Masts and Towers in Building Structures.

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.

Traditional long-span structure
Fig. 5.6 Traditional long-span structure



 Suspension structure
Fig. 5.7 Suspension structure


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.

Types of perimeter support.
Fig. 5.8 Types of perimeter support.




Fig. 5.9 Central support

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).

 Longitudinal stability
Fig. 5.10 Longitudinal stability

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