BRIDGE 



BRIDGE 



rock carved by the wind and rain. (See the 

 article NATURAL BRIDGE for pictures.) Timber 

 foot-crossings were probably the first made by 

 man, though possibly suspension bridges of 

 the grape-vine type, such as those constructed 

 in modern times by the Indians of British 

 Columbia and other primitive peoples, are 

 even older. 



The Romans were the first great bridge 

 builders. Besides braced timber structures like 

 that defended by Horatius, they constructed 

 magnificent arches of stone, some of which re- 

 main to this day (see AQUEDUCT). After the 

 fall of Rome little progress was made until 

 the era of iron and steel, though the length of 

 both timber and arch bridges was increased. 



Timber Bridges. The simplest form of tim- 

 ber bridge, a log or plank with one end on 

 each bank of a stream, is of course limited in 

 span (the distance between the points of its 

 support) by the length of the log or plank. 

 Moreover, everyone has noticed, in crossing 

 such a plank bridge, that it bends more and 

 more as the center is approached, and that the 

 longer the span the weaker the bridge. This 

 is because each point of support acts as the 

 fulcrum of a lever (which see). It is obvious, 

 then, that to lengthen or to strengthen a 

 wooden bridge it is necessary to have addi- 

 tional points of support. But the stream or 

 valley may betoo deep for a pier, or the cur- 

 rent too swift for one to be built with ease. 

 In such a case added support must be a part 

 of the structure itself. 



In an ordinary wagon bridge such as that 

 represented in Fig. 1, the strength, of course, 

 depends not on the planks but upon the frame- 

 work which supports them. If the stream to 

 be crossed is seven yards wide, a frame of tim- 



bers unsupported except at the ends would 

 perhaps break under heavy loads. In the sim- 

 ple truss shown in Fig. 1, any weight upon the 

 bridge exerts a downward pull upon the post 

 at the center, which in turn distributes the 

 load between the two slanting supports in an 

 outward thrust upon the foundations. The 

 post is said' to be subject to tension, or 

 stretching, and the slanting supports to com- 



pression, or squeezing. It will be shown later 

 that the horizontal timbers are subject to 

 both tension and compression. In Fig. 1 the 

 span of seven yards has been safely bridged 

 with timbers none of which is over four yards 

 in length. The exact strain which each one 

 will be called upon to bear under a given load 



can be accurately determined (see COMPOSITION 

 OF FORCES). 



In many bridges the truss is underneath the 

 roadway, as in Fig. 2. Here the upright posts 

 are subject to compression and the horizontal 

 and two diagonal braces to tension. That this 



is so can be seen from Fig. 3, which shows 

 that if a weight is placed on the bridge heavy 

 enough to bend it, the distance between the 

 points connected by the truss timbers is in- 

 creased. 



By the construction of more elaborate trusses, 

 timber bridges may be greatly extended in 

 length. One erected at Wittingen, Germany, 



in the eighteenth century had a span of 390 

 feet, probably the longest in any wooden 

 bridge. A truss frequently seen on timber 

 bridges is the Howe truss, shown in Fig. 4, in 

 which the diagonals are of timber and the 

 vertical tie-rods (so called because they tie 

 together the parts which weight tends to pull 

 apart) are of iron. 



Wooden bridges have the great disadvantage 

 of being rapidly worn by weather and easily 

 burned. They are serviceable only where 

 those made of other materials would be at 

 trie time too expensive. Railroad trestles, 

 bridges of many small spans which cross val- 

 leys, are commonly built of wood, and seldom 

 resist decay for more than fifteen j'ears. 



