Dq32729732

A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 3, Issue 2, March -April 2013, pp.729-732
729 | P a g e
Optimal Bracing System for Steel Towers
A. Jesumi*, M.G. Rajendran**
*(PG student, School of Civil Engineering, Karunya University, Coimbatore –641 114)
** (Professor, School of Civil Engineering, Karunya University, Coimbatore –641 114)
ABSTRACT
The major system providing lateral load
resistance in steel lattice towers is bracing system.
There are different types of bracing systems for
steel lattice towers. The heights of these towers
vary from 20 to 500 meters, based on the practical
requirements. This study has focused on
identifying the economical bracing system for a
given range of tower heights. Towers of height 40m
and 50m have been analyzed with different types
of bracing systems under wind loads. The diagonal
wind has been found to be the maximum for
towers. The optimal bracing system has been
identified and reported.
Keywords - bracing system, steel lattice towers,
wind analysis
1. INTRODUCTION
Towers are tall steel framework construction
used for different purposes such as communication,
radio transmission, satellite receptions, air traffic
controls, television transmission, power transmission,
flood light stands, oil drilling masts, meteorological
measurements, etc. The present paper discusses
microwave transmission towers. Lattice towers act as
vertical trusses and resists wind load by cantilever
action. The bracing members are arranged in many
forms, which carry solely tension, or alternatively
tension and compression. The bracing is made up of
crossed diagonals, when it is designed to resist only
tension. Based on the direction of wind, one diagonal
takes all the tension while the other diagonal is
assumed to remain inactive. Tensile bracing is smaller
in cross-section and is usually made up of a back-to-
back channel or angle sections. The bracings behave
as struts, when it is designed to take compression.
One of the most common arrangements is the cross
bracing. The most significant dimension of a tower is
its height. It is normally several times larger than the
horizontal dimensions. The area which is occupied at
the ground level is considerably limited and so,
slender structures are commonly used.
The tapered part of the tower is advantageous with
regard to the bracing, as it reduces the design forces.
The greater the height of the tower, greater will be the
distance it can transmit radio signals. Towers are
classified as Self-supporting towers and Guyed
towers. Self-supporting towers are generally preferred
since they require less base area. Towers are subjected
to gravity loads and horizontal loads.
Bracings hold the structure stable by transferring the
loads sideways (not gravity, but wind or earthquake
loads) down to the ground and are used to resist
lateral loads, thereby preventing sway of the structure.
Bracing increases the resistance of the structure
against side sway or drift. The higher the structure,
the more it is exposed to lateral loads such as wind
load, since it has higher tendency to sway. If the
bracing is weak, the compression member would
buckle which leads to failure of the tower. Diagonal
braces are efficient elements for developing stiffness
and resistance to wind loads. There are different types
of bracing systems in common use such as Single
diagonal bracing, double diagonal (X-X) bracing, X-B
bracing, XBX bracing, arch bracing, subdivided V
bracing, diamond lattice system of bracing, K, Y, W,
X bracings, etc.
K. Agarwal and K. Garg (1994) have assessed free-
standing lattice towers for wind loads. It has been
found that large variations have occurred in wind
loads on towers and there are several gaps in the
present recommendations which are to be answered
by more rigorous wind tunnel investigations. The
behavior of cross-bracings in latticed towers was
studied by Alan R. Kemp and Roberto H. Behncke
(1998). The cross bracings have shown complex
behavior and the number of bolts in the connection of
the bracing to the main legs have been apparent in the
results. M.Selvaraj, S.M.Kulkarni and R.Ramesh
Babu (2012) have investigated on the behavior of
built up transmission line tower from Fiber
Reinforced Polymer (FRP) pultruded sections. They
have discussed experimental studies carried out on an
X-braced panel of transmission line tower made from
FRP pultruded sections. The upgradation of
transmission towers using a diaphragm bracing
system was experimented by F. Albermani, M.
Mahendran and S. Kitipornchai (2004). Their results
showed that considerable strength improvements were
achieved with diaphragm bracings. The upgrading
system using the most efficient diaphragm bracing
type has been successfully implemented on an
existing 105 m-height TV tower. F. Al-Mashary, A.
Arafah and G. H. Siddiqi [5] have investigated on the
effective bracing of trussed towers against secondary
moments. The study showed that improper bracing
configuration of the main and/or secondary braces
induced high secondary moments. Thus, it is
necessary to identify the economical bracing system
for a given range of tower height.

730 | P a g e
The main purpose of the paper is to demarcate the
economical bracing system of steel lattice towers.
Investigations carried out to analyze towers with
different heights and different bracing configurations
have been presented. The towers have been analyzed
for wind loads with STAAD Pro., to compare the
maximum joint displacement of each tower.
Optimized design has been carried out to estimate and
to compare the weight of each tower. The results have
been used to identify the economical bracing system
for a given range of height of towers.
2. GENERAL CONSIDERATIONS
In this study, five steel lattice towers with
different bracing configurations such as the X-B,
single diagonal, X-X, K and Y bracings have been
modeled for a given range of height. The heights of
the towers are 40m and 50m with a base width of 2m
and 5m respectively. The tower of height 40m has 13
panels and the tower of height 50m has 16 panels. 70-
72% of the height is provided for the tapered part and
28-30% of the height is provided for the straight part
of the tower.
3. TOWER ANALYSIS
The towers have been modeled using
geometric coordinates. The member property was
assigned to each member of the structure. The leg,
diagonal bracings and horizontal bracings are
provided with angle sections. Elastic modulus,
Poisson’s ratio, density, alpha and damping are the
material properties used for the analysis. A
hemispherical dome was assumed to be mounted at
the top panels of the towers. The towers were
analyzed considering it to act as a space structure with
pin joints, for dynamic wind loading and optimized
design was carried out. The displacements and
weights of the towers obtained were compared to
arrive at an optimal solution.
Fig. 1: Models of the towers with different bracing
configurations for 40m height
The elevations of the towers of height 40m and 50m
with different bracing configurations are shown in
fig.1 and 2 respectively.
Fig. 2: Models of towers with different bracing
configurations for 50m height
4. LOADING
The loads act in three mutually perpendicular
directions such as vertical, normal to the face and
parallel to the face of the tower. The loads applied to
the towers are based on the codal provisions in IS:
875-1987 part 3. Since towers are tall and flexible, it
is critical under wind load. The wind load is
determined by dividing the tower into different panels
of equal heights. The calculated wind load is
transferred on each joint of the exposed face of the
tower. For square steel lattice towers, the maximum
load occurs when wind blows diagonally. Therefore,
IS: 875-1987 part 3 recommends the diagonal wind to
be 1.2 times the wind blowing normal to the face. The
vertical loads act centrally and are distributed equally
among the four legs. Bending moments produce an
equal compression in the two legs of one side, and
equal tension in the two legs of the other side when
the wind is considered acting in any one direction.
The shear forces are resisted by the horizontal
component of the leg forces and the brace forces.
Thus, the taper has a major influence on the design of
the bracing.
The design wind pressure is calculated at increasing
heights of a tower from the mean ground level with
the following equation:
PZ = 0.6VZ
2
(1)
Where,
PZ is the design wind pressure in N/m2
VZ is the design wind speed in m/s
The design wind speed is obtained taking into
account, factors such as risk coefficient, terrain
roughness, height, size of the structure and local
topography. It is expressed as:
VZ = Vbk1k2k3 (2)
Where,
Vb is the basic wind speed in m/s

731 | P a g e
k1 is the risk coefficient
k2 is the terrain, height and structure size
factor
k3 is the local topography factor
The basic wind speed for the proposed location was
39m/s. The wind load is the product of the dynamic
wind pressure, the overall force coefficient and the
effective exposed area of the tower. The force
coefficient for the exposed surface depends on the
solidity ratio. It is expressed as:
F = CF.A.PZ (3)
Where,
F is the force acting on the structure
CF is the force coefficient
A is the exposed surface area of the structure
PZ is the design wind pressure
5. LOAD COMBINATION
The towers have been analyzed for the following
load combinations according to IS: 875-1987 part 3.
1. DL + WL with wind blowing normal to the
face of the tower
2. DL + WL with wind blowing diagonal to the
face of the tower
3.
6. ANALYSIS
The steel lattice towers have been analyzed,
idealizing it as a 3D truss as per IS: 800-2007, with
various bracing configurations. The dead loads acting
on the tower are self weight of the tower and self
weight of antenna. The wind loads are considered
acting both normal and diagonal to the face of the
tower. Wind loads have been computed by MS Excel
and have been incorporated in the analysis done by
STAAD Pro.V8i. The joint displacement with respect
to normal wind and diagonal wind has been obtained
from the analysis for each tower and has been
tabulated and shown in table 1. The sizes of leg and
bracing members have been checked for the
maximum forces computed from the analysis and
optimized designed has been done as per IS: 800-2007
and the weight of each tower has been noted and
shown in table 2.
7. RESULTS AND DISCUSSION
Table 1: Maximum joint displacement of the tower
Table 2: Results showing the weight of the tower
0
50
100
150
200
40 50
Weightofthetower
Height of the tower
X-B
Single Diagonal
X-X
K
Y
Fig. 3: Weight of each tower
From the above results in table 2 and fig. 3, it has
been found that Y bracing is the most appropriate
arrangement of bracing system that resists lateral
loads for the given range of heights of the towers, as it
shows comparatively lesser weight than the other
bracing systems. The demarcated economical bracing
system is shown in fig. 4.
Fig. 4: The economical bracing system of tower
The displacement undergone by the tower is shown in
fig. 5.

732 | P a g e
Fig. 5: Displacement of the tower
8. CONCLUSION
In this paper, analytical studies have been
presented to find the most appropriate arrangement
and cost-effective bracing system of steel lattice
towers for the effective resistance against lateral
forces. The joint displacement and weights are the
significant parameters obtained from the analysis.
However, there is no sufficient data regarding the
permissible displacement for towers. From the results
obtained, Y bracing has been found to be the most
economical bracing system up to a height of 50m.
Further, the study will be carried out for towers of
greater heights.
ACKNOWLEDGEMENT
The authors thank the management of Karunya
University for having provided the necessary facilities
to carry out this work.
REFERENCE
[1] K. Agarwal and K. Garg, Wind Load
Assessment on Free Standing Lattice Towers,
Indian Institute of Engineers Journal, vol 75,
November 1994, pp. 171-177.
[2] Alan R. Kemp and Roberto H. Behncke,
Behavior of Cross-Bracing in Latticed Towers,
Journal of Structural Engineering, April 1998,
pp. 360-367.
[3] M.Selvaraj, S.M.Kulkarni and R.Ramesh Babu,
Behavioral Analysis Of Built Up Transmission
Line Tower From FRP Pultruded Sections,
International Journal of Emerging Technology
and Advanced Engineering, Volume 2, Issue 9,
September 2012, ISSN 2250-2459.
[4] F. Albermani, M. Mahendran and S.
Kitipornchai, Upgrading of Transmission
Towers Using a Diaphragm Bracing System,
Engineering Structures, 26, pp. 735-744.
[5] F. Al-Mashary, A. Arafah and G. H. Siddiqi,
Effective Bracing of Trussed Towers against
Secondary Moments,
faculty.ksu.edu.sa/2639/Publications%20PDF/T
OWER-BR.DOC.
[6] Indian Standard Code of Practice for design
loads (other than earthquake) for buildings and
structures IS: 875-1987, part 3 Reaffirmed 1997
Second Revision), 1989. Bureau of Indian
Standards, New Delhi.
[7] Indian Standard Code of Practice for design
loads (other than earthquake) for buildings and
structures IS: 875-1987, part 5 Reaffirmed 2008
Second Revision), 1988. Bureau of Indian
Standards, New Delhi.

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Dq32729732