ERECTION OF STRUCTURES 69. The Marking Diagram. Before the drawing of each piece of the structure leaves the drafting room, it is given a mark; and this mark is put on an outline drawing of the structure. In most cases, the letter which begins the name of the piece is put upon it, and this letter has a number which indicates which member it is. Thus columns are indicated C.), etc.; beams B.), etc.; girders etc.; and other members are designated in a similar manner.
In case of trusses, the lower joints are marked with the letter L with its subscript, and the upper joints have the letter U as in Fig. 87.
The great advantage of a marking diagram is that pieces are immediately recognized in the stock pile. In the case of trusses, the notation used has an added value, since, by the mark on the member, it can be determined at once just where that member belongs. Thus is the second panel of the lower chord; is the third panel of the upper chord; L1 is the first vertical; and is a diagonal in the third panel, as is also 70. Erection of Beams. Beams are of such light weight that almost any light derrick is suf ficient to raise them to position. Once in posi tion, they are held there while the workmeL connect them up to the other members with a few bolts, and they stay in this way until the riveters rivet them up.
In railroad work four methods present them selves for consideration. These are: (1) Trestle falsework ; (2) Gin pole; (3) Derrick cars or locomotive cranes; (4) Derrick; (5) Gallows frames.
(1) In the first method, the timber trestle work is built up in a manner similar to that for trusses (Plate 24), and the track is laid out upon it. The girder is then either run out
on a car, and skidded off the car into place; or it is unloaded by a gallows frame, or by a crane or derrick, and lowered into place (Plate 24).
(2) The gin pole method is applicable only to very small spans, and to locations where the surface of the ground is not far from the top of the rail. It is not much used.
'• (3) In case the structure is double-track, two derrick cars or two locomotive cranes, or one of each, may be used to lift the girder from one track into position on the other. Plate 25 shows a good example of this. In some cases the girder is picked up at the center and run out until the car nears the end of the track. The girder is then lowered into place. Plate 27 shows such a case.
(4) In cases where the stream is narrow or is dry, an ordinary derrick may be set up on the ground by the side of the track, and the girder erected as shown in Plate 26.
(5) Gallows frames are much used in the erection of girders. They are convenient and economical, especially where they can be erected on the ground (Plate 24 and Fig. 88).
72. Erection of Simple Truss Bridges. Simple truss bridges may be erected in four ways, namely: (a) By gantry traveler and falsework; (b) By derrick cars and falsework; (c) By cantilever method, without falsework; (d) By truss travelers.
(a) Plates 28 and 29 show good examples of the use of the gantry traveler. Piles are driven; and on top of these, timber bents are built. Upon the top of the bents are placed joists, and on these a floor of heavy timbers is placed. At the outer edges of the floor is a track; and along this track moves the gantry traveler, which is nothing more than two gallows frames connected.
Several hoists are connected at the top of the gantry, and the various members which have been run out to the gantry on cars on the rail road track are taken and held in place while they are being connected. Plate 29 shows an end-post being held in while the pin is being driven at the joint (b) In case the gantry is not used, derrick cars may take its place (Plate 30).