certain type of structure could be built in a certain limited time while another type of structure could not. Again, the length or shortness of the working season that is entirely free from danger of washout of false-work could be a sufficient reason for changing materially the layout—for instance, by necessitating pin-connected spans instead of riveted ones, or steel truss-spans instead of concrete-arch ones, or semi-cantilevering of certain spans instead of falsework erection throughout.

Stream Conditions

The various influences of the stream that is to be crossed are more potent than most other factors in affecting the layout. The high-water- and the low-water-elevations are important features in the designing of the piers; the amount and character of the drift determine the minimum vertical distance between high water and the bottom of the superstructure, and, therefore, aid in settling the pier height; and the amount and consistency of the passing ice constitute an important factor in the design of the piers, especially in respect to their length and the character of their end finish; and any increasing of the cost of the piers tends, for economic reasons, to lengthen the spans.

The clear waterway required to pass the probable maximum flood will often settle the total length of structure; and it may result in raising the high-water mark that was determined in some other manner. The profile of the river-bed and the probable scour of the materials of which it is composed are likely to affect the layout, especially if the piers require expensive protection of mattress work and rip-rap to check the said scour. The frequency and extent of the floods will influence the cost of building the piers—hence also the determination of the layout-as will also the questions of rise and fall of tides, velocities of the passing water, reversal of current, and the existence or possible future building of levees.

A most important factor is the possibility of the permanent shifting of the channel from one side of the river to the other. If this possibility exist, one of three things must be done, viz.: first, two movable spans must be provided; second, some effective method of retaining the channel in one position must be arranged for; or, third, the design must be so made that any fixed span of the structure may at any time be converted into a vertical-lift span.

Foundation Considerations

Important also in the determination of layout are the character and the depth of the substructure foundation. The deeper the piers have to go the longer will be the economic lengths of the spans. Again, the more difficult it is to penetrate the materials overlying the bed-rock or final foundation, the greater the cost of the piers, and the longer the economic spans. The ultimate  depths  to  foundation  and  the  materials  to  be  pene-