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Steel Designers' Manual - 6th Edition (2003)
Mobility for non-mobile cranes
Non-mobile cranes can be made mobile by mounting them on rails. This has two
advantages: the positioning of the crane can be more easily dictated and controlled,
and the loads transmitted by the rails to the ground act in a precisely known location.
Many cranes have collapsed because of insufficient support underneath.
However, most rail-mounted crane failures have occurred from overloading. Where
the crane is to work over complex plant foundations the rail can be carried on a
beam supported, if necessary, on piles especially driven for the purpose. If the rail
is supported only by a beam on sleepers in direct contact with the ground, the load
can be properly distributed by suitable spreaders. In either case conditions must
be properly considered and designed for. Problems often occur when too much
faith is invested in the capability of the ground to support a mobile crane and its
outriggers.
Non-mobile cranes
Cranes and craneage 993
Non-mobile cranes are generally larger than their mobile counterparts. They can
reach a greater height, and are able to lift their rated loads at a greater radius.
There are two main types of non-mobile crane: the tower crane and the (now
rare) derrick. Due to their great size, the cranes must arrive on site in pieces. Thus
the disadvantage of a non-mobile crane is that it has to be assembled on site. Having
been assembled, the crane must receive structural, winch and stability tests before
being put into service.
A tower crane with sufficient height and lifting capacity (see Fig. 33.11) has
several advantages:
(1) It requires only two rails for it to be ‘mobile’.These two rails, although at a wide
gauge, take up less ground space than a derrick.
(2) It carries most of its ballast at the top of the tower on the sluing jib/counter
balance structure, and so very much less ballast is needed at the bottom. Indeed,
in some cases, there is no need for any ballast at the tower base or portal.
(3) Because the jib of a tower crane is often horizontal, with the luffing of a derrick
jib replaced by a travelling crab, the crane can work much closer to the
structure and can reach over to positions inaccessible to a luffing jib crane.
(4) A tower crane is ‘self-erecting’ in the sense that, after initial assembly at or
near ground level, the telescoping tower eliminates the need for secondary
cranes.
(5) As shown in Fig. 33.12 a tower crane can be tied into the structure it is erecting,
thus permitting its use at heights beyond its free-standing capacity.
There exist several types of tower crane, e.g. articulated jib, luffing and saddle
cranes as illustrated in Fig. 33.13. It is essential that manufacturers or plant hirers
are consulted in order to make the most appropriate choice of crane.