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Steel Designers' Manual - 6th Edition (2003)
174 Other structural applications of steel
The influence of height and topography on wind speed can be significant; this
is covered in some detail in both codes. Ice loads and types of ice are also covered
but neither mentions the very significant influence of topography on the formation
of ice. This has not yet been subject to systematic study but some hill sites are
known to be subject to icing well in excess of the code requirements. The combination
of wind and ice loads is even less well understood although some guidance
is given.
5.1.4 Analysis
In the analysis of towers the largest uncertainty is accurate knowledge of the wind
loads. Highly sophisticated methods of analysis cannot improve this. A static linear
three-dimensional structural analysis is sufficient for almost all lattice tower
structures.
For transmission towers, line break conditions can also be critical. Line breakage
will in general induce dynamic loads in addition to any residual static loads. Detailed
consideration of transmission tower loading is outside the scope of this section.
For lattice towers with large complicated panel bracing, the secondary bracing
forces can be significantly altered by non-linear effects caused by curvature of the
panels under the influence of the design loads. Generally the rules in the codes are
sufficient, but where structures are of particular importance or where there is much
repetition of a design, a non-linear analysis may be necessary.
Dynamic analyses of self-supporting lattice towers are rarely necessary unless
there are special circumstances such as high masses at the top, use as a viewing
platform, or circular or almost solid sections of mast which could be responsive to
vortex shedding or galloping. Knowledge of the dynamic response is also necessary
for assessment of fatigue of joints if this is significant.
For guyed towers the non-linear behaviour of the guys is a primary influence and
cannot be ignored. The choice of initial tension, for example. can have a very great
effect on the deflections (and dynamic behaviour). The effects of the axial loads in
the mast on column stiffness can be significant. Methods of static analysis are given
in the main international codes for the design of guyed towers. Guyed towers can
also be particularly sensitive to dynamic wind effects especially those with cylindrical
or solid sections. 2
General guidance on the dynamic responses and aerodynamic instabilities of
towers can be obtained from References 3, 4, 5 and 6.
5.1.5 Serviceability
Serviceability requirements vary greatly depending on the purpose of the structure
and its location.