piers are shorter than those for through or half-through spans, and, generally, the spans are narrower, thus causing a saving of metal in both the cross-girders and the lateral bracing. The clear headway required, especially for short spans, is likely to influence the layout more or less.

The possibility of using buried piers and protecting the feet of the embankments near them by rip-rap will not only affect the physical appearance of the bridge, but also it will modify the economics of the crossing.

In case a bridge is to cross a navigable stream, the layout of spans will depend primarily upon whether a swing, bascule, or vertical-lift span is adopted for the opening. If a swing is employed, it will generally require an expensive draw protection, while for a bascule or a vertical lift some comparatively inexpensive dolphins, either with or without cheap fender-walls of sheathed piles, will suffice.

The possibility of building an arch, a cantilever, or a suspension bridge instead of a simple-span structure would affect the layout in many ways— physically, aesthetically, and economically.

Again, the material adopted for construction whether masonry, concrete, steel, or timber-will have a similar influence.

The matter of shore protection is not likely to affect directly the layout for a bridge, although its use certainly does increase the total cost; but it might be the reason for shifting the crossing to a location where the bank is better protected by nature against scour.

Finally, the layout is affected by the character of the approaches; for they may be of earth embankment, timber or pile trestle, steel viaduct, or reinforced- concrete girders or arches.

Future Enlargement

The possibility of future enlargement of structure ought to receive consideration; and if it be decided that it is at all probable, a study of the layout should be made so as to determine how best to accomplish such enlargement when the time comes for so doing. The points to consider are whether it will be best to build an entirely separate new bridge close alongside, or to put a double-track superstructure on the old, single-track piers by enlarging them or expanding their tops, or, at the outset, to put in large piers and build the superstructure in such a manner that the trusses can be doubled in the future.

Again, it would frequently be good engineering to provide at first only the floor systems necessary to suffice for traffic requirements at the outset, but to design the trusses and substructure so that additional roadways and tracks can be added in the future when needed.

Time Considerations

The time allowed for completing the substructure or the superstructure or the whole bridge may affect the layout, for it is understandable that a