Such a general condition existed on the line of the Canadian Northern Pacific Railway as it followed up the Fraser and the Thompson rivers, crossing them nine times with only one structure entirely on the square.

In some skew crossings, especially when the obliquity is small, it is permissible to square the piers to the structure, thus saving considerable masonry; but this practice is not always advisable because of the damming of the water by the large area of the substructure that is opposed to the current.

The layout of any bridge on a curve, or which has its approaches on curve, is greatly affected by the curvature, in that it has a tendency to shorten the span-lengths in the effort to avoid excessive width of superstructure and undue increase in length of piers.

The determination of the best grade to use for the approaches to a bridge is an economic problem of major import. It is of much more consequence in railroad bridges than in highway structures, because of the far steeper grades which are permissible in the latter; for, of course, the steeper the grade the shorter the approach and the less its cost. It is sometimes practicable to put a grade on the river spans of a bridge, leading up to a movable span or to a single, high-level channel-span; and this should always be done when practicable, notwithstanding the fact that the grade may have to be less than that allowable for the approaches, because even inches in elevation on the land construction often count materially in determining the length and cost thereof.

In railroad bridges the fixing of the approach grades sometimes involves the economic solution of the question of a steep grade with pusher engines versus an easy grade without them. In this case it is necessary to add to the first cost of the former, the first cost of all the pusher engines needed, plus the capitalized value of the annual cost of their operation and deterioration, and compare the sum with the first cost of the latter. Moreover when figuring the annual cost of operation, it is necessary to include therein the annual expense due to delay of trains caused by stopping, attaching the pushers, and regaining speed.

Geographical Conditions

The layout of a bridge is sometimes influenced to a certain extent by its geographical location, because a structure suitable for the heart of a city might not be appropriate in a country district, and vice versa. Generally the variation involved would be a question of aesthetics, or possibly one of flooring, for sometimes it is necessary to cover over the deck of a railroad bridge so as to permit it to take care also of highway traffic. In mountainous districts where the transportation of large, heavy pieces is either very expensive or altogether impracticable, the layout would be governed by this condition.