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For elevated railroads it is necessary to determine the following:
First. The number of tracks on the various portions of the line, and the clearances over streets and alleys.
Second. The live load per track to be carried by the structure.
Third. The location of the line, whether in the streets or on private property.
Fourth. The style or styles of girder construction. In some locations the City Ordinances may require open-webbed girders, as these shut out less light than do solid plate girders, while in other locations the plate girders would be permissible.
Fifth. The location of columns, whether in the street or on the curbs, also, for location on private property, the number of columns per bent.
Sixth. The economic span length. As indicated in Chapter III, the greatest economy will exist when the cost of the longitudinal girders is equal to the cost of the cross-girders, columns, and pedestals. Where the columns are located in the street or on the curbs, due consideration mast be given to the probable cost of removing underground obstructions, such as water-pipes, gas-mains, etc.
With these points all settled, the calculations for proportioning all parts of the structure may be proceeded with.
Where the structure is on a curve, it is best to make the bents radial whenever practicable. The exact location of each column should be figured from certain known lines, and all ordinates for same should be indicated on the layout. Much careful study should be given to the work of establishing each feature of the layout; for, if mistakes be made therein, they are liable to cause great delay and expense later on.
Roof-trusses and steel buildings will not be treated in this book, as it deals mainly with bridges, viaducts, and elevated railroads. The office work con- nected with the designing of roofs and steel buildings will, however, not differ
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