Environmental Loading - Tower Structures.

The primary environmental loads on tower structures are usually due to wind and ice, sometimes in combination. Earthquakes can be important in some parts of the world for structures of high mass, such as water towers. Loading from climatic temperature variations is not normally significant but solar radiation may induce local stresses or cause significant deflections, and temperatures can influence the choice of ancillary materials.

Most wind codes use a simple quasi-static method of assessing the wind loads, which has some limitations for calculating the along-wind responses but is adequate for the majority of structures. Tower structures with aerodynamically solid sections and some individual members can be subject to aeroelastic wind forces caused by vortex shedding, galloping, flutter and a variety of other mechanisms which are either poorly covered or ignored by current codes of practice. Such factors have been responsible for more tower collapses and serviceability failures worldwide than any shortfall in resistance to along-wind loads.

Most national and international design codes now specify wind loads in terms of design wind speeds, either mean hourly or gust, that will recur on average once in a 50 year period (i.e. with an annual probability of 2%). Guidance is sometimes given on wind shape factors for typical sections and lattices. Consideration of dynamic response to the wind is not always covered in depth and there is still a mixture of limit state and working stress codes. BS 81001  is a recent code in a limit state format specifically written for towers. Wind loads are specified in terms of a ‘50 year return’ mean hourly wind pressure together with gust factors which convert the forces to an equivalent static gust.The overall wind forces calculated using BS 8100 are substantially  similar to those that would be obtained using earlier codes such as CP32 but forces near the tops of towers are relatively higher due to an allowance for dynamic response. The code also gives guidance on means of allowing for the importance of particular structures by adjusting the partial factor on the design wind speed.

Guidance is limited for structures that have a significant dynamic response at their natural frequencies, and gust factors for guyed towers are specifically excluded from the scope of the code. Part 4 of BS 8100 is intended to address these aspects.

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.

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