Engineering Wonders of the World
Volume I

THE DEVELOPMENT OF THE BRIDGE. 105 of embellishment is superfluous, and may easily amount to bad taste. Most large girders consist of open framework, the continuous plate web being replaced by a bracing of “ struts ” and ties, containing a minimum amount of material, by which the stresses are transmitted to the flanges. Such a framework is sometimes called a “ truss.” Although the complete structure may be sub- ject to bending moments, the struts and ties are in compression or tension. Fig. 10 is an example of a trussed beam. From this it is evident that the beam and the strut are in compression and the two ties in tension. If fitted with one strut, the truss is known as a “ king ” truss ; if with two (see dotted lines), as a “ queen ” truss. Inverting a truss reverses the stresses, and renders necessary a disposi- tion of material, as shown in Fig. 11. Nearly every braced girder is a development of either the king or queen truss. Thus the “ Fink ” truss—much used in America—is merely a multiplication of king trusses (Fig. 12). Fig. 13 is an outline of the “ Warren ” truss, divided by its bracing into a series of equilateral triangles, the sides of which act as struts or (Fig. 12.) ties according to the nature and position of the load upon it. The load depends upon the traffic, but nearly always consists of a moving or “live” load more or less concentrated. (Fig. 13.) added to the distributed dead weight of the structure itself, and to that of the road or railway over it. A dense crowd of people on foot, extending from one end to the other, would constitute a distributed live load ; while a heavy vehicle, such as a traction engine, or, in the case of railway bridges, of a locomotive, would be a concentrated live load. Besides the loads just mentioned, allowance has to be made for the stresses due to wind pressure and, usually, for the weight of snow. A distributed load would place the members b, d, f, g, i, and k in compression, and a, c, e, h, j, I in tension. But suppose a moving concentrated load arrived at a, for instance, its effect would be to impose further com- pression on b and compress c also. The same would be the case with any other pair, or pairs, of members that the load happened to be over ; from which it follows that each diagonal may become alternately a strut or a tie, according to the position of the load. The top flange or “ boom ” of the truss is, of course, permanently in compression and the bottom boom in tension. The “ lattice ” girder (Fig. 14) is virtually a combination of two Warren girders. By pin- ning the diagonals at their intersection it can be made stiffer than a Warren. A further