TRAVELING CRANES BASIC AND TUTORIALS


A traveling tower crane is erected freestanding on a base frame and ballasted by the user to accommodate in service loads. When out of service, it may need to be parked and anchored down to prearranged storm ballasting blocks or guyed to resist storm winds. An installation might be on the ground or mounted on another structure such as a building roof.

The base travels on railroad-type rails set to a very wide gauge. At each corner of the base one or more wheels is provided; when more than one wheel is used, they are mounted in a bogie that will equalize the load on all wheels at any one corner (Figure 6.39).



Some crane manufacturers offer options on the number of wheels to be placed at each corner of any one crane model. As the number of wheels increases, the weight of track and the number of track supports need to decrease. This can have significant ramifications for installation cost, particularly if soil conditions are poor.

Crane rails can be supported in a number of ways, including wooden ties on stone ballast (in this case the term ballast is used to refer to the bed of material placed between the tie and the native soil or sand base), a continuous steel beam on wooden ties and stone ballast or on concrete footings, or a continuous concrete footing or concrete sleepers on stone ballast.

The best system is that which will support the crane properly at least cost; this will be a function of crane wheel loads, soil conditions, and availability and cost of the materials at the jobsite. The crane manufacturer provides the wheel-load data, but the installation designer must make the decisions from that point on.

The spacing of sleepers or ties can be determined from rail strength and the wheel loads. For multiple-wheel arrangements, some continuity can be taken into account, but we suggest that supports outside of the bogie should be taken as simple. Deflection should also be checked to avoid lifting the ties off their beds.

Track splices are designed to carry only shear loads, so that splices must be centered between close-spaced supports or placed directly over a support. The spacing must be set so that the two rail ends do not differ in elevation (as a result of deflection or any other cause) as the wheel passes over the splice. This will prevent horizontal impact forces from occurring, forces that can be quite significant given the inertia of the tall crane above.

Rails must be laid to comply with the tolerances given by the manufacturer or specified by code. There are strict limits to variations permitted in gauge, in elevation along the tracks and between the tracks, in straightness, and in slope.

Crane rails can be laid to curves but only if the bogies are designed to permit it. Centrifugal forces which develop as the crane travels a curve can have an important effect on stability. The manufacturer must supply data for minimum radius of curvature that will permit safe travel at the speed the crane is capable of attaining.

Curved track as well as slewing forces, wind, and rail misalignment induce lateral forces on the rails. Rail strength and anchorage must be sufficient to restrain these forces. Magnitudes, however, are not easily determined; the crane manufacturer’s recommendations should be sought and followed.

On poor soils, track differential settlements can be a problem as they may cause track elevations to deviate from permitted tolerances and endanger operations. It would be wise to monitor elevations at marked points. This will show whether settlement is ongoing or stabilizing.

With wooden ties, settlements can be corrected by jacking the rail and tie and resetting the stone ballast. For concrete supports it may be necessary to install steel shim plates with sufficient contact area to prevent the concrete from being crushed.

The parking area must be designed and constructed in advance of crane erection. It will consist, for most cranes, of an area with close support spacing for the rails that will be capable of resisting the storm-wind compressive wheel loads. In addition, there must be four buried ballast blocks to which the crane can be tied down by means of cast-in fittings.

In U.S. practice, the buried ballast together with the traveling ballast must be capable of counterbalancing 1.5 times the maximum overturning moment. At the ends of the tracks, trippers are set that will automatically cause the crane travel brakes to engage. At a distance somewhat beyond the crane stopping distance, end stops, or bumpers, are installed as a last means to prevent the crane from running off the rails.

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