This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The ordinary contractor, engaged in work under Class B, requires to add to the plant of the jobbing builder a considerable amount of lifting tackle and yard machinery, and as his business further develops will also need various machines referred to presently under Class 3.
The Derrick in its simplest form consists of a single straight pole, placed vertically in a convenient position, generally within the building. The lower end should be either firmly embedded in the ground or securely fixed to some substantial portion of the structure.
The upper end is held in position by three stays or guy-ropes fastened to the top, and carried away as far as possible, and, as nearly as can be arranged, making an angle of 120 degrees with one another (Fig. 219). The addition of a sling chain, bound tightly round the top just below the point where the guy-ropes are secured, and having its terminal ring hanging free to receive the hook of the top block of a set of pulley blocks, completes the derrick.
The pole selected for this implement should be a good sound one, as free as possible from knots and shakes. There is no hard-and-fast rule as to what weight should be placed on any given sized derrick, but loads up to 1 ton can be safely raised 15 to 20 feet on a sound scaffold pole properly guyed. When heavier loads are to be dealt with it is advisable to use a die-square balk of either deal or, better still, pitch-pine; a 12-inch square balk of which latter, 30 feet high, can be safely trusted with a load of 14 or 15 tons. Although it is essential that the single-pole derrick should be nearly vertical in order to ensure its working under the best possible conditions, it may, in cases where the load is not great, be found convenient to let the top swing over in one or other direction, by means of loosening one of the guy-ropes and at the same time tightening the other two. In this way a load may be raised vertically through a small opening left in an upper floor or stage, and then, by letting the top of the derrick swing over a few feet, landed on the floor or stage itself, or even, if it be, say, a girder or constructional stone, placed in its final position without further handling.
Another form in which the derrick may be constructed consists of two poles, bound together with a stout cord about 18 inches from the top, and inclined towards each other at this point at an angle of about 30 degrees. Two guy-ropes are all that are required (Fig. 220), and by loosening one of these the load may be made to travel a much greater distance than with the single-pole derrick without any risk of buckling the poles. The extra space needed for this arrangement, however, renders it less convenient for use in the interior of buildings in course of erection. It is most useful for raising heavy pieces of material from carts, etc., as the cart containing the load can be drawn directly under the blocks, the load raised and the cart withdrawn empty. The load can then be lowered on to a trolley or rollers and taken within the building to the other lifting apparatus, of whatever kind it may be.