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But little need be said in this chapter concerning the designing of trestles and viaducts, as that subject is fully covered in Chapters XIV and XVI. However, as the latter chapters are specifications, and are written in very concise form, it seems advisable to give here certain explanations of the reasons for the rules and directions therein formulated, even at the risk of repetition of a few matters.
The best layout for a trestle or viaduct is the one which will make the cost of the structure a minimum, provided that the specifications used in designing will insure for any layout the requisite strength and rigidity.
As stated in Chapter III, the greatest economy will exist when the cost of the bents and their pedestals is equal to the cost of the longitudinal girders and longitudinal bracing. On this account it is advisable to make the tower spans shorter than the intermediate spans, taking care, however, not to have the former too short for either appearance or proper resistance to traction. In general, tower spans should vary in length from twenty to thirty feet, although for very low structures it may sometimes be advisable to go a few feet below twenty. For the intermediate spans the length generally varies from thirty to sixty feet; but for very low structures with heavy rolling loads the economic length may be found to be less than thirty feet, in which case it will be perfectly legitimate to reduce the span length to suit the economic conditions.
The reason for adopting sixty feet for the superior limit is because trestles and viaducts are nearly always erected without falsework by starting erection at one end of the structure and dropping the members down by means of an overhanging
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