Engineering Structures 26 (2004) 735744

www.elsevier.com/locate/engstruct

Upgrading of transmission towers using a diaphragm bracing system

F. Albermani a, M. Mahendran b, S. Kitipornchai c,

a
Department of Civil Engineering, University of Queensland, Brisbane, Australia
b
School of Civil Engineering, Queensland University of Technology, Brisbane, Australia
c
Department of Building and Construction, City University of Hong Kong, Hong Kong, China

Received 6 October 2003; received in revised form 11 December 2003; accepted 6 January 2004

Abstract

Many older transmission towers are designed based on tension-only bracing systems with slender diagonal members. However,
the increased demand in power supply and changing global weather patterns mean that these towers require upgrading to carry
the resultant heavier loading. The failure of a single tower can rapidly propagate along the line and result in severe damage that
costs many millions of dollars. Hence, this research project aimed at developing ecient upgrade schemes using diaphragm bra-
cings. Tower strength improvement was investigated by adding a series of diaphragm bracing types at mid-height of the slender
diagonal members. Analytical studies showed that considerable strength improvements could be achieved using diaphragm bra-
cings. They also showed the eects of dierent types of bracings, including those of joining the internal nodes of diaphragm mem-
bers and the location of diaphragms. Experimental studies were undertaken using a tower sub-structure assembly that was
strengthened with a variety of diaphragm bracings under two types of loading. The results conrmed the analytical predictions
and allowed recommendations on the most ecient diaphragm bracing types. An upgrade scheme that used the most ecient dia-
phragm bracing type was successfully implemented on an existing 105 m high TV tower. This paper presents the details of both
the analytical and experimental studies and their results.
# 2004 Elsevier Ltd. All rights reserved.

Keywords: Transmission-towers; Diaphragm bracing; Nonlinear analysis

1. Introduction towers. The failure of one tower can rapidly propagate

along the line through cascading and lead to severe
In the transmission line industry, many older towers damage to the entire power line. The estimated cost
were designed based on tension-only bracing systems of repairing and/or replacing assets for 40 km of a
with diagonal bracing systems that had high slender- double circuit line is over $30 million [4]. A similar
ness ratios of around 250. In recent times, there has situation is evident in telecommunication towers, for
been a signicant increase in the demand for power which the demand for more antennae on top has
supply, and many of these older transmission lines are increased considerably.
required to carry heavier conductors. There have also The failure of these old transmission and telecom-
been many changes in global weather patterns (extreme munication towers could be eliminated through the
winds, heavy ice, etc.). Hence, some of the slender bra- development of a cost-eective structural upgrading
cing diagonals have developed fatigue cracks due to scheme, but this would be a dicult task. The tension
only design makes it dicult to model these towers
cyclic wind loading. All of these factors mean that old
using conventional software. Accurate analysis of the
transmission towers are not only subjected to increased
towers is complicated because the structure is three-
loading, but also to the degradation of some of their dimensional, is comprised of eccentrically connected
critical members. This can lead to the failure of the asymmetric angle section members, and is subject to
complex loadings. The inuences of material and geo-


Corresponding author. Tel.: +852-2788-7609; fax: +852-2788- metric nonlinearities play a very important role in
7612. determining the ultimate behavior of these towers.
0141-0296/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.engstruct.2004.01.004
736 F. Albermani et al. / Engineering Structures 26 (2004) 735744

Kitipornchai et al. [1] and Albermani and Kitipornchai

[2] have developed a compact and practical nonlinear
method for simulating the global structural response of
transmission towers. The program developed,
AKTower, has been used to predict the behaviour of
many full-scale transmission towers, and its predictions
have agreed well with full-scale test results. Hence, the
program was used in this research to investigate suit-
able upgrade schemes based on the concept of adding a
series of diaphragm bracings.
A tower sub-structure with a variety of diaphragms
at mid-height of the slender diagonal members was
considered for this purpose. Analytical studies showed
that considerable strength improvements could be
achieved using diaphragm bracings. They also showed
the eects of dierent types of bracings, including those
of joining the internal nodes of diaphragm members
and the location of diaphragms. Experimental studies
were then undertaken to validate analytical predictions
under dierent types of loading. This paper presents
Fig. 1. Tower sub-structure.
the details of both the analytical and experimental stu-
dies and their results.
was calculated to be 4.35 kN (each P) in bending and
2. Analytical studies 4.38 kN in torsion. This corresponds to critical axial
load in the diagonal of 5.31 kN.
Albermani and Kitipornchai [2] developed a non- The eect of strengthening the diagonal bracings by
linear analysis technique for predicting and simulating adding various types of diaphragms or plane-bracing
the ultimate structural behavior of transmission towers. systems at mid-height (y=H 0:5) was investigated.
This technique accounts for geometric and material The diaphragm types examined in this research are
nonlinear eects, asymmetric section properties, and shown in Fig. 2 and are referred to as types 1 to 3.
eccentricities. The geometric nonlinearity is incorpor- Fig. 3 shows the sub-structure with diaphragm type 1a
ated through initial-stress and deformation stiness implemented. The sub-structure was modeled as a
matrices, and through the continuous updating of the frame structure with the diagonal and diaphragm mem-
structural geometry. Material nonlinearity is accounted bers modeled as truss members, i.e. the member ends
for through a lumped plasticity approach coupled with were modeled as both in-plane and out-of-plane pin-
the concept of yield surface in force space. A predictor- ned. This is consistent with common practice in trans-
corrector solution strategy is used to follow the mission tower structures.
load-displacement response of the structure up to its The eect of joining the internal nodes of the dia-
ultimate capacity. This approach can be applied eec- phragm members at their crossing point was also inves-
tively to predict the response of practical large-scale tigated. A diaphragm such as type 1a, with its members
structures such as transmission towers [3]. The same joined together at the internal node (in this case at the
approach was adopted in analyzing the tower sub- centre of the tower), will be referred to by adding the
structure shown in Fig. 1, and was used to investigate letter c to the type name (1ac in this case). The ratio of
various strengthening schemes. The legs and horizontal the buckling load (both bending and torsion load
members were taken as angle sections (45  45 cases) for the sub-structure with one of the diaphragm
5 mm). Solid circular rods of 16 mm in diameter were types implemented to the buckling load of the original
used for diagonals and diaphragms. This gave a slen- sub-structure (Fig. 1) is given in Table 1. The benecial
derness ratio of 274 for the diagonal members of the eect of adding a diaphragm is clear from this table for
substructure shown in Fig. 1, which is consistent with both bending and torsional loading, under which the
the tension-only design. The sub-structure was sub- buckling capacity increases as much as 1.84.5 times.
jected to lateral loading, P, at two corners, as shown in The eect of introducing diaphragm types 1a, 2a,
Fig. 1. This loading condition will be referred to as the and 3a at various heights (y/H) was also investigated
bending case. A second loading condition was also for bending and torsional loading. Fig. 4 shows the
investigated, and will be referred to as the torsion case. variation of buckling capacity with diaphragm location
For the torsion case, one load P was applied at one for bending loading, while Fig. 5 shows the variation
corner only. The buckling load for this sub-structure for torsional loading.
 F. Albermani et al. / Engineering Structures 26 (2004) 735744 737

Fig. 3. Tower sub-structure with type 1a diaphragm bracing.

shown in Fig. 1 were 877 and 1080 mm, respectively, in

the test structure. The vertical legs and horizontal
members were 45  45  5 mm equal angles. Diagonal
rods of 16 mm in diameter were used in the prelimi-
nary test series, as was assumed in the analytical stu-
dies, but in the nal test series they were 10.3 mm. All
of the members were made of grade 43A steel with a
Fig. 2. Types of diaphragm bracings. design yield stress of 275 MPa.
Both ends of the diagonal members were welded to
short rectangular plates of 12 mm in thickness. This
For bending loading, the buckling capacity is not allowed the diagonal rods to be lapped and bolted to
very sensitive to the diaphragm location for type 2a, the vertical side of the horizontal angles using M10 8.8
but it is quite sensitive for type 3a with an optimum grade bolts and washers. The bolt locations were 20
position of y=H 0:5. The capacity is less sensitive for
diaphragm type 1a with an optimum location of
Table 1
y=H 0:63. Comparison of buckling capacity ratios with various diaphragm
For torsional loading, the buckling capacity is not types (diaphragm at y=H 0:5)
very sensitive to the diaphragm location for type 1a,
Diaphragm type Buckling capacity ratio
but it is quite sensitive for type 2a and to a lesser
extent for type 3a. For the three types, the optimum Bending Torsion
location of the diaphragm is y=H about 0:5. None 1 1
1 1 1.001
1a 2.264 1.815
2 1.822 1.817
3. Experimental studies 2a 2.115 3.904
3 1.444 1.575
3.1. Test structure 3a 3.708 2.789
1c 1.418 1.470
Experimental studies considered the tower sub-struc- 1ac 3.141 4.103
ture that was described in Section 2 and used in the 2c 4.331 4.347
2ac 4.456 4.473
analytical studies. Fig. 6 shows the test structure and
3ac 4.338 4.349
the details of its components. The dimensions B and H
738 F. Albermani et al. / Engineering Structures 26 (2004) 735744

Fig. 4. Buckling load vs. diaphragm location for bending case.

mm above the bottom horizontal edge of the horizon- Fig. 6b shows the details of the diagonal rod to
tal angles. The other end of the diagonal rod was bol- horizontal angle connections.
ted directly to the vertical angle legs at a distance of 35 Analytical studies showed that the xity of base con-
mm above the top of the base plate. Both rods were nections did not aect the buckling of diagonal rods.
placed outside the angle and hence were eccentrically Hence, no attempt was made to create a pinned base
connected. This is similar to the practice used in trans- connection. The vertical angle legs were welded to 12
mission line towers, whereby diagonal angle members mm plates. These plates were then bolted to thicker
are often lapped outside and bolted to horizontal mem- plates that were clamped to the supporting beams. The
base connection used in the test structure as shown in
bers. The inner rod was 8 mm away from the angle
Fig. 6c should therefore be considered as semi-rigid.
face whereas the outer rod was at a distance equal to
To simplify the fabrication of the test structure,
its diameter plus 8 mm from the angle face. The M10
6 mm gusset plates were rst welded to the top of verti-
bolts were wrapped with rubber sleeves and special cal angles as shown in Fig. 6d. The horizontal angles
care was taken to eliminate any friction in these joints, were then simply bolted to the plates on both legs using
i.e. between the end plates and between the bolt and M10 8.8 grade bolts. In this way, the corner connections
rod at the bolt holes. Hence, the connections can were made rigid as was assumed in the analyses.
be considered as in-plane pinned rather than As stated in Section 2, two load cases were con-
out-of-plane pinned. Indeed, they were closer to sidered. In the rst case, referred to as the bending
out-of-plane xed (some buckling modes tended to case, equal horizontal loads were applied at the cor-
indicate that they were not quite xed out-of-plane). ners of the test structure using a spreader beam, as

Fig. 5. Buckling load vs. diaphragm location for torsion case.

 F. Albermani et al. / Engineering Structures 26 (2004) 735744 739

Fig. 6. Details of test structure: (a) Overall test structure; (b) diagonal rod to horizontal angle connections; (c) base connections; (d) corner con-
nections; (e) load application for the bending case; (f) load application for the torsion case.
740 F. Albermani et al. / Engineering Structures 26 (2004) 735744

shown in Fig. 6e. A horizontal load was applied at 3.3. Results and discussion
only one corner in the second load case, referred to as
the torsion case (see Fig. 6f). The load distributors Table 2 compares the test results with the analytical
predictions. The diagonal and diaphragm members in
were located along the centroidal line of the horizontal
the analysis were modeled with end conditions as in-
angles. A 50 kN load cell was used in series with the
plane pinned and out-of-plane xed. This allowed
actuator to measure the applied load. accurate simulation of the test structure conditions
In most tests, at least a pair of strain gauges was reported in the previous section. The use of these end
used in the diagonal compression rods at diametrically conditions mean that the analytical results presented in
opposite locations (see Fig. 6a). This allowed the moni- Table 2 are dierent to those reported in Table 1 and
toring of axial strains in these rods during the test, i.e. Figs. 4 and 5 in Section 2.
using the average of the two strain gauge readings. Due The results of the preliminary test series using the
to the eccentric connections, there was noticeable out- larger 16 mm diagonal rods are also presented in
of-plane bending in the rods before buckling. To moni- Table 2. However, the results for the cable diaphragm
tor this eect, two pairs of strain gauges were used in system are not presented because the cable system was
some tests at diametrically opposite locations. These unable to provide much strength improvement. There
strain readings not only allowed the monitoring of were also practical diculties in installing the cable
system.
axial strain, but also the in-plane and out-of-plane ex-
There was considerable eccentricity in all of the con-
ural strains caused by eccentric connections.
nections, and with the unavoidable imperfections in the
test structure and its components, both the diagonal
3.2. Test program compression rods were bending either in-plane or out-
of-plane before they reached buckling. In some cases,
The rst test series was undertaken using 16 mm
they were bending about both axes. Attempts were
diagonal rods and without any diaphragm bracings. made to minimize these eects so that such premature
This was to verify the test set-up and loading process. bending and yielding could be eliminated. Despite these
In all the tests, the test structure was loaded until one attempts, the diagonal compression rods were always
or both the slender diagonal compression members bending in plane or out-of-plane during the tests.
buckled. Following this, dierent types of diaphragm Therefore, as indicated in Section 3.1, strain gauge
bracings described in Fig. 2 were added one by one and measurements were used to determine the point of
tested until buckling. With the addition of diaphragm buckling of the diagonal rods. The buckling load was
bracings, the buckling capacity of diagonal rods calculated based on the measured maximum axial com-
increased considerably. Hence, the test structure could pression strain in the rods (average of the strain read-
not be loaded until buckling as there were local failures ings from the two strain gauges located diametrically
opposite on the rods). The axial compression strain
at connections. Hence, the diameter of the diagonal
remained constant or decreased after the buckling of
rods was reduced to 10.3 mm and the tests were
the compression rods. The test structure was able to
repeated. The corresponding diaphragm bracings were carry further load as the load was now shed to the ten-
only 5.3 mm in diameter, although in the preliminary sion diagonal rods. Table 2 reports the test buckling
test series both the diagonal and diaphragm bracing strain and load determined in this manner. In some
rods were 16 mm in diameter. In some tests, a cable tests, both diagonal compression rods buckled, and the
diaphragm system was used instead of rods because lower buckling load is reported in Table 2.
analyses showed very small forces in the diaphragm Table 2 also reports the applied load P at each cor-
bracing members. The same angle members were used ner of the test frame at the point of buckling. However,
in all of the tests because they were only stressed to this was not used in the comparisons with analytical
levels well below their yield stress. Strain gauge and predictions for the following reason. There appeared to
displacement transducer readings were monitored be some rigid frame action in the test structure even
with the presence of diagonal rods, in particular when
throughout all of the tests. Fig. 7 shows the test struc-
smaller rods (10.3 mm) were used for the bending case.
ture with various diaphragm bracings. The diaphragm
Hence, there was limited correlation between the mea-
bracing rods were simply welded to the diagonal rods
sured load in the rods and the applied load P. How-
at mid-height. The internal connections of the dia- ever, this situation was improved in the cases of larger
phragm were also welded, but in some cases they were diagonal rods (16 mm) and the torsion case.
joined together with steel ties. Table 2 presents the During the initial tests, considerable friction was
details of the tests that were carried out in this observed at the diagonal rod to angle member connec-
research. tions. However, the use of rubber sleeves and other
 F. Albermani et al. / Engineering Structures 26 (2004) 735744 741

Fig. 7. Diaphragm bracings used in the test structure: (a) Type 1a diaphragm; (b) type 2 diaphragm; (c) type 2a diaphragm; (d) type 3 dia-
phragm; (e) type 3ac diaphragm.

means to reduce the friction at these connections The diagonal rods always buckled in-plane in these
appeared to have worked, as seen from the good agree- tests, as predicted by analysis (see Fig. 8a). Therefore,
ment between test and analytical results obtained in the the test conditions for the connections can be con-
case of test structure without any diaphragm bracings. sidered equivalent to in-plane pinned.
742 F. Albermani et al. / Engineering Structures 26 (2004) 735744

Table 2
Details of test program and results

Diag. Diaph. Load case Applied Test bucklinga Analysis Test Pcr Buckling mode Pcr/Pcrnb
Rod Type load P
Strain Load Pcr Pcr Analysis Pcr Test Analysis Test Analysis
(mm) (kN)c
(x106) (kN) (kN)
16.0 None Bending 5.75 146 5.87 5.31 1.11 IP-1 IP-1 1.00 1.00
16.0 1 Bending 5.75 5.31 IP-1 IP-1 1.00 1.00
16.0 1a Bending >13.5 12.01 OP-2 IP-1 >2.35 2.26
16.0 None Torsion 5.90 5.34 IP-1 IP-1 1.00 1.00
16.0 1 Torsion 5.70 5.34 IP-1 IP-1 0.97 1.00
16.0 1a Torsion >15.0 9.70 OP-2 IP-1 >2.54 1.82
10.3 None Bending 1.78 51 0.85 0.81 1.05 IP-1 IP-1 1.00 1.00
10.3 1ac Bending 4.52 175 2.91 3.26 0.89 OP-2/IP-2 IP-2 3.42 4.02
10.3 2 Bending 3.06 131 2.18 2.34 0.93 OP-2/IP-2 IP-2 2.56 2.89
10.3 2c Bending 4.50 193 3.21 3.26 0.98 OP-2/IP-2 IP-2 3.78 4.02
10.3 2ac Bending 4.53 199 3.31 3.28 1.01 OP-2/IP-2 IP-2 3.89 4.05
10.3 3 Bending 4.56 155 2.58 3.26 0.79 OP-2/IP-2 IP-2 3.04 4.02
10.3 3a Bending 4.57 174 2.90 3.28 0.88 OP-2/IP-2 IP-2 3.41 4.05
10.3 3ac Bending 4.80 172 2.87 3.26 0.88 OP-2/IP-2 IP-2 3.38 4.02
10.3 None Torsion 2.83 53 0.88 0.81 1.09 IP-1 IP-1 1.00 1.00
10.3 1a Torsion 5.31 157 2.62 3.26 0.80 OP-2/IP-2 IP-2 2.98 4.02
10.3 1ac Torsion 5.04 150 2.50d 3.25 0.77 OP-2/IP-2 IP-2 2.84d 4.01
10.3 2ac Torsion 5.04 187 3.11 3.25 0.96 OP-2/IP-2 IP-2 3.53 4.01
10.3 3 Torsion 5.05 158 2.63 3.26 0.81 OP-2/IP-2 IP-2 2.99 4.02
10.3 3a Torsion 5.05 170 2.83 3.26 0.87 OP-2/IP-2 IP-2 3.22 4.02
10.3 3ac Torsion 5.60 169 2.81 3.25 0.86 OP-2/IP-2 P-2 3.19 4.01
a
Test buckling load Pcr is the smaller buckling load of the two diagonal rods and is based on measured axial strains.
b
Pcr/Pcrn is the ratio of the buckling loads of diagonal rods with and without diaphragm braces.
c
P is the applied load at each corner of the frame at the point of buckling of diagonal rods.
d
Diaphragm member was deformed and hence did not adequately restrain the diagonal compression member.

With the addition of diaphragm bracings, consider- range of 2.563.89 (i.e. a 156289% increase). They
able strength improvements were obtained, as seen in also agreed reasonably well with corresponding analyti-
the results reported in Table 2. Depending on the dia- cal predictions (2.894.05). The analytical predictions
phragm bracing type used, the buckling loads of diag- were based on an idealized test structure and simplied
onal compression rods increased by a factor in the end conditions, hence the reason for some of the dier-

Fig. 8. Buckling mode of diagonal rods: (a) Without diaphragm bracing; (b) With diaphragm bracing.
 F. Albermani et al. / Engineering Structures 26 (2004) 735744 743

ences. The buckling mode appeared to be the in-plane

second mode as predicted by the analysis, but often it
was a mix of out-of-plane and in-plane second modes,
as shown in Fig. 8b. However, experiments were able
to conrm the strength improvements due to the use of
diaphragm bracings as predicted by the analysis. The
reasonable agreement between the test and analytical
results as shown in column 8 of Table 2 mean that
analysis alone can now be used to further study the
behaviour of tower structures with diaphragm bra-
cings.
As predicted by the analysis (see Table 1), connect-
ing the internal nodes of the diaphragms led to con-
siderable strength improvements, particularly when the
pinned connection (both in-plane and out-of-plane) of
diagonal bracing members was used. Most importantly,
it provided a more rigid structure with all of the diag-
onal rods tied together very eectively.
Based on the test results and the experience in instal-
ling the various diaphragm bracings, it can be con-
cluded that diaphragm type 2c is the most ecient
system. Type 1ac also performed well in the test series,
but some of the earlier analytical studies showed that
they might not provide higher strength improvements
for certain loading and end conditions. Type 3 was not
found to be as eective as Type 2c. Types 2ac and 3ac
were also very eective from the strength improvement
point of view, but not from the practical point of view.
The additional diaphragm rods and connections that
are needed in these two types of diaphragms cannot be
justied when type 2c can provide similar strength
improvements.

Fig. 9. Upgrading of TV tower using type 2c diaphragm bracing.

4. Practical implementation

The upgrading scheme presented in the previous sec- Nonlinear analysis of the tower was undertaken
tion was implemented in strengthening a 30-year-old using the program AKTower. Twenty-ve load cases
TV tower. Fig. 9a shows an isometric view of this that account for the new antennas, various directions
tower. The lower part of the 105 m high tower has a of wind and ice were considered in the analysis. In
15  15 m square-base, while its upper 21 m is a tri- these load cases, the ultimate design loads in the trans-
angular mast. The self-weight of the tower is about 687 verse and longitudinal directions of the tower are of
kN. It was intended to add a number of new antennas the order of 1920 kN and 700 kN acting along the
to this tower, which was already displaying excessive height of the tower. Based on the nonlinear analysis
deection and rotation that was aecting its trans- results, a Type 2c diaphragm bracing system (see Fig. 2)
mission performance. The authority responsible for the was proposed at a number of levels along the tower, as
tower was planning to replace many of the diagonal shown in Fig. 9b. This strengthening scheme used less
members with heavier sections. Most of these diagonal steel and was easier to implement than the replacement
members were composed of back-to-back double of the existing diagonal bracings, and made a signi-
angles with typical sections being 127  89  9:5 mm cant improvement to the tower response in terms of
(5  3:5 3=8 inch) or 76  64  4:8 mm both strength and stiness. Fig. 10 compares the tower
(3  2:5  3=16 inch). The members were made of steel deected shape at collapse under one of the load cases.
with a yield stress of 250 or 350 MPa. A typical length The upper triangular mast of the tower is not shown in
of these diagonals is in the range of 7 to 11 m with a this gure so that it can oer a clearer comparison of
number of secondary bracings along the length, as the deected shapes. Fig. 10a shows the original tower
depicted in Fig. 9a. at collapse (load factor of one) and Fig. 10b shows the
744 F. Albermani et al. / Engineering Structures 26 (2004) 735744

5. Conclusions

This paper has presented the details of an investi-

gation of the upgrading of transmission towers using
diaphragm bracings. The investigation included analyti-
cal and experimental studies of a tower sub-structure
assembly that was strengthened with a variety of dia-
phragm bracings under two types of loading. The ana-
lytical and experimental results agreed reasonably well
and showed that simple diaphragm bracing systems can
be very eectively used in the upgrading of older trans-
mission towers. Diaphragm Type 2c should be used for
this purpose, as it appears to be the most ecient bra-
cing system. The numerical program AKTower can con-
rm the use of a suitable diaphragm bracing system,
depending on the tower structure and loading con-
ditions. An upgrade scheme using diaphragm bracings
was successfully implemented on an existing 105 m high
TV tower. This scheme used less steel than the replace-
ment of the existing diagonal bracings, was easier to
implement in practice, and led to improved tower per-
formance. Although no dynamic assessment of a tower
retrotted with diaphragm bracings was conducted, it is
expected that such retrotting will improve the towers
dynamic response since it enhances the stiness without
too much increase in mass.

Fig. 10. Deected shapes of original and upgraded TV towers at

collapse. References

[1] Kitipornchai S, Albermani F, Chan SL. Elasto-plastic nite

upgraded tower under the same load case at collapse, element models for angel steel frames. J Struct Eng, ASCE
which takes place at a load factor of 1.37. With this 1990;116(10):256781.
upgrading scheme implemented, the tower ultimate [2] Albermani FGA, Kitipornchai S. Non-linear analysis of trans-
load under this load case increased by nearly 40%, with mission towers. Eng Struct 1992;14(3):13951.
[3] Albermani F, Kitipornchai S. Numerical simulation of structural
much inhibited deformation in comparison with the
behaviour of transmission towers. Thin-Wall Struct 2003;41(23):
original tower. This upgrading was successfully imple- 167177.
mented and the tower has been functioning satisfac- [4] Behncke R. A strategy for major overhead line failures. Power
torily for over 3 years. Engineers Line Conference, Sun Valley, Idaho, March 2002.