In some cases two travelling cranes of equal power may be temporarily coupled together to lift a load equal to twice the load lifted by one traveller alone. In this case the disposition and spacing, and the total wheel-base of the wheel loads, will be determined by the dimensions of the end cradles, and that position of the total load must be ascertained which gives rise to the maximum reaction in the supporting column, as well as the maximum bending moments in the girder.

The crane manufacturer in the design of a traveller of this class will always require certain important minimum dimensions or clearances to be maintained in the structure in which the crane is to be employed.

One of these is the distance to be maintained between the centre of the rail and the face of the wall, pier, buttress, column, or other projection past which the end cradles have to travel. This dimension is required by the details of the end cradle, and the bearings of the axles of the cradle wheels.

In small travellers of short span and light load the clearance required is about 6½ inches; a clearance of 9 inches will suffice for cranes of about 35 tons lifting power, while 11 to 12 inches will cover most ordinary cases of heavier cranes. This dimension will be found to exercise a considerable influence over the design of columns to support traveller roadways, as will be seen by the examples referred to in Chapter IV (On The Practical Design Of Columns And Struts).

The other dimension referred to is the headway required, usually measured from the rail level, over the crane, to the lowest fixed portion of the roof or floor above; such, for example, as the distance to be maintained between the level of rails and the underside of the tie-rod or tension member of a roof principal.

A distinct understanding on this point should always be maintained with the crane manufacturer, who should be asked to state his requirements, while it is judicious at the same time to allow some small margin over the precise figure asked for. In most cases a headway of not less than 7 feet 6 inches will be required for cranes of, say, 30 tons power, the precise amount being regulated by the details of the crab, the diameter and height above rail of the great wheel and main barrel, or possibly, in the case of a steam crane, by the dimensions of the boiler used.'

In cranes of small power worked by hand, arrangements are sometimes made by which the traveller can be arranged beneath the roof or floor above with but a few inches of clearance.

The preservation of the truth of the gauge between the rails of a traveller road, from end to end of the distance to be traversed, must at all times claim the attention of the designer of the structure, especially when the spans of the traveller are large, and the supporting columns are lofty.

This is frequently attained in the design of the superstructure, as, for example, in cases where the tension member of the roof principal overhead is designed to act as a tie or a strut, and in such wise rigidly maintain the gauge of the road. An example of this form of construction is given in Figs. 236, 237.

Where the crane road is lofty, and the supporting columns of corresponding height, the longitudinal stability of the row of columns and girders must also be considered, and provided for, by bracing, bracketing of the girders, bolting down to foundations, or the like. See Figs. 216, 237.

Where the traveller road is carried by the walls of a building, the stiffness and stability of the masonry or brickwork should be considered in the case of very heavy cranes, and buttresses or piers arranged for as the case may require, the offsetts in the walls being arranged to suit the details of the crane-road and the clearance for cradle above mentioned. In certain types of foundry cranes the stability of the crane is often really dependent upon the stiffness and stability of the enclosing walls of the foundry and its roof framing.

The type and section of girder to be used for a traveller road will be determined by the power and span of the crane to be carried, and the span of the opening to be bridged.

Thus the girder may be of double-webbed or box type, or in some special cases, where the shearing stresses are exceptionally severe, may be of the three-webbed type, while the single-webbed girder may be of any section from a heavy riveted girder to an ordinary rolled joist of light section.

Latticed web or triangulated girders may also be employed subject to great care being taken that the maximum web stresses arising from the rolling load are amply provided for, and also, which is of equal importance, that the upper flange between the apices of the triangulations has sufficient transverse resistance as against the concentrated wheel loads. These considerations generally lead to the adoption of the plate-web type for small spans.

The plate girder may be constructed of uniform depth with parallel flanges, or of the fish-bellied form shown in Fig. 216.

In girders of uniform depth, where the loads and span are considerable, the necessary piling up of the plates towards the centre of the span requires an uniform seating for the rail, which is obtained either by the use of iron packings at intervals, or a continuous strip under the rail, or, as in Fig. 245, by a timber bolster or packing notched to the stepping up of the plates, and trimmed off to a true surface on the top to receive the rail, which may be coach-screwed or spiked to it. The fish-bellied girder, on the other hand, though perhaps rather more troublesome and expensive to construct, possesses the advantage that the sections of the top and bottom flanges being practically uniform from end to end, the upper surface of the top flange plates offers a continuous and even bearing for the rail without the use of packings.

The necessity of a secure fastening for the rail in order to prevent the rolling-up tendency of heavily concentrated wheel loads has already been pointed out.