EVOLUTION OF THE MODERN RAILWAY BRIDGE. 471 



The next form of truss is that known as the Bolman truss 

 (Fig. 20). In this also, as in the Fink truss, there is no bottom 

 chord necessary. The distinctive characteristic of the Bolman 

 truss is that from the lower end of each vertical compression 

 member the tension members run directly to each abutment, 

 differing in this respect from the Fink truss, where most of the 

 tension members run across simply one or two panels of the 

 bridge. In this way any load coming upon the top of one of the 

 panels in the Bolman truss passes down the vertical compression 

 member and is at once carried to the abutments by means of the 

 tie-rods. Theoretically, this bridge is one of the most simple that 

 can be constructed; but when the span becomes of any great 

 length, the length of these tie-rods becomes so great as to render 

 them unmanageable, and within very small limits they become 

 impracticable for that reason. Hence, the Bolman truss has not 

 been used to any considerable extent. By the addition of the bot- 

 tom chord to support the floor timbers of the bridge, either the 

 Bolman or the Fink bridge can be used as a through bridge as 

 well as a deck bridge, although to achieve the utmost economy in 

 their use they are both eminently deck bridges. 



We will now take up the different kinds of trusses that are 

 used in ordinary railroad work, all of which are simply some 

 combination of the king-post trusses, either upright or inverted. 

 The first and most common form in this country is what is known 

 as the Howe truss (Fig. 21). In this the braces are diagonal and 

 the tension members are vertical. 



This form of truss has probably been built a hundred times 

 more than any other form that is in use. It is not in every re- 

 spect an economical truss ; but the reason of such general use is 

 the fact that it is one of the most simple to construct. The full 

 lines are those in compression, and are usually built of wood. In 

 the Howe truss, the lower chord, which is in tension, is also built 

 of wood, while the only iron- work about it is the vertical rods 

 and cast-iron blocks for the ends of the post. You will thus see 

 the advantages of this truss in a country where wood is very 

 plenty and iron is scarce. The construction of the iron-work 

 is very simple, and the parts are in pieces, so that they can be 

 easily handled by one gang of men with the ordinary block and 

 tackle. The angle blocks are all duplicates, so that, after a pattern 

 has once been made, a great many similar pieces can be made 

 from it ; and this, in the absence of skilled labor or proper shops 

 for doing bridge work, is a great saving of time. 



In all Howe trusses a very large " factor of safety " has to be 

 used in order to take into account the uncertain character of the 

 wood. By a factor of safety we mean this : you have a given 

 load which is to be supported by a bridge ; if all the material 



