Travanca and André
178
is possible that the highest values of wind effects occur for wind velocities smaller than
the design wind velocity.
Some structures have attached several ancillaries. In these cases, the ancillaries’
arrangements provide a certain degree of internal shielding to the structural elements.
This positive effect is accounted for in BS EN 1993‐3‐1 + UK NA by reduction factors,
K A , used to determine the force coefficients of the structural elements (see Table 8.2).
Disturbed flows occur when wind flows past a bluff body causing the flow to separate
from the surface of the structure rather than follow the body contour. At relatively low
wind velocities, disturbed flows result in spiral vortices that are created periodically and
symmetrically on either side of the body. However, for velocities higher than a limiting
value, the vortices are shed alternatively, that is on one side first then the other. As a
result, alternating low pressure zones are formed on the downstream side of the body
and a fluctuating transverse force is created. This dynamic phenomenon is called vortex
shedding [39,40] and may be significant for very slender and tall monopoles with circular cross‐sections. Galloping is a self‐induced dynamic phenomenon of flexible bodies
and occurs when there are large amplitude lateral or torsional oscillations due to aerodynamic forces that are in‐phase with the motion of the body [39,40]. Guyed masts are
more susceptible to this phenomenon. Flutter is also a self‐induced dynamic phenomenon that occurs when the natural frequencies of the torsional and lateral modes are
very similar [39,40]. A famous example of a structural collapse due to flutter is the
Tacoma Narrows Bridge failure of 1940.
BS EN 1991‐1‐4 [36,37] offers in Annex E expressions for determining the equivalent static wind load distribution due to the resonant response in the crosswind
direction. These loads should also be accounted for in the structural analysis. It is
conservative to simply combine the maximum values of the loads due to the along
wind (see Equation (8.4)) and due to the crosswind. A well‐known rule of thumb is to
determine the fundamental natural frequency of the structure, and if this value is
below 1 Hz, the resonant response may be significant [39,41,42]. Through most monopoles having a fundamental natural frequency lower than 1 Hz, dynamic analysis
should therefore be performed.
In the case of lattice towers, the simplified quasi‐static design procedures are in general conservative. For guyed masts, however, such a procedure may not be appropriate,
as the vibration modes are not well separated and several modes can all contribute significantly to the response of the structure under a fluctuating wind action. Since a full
Table 8.2 Reduction factor, K A , for ancillary items.
Position of
ancillaries
Ancillaries conforming to BS EN
1993‐3‐1, B.2.3(2)
Ancillaries not conforming to BS EN
1993‐3‐1:2006, B.2.3(2), and circular
sections in supercritical flow
Square or rectangular
plan form
Triangular
plan form
Internal to
the section
0.6
0.5
1.0
External to
the section
0.7
0.6
1.0
178
is possible that the highest values of wind effects occur for wind velocities smaller than
the design wind velocity.
Some structures have attached several ancillaries. In these cases, the ancillaries’
arrangements provide a certain degree of internal shielding to the structural elements.
This positive effect is accounted for in BS EN 1993‐3‐1 + UK NA by reduction factors,
K A , used to determine the force coefficients of the structural elements (see Table 8.2).
Disturbed flows occur when wind flows past a bluff body causing the flow to separate
from the surface of the structure rather than follow the body contour. At relatively low
wind velocities, disturbed flows result in spiral vortices that are created periodically and
symmetrically on either side of the body. However, for velocities higher than a limiting
value, the vortices are shed alternatively, that is on one side first then the other. As a
result, alternating low pressure zones are formed on the downstream side of the body
and a fluctuating transverse force is created. This dynamic phenomenon is called vortex
shedding [39,40] and may be significant for very slender and tall monopoles with circular cross‐sections. Galloping is a self‐induced dynamic phenomenon of flexible bodies
and occurs when there are large amplitude lateral or torsional oscillations due to aerodynamic forces that are in‐phase with the motion of the body [39,40]. Guyed masts are
more susceptible to this phenomenon. Flutter is also a self‐induced dynamic phenomenon that occurs when the natural frequencies of the torsional and lateral modes are
very similar [39,40]. A famous example of a structural collapse due to flutter is the
Tacoma Narrows Bridge failure of 1940.
BS EN 1991‐1‐4 [36,37] offers in Annex E expressions for determining the equivalent static wind load distribution due to the resonant response in the crosswind
direction. These loads should also be accounted for in the structural analysis. It is
conservative to simply combine the maximum values of the loads due to the along
wind (see Equation (8.4)) and due to the crosswind. A well‐known rule of thumb is to
determine the fundamental natural frequency of the structure, and if this value is
below 1 Hz, the resonant response may be significant [39,41,42]. Through most monopoles having a fundamental natural frequency lower than 1 Hz, dynamic analysis
should therefore be performed.
In the case of lattice towers, the simplified quasi‐static design procedures are in general conservative. For guyed masts, however, such a procedure may not be appropriate,
as the vibration modes are not well separated and several modes can all contribute significantly to the response of the structure under a fluctuating wind action. Since a full
Table 8.2 Reduction factor, K A , for ancillary items.
Position of
ancillaries
Ancillaries conforming to BS EN
1993‐3‐1, B.2.3(2)
Ancillaries not conforming to BS EN
1993‐3‐1:2006, B.2.3(2), and circular
sections in supercritical flow
Square or rectangular
plan form
Triangular
plan form
Internal to
the section
0.6
0.5
1.0
External to
the section
0.7
0.6
1.0