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¨ The required separation distance from the object to be pro- Stability is achieved when the ratio between the load torque
tected must be maintained even under wind load (preven- and the tilting moment is > 1. The following rule applies: The
tion of intolerable bending) greater the ratio between the load torque and the tilting mo-
ment, the greater is the stability.
Determination of the tilt resistance The required stability can be achieved as follows:
The wind forces acting on the areas of the air-termination ¨ In order to keep the area of the air-termination rod that is
rod that are exposed to wind generate a line load q‘ on the exposed to wind small, the cross-sections used have to be
surface which generates a corresponding tilting moment M T as small as possible. The load on the air-termination rod is
on the self-supporting air-termination rod. To ensure stability reduced, but, at the same time, the mechanical stability of
of the self-supporting air-termination rod, a load torque M L the air-termination rod decreases (risk of breakage). It is
generated by the post must counteract the tilting moment M T . therefore vital to make a compromise between the smallest
The magnitude of the load torque M L depends on the stand- possible cross-section to reduce the wind load and the larg-
ing weight and the radius of the post. If the tilting moment is est possible cross-section to achieve the required stability.
greater than the load torque, the air-termination rod falls over
due to the wind load. ¨ The stability can be increased by using larger standing
The stability of self-supporting air-termination rods is proven in weights and / or larger post radii. This often conflicts with
static calculations. In addition to the mechanical characteristics the limited installation space and the general requirement
for a low weight and easy transport.
of the materials used, the following is included in the calculation:
¨ Area of the air-termination rod exposed to wind Implementation
Determined by the length and diameter of the individual In order to provide the smallest possible area exposed to wind,
sections of the air-termination rod. the cross-sections of the air-termination rods were optimised
¨ Area of the braces exposed to wind according to the results of the calculation. To facilitate trans-
Extremely high self-supporting air-termination rods are sta- port and installation, the air-termination rod consists of an
bilised with three braces which are mounted equidistantly aluminium tube (separable, if required) and an aluminium air-
around the circumference. The area of these braces that is termination rod. The post for the air-termination rod is hinged
exposed to wind is equal to the area of these braces pro- and is available in two sizes. Thus, roof pitches up to 10 ° can
jected onto a plane at a right angle to the direction of the be compensated.
wind, in other words the brace lengths are shortened ac-
cordingly in the calculation. Determination of the break resistance
¨ Weight of the air-termination rod and braces Not only the stability, but also the break resistance of the air-
The own weight of the air-termination rod and the braces termination rod must be verified since the wind load exerts
are taken into account when calculating the load torque. bending stress on the self-supporting air-termination rod. The
¨ Weight of the post bending stress must not exceed the maximum permissible
The post is a tripod loaded with concrete blocks. The weight stress and increases if longer air-termination rods are used.
The air-termination rods must be designed to ensure that wind
of this post consists of the own weight of the tripod and the loads which can arise in wind zone 2 cannot cause permanent
individual weights of the concrete blocks. deformation of the rods.
¨ Tilt lever of the post Since both the exact geometry of the air-termination rod and
The tilt lever describes the shortest distance between the the non-linear performance of the materials used must be
centre of the tripod and the line or point around which the taken into account, the break resistance of self-supporting
whole system would tilt. air-termination rods is verified by means of an FEM calcula-
tion model. The finite elements method (FEM) is a numerical
Stability is proven by comparing the following moments: calculation method for calculating stress and deformation of
¨ Tilting moment complex geometrical structures. The structure under examina-
Formed by the wind-load-dependent force on the air-termi- tion is divided into so-called “finite elements” using imaginary
nation rod, braces and the lever arm of the air-termination surfaces and lines and these “finite element” are intercon-
rod. nected via nodes.
¨ Load torque The following information is required for calculation:
Formed by the weight of the post, the weight of the air- ¨ FEM calculation model
termination rod and braces, and the length of the tilt lever The FEM calculation model corresponds in a simplified form
related to the tripod. to the geometry of the self-supporting air-termination rod.

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