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Steel Designers' Manual - 6th Edition (2003)
Steel towers and connections arc normally galvanized and are also painted with
a durable paint system if the environment is likely to be polluted or otherwise
corrosive. It is important that regular maintenance is carried out; climbing access
is normally provided for inspections.
Deflections of towers are generally significant only if they would result in a loss
of serviceability. This can be critical for the design of telecommunication structures
using dish antennas. In the past signal losses due to deflection have often been
assessed on the misunderstanding that the deflections under the design wind storm
would occur sufficiently often to affect the signals. Studies have demonstrated that
short periods of total loss of signal during storms smaller than the design wind storm
have a negligible effect on the reliability of microwave links compared with losses
due to regular atmospheric conditions.
5.1.6 Masts and towers in building structures
Towers and masts 175
Consideration of a masted solution arises from the need to provide a greater flexibility
in the plan or layout of the building coupled with its aesthetic value to the
project as a whole. At the same time it offers the opportunity to utilize structural
materials in their most economic and effective tensile condition.The towers or masts
can also provide high-level access for maintenance and plant support for services.
The plan form resulting from a mast structure eliminates the need for either internal
support or a deeper structure to accommodate the clear span. By providing span
assistance via suspension systems the overall structural depth is minimized giving a
reduction in the clad area of the building perimeter. The concentration of structural
loads to the mast or towers can also benefit substructure particularly in poor ground
conditions where it is cost effective to limit the extent of substructures (Figs 5.6 and
5.7). However, differential settlement can have a significant effect on the structure
by relaxing ties on suspension systems. The consequent load redistribution must be
considered.
Most tension structure building forms consist of either central support or perimeter
support, or a mixture of the two. Any other solutions are invariably a variation
on a theme. Plan form tends to be either linear or a series of repetitive squares.
The forces and loads experienced by towers and masts are illustrated in Figs 5.8
and 5.9. In all cases it is advantageous but not essential that forces are balanced
about the mast. Out-of-balance loads will obviously generate variable horizontal
and vertical forces, which require resolution in the assessment of suitable structural
sections.
Suspension ties must be designed to resist not only tension but also the effects of
vibration, ice build-up and catenary sag. Ties induce additional compressive forces
in the members they assist. These forces require careful consideration, often necessitating
additional restraints in the roof plane in either the open sections or top
chord of any truss.
Longitudinal stability is created either by twinning the masts and creating a vertical
truss or by cross bracing preferably to ground (Fig. 5.10).