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The derricks used for erection operations in construction are built to handle heavy loads from a fixed position. The support frame-consisting of the base, mast, A-frame, and all other than the boom-remains in a stationary position relative to the ground or supporting structure during its lifting operation. The boom is generally rigged with sheave blocks and fall fines to handle a load just like a mobile crane.
The basic difference between a derrick and crane is in the topping-lift connection to support the upper end of the boom. On a derrick the lines go to a mast of the A-frame at a point anywhere from mid-height to higher than the highest point of the boom's vertical reach. In this way, the topping lift line makes a much larger angle with the boom axis than in the case of the crane. This allows a derrick to carry much greater load than a mobile crane can without requiring many parts of topping-lift support lines. If there are not too many parts of the support and lift lines to move, the angle can be changed much more rapidly. Generally, derricks are custom-made for a given job. Details of the dimensions, rigging, and operation of derricks are given in the specifications by the manufacturer.
The stiff-leg derrick is one equipped with a vertical mast that is shorter than the boom. This support member is tied into the framework of other members, which are the angling legs extending from the peak of the mast toward the boom to form a triangle with the mast's base at the bottom frame level.
This derrick is a piece of erection equipment to consider where there is a strong base level to support the entire horizontal under-frame. It may be used on level ground, a boat deck, or the more-or-less horizontal chord members of a bridge. In the latter case the derrick's width between the bottom of the angling support legs would have to equal the bridge width between trusses with a cross member equal to that width. A stiff-leg derrick could also be used for higher lifts by having it supported at a higher level on a triangular tower with vertical legs under the mast and bases of the angling support legs. This would be called a tower derrick. Generally, the whole supporting frame for the derrick is moved on rollers by winching under its own power along beams or rails, but not when it is hoisting a load.
In order to avoid taking too much area at the base level and to keep the supporting legs relatively short, it will be necessary to use counterweights or ballast on the base frame around the mast bottom of the stiff-leg derrick. This is somewhat similar to the arrangement for a mobile crane. It is done to avoid negative reaction under the mast that would indicate the derrick is about to tip over in the direction of its boom.
Any negative reactions must be counteracted by the derrick weight and counterweights on the base supporting frame. These reactions, which can be readily calculated, act as vertical loads on the bearing surface. They are vertical forces, but the axial forces in the angling support legs will be larger. In the case of a tower derrick the vertical legs will have to support these reaction loads as their axial force.
A guy derrick generally has a mast that is taller than the boom so that the boom can turn under the guys supporting the mast. These guys are somewhat stiffer cables than the lifting lines that run from the top of the mast to solid hold-down points on adjacent buildings or whatever solid hold down points are available. There are generally about six guys to give the mast stability in all directions. The base for the mast takes very little space and is supported on a "bull wheel" turntable powered by cable turning the wheel in a horizontal plane.
The guy derrick has the largest angle of all erection equipment between the topping lift line and the boom. This means that it has the best load capability for the lightest topping lift lines. Guy derricks also represent the least equipment structure for erection, so they should be the most economical in cost per pound hoisted. They are used generally for tall buildings where there is plenty of opportunity for tie-down supports of the guys surrounding the building to be erected. The guy derrick can move up the building as it is erected, using the derrick's own power and parts, that is. its mast and boom like a caterpillar moving up a tree.
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