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FAQ >> Mobile Crane Clearances

Mobile Crane Clearances

With each work cycle, a crane picks up a load, swings it and places it where needed. The path of both load and boom must clear obstructions through the full range of movement. The boom must be long enough to raise the load to the required height without danger of collision between the load and the boom, but not so long that it will swing into nearby structures. The tail end of the superstructure requires a clear swing path, too. And, after completing the work, the crane must be able to extricate itself and lower its boom for dismantling. Far from all mobile-crane lifts run against these constraints, but they are common enough to demand attention.

It is embarrassing, to say the least, to send a crane to a job only to find that it is incapable of placing the loads where needed. Embarrassment may be only the first problem in a series, as some accidents come about because field crews try to improvise and work around an unexpected limitation.

There are numerous ways that a crane might come up short. Shortness of reach or lifting capacity is an obvious example. This is best averted by diligent review of loads, radii, and crane capabilities. But there are less obvious shortcomings just as debilitating to an operation as the straightforward ones that might be ferreted out only by careful study with the benefit of an experienced eye. The more subtle shortcomings often are interferences between the moving crane or load and other objects. These are referred to as clearance deficiencies. Clearance problems can come up in various ways.

The lifted load is at risk of fouling the boom or jib. A wide load lifted close to the head machinery at a high boom angle could be at risk. This is sometimes a dilemma faced by riggers placing unwieldy vessels or machinery. Several styles of boom heads such as the hammerhead tip or offset tip are configured to lessen the possibility of fouling. The head sheaves of telescopic booms are often similarly offset, too.

  • The suspended load or the boom tip fouls a nearby building or other obstruction. In tight quarters, the boom might not be capable of being raised high enough to swing past the obstruction. Urban and industrial sites can be places of heightened exposure to this peril.
  • The boom head is obstructed from above by an overhanging structure or object.
  • Reaching some distance past the leading edge or parapet of a building, the underside of the boom or jib cannot clear it. The authors refer to this as a swing clearance problem.
  • The hook cannot reach sufficient height to place the load. This is known as a drift deficiency. It is sometimes caused by an underestimation of the length slings or height of the rigging supporting the load.
  • The aft end of the crane superstructure cannot clear an obstruction. The counterweights, for instance, interfere with a tree or a live mast fouls a building.
  • Any part of the crane or the suspended load encroaches within a restricted zone surrounding a power line.

Basic clearance checking can be done using the range diagram provided with the crane documentation, a copy of which is often also mounted in the crane cab. For most work, the rough data obtainable with a range diagram are sufficient to verify that the job can be done and that boom and jib lengths and jib offset will offer satisfactory clearance. A clearance problem is shown in Figure 5.4 where crane reach is obstructed by the building in front of it. When conditions appear to be too close to permit reliance on the diagram, calculations or a more comprehensive graphical approach might be used. In many instances, a conventional CAD drawing, accurately dimensioned and detailed, is a suitable tool for clearance checking. These problems can be difficult to visualize with two-dimensional tools even for experienced planners. As this manuscript is being prepared, technological aids to assist lift planners are in existence but with limited utility. Building Information Modeling (BIM), in particular, is promising, but presently the lead time and costs needed to generate a useful model make sense only for a select few projects. No doubt some of its limitations will disappear over time, and other advanced planning aids will also come into being. Until the improbable arrival of the day when a planner will be able to don virtual reality goggles and walk through a comprehensive 3-D model of a crane set up on the site, it will be necessary to hone visualization skills and make the best possible use of more rudimentary analytical tools such as those that follow. And no matter what mathematical model is used, obviously it is only useful to the extent that it matches the actual field conditions.

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