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Steel Designers' Manual - 6th Edition (2003)
996 Erection
Fig. 33.13 Tower cranes used in the construction of Citigroup Tower, London (courtesy of
Victor-Buyck Hollandia)
the whole cost is targeted at the one job for which it has been initially designed (see
Fig. 33.14).
Where the frame is single-storey, and at the cost of only a slight increase in time
and labour, it is possible to do without an on-site crane. With the help of a winch
(powered by either compressed air or an internal combustion engine, and some
blocks and tackle), a light lattice-guyed pole can be used to give very economic erection
(see Fig. 33.15). In this instance the pole is carried in a cradle of wires attached
at points on the tower. These connection points need to be carefully designed to
ensure that they will carry the load without crippling the tower structure.
It is vital that all poles are used in as near a vertical position as possible, since
capacity drops off severely as the droop increases. This requires careful planning
and the employment of a gang of men experienced in the use of the method.
In a different context, pairs of heavier poles provided with a cat head to support
the top block of the tackle can be used inside existing buildings to erect the components
of, for example, an overhead travelling crane, or to lift in a replacement
girder. The arrangements for a pole and its appurtenances take up much less floor
space in a working bay than a mobile crane. This is because a mobile crane needs
a wide access route and adequate space to manoeuvre itself into position – particularly
useful where headroom is restricted.