Page:Popular Science Monthly Volume 36.djvu/487

Rh 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 bottom 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 respect 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