The economics of approaches to bridges will involve the question whether it is best and cheapest to build earth embankments, timber trestles, steel viaducts, reinforced-concrete viaducts, or any combination of these, and at what heights it would pay to change from one type of construction to another.

In determining the economics of the different kinds of structure it does not suffice to compare merely their first costs; for it is necessary to take into account the items of depreciation, maintenance, and repairs by computing the annual expenses for these, finding the sums of money which, at the governing rate for simple interest, would produce these annual amounts, and adding the results to the first costs.

In certain cases it might not be best to adopt the theoretically-economic kind of structure, because the requisite funds for building it may not be available; and in such cases the cost of renewals should receive due consideration by taking cognizance of the probable increase in the future prices of perishable materials, such as timber, as well as of the special danger to the structure from fire or washout due to the employment, either permanently or temporarily, of such inferior construction. As indicated in a previous chapter, the danger from fire to a structure built either wholly or partially of timber is a serious matter. It may be permissible under certain conditions to risk losing an approach to a bridge by either fire or flood; but if the danger extends also to the main structure, the cheapening expedient is not permissible.

Again, due consideration should be given to the question of the expense caused by the interruption of traffic by putting out of commission either one or both of the approaches. Generally speaking, it does not pay to take any chance of even temporary disaster to the structure; but, as before pointed out, it sometimes appears to be unavoidable.

In the case of embankments when earth is expensive at the outset and can be brought to the site much more cheaply after the bridge is finished and the railroad line that it carries is in operation, it will generally pay to build, as inexpensively as possible, a timber trestle; and later, just before it begins to need expensive repairs, fill around it and construct an embankment by dumping earth from above by means of a construction train.

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