DYNAMIC ANALYSIS OF 4-LEGGED STEEL …iaeme.com/MasterAdmin/uploadfolder/IJCIET_10_01... · Shwetha Shetty M R, Anusha M, Ashwini A and Rajiv T 1536 [email protected] 1. INTRODUCTION

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International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 01, January 2019, pp. 1535-1550, Article ID: IJCIET_10_01_141

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=01

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

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DYNAMIC ANALYSIS OF 4-LEGGED STEEL

TELECOMMUNICATION TOWER

Shwetha Shetty M R

Assistant Professor, Department of Civil Engineering, Sri Venkateshwara College of

Engineering, Bengaluru, India

Anusha M



Ashwini A



Rajiv T

Assistant Professor, Department of Civil Engineering, Sapthagiri College of Engineering,

Bengaluru, India

ABSTRACT

The four legged self-supporting towers are widely used worldwide for the

telecommunication purposes. The communication industries have seen a tremendous

increase in last few years which have resulted in installation of large number of towers

to increase the coverage area and network consistency. These are lifeline structures;

they play a significant role in wireless communication network. Hence failure of such

structures in a disaster like wind and earthquake is a major concern. Therefore utmost

importance should be given in considering all possible extreme conditions for

designing these towers.

The design of these steel telecommunication towers are done by the STAAD.PRO,

for this “automatic selection of members based on reanalysis procedure with fixed

group select optimized section” method is adopted.

Keywords- Bracings, Natural frequency, STAAD Pro, Response spectrum analysis,

Telecommunication towers

Cite this Article: Shwetha Shetty M R, Anusha M, Ashwini A and Rajiv T, Dynamic

Analysis of 4-Legged Steel Telecommunication Tower, International Journal of Civil

Engineering and Technology, 10(01), 2019, pp. 1535–1550

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=01

Shwetha Shetty M R, Anusha M, Ashwini A and Rajiv T


1. INTRODUCTION

Telecommunication towers are tall structure usually designed for supporting parabolic

antennas which are normally used for sending radio signals, also used for microwave

transmission for communication, and for television signals to remote places and they are

installed at a specific height. This thesis concentrates on the study of “self-supporting towers”.

Self-supporting telecommunication towers are classified as three-legged or four-legged space

truss structures of varying heights and base widths. These towers consist of main legs,

horizontal and transverse bracings. The main legs are typically composed of 90° angles (in

four-legged towers), 60'xhifflerized or cold-formed angles (in three-legged towers), or tubular

hollow sections. Various bracing patterns are used but the most common ones are the chevron

and the cross bracing. It is a challenging job for structural engineer to design and construct a

telecommunication tower to resist vertical loads (support platform, steel ladder and antenna

loads) in open weather with high degree of reliability.

Due to the relative small weight of these towers and having wide-area components at the

top of them like dishes, the major load on them is generally wind load. Seismic design of

communication towers is important in the earthquake vulnerable areas. Since more than half of

the Indian Sub-continent is prone to moderate to severe earthquakes it has become more

important to design the communication systems for seismic safety also.

1.1. Classification of towers

Towers are classified into three major groups based on the structural action. They are

1) Monopoles

2) Guyed towers

3) Self-supporting towers

Monopole-It is single self-supporting pole. Monopoles are designed for use with cellular,

microwave, broadcast, and other applications. It is generally placed over roofs of high raised

buildings, when number of antennae required is less or height of tower required is less than

9m. Figure 1-2(a) shows a monopole.

Guyed Towers-The guyed towers provide height at a much lower material cost than the

self-supporting towers due to the efficient use of high-strength steel in the guys. The guyed

towers are normally guyed in three directions over an anchor radius of typically 2/3 of the tower

height and have a triangular lattice section for the central mast as shown in figure 1-2(b).

Self-Supporting Towers-The towers that are supported on ground or on buildings are called

as Self-supporting towers. Though the weight of these towers is more they require less base

area and are suitable in any situations. Most of the Power Transmission, microwave, TV, and

flood light towers are self-supporting towers. The figure 1-2(c) shows a typical self-supporting

tower.

Mainly six types of bracing systems are seen in tower structures, namely “K, XX, XB, Y,

W and Arch bracing” system. The XX bracing system consists only X bracing, and no

horizontal member. Therefore it is statically determinate for each panel. XB bracing system

consists, X bracing with horizontal member. The horizontal members are the redundant

members and carry only nominal forces, hence it is statically indeterminate.

K and Y bracing systems are statically determinate and they provide better head rooms.

Hence they are used in the panels next to ground. W bracing system is a kind of overlapping

panels, and it is also statically determinate. An arch bracing configuration can be adopted in

wide panels.

Dynamic Analysis of 4-Legged Steel Telecommunication Tower


To study the effective and economic performance of bracing system, in this dissertation the

concept of “Combined or Hybrid models” are considered. The different types of bracing

systems are used in the tapering and straight portion of the tower.

In this dissertation mainly three types of bracing systems are used with different

combination, namely K, XX and XB bracing system shown in Figure 1-3(a), 1-3(b) and 1-3(c)

respectively.

(a) K bracing b) XX bracing (c) XB bracing

2. PROJECT SPECIFICATION

2.1. Objective

1) To generate 3D frame model of telecommunication tower using FE software to

carry out modelling and analysis.

2) To study the effects on telecommunication tower due to change in tower parameters,

like tower height, base width and different types and combination of bracings used.

3) To study the effect of wind load on telecommunication tower structures for different

wind zones as per Indian Standard code of practice, IS875 (part-3): 1987 and IS802

(part-1/sec-1): 1995.

4) To study the variation of natural frequency using the concept of modal analysis for

the tower models.

5) To study the seismic behaviour on telecommunication tower structures, by

equivalent static and response spectrum analysis, for all four different seismic zones

of India as per Indian Standard code of practice IS1893 (part-1): 2002.

2.2. Methodology

Modelling of 3D frame telecommunication tower structure is done using the software for five

different types of combination of bracings in three different heights 30m, 45m and 60m with

the base width of6m, 7m, and 8m respectively. The model descriptions with the bracing

arrangements are given in table 2-1



Table 2-1 Model descriptions with the bracing arrangements

Sl. No. Model Tapered portion Straight portion

1 M1 K-XX K XX

2 M2 XX-K XX K

3 M3 K-XB K XB

4 M4 XB-K XB K

5 M5 XB-XX XB XX

Calculation of the wind load is done as per Indian code of practice, IS-875 part -3:1987 &

the gust factor is considered for wind load calculation as per IS-802 Part 1/ sec1: 1995. Both

gust wind (θ=00) and cross wind (θ=450) is considered in the wind analysis on

telecommunication towers. These calculated loads are applied on the structures and its effects

are studied.

The seismic analysis is carried out by using equivalent static and response spectrum

analysis for all zones as per IS 1893 (part 3)-2002 code of practice. The modal analyses of all

the models are carried out to get the natural frequencies and mode shapes. Then equivalent

static analyses of all the structures are carried out and the results are obtained after that the

response spectrum analysis is carried out to know its effect on the structure.

Natural frequencies and joint displacements obtained from the FE analysis for all the

models are tabulated, compared and the conclusions are drawn.

2.3. Tower details

Towers under study are four legged self-supporting telecommunication towers. The parameters

considered are,

• Different heights and base widths

• Different types and combination of bracings

• Different types of load conditions acting on it.

2.3.1. Tower configuration

The lattice tower is designed for three different heights of 30m, 45m and 60m with the base

width 6m, 7m and 8m respectively. The towers are provided with different types and

combination of bracings. All the support conditions applied to the models in the analysis are

fixed.

A platform load of 0.82kN/m2 is applied at 26m, 41m, and 56m respectively for 30m, 45m

and 60mtower. The weight of the ladder and cage assembly is assumed to be10% of total

weight. The antenna loads are summed up and distributed evenly to the nodes at the considered

heights

The angle properties assigned to all the tower structures are given in the table 2-2

Table 2-2 Member details of the towers

Sl.no. Height

Bracing System Section 30m 45m 60m

1 0-12m 0-16m 0-24m

Bottom leg ISA200X200X25

2 Bracing ISA150X150X18

3 12-20m 16-33m 24-46m

Upper leg ISA200X200X15


5 20-30m 33-45m 46-60m

Top leg ISA110X110X12




2.3.2. Load combinations

The following load combinations considered for the analysis of the telecommunication towers:

• Dead Load + Live Load

• Dead Load + Live Load + Wind Load at θ = 00

• Dead Load + Live Load + Wind Load at θ = 450

• Dead Load + Live Load + Equivalent Static Load

• Dead Load + Live Load + Response Spectrum

2.4. FE ANALYSIS AND DESIGN OF TOWER

Tower model is considered for the FE analysis with the different combination of bracing

system. Models are analysed considering wind and earthquake loads with the help of

STAAD.PRO. Mainly five types of models are created in each tower; the models are of

different combination of bracings. Modal analysis is carried out on the developed FE model to

obtain the natural frequency and mode shape. This is followed by equivalent static and response

spectrum analysis using the generated response spectrum plot for all the zones as per IS 1893-

2002 to obtain the maximum forces on the structure.

The various parameters considered for the 30m tower model are given in the table2-3.

Table 2-3 Details of 30m tower models

Parameters Model(m)

Height of tower 30

Height of straight portion at top of tower 10

Height of inclined portion 20

Base width 6

Top width 2

Number of 4m high panels 5

Number of 2m high panels 5

Geometry of the 30m tower-The geometry of the structure with the nodes and beams display

for all five models namely M1: (K-XX), M2: (XX-K), M3: (K-XB), M4: (XB-K) and M5:

(XB-XX) are shown in the Figure 2-4-1. The details of the nodes and the beams of all the

modes are tabulated in the table 2-2.



Figure 2-4-1 geometry of the 30m M1: (K-XX), M2: (XX-K), M3 :( K-XB), M4: (XB-K) and M5:

(XB-XX) tower respectively.

Table 2-4 Details of Nodes and Members in 30m tower models

Hei

gh

t

of

To

wer

(

m)

Model Base width

(m)

Top width

(m)

Number of

Nodes

Number of

Elements

30

M1 K-XX 6 2 64 164

M2 XX-K 6 2 64 164

M3 K-XB 6 2 64 184

M4 XB-K 6 2 64 184

M5 XB-XX 6 2 44 144

FE analysis results

2.4.1 Wind along x direction (θ = 00) on 30m tower

Figure 2-4-2 Definition of along wind load (θ = 00) for 30m tower M1: (K-XX)



2.4.2 Cross wind (θ = 450) on 30m tower

In the similar pattern the cross wind is applied to the structure as nodal load in X and Z

both direction, as shown in figure 2-4-2

Joint displacements obtained as a result of wind analysis. The results obtained from normal

wind and cross wind analysis are individually tabulated and comparison is made between two

cases. The joint displacements in mm are tabulated in table2-4 and 2-5.

Table 2-4 Comparison of Joint Displacements of 30m Tower in mm (θ = 00)

Hei

gh

t

of

To

wer

(m

)

No

de

hei

gh

t (m

)

Case1: V= 55m/s Case2: V= 33m/s

M1 M2 M3 M4 M5 M1 M2 M3 M4 M5

K-X

X

XX

-K

K-X

B

XB

-K

XB

-XX

K-X

X

XX

-K

K-X

B

XB

-K

XB

-XX

30

4 0.77 0.75 0.77 0.99 0.97 0.29 0.28 0.27 0.38 0.37

12 6.34 5.86 6.61 7.52 7.39 3.33 3.09 3.22 3.24 3.17

20 17.13 16.37 18.51 22.44 22.19 9.86 9.41 9.73 9.13 8.53

30 39.29 40.86 44.41 55.81 56.99 24.27 22.56 23.62 21.48 20.55



Table 2-5 Comparison of Joint Displacements of 30m Tower in mm (θ = 450) H

eig

ht

of

To

wer

(m

)

No

de

hei

gh

t (m

) Case1: V= 55m/s Case2: V= 33m/s

M1 M2 M3 M4 M5 M1 M2 M3 M4 M5

K-X

X

XX

-K

K-X

B

XB

-K

XB

-XX

K-X

X

XX

-K

K-X

B

XB

-K

XB

-XX

30

4 0.54 0.87 0.54 0.87 0.87 0.20 0.37 0.19 0.33 0.32

12 4.49 4.42 2.38 5.53 5.55 2.35 2.35 2.27 2.35 2.34

20 12.10 11.56 8.25 15.94 16.04 6.96 6.65 6.89 6.46 6.14

30 27.79 28.86 17.70 37.87 39.41 17.16 15.96 16.70 15.19 14.69

2.5. Free vibration analysis-

The natural frequencies of the 30m tower of all the models are tabulated in table 2-6.The

respective mode shapes of all the models with the height of 30m are shown in figure 2-5-1 to

2-5-5. The first mode is the natural excitation of the structure in X direction, second mode in z

direction and the torsion mode shows the response of structure under torsion.

Table 2-6 Natural frequencies of the 30m tower structures

Height

of Tower

(m)

Mode

Natural frequency in Hz

M1 M2 M3 M4 M5

K-XX XX-K K-XB XB-K XB-K

30

Mode 1 5.174 5.332 4.948 4.743 4.703

Mode 2 5.175 5.333 4.949 4.869 4.809

Torsion 12.910 17.457 12.675 16.520 14.290

1st mode 2nd mode Torsion mode

f1 = 5.174 Hz f2 = 5.175 Hz f = 12.910 Hz

Figure 2-5-1 Modes shapes of 30m tower, M1: (K-XX)




f1 = 5.174 Hz f2 = 5.175 Hz f = 12.910 Hz

Figure 2-5-1 Modes shapes of 30m tower, M1: (K-XX)


f1 = 4.948 Hz f2 = 4.949 Hz f = 12.675 Hz

Figure 2-5-3 Modes shapes of 30m tower, M3: (K-XB)


f1 = 4.743 Hz f2 = 4.869 Hz f = 16.520 Hz

Figure 2-5-3 Modes shapes of 30m tower, M4: (XB-K)




f1 = 4.703 Hz f2 = 4.809 Hz f = 14.290 Hz

Figure 2-5-5 Modes shapes of 30m tower, M5: (XB-XX)

3. RESULTS AND DISCUSSION

The 3D frame telecommunication model subjected to dynamic loads are evaluated by

considering fifteen models with three types of bracings, five combination of bracing systems

for three different heights. The FE analysis has been carried out to study the behaviour of these

models under the seismic and wind induced dynamic loads. The results obtained from FE

analysis are discussed.

3.1. Wind analysis results

The wind analysis is performed by considering two different cases with respect to gust factor,

along wind (θ = 00) and cross wind (θ = 450). As a result of which Joint displacements of the

tower structures are worked out.

Table 3-1 Variation of joint displacements (mm) at the top of tower (along wind)

Joint Displacement (mm) (normal wind θ = 00 ) at Top of Tower

To

wer

Hei

gh

t(m

)

Case1: V= 55m/s Case2: V= 33m/s

M1 M2 M3 M4 M5 M1 M2 M3 M4 M5

K-

XX

XX

-

K

K-

XB

XB

-

K

XB

-

XX

K-

XX

XX

-

K

K-

XB

XB

-

K

XB

-

XX

30 39.29 40.86 44.41 55.81 56.99 24.27 22.56 23.62 21.48 20.55

45 179.1 171.44 175.15 169.8 120.9 34.50 32.86 36.28 33.76 33.30

60 275.7 267.18 288.80 274.5 267.3 93.83 90.91 98.26 93.41 92.34



Figure 3-1 (a-e) Variation of joint displacements (mm) at the top of tower (along wind)

Table 3-2 Variation of joint displacements (mm) at the top of tower (cross wind)

Joint Displacement (mm) (normal wind θ = 450 ) at Top of Tower

To

wer

Hei

gh

t(m

) Case1: V= 55m/s Case2: V= 33m/s

M1 M2 M3 M4 M5 M1 M2 M3 M4 M5

K-

XX

XX

-

K

K-

XB

XB

-

K

XB

-

XX

K-

XX

XX

-

K

K-

XB

XB

-

K

XB

-

XX

30 27.79 28.86 17.70 37.87 39.41 17.16 15.96 16.70 15.19 14.69

45 126.66 121.20 123.83 120.10 85.48 24.39 23.23 25.65 23.87 23.55

60 275.77 188.89 288.80 194.10 189.02 93.83 64.28 98.26 66.05 65.29



Figure 3-2(a-e) Variation of joint displacements (mm) at the top of tower (Cross wind)

From the figures 3-1(a-e) and 3-2(a-e), it can be seen that with the increase in tower height

the displacement increases in all the cases. From figure 3-1(a) it can been that when wind speed

in increases from 33m/s to 55m/s, the displacement of 3m tower increases around 16mm, in

45m tower it increases around 145mm and in 60m tower around 190mm.Among the considered

tower models the Model3 (M3) with K-XB bracing has height displacement and Model2 (M2)

with XX-K bracing has lowest displacement.

3.2. Free vibration analysis

The comparison of natural frequency for 1st mode of all the tower models obtained from free

vibration analysis is tabulated in table 3-3



Table 3-3 Natural frequency (Hz) of tower (1st mode) H

eig

ht

of

To

wer

(m

) Tower models

M1 M2 M3 M4 M5

K-XX XX-K K-XB XB-K XB-K

30 5.174 5.332 4.948 4.743 4.703

45 3.129 3.174 3.167 3.201 3.177

60 1.913 1.928 1.827 1.879 1.821

Figure 3-3 Comparison of Natural Frequency of Towers

The mass of the tower plays major role than stiffness when the height of the tower increases,

by reducing the natural frequency of the tower. As we can see from the figure 3-3 a plot of

natural frequency against tower height in Model1 (M1) with K-XX bracing, the natural

frequency of the tower decreased from 5.174 Hz to 3.129 Hz when height of the tower increases

from 30mto 45m and it is further reduced to 1.913 Hz when height of tower increased from

45m to 60m. The similar pattern can be seen in all the models. The Model5 (M5) with XB-XX

bracing have the least natural frequency and Model2 (M2) with XX-K have the highest natural

frequency amongst the considered models at 60m height tower.

Response spectrum analysis: Equivalent static approach as per IS 1893 (part 1) – 2002 the

response spectra are generated in account of all the seismic zones shown in table 3-9.



Table 3-4 Joint Displacement (mm) at Top of Tower (Response spectrum analysis)

Height of

Tower (m) Zone

Joint displacement (mm)

M1 M2 M3 M4 M5

K-XX XX-K K-XB XB-K XB-XX

30

II

0.627 0.633 0.677 1.691 1.662

45 1.709 1.718 1.678 1.765 1.999

60 5.297 5.065 5.580 5.668 5.884

30

III

0.990 1.000 1.068 2.137 2.121

45 2.699 2.713 2.649 2.786 3.157

60 8.340 7.998 8.786 8.949 8.929

30

IV

1.485 1.500 1.602 2.744 2.746

45 4.049 4.069 3.974 4.179 4.736

60 12.489 11.997 13.158 13.424 13.394

30

V

2.228 2.250 2.403 3.654 3.685

45 6.073 6.103 5.961 6.269 7.104

60 18.714 17.996 19.717 20.136 20.091

Figure 3-4 (a-e) shows that with the increase in height of towers the displacements increases

irrespective of the zones.



4. CONCLUSION

Based on the results obtained from the analysis it can be seen that the wind is the dominate

factor in the tower modelling than the seismic forces but the seismic effect cannot be neglected

as observed from the results.

All the models have been checked for Indian standard code. The 60m tower Model2 (M2)

with XX-K bracing, Model4 (M4) with XB-K bracing and Model5 (M5) with XB-XX bracing

fails in wind load for 55m/s and all the other models pass in both the wind cases and seismic

zones. The failure can be avoided with higher grade of steel or the structure can be redesigned.

There is no seismic failure in the models studied. The leg members are more predominant

in taking the loads than horizontal and diagonal members, which has been reflected in the

utilization ratio.

REFERENCE

[1] Mikus, J: Seismic analysis of self-supporting telecommunication towers. M.Eng. Project

Report G94-10, Department of Civil Engineering and Applied Mechanics, McGill

Universiyt, Montreal, Que (1994).

[2] G. GhodratiAmiri, M.A. Barkhordari, and S. R. Massah: “Seismic Behavior of 4-Legged

Self-Supporting Telecommunication Towers”, 13th World Conference on Earthquake

Engineering Vancouver, B.C., Canada, (August 2004).

[3] G. GhodratiAmiri and S. R. Massah: “Seismic Response of 4-Legged Self-Supporting

Telecommunication Towers”, International Journal of Engineering, Volume 20, No. 2,

(August 2007).

[4] NitinBhosale, Prabhat Kumar and Pandey.A.D: “Influence of Host Structure

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[5] SiddharthBehera and Achal Kumar Mittal: “A Comparative Study of Wind Forces on Tall

Building and Towers as Per Is 875-Part- III (1987) and Draft Code (2011)” VI National

Conference on Wind Engineering, CSIR-Central Building Research Institute, Roorkee,

(2012).

[6] Venkateswarlu. B., Harikrishna. P., Rajan. S. and Kumar. M: “Stochastic gust response of

microwave lattice towers”, Computers and Structures, Vol. 52, No.5, (1994, 1031-1041).

[7] Jithesh Rajasekharan1, S Vijaya: ANALYSIS OF TELECOMMUNICATION TOWER

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CODES / STANDARDS

[9] IS800:1984, Indian Standard Code of Practice for General Construction in Steel, Bureau of

Indian Standards, New Delhi.

[10] IS: 802 (part1/sec1): 1995, Indian Code of Practice for Use of Structural Steel in Overhead

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for Buildings and Structures, Part 1: Dead Loads. Bureau of Indian Standards, New Delhi.


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Documents

DYNAMIC ANALYSIS OF 4-LEGGED STEEL …iaeme.com/MasterAdmin/uploadfolder/IJCIET_10_01... · Shwetha Shetty M R, Anusha M, Ashwini A and Rajiv T 1536 [email protected] 1. INTRODUCTION