The latter may have something to say about the angle of skew, as the United States Engineer Corps always advocates a square crossing, if it be practicable; hence the bridge engineer who desires to obtain approval for a bridge on a skew of any magnitude must be prepared to show good reason for his request; and even then it may not be granted, because, like the author, the Government engineers look upon a skew bridge as an abomination.

While the Department does not pay much attention to the character of the draw protection, it is likely to insist that it be not omitted and that its dimensions be satisfactory.

Ordinarily, also, it does not concern itself with the dimensions of the substructure; but sometimes, especially in case of a skew bridge, objection is raised to placing too much rip-rap around the piers and thus obstructing the flow of water in the channel.

Grade and Alignment

In most cases the grade and the alignment of the railroad or travelway are determined before the bridge engineer is called in, but sometimes it is otherwise; and there arise occasionally conditions which compel a conscientious bridge specialist to insist upon a change in either the grade or the alignment — or in both.

The ideal way to adjust the grade on a structure is to carry it over unbroken and, preferably, level in the case of railroad bridges, thus avoiding either a sag or a hump, as either of these objectionable conditions involves loss of power due to the climbing of unnecessary grades. Again, any great sag causes traction stresses and a shock that might better be avoided, if practicable. In a highway bridge this is not so important, and a rise from ends to centre of structure is permissible, especially as it facilitates drainage and improves appearance, notably in long-span suspension-bridges.

The ideal alignment for a structure is not only to have it on tangent throughout its entire length, but also to continue the said tangent quite a distance away from the bridge at each end. Sharp curves constitute an invitation for derailment; and a derailment on a bridge, or near the end of one, is liable to prove disastrous. A reverse curve on a structure, or on an approach thereto, is not permissible, if it can possibly be avoided. Where two curves in opposite directions come close together, there should be a stretch of tangent between them; and when this tangent is on a bridge, it should be made as long as possible. Sometimes it is entirely impracticable to avoid curvature on bridges and their approaches, as in the case of a rail- road following the course of a river that runs between high banks and having to cross it from time to time in order to avoid heavy excavations and tunneling. In such cases curves on the approaches are unavoidable, and often it is necessary to put a part or even the whole of the structure itself on curve.