Tenth. Shifting channel.

Eleventh. High wind pressures to be provided for.

Twelfth. Wide deck.

Thirteenth. Necessity for quick operation.

A low vertical clearance is evidently favorable to the vertical lift. The real factor in this case is the required vertical movement of the lift span. A greater clearance above the water when the span is down favors the vertical lift; since, for any required clear height with the span raised, the vertical movement is reduced.

A large horizontal clearance favors the vertical lift in comparison with the bascule. For a given weight of moving span, the towers, counter-weights, and machinery of a, vertical-lift bridge are independent of the span-length, while those items for a bascule vary nearly directly therewith.

As will be explained fully later on in this chapter, the ratio of vertical and horizontal clearances for equal costs of bascules and vertical lifts is generally about unity, being somewhat less for short and light spans, and materially greater for long and heavy ones.

Increased weight of span is favorable to the vertical lift. This is chiefly due to the weight of the rear legs and bracing of the towers, which, for a given height thereof, are nearly as heavy for light spans as for heavy ones. For a very light span and high vertical clearance, the weight of the towers may nearly equal that of the span; whereas, for a heavy span and the same vertical clearance, it may be only one-third of the said weight. There is no such variation in the case of the bascule, since the weight of the bracing is a smaller proportion of the total weight of the towers and counterweight trusses.

A layout in which the economic length of the flanking-spans is much greater than the proper length of a bascule tower-span favors the vertical lift. In such a case the rear legs of the vertical-lift towers rest on the flanking-spans without producing any material stresses therein. But in bascules with overhead counterweights it will be necessary to put in an additional pier, or to carry the weight of the counterweight on one of the flanking-spans, or to put the counterweight trunnion over the pier and cantilever the flanking-span out to support the trunnions of the moving span. The first method is most economic where the substructure is cheap, and the third generally where the substructure is expensive. The third scheme requires ample fenders to protect the cantilevered portions from passing vessels. In deep water these fenders may be impracticable or very costly, thus making the second arrangement the best.

Over a canal, or a small canalized river, the layout often calls for a movable span and two short approach spans. In such a case four piers will be required for either the bascule or the vertical lift. This case is nearly always less favorable to the vertical lift than the layout where long flanking-spans are called for.

Deep foundations and expensive piers are favorable to the vertical lift,