Civil Engineering Reference
In-Depth Information
intersected by any tornado), then the total number of intersections with
any
tower along
the line per year is given by:
r
t
=
(υwℓ) (L/s)
(13.10)
It should be noted that the rate of intersection is independent of the intersection angle,
β.
Equation (13.10) may also be written as:
r
t
=
n (a/A)N=υaN
(13.11)
where
n
is the number of events per year in an area,
A; a
the area of tornado path; and
N
the number of towers in the area.
Example
Assume
L
=500km;
s=
0.5 km;
ℓ=5
km;
w=
0.1 km;
υ=
10
−4
per km
2
per year. Then,
from Equation (13.10), the number of intersections with this line per year=10
−4
×
0.1×5×(500/0.5)=0.05, i.e. an average of 1 intersection every 20 years.
13.3.2
Downburst risk model
Damage 'footprints' produced by severe thunderstorm downbursts (Section 1.3.5) are
usually wider than those produced by tornadoes. However, the lengths of the damaged
areas produced by downbursts are generally shorter than those of tornadoes. The
increased width usually results in several transmission line spans being enveloped by
damaging winds, and several adjacent towers often fail as a group. The direct wind load
on the conductors themselves is therefore a significant component of the overall wind
load in downburst events. This must be incorporated into a risk model.
Oliver
et al.
(2000) describe a downburst risk model for transmission lines, which
allows the prediction of an event frequency, where an event is the intersection of a region
of wind above a given or design wind speed with a line of some defined length. The
probability of such an event is dependent on:
• the overall length of the line,
L
;
• the relative angle,
θ−φ,
between the direction of the downburst path,
θ,
and the line
orientation,
φ;
• the probability of exceedence of the threshold wind speed of interest,
U,
at any point in
the surrounding region, derived from the anemometer records; and
• the width of the path of winds above the threshold,
w
u
.
The return period,
R
U,L
,
of the event was shown (Oliver
et al.,
2000) to be given by:
(13.12)
