it is modified bycurving the upper chord and takes the form ofa single intersection truss. For longer spansranging from 250 to 450 feet the newer simpletruss bridges employ the Petit or the Balti-more trusses. span pfn-connected truss for a single trackthrough bridge, and will serve to convey a veryaccurate idea of the design of such a structure. Draw bridges* may be classified as swingbridges,* rolling bridges, and lift bridges.**Swmg bridges are of the simple or double typeand may be constructed of trusses or girders,so as to be wholly or partially continuous. Theearlier forms had a tower over the central pierfrom which chains extended to the ends of thetruss, or the platform. These ends rested freelyupon the abutments so that a live load on one Fig. 3S. vf^. Fig. 36. ^ end lifted the other, and the arrangement wasentirely bad. approximating to that of the tri-angular truss shown in Fig. 35, and is now obso-lete, while the modern swing bridges are of the BRIDGE CONSTRUCTION type snown in Fig. 36. In this design, whenthe bridge is fully loaded the position of themembers (ab) is such that the members (be)carry the greater part of the load to the turn-table, and insure a better distribution of theload thereon. They are generally known as?centre-bearing* or rim-bearing,* according tothe method of supporting the turn-table on thecentre pier. Fig. 37 shows the centre-bearingmethod where the entire weight is carried onthe central pivot. It is applicable to short 38 shows the rim-bearing method where theentire load is carried on a circular drum whichmoves upon a series of wheels or ro
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectencyclo, bookyear1908
