EVOLUTION OF THE MODERN RAILWAY BRIDGE. 461 



THE EVOLUTION" OF THE MODERN RAILWAY 



BRIDGE. 



By Peof. CHAKLES DAVIS JAMESON, 



OF THE STATE UNIVEESITY OF IOWA. 



ABRIDGE is a structure over a river, ravine, or other open- 

 ing, for the purpose of sustaining a moving load. This, in 

 the case of a railroad bridge, consists of a heavy locomotive and 

 train coming on at one end, rushing rapidly over the bridge, and 

 off at the other end. This fact, that the load to which bridges are 

 subjected is a moving load applied for only a short period of time 

 and then removed, is a most important factor to be considered in 

 calculating the necessary strength of the various members, as the 

 strain produced in any piece of material by the application of a 

 load is nearly doubled when the load is applied quickly as com- 

 pared with that produced by the same load when applied grad- 

 ually. 



Bridges may be divided into the following classes : 1. The 

 beam or girder. 2. The framed truss. 3. The arch. 4. The sus- 

 pension bridge. 



The most ordinary form which we see in this country, and the 

 one most generally used for the purpose of railway bridges, is 

 the framed truss, and that is the one the development of which it 

 is our purpose to show. 



The one point to be carefully studied in all bridge construc- 

 tion is economy that is, to get as much strength with as little 

 material as possible; in other words, the maximum amount of 

 strength with the minimum amount of material. 



The simplest method of crossing any opening where the dimen- 

 sions of the opening are not so great, or the load so heavy, as to 

 forbid its use, is by means of a plank placed from one side to the 

 other, making the plank of such a length that the ends may have 

 sufficient bearing upon each side of the opening (Fig. 1). 



Fiff.l 



In crossing an opening by means of a simple plank or beam, 

 supposing we make the beam large enough, it answers every pur- 

 pose and will hold up the required load. But in this there is 

 great waste of material. We will take, for example, a plank 

 twelve inches wide, and three inches deep, over an eighteen-foot 

 opening that is, the plank would have to be about twenty-one or 

 twenty-two feet long, in order to allow the ends sufficient bearing 

 surface upon the masonry on each side. This plank would hold 

 up a certain amount of weight, but, as the weight is increased, in 



