Safety of 5G Network Physical Infrastructures 175
The UK NA adopts the Deaves and Harris model to express the change of mean
wind velocity with height and gives charts for the roughness factor that depend on
three terrain types, instead of five categories suggested in BS EN 1991‐1‐4: Sea,
Country terrain and Town terrain. When there is the choice between two or more
terrain types in a given area, then the lowest value of terrain roughness length should
be used. The UK NA reduces the complexities of orography assessment and effects on
wind velocity by implementing the altitude factor, c alt . Orography effects must only be
evaluated according to the rules specified in Annex A.3 of BS EN 1991‐1‐4 for sites
that fulfil the criteria specified in Figure NA.2 of UK NA [37]. The effects of interference between neighbouring structures are also addressed in BS EN 1991‐1‐4. For
example, when a structure is located close to another structure that is at least twice its
height, it could be exposed to higher wind velocities for certain wind directions. Annex
A.4 of BS EN 1991‐1‐4 gives a conservative method to estimate this effect. Additionally,
when groups of structures of similar height are packed closely together, they provide a
shielding effect against wind action in a zone extending from the ground to about the
average height of the top level of each structure: termed displacement height, h dis .
Therefore, the effective height to be considered for wind action assessment is z – h dis .
Rules to obtain the value of h dis are given in BS EN 1991‐1‐4. The use of the latter
concept implies that the neighbouring structures can only be removed after the end of
the work period of the structure under consideration, and that the shielding effect is
uniform in plan.
In the UK NA, the fluctuating component of the wind velocity is simulated by a turbulence intensity parameter, I v , whose value for flat terrains can be determined from
one of the several charts provided. Correction factors are also given to calculate the
value for other types of terrain roughness. For sites where orography is important (see
Figure NA.2 of the UK NA), the values of I v must be divided by the orography factor, c o ,
given in Section 4.3.3 of BS EN 1991‐1‐4. The two components of the wind, mean and
fluctuating, are then combined to obtain the maximum peak gust velocity, ˆ
V :
( )
( )
( )
= + ⋅
⋅
v
m
ˆ
1
V z
g I z V z
(8.3)
In the UK NA, a value of peak factor, g, equal to 3.0 is specified. Having determined
the wind velocity, the wind effect on a body (element/structure) can be calculated. The
simplest analysis is to analyse the along wind structural response, with more complex
analyses needed for the across wind and torsional responses [38]. All modern design
codes require the effects of wind on a structure to be accounted for, either by wind
pressures or by wind forces. For mobile communication structures, the latter method is
preferred.
The complex processes and relationships of wind engineering have been translated to
code rules by assuming simplifications. One of the most important assumptions made
in most of modern design codes dealing with wind effects is the quasi‐static hypothesis,
which assumes [35]:
1) the wind turbulence intensity is low;
2) the fluctuations of the effects of the wind action follow the variations of the direction
of the mean wind velocity; and
3) there is a perfect correlation between fluctuating sectional forces.
The UK NA adopts the Deaves and Harris model to express the change of mean
wind velocity with height and gives charts for the roughness factor that depend on
three terrain types, instead of five categories suggested in BS EN 1991‐1‐4: Sea,
Country terrain and Town terrain. When there is the choice between two or more
terrain types in a given area, then the lowest value of terrain roughness length should
be used. The UK NA reduces the complexities of orography assessment and effects on
wind velocity by implementing the altitude factor, c alt . Orography effects must only be
evaluated according to the rules specified in Annex A.3 of BS EN 1991‐1‐4 for sites
that fulfil the criteria specified in Figure NA.2 of UK NA [37]. The effects of interference between neighbouring structures are also addressed in BS EN 1991‐1‐4. For
example, when a structure is located close to another structure that is at least twice its
height, it could be exposed to higher wind velocities for certain wind directions. Annex
A.4 of BS EN 1991‐1‐4 gives a conservative method to estimate this effect. Additionally,
when groups of structures of similar height are packed closely together, they provide a
shielding effect against wind action in a zone extending from the ground to about the
average height of the top level of each structure: termed displacement height, h dis .
Therefore, the effective height to be considered for wind action assessment is z – h dis .
Rules to obtain the value of h dis are given in BS EN 1991‐1‐4. The use of the latter
concept implies that the neighbouring structures can only be removed after the end of
the work period of the structure under consideration, and that the shielding effect is
uniform in plan.
In the UK NA, the fluctuating component of the wind velocity is simulated by a turbulence intensity parameter, I v , whose value for flat terrains can be determined from
one of the several charts provided. Correction factors are also given to calculate the
value for other types of terrain roughness. For sites where orography is important (see
Figure NA.2 of the UK NA), the values of I v must be divided by the orography factor, c o ,
given in Section 4.3.3 of BS EN 1991‐1‐4. The two components of the wind, mean and
fluctuating, are then combined to obtain the maximum peak gust velocity, ˆ
V :
( )
( )
( )
= + ⋅
⋅
v
m
ˆ
1
V z
g I z V z
(8.3)
In the UK NA, a value of peak factor, g, equal to 3.0 is specified. Having determined
the wind velocity, the wind effect on a body (element/structure) can be calculated. The
simplest analysis is to analyse the along wind structural response, with more complex
analyses needed for the across wind and torsional responses [38]. All modern design
codes require the effects of wind on a structure to be accounted for, either by wind
pressures or by wind forces. For mobile communication structures, the latter method is
preferred.
The complex processes and relationships of wind engineering have been translated to
code rules by assuming simplifications. One of the most important assumptions made
in most of modern design codes dealing with wind effects is the quasi‐static hypothesis,
which assumes [35]:
1) the wind turbulence intensity is low;
2) the fluctuations of the effects of the wind action follow the variations of the direction
of the mean wind velocity; and
3) there is a perfect correlation between fluctuating sectional forces.