Transportation on Land 281 



girders covered with planks. The principle upon which the 

 structure depends may be illustrated in a very simple way. 

 Lay a stick of wood upon two end supports, resembling the 

 end piers of a bridge. Press with the hand upon the middle 

 of the stick, gradually bending and finally breaking it. Ob- 

 serve closely the ways in which the force is applied and the 

 resistance encountered. The wood fibers in the upper layers 

 of the stick are crushed together, and those in the lower layers 

 are pulled apart. Engineers speak of the force applied above 

 as the crushing force, and of the resistance of the material to 

 this force as the crushing strength. The pull or force applied 

 to the lower layers is called the tensile force, which is met by 

 the tensile strength or resistance of the material. 



These are the essential facts in all bridge construction, of 

 the simplest as well as of the most complex form of structure. 

 A study of the familiar types of bridges will make these facts 

 clearer, and it will show also how the crushing and tensile 

 strength is distributed among different parts in order to secure 

 greater strength and at the same time to avoid increasing 

 greatly the weight of the bridge. 



120. The truss bridge. Simple girders of wood or steel 

 may meet all ordinary requirements over short spaces or spans. 

 But longer spans are very often necessary, over which simple 

 girders cannot be placed without increasing excessively the 

 weight, and decreasing the supporting strength of the structure. 

 Evidence of this you may find in bridges in your neighborhood. 

 For long spans a type of bridge known as the truss is often con- 

 structed ; this you will perceive is but a modified form of the 

 girder type. 



The chief features of the truss are illustrated in diagram B, 

 Fig. 84. Note that the crushing and tension layers of the 

 girders are now actually divided into two sets of members, 

 which are called the compression and the tension members. 

 Compare diagrams B and A . In the triangle type the girder 

 may be considered as twice as long as that in A, but divided 



