142 



LECTURE 



arrangement shown in Fig. 27, in which the tie A C is added, to pre- 

 vent the, roof from spreading and pushing out the walls. 



Fig. 27. 



There are'innumerable forms of roofs, some entirely of wood, others 

 entirely of iron, others mixed, which take different forms, as the braces 

 are made either to resist compression or extension, for, as we have seen 

 in Figs. 18 and 19, we may always substitute for a tensible brace one 

 which acts as a strut. All well designed roof trusses will, however, 

 bear the test of an analysis, based on the principles just enunciated. 



One more example may be given in which this simple form of truss 

 is extended to adapt it to the heaviest bridges with great success. 



The iron bridges on the Baltimore and Ohio railroad, and else- 

 where, known as Bollman's bridges, are made, as shown in Fig. 28, 

 where the struts c d efg h i, and the tie rods belonging to them, sup- 

 port the beam A B at these points. 



A c d e f g h 



In an improvement by Fink, shown in Fig. 29, the tie rods on each 

 side of each strut are of the same length, and therefore equally effected 

 by changes of temperature, which is an important matter, since in 

 Fig. 28 the struts near the ends are subjected to side strains from the 

 unequal changes of length of the rods. This arrangement of Fink's 

 permits, moreover, the use of much lighter tie rods for the lesser parts 

 of the system, as indicated in the figure, and no more material is there- 

 fore used than is absolutely necessary. 



Fig. 29. 



