With regard to the towers, the vertical lifts may be divided into three classes, viz.,

A. Structures with towers having inclined rear legs.

B. Structures with towers having vertical rear legs.

C. Structures having towers, each composed of a single bent, and generally, but not necessarily, connected at their tops by a span or strut crossing the opening.

In respect to the economics of these three classes, it may be stated that, for a combination of a short span and a moderate vertical clearance, Class C is the most economic; but it is not compatible with rigid construction for high clearances also that Class A is always more economic than Class B, because the latter involves the doubling of the number of sheaves and a considerable increase in the weight of the towers, as well as a small extra amount of wire rope. It is found advantageous, however, in the case of very-badly-skewed structures, because it throws the large and clumsy counterweights entirely outside of the towers and permits of a thorough system of internal sway bracing for the latter.

Economics in Detailing of Vertical Lifts

There are a few economic problems that arise in the detailing of vertical-lift bridges, the principal of which are the kinds of materials for counterweights, the use or non-use of counterbalancing chains, the employment or omission of buffers and the best type of same, the character of pavement base, the location of the machinery house, and the determination of the number and the size of the supporting ropes which will make the combined cost of ropes and sheaves a minimum.

In respect to the cheapest material for counterweights, in most cases it is ordinary concrete; but sometimes, when the space is limited, it pays to make the mass heavier by incorporating in it materials of greater density than that of ordinary stone, such as iron ore or pig iron. The latter was employed entirely for the counterweights of the Halsted-Street Lift-Bridge, the first structure of the type on a large scale ever built; but its utilization was not economic, consequently in subsequent structures its employment was abandoned.

As to whether it is advisable to use chains for the purpose of keeping the main cables always counterbalanced, that is an economic problem which is dependent upon the kind of power used, how often the bridge is to be operated, and the extent of the span movement. They should not be employed for low lifts, as in these the unbalanced rope-load is just about right to hold down the span properly. The author favors the adoption of such chains for most cases where the vertical movement is large, so as to

make the peak load of power a minimum and thus keep down the cost of both installation and operation; but he recognizes that, when the price of power is low and the bridge is not to be opened often, it