This car is b0 feet long, 60 feet between centers of trucks, and is as wide as the clearance diagram, 14 feet for single track. It is evident that the trusses cannot be spaced so as to interfere with the clear ance line of the body of the car and its projecting ends. These clearance lines are represented as broken lines in Fig. 120, and are marked c–c. Note that the center of the track is seldom in the center of the floor-beam. Also, it is evident that the sharper the curve, the greater the required distance between trusses, and accordingly the greater the floor-beams in length. This varies the moment in the different floor-beams and therefore makes them more costly. The stringers, also, are more costly, on account of the fact that their ends are skewed. On account of the eccentricity of the track, one truss takes more of the load than the other, and therefore the trusses are not the same—a fact which further increases the cost.
From the above it is seen that almost all conditions incident to the building of a bridge on a curve tend to increase the cost; and hence a fundamental principle of bridge engineering: Avoid build ing bridges on curves.
63. Weights and Loadings. For the weight of steel in any particular span, and for the loading required for any particular class of bridge, see Articles 20 to 23, Part I. The weight of the ties and the rails and their fastenings is usually set by the specifications at 400 pounds per linear foot of track. For highway bridges the weight of the wooden floor is usually taken at 41 pounds per square foot of roadway for every inch in thickness of floor.
Highway bridges are divided into different classes according to their loadings (see Cooper's Specifications). The decision as to the class to be employed depends somewhat upon the distance to the nearest bridge across the same stream. In case the nearest bridge is only a few miles away and is of heavy construction, it is not actually necessary to construct a heavy bridge at the proposed site, the heavier traffic being required to pass over the other bridge. In case a heavy bridge is not in the neighborhood, then one should be constructed at the proposed site. If the proposed site is on a road connecting adja cent towns of large size, then a heavy bridge should be constructed and provision made for future interurban traffic, even if none is at that time in view, since it will be more economical to do this than to erect a new bridge in the future.
In the case of railroad Bridges, new ones are nearly always con structed to carry the heaviest main line engines. These are usually of a class corresponding to Cooper's E 40 or E 50. In some localities branch-line bridges are built for the same live loadings; but in the majority of cases the branch-line bridges consist of the old bridges from the main line.
64. Specifications. For any particular bridge the specifica tions are either written by the engineer in charge, or some of the very excellent general specifications which are on the market in printed form are used. Some railroads use these general specifications with the addition of certain clauses which are desired by the chief or bridge engineer. The principal differences in these general specifications are in regard to the allowance for impact.
Whenever highway design is mentioned in this text, it is to be in accordance with Cooper's Highway Specifications (edition of 1901). Wherever plate-girder design is given, it is in accordance with Cooper's Railway Specifications (edition of 1006); and wherever truss design is given, it is in accordance with the general specifications of the American Railway Engineering & Maintenance of Way Association (second edition, 1906).
65. Stress Sheet. Before the sections are designed, the com puter makes a skeleton outline of the truss, and on this places the dead-load and live-load stresses, and, in case the wind should be considered, the wind-load stresses. This is sent to the designer. The designer determines the various sections, and also the moments and shears in the stringers and floor-beams. These are placed on a sheet usually 17 by 23 inches. This is called a stress sheet. This sheet is now given to the draftsman, who makes a shop drawing. The stress sheets for railroad bridges are usually more elaborate than those for highway bridges. Plate I is the stress sheet of a highway bridge; and Plate II (Article 78) and Plate III (Article 93) are examples of the best modern practice in the making of plate-girder and truss bridge stress sheets.
66. Floor System. Perhaps no part of bridge design is better standardized than the construction of the open steel floors for railroad bridges. The stringers are usually placed 6 feet 6 inches apart, and consist of small plate-girders, or, if the panel length is short, of one or more I-beams. I-beams are economical in regard to first cost, but are disadvantageous on account of the eccentric con nections which necessitate heavy brackets to resist part of their re action. They are also somewhat undesirable on account of the fact that, the ties deflecting, most of the load is carried by the inner I-beam. However, 1-beams for stringers and for short-span bridges (see Fig. 121) are much used in present practice, and give good re sults. Figs. 121 to 127 show the standard open floor sections of the Lehigh Valley Railroad. Table XIX gives the required number of I-beams, together with their weight, which are to be used for short span bridges or as stringers in panels of given length.
Solid floors consist of angles and plates, channels and plates, or other shapes. They extend transversely across the bridge from truss to truss, the lower chords, in case of truss bridges, be ing made heavy enough to act as girders as well as tension members. Figs. 128 to 130 show sections of solid floors. The ballast is laid di rectly upon these solid floors, which are first covered with a good damp proof paint. The floors should also be supplied with good drainage fa cilities.
' Concrete is sometimes laid rectly upon the steel floor, and the ballast put upon this concrete, which has previously had a layer of some good waterproof ing applied on its upper sur face.
67. Practical Considera tions. The possibilities of the rolling mill and the various shops of a bridge company, such as the drafting room, forge, foundry, templet shop, assembling shop, and riveting and finishing shop, and also the shipping and ereet'ng facil ities, should be well known in order to make the most eco nomical use of them. This requisite knowledge comes only from experience. The best way to obtain this experi ence without being actually employed in the shops, is to go into the shops every chance that presents itself, keep your eyes and ears open, and ask all the questions you can. The use to be made of handbooks of the various steel manu facturers is given in Part I of "Steel Construction," and should be thoroughly studied before going further. Some one of these handbooks is in dispensable to persons design ing steel structures. That of the Carnegie Steel Company (edition 1903) is one of the best, and will be frequently referred to in the present text.
Copies may be procured from the Carnegie Steel Company, Frick Building, Pittsburg, I'a. The usual price to students is 50 cents, to others $2.00.