ANALYSIS OF TELECOMMUNICATION TOWER SUBJECTED TO

Jithesh,Vijaya Page 68

ANALYSIS OF TELECOMMUNICATION TOWER SUBJECTED TO

SEISMIC & WIND LOADING

Jithesh Rajasekharan1, S Vijaya2

¹PG Student, Department of Civil Engineering, Dr. A.I.T, Bengaluru, VTU, Karnataka, India

²Professor, Department of Civil Engineering, Dr. A.I.T, Bengaluru, VTU, Karnataka, 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. In wireless communication

network these towers play a significant role hence failure of such structure in a disaster is a major concern. Therefore

utmost importance should be given in considering all possible extreme conditions for designing these towers. In most

studies the researches have considered the effect of wind only on the four legged self-supporting towers. In this

dissertation, studies are been carried out on models of varying heights with different bracing for seismic along with the

wind effect. The wind effect on the structure is studied by using the gust factor method and the seismic effect on the

structure is studied by carrying out the modal analysis and response spectrum analysis. The results obtained from the

above analysis are tabulated, compared and conclusions are drawn.

Keywords — Telecommunication towers; Bracings; Wind Analysis; Gust factor method; Modal analysis; Response spectrum

analysis

I. INTRODUCTION

Communication towers or lattice towers are classified into three categories that are Guyed masts,

monopole and self-supporting towers. The structure engineer faces the challenging job of designing and

constructing telecommunication towers to support antenna loads, platform as well as steel ladder loads in

open weather with high degree of reliability. The major cause of failures of telecommunication tower

throughout the world though still remains to be high intensity winds (HIW). The major problem faced is

the difficulty in estimating wind loads as they are based on a probabilistic approach. There has been

several studies in telecommunication towers taking into consideration the wind as well as dynamic effect.

Lefort investigated the effects of wind and earthquake loads on the self-supporting antenna towers and it is

reported that for towers, seismically induced member forces may exceed forces obtained from service and

wind load calculations. . Amiri and Booston (2002) studied the dynamic response of antenna-supporting

structures. In this regard, self-supporting steel telecommunication towers with different heights were

evaluated considering the wind and earthquake loads. A comparison is made between the results of wind

and earthquake loading. These comparisons resulted in the necessity of considering earthquake loads in

tower analysis and design.


Nitin Bhosale et al. (2012) have carried out the seismic response of 4 legged telecommunication

towers under the effect of design spectrum from the Indian code of practice for zone – IV. The axial forces

of the tower member is considered, comparison is made between roof top tower and ground tower. Richa

Bhatt et al. (2013) have carried out study on the influence of modelling in lattice mobile towers under

wind loading where in the towers are analysed for gust factor wind. Displacements, Member forces and

maximum stress have been compared to find out the effect on towers. Siddesha. H (2010), carried out the

wind analysis on antenna towers with static & gust factor method, the displacements at the top of the tower

with angle and square sections are considered.

The objective of this dissertation work is to study the effect of wind load on tower structures for

different wind zones using gust factor and also to study the seismic effect on the tower structures by

carrying out the modal analysis and response spectrum analysis. This paper helps in understanding the

effect of both wind and seismic forces on the tower structures by considering different conditions.

II. MODELLING AND LOADING DETAILS

A. Modelling of Towers

The lattice tower is designed for three heights of 30m, 40m and 50m. The towers are provided with

different types of bracings such as K, XB, XX, and Y. The tower have been modelled as 3D space frame

using the finite element in STADD. Pro 2007 software. The details of the towers are given in table I for

various heights. The fig 1 shows 30m towers with different types of bracing considered in this study. The

table II shows the member properties assigned to the towers.

TABLE I

Details of Different Towers

Height of Tower 30 40 50

Height of Slant Portion 20 28 36

Height of Straight Portion at Top of Tower 10 12 14

Base Width 5 5 5

Top Width 2 2 2

No. of 4m Panel 5 7 9

No. of 2m Panel 5 6 7

Fig 1: 30m Towers with Different Bracings Considered


TABLE II Member Details of Tower

B. Loads Acting on Tower

A platform load of 0.82 kN/m2 is applied at 26m, 36m, and 46m respectively for 30m, 40m and 50m

tower. The weight of the ladder and cage assembly is assumed to be 10% of total weight. The antenna

loads are summed up and distributed evenly to the nodes at the considered heights. The details of the

antenna provided on the tower are given in the table III. TABLE III

Antenna Loading for the Towers

Sr.

no

Item Qty Dia (m)

(wxdxh)

Weight/

Antenna

(kg)

Location

from Base

(30m

tower)

Location

from Base

(40m

tower)

Location

from Base

(50m

tower)

1 CDMA 6 0.26 x 2.5 20 28m 38m 48m

2 Microwave 1 1.2 77 24m 34m 44m

3 Microwave 1 0.6 45 24m 34m 44m

4 Microwave 2 0.3 25 24m 34m 44m

C. Wind Load

The wind load on the tower structure is calculated by using IS 875 (part 3): 1987 and IS 802 (Part 1:

Sec1)-1995. For the calculation of the wind load by the gust factor method the parameters considered are

as follows:

Wind zone V and VI , basic wind speed is 50m/s and 55m/s, the risk coefficient factor k1 considered is

1.08 (considering design life of 100 years), topography factor k3 = 1.0 (flat terrain), the value of terrain

and height factor k2 and the gust response factor G is calculated for category 1 that is open terrain

category. The table IV shows the wind load calculated for 30m tower similarly the wind loads can be

calculated for 40m and 50m tower. TABLE IV

Wind Load Acting on 30m Tower (kN)

Panel

No Distance (m)

Case 1 (50m/s) Case 2 (55m/s)

K XB XX Y K XB XX Y

1 4 22.13 23.88 21.15 24.50 26.78 28.90 25.59 29.64

2 8 20.65 23.13 19.78 22.68 24.99 27.99 23.93 27.44

3 12 20.18 20.13 19.38 21.91 24.42 24.36 23.44 26.51

4 16 20.26 16.83 19.59 21.63 24.51 20.36 23.71 26.18

5 20 18.64 14.49 18.21 19.49 22.56 17.53 22.03 23.58

6 22 7.42 7.83 7.09 8.07 8.98 9.47 8.58 9.77

7 24 9.82 10.24 9.48 10.49 11.88 12.39 11.47 12.69

8 26 7.81 8.24 7.47 8.50 9.45 9.97 9.03 10.28

9 28 11.26 11.70 10.91 11.97 13.62 14.16 13.20 14.48

10 30 8.21 8.66 7.85 8.93 9.93 10.48 9.50 10.81

Sr.

No.

Tower Elevation (m) Member Description Section

30 m 40m 50m

1 0-12 0-16 0-20

Leg member ISA 200 x 200 x 25

2 Bracing ISA 150 x 150 x 10

3 12-20 16-28 20-36


4 Bracings ISA 130 x 130 x 10

5 21-30 28-40 36-50


6 Bracings ISA 90 x 90 x 10


D. Dynamic load

The dynamic loads are applied on the tower structure using IS 1893: part 1, 2002. For the analysis

purpose the acceleration spectrum have been prepared for zone II to zone V assuming soil condition as soft

and damping as 5%. The important factor (I) = 1.5 and response reduction factor (R) = 5. The fig 2 shows

the design response spectra for all the seismic zones.

Fig 2: Design Response Spectra for All the Seismic Zones

III. FE ANALYSIS AND RESULT DISCUSSIONS

A. FE Analysis of Towers

In this a typical structure of telecommunication tower of different heights and bracings is

considered for the analysis. The structural system consists of leg member, primary bracings, and

secondary bracings. The varying heights such as 30m, 40m and 50m with the combination of bracing

systems like K, XB, XX and Y are considered for the analysis. The end conditions are considered as fixed

end condition and the properties assigned to the models are kept same for all the models. STADD software

is used in the analysis.

Initially the wind analysis is carried out on the developed model. The joint displacements at the top

of the tower and the member stress at the base leg members are compared for different wind zones. Then

the modal analysis is carried out on the models to obtain the natural frequency and mode shape. The

response of the structure is different at each of the different natural frequencies. These deformation

patterns are called mode shapes. After that the response spectrum analysis is carried out from the

generated spectra shown in fig 2. The displacement at top and member stress at bottom leg member as

well as at change point is compared for all models.

B. Wind Analysis Results

Wind analysis is carried out for two wind zones of basic wind speed 50 m/s and 55 m/s. The gust

wind factor is taken into consideration for the analysis. The combination of dead load, antenna load and

wind load is the load take for the analysis of the models. The joint displacement comparison is done in

table V and member stress comparison is done in table VI. The fig 3 (a-d) shows the variation in

displacement at top for different towers and the fig 4 (a-d) shows the variation in stress at the bottom leg.


TABLE V

Comparison of Displacement at Top with Different Bracing

(

(a) (b)

(c) (d)

Figure 3 (a-d): Variation of Displacement (mm) at Top for Different Tower

Fig 3 (a-d) indicates that the displacement increases as the height of masts increases. As the mast

height increases from 30m to 40m the displacement increases by 68% and from 40m to 50m the

displacement increases by 60%. When the wind speed increases from 50m/s to 55m/s then the joint

displacement of tower at top increases by about 15% for all the models. The tower with Y bracing has the

highest joint displacement in the considered models whereas K bracing have the least displacement.

TABLE VI

Comparison of Member stresses (N/mm2) at Bottom Leg

Tower

Height (m)

Case 1 (50 m/s) Case 2 (55 m/s)

Bracing Bracing

Tower

Height

(m)

Case 1 (50 m/s) Case 2 (55 m/s)

Bracing Bracing

K XB XX Y K XB XX Y

30.00 27.02 27.33 29.24 28.85 32.70 33.07 35.36 34.92

40.00 84.67 90.62 92.07 92.60 102.39 107.15 108.84 109.36

50.00 211.68 218.88 222.27 226.14 256.18 264.86 269.13 272.80


K XB XX Y K XB XX Y

30.00 31.51 31.44 34.22 34.27 37.36 37.09 40.51 40.53

40.00 58.08 62.35 63.88 63.71 69.23 73.21 75.00 74.76

50.00 95.14 100.33 104.05 102.64 113.82 120.00 124.57 122.92

(a) (b)

(c) (d)

Figure 4 (a-d): Comparison of Member Stress (N/mm2) at Bottom Leg

The figure 8.2 (a-d) shows the variation in stress pattern with respect to height of structure. The

member stress at bottom leg of XX bracing tower has higher stress as compared to other tower models.

The increment in stress is about 45% when tower height increases from 30m to 40m whereas when the

tower height increases from 40m to 50m there is an increase of 40% in member stress at the bottom leg

member. When wind load increases from 50ms to 55 m/s there is an increase of 15% in member stress.

C. Modal Analysis Results

In this analysis the overall mass and stiffness of a structure is used to find the various periods at

which it will naturally resonate. Modal analysis of the tower structures are carried out and the modal

parameters such as natural frequency and mode shapes are obtained. The natural frequency are tabulated

in table VII. The fig 5 shows the first mode shape of 50m tower with different bracings.


Table VII

Natural Frequencies of Telecommunication Towers (Hz)

Height Mode Bracings

K XB XX Y

30

Mode 1 4.994 4.822 4.876 4.609

Mode 2 4.994 4.822 4.877 4.609

Torsion 10.776 15.895 15.431 9.965

40

Mode 1 2.954 2.849 2.899 2.725

Mode 2 2.954 2.849 2.899 2.725

Torsion 8.312 12.202 13.126 7.707

50

Mode 1 1.941 1.874 1.914 1.789

Mode 2 1.941 1.874 1.914 1.789

Torsion 6.755 9.879 10.660 6.276

K XB XX Y

f = 1.941 Hz f = 1.874 Hz f = 1.914 Hz f = 1.789 Hz

Fig 5: first mode shape of 50m tower

The comparison shows that the frequency of the tower with Y bracing have the least natural

frequency since its stiffness is higher as the weight of the structure is more as compared to others. As the

tower height increases the mass starts to play a major role than the stiffness of the structure there by

reducing the natural frequency of the structure.

D. Response Spectrum Analysis Results

The Response spectra are generated as shown in fig 2 for the different seismic zones as specified

by the IS 1893(part 1):2002 and the FE analysis is carried out. The Displacements and the member

stresses at bottom leg obtained from the Response Spectrum Analysis are tabulated in table VIII and table

IX respectively. The stress at change point is tabulated in table X. The comparison of displacement is done

in fig 6 and that of member stress at bottom leg is done in fig 7. Comparison of member stress at change

point is done in fig 8.

Table VIII

Joint Displacement (mm) at Top of Tower

Tower Height (m) Zone Bracing

K XB XX Y

30 II 1.94 2.25 2.06 2.32


40 4.49 4.85 4.69 5.26

50 10.46 11.26 10.80 11.26

30

III

3.30 3.58 3.50 3.62

40 7.46 8.08 7.80 8.75

50 16.73 18.00 17.28 19.63

30

IV

4.64 5.04 4.92 5.07

40 10.75 11.64 11.23 12.59

50 23.00 24.75 23.76 27

30

V

6.96 7.56 7.38 7.60

40 16.12 17.46 16.85 18.89

50 34.50 37.14 35.65 40.50

(a) (b)

(c) (d)

Fig 6 (a-d): Comparison of Joint Displacement (mm) at Top of Tower

The Fig 6 (a-d) shows the displacement pattern of joints with respect to different zones of

earthquake. It can be observed from the graph that from zone II to zone III there is an increment in

displacement of about 38% and from zone III to zone IV the displacement increases by 28%. As the zone

varies from IV to V there is an increase in displacement of about 33% in 30m and 40m tower where as in

50 m tower there is an increase of about 40%. From the fig 6 it can be seen that from 30m to 40m the

increase is nearly linear but as the height increases from 40m to 50m there is a steep increase in the

displacement in all zones.


Table IX

Member Stresses (N/mm2) at Bottom Leg


K XB XX Y

30

II

5.46 5.51 5.15 6.59

40 7.35 7.68 7.02 8.75

50 9.90 10.46 9.65 11.39

30

III

6.70 6.60 6.36 7.08

40 8.97 9.23 8.45 10.68

50 12.19 12.66 11.56 14.45

30

IV

7.91 7.90 7.55 9.11

40 10.78 10.95 10.24 12.80

50 14.48 14.86 13.85 17.14

30

V

10.01 10.14 9.60 11.41

40 13.74 14.11 13.17 16.26

50 18.67 19.36 18.08 22.08

(a)

(b)

(c) (d)

Fig 7 (a-d): Member Stresses (N/mm2) at Bottom Leg

The figure 7 (a-d) shows the variation of member stress at bottom leg member for different heights

of tower. In zone IV as the tower height increases from 30m to 40m there is an increase in stress of about

28% whereas from 40m to 50m the increase is about 25% and in zone V the increase in stress from 30m to

40m is about 30% whereas from 40 to 50m it is about 25%. In zone V when the height increases from 30m

to 50m the increase in stress is about 45%. The tower with Y bracing is having the highest stress which is

nearly 18% more than that of XX bracing which is having the least stress at bottom leg.


Table X

Member Stresses (N/mm2) on Leg at Change Point


K XB XX Y

30

II

5.77 5.82 6.89 6.93

40 6.86 6.94 8.29 8.14

50 8.67 8.78 10.59 9.50

30

III

7.42 7.44 8.95 8.53

40 8.84 9.04 10.25 10.10

50 11.22 11.42 13.60 13.20

30

IV

9.04 9.21 10.97 10.28

40 10.72 10.09 13.27 12.68

50 13.76 14.06 16.62 16.16

30

V

11.58 12.25 14.64 13.35

40 14.57 15.11 17.54 17.09

50 18.42 18.71 22.15 21.58

By comparing the table X and IX it can be noted that the member stress for XX bracing is more at

the change point making it the critical section. The stress at change point is nearly 15% more than that at

leg member. This is due to the absence of horizontal bracing in the tower. It can also be noted that the

tower of 30m height is also showing the stress at the change point more than that of leg member in all

zones. This is due to the presence of antenna structures close to the change point and also the change in

properties at the top portion of tower.

IV. CONCLUSION

The following are the major conclusions drawn from this study:

• From the wind analysis it can be observed that the increase in joint displacement is nearly 68%

when tower height increases from 30m to 40m and when tower height increases from 40m to 50m

the displacement is likely to increase by 60%.

• The change in stress when height increases from 30 to 40m is about 45% and from 40 to 50m is

39% on both cases of wind speeds.

• For an increase in wind speed from 50 to 55 m/s with no change in direction the displacement as

well as the member stresses increase by 15% to 17%.

• In wind analysis the joint displacement is more for the tower with Y bracings whereas the member

stress at bottom leg is more for the tower with XX bracing due to the absence of horizontal bracing.

• There is gradual decrease in the natural frequency of the structure as the height of tower increases.

This is due to the influence of mass as the height increases the mass starts to play predominate role

than stiffness there by reducing the natural frequency of the structure.

• In the response spectrum analysis the increase in displacement is about 38% from zone II to zone

III whereas from zone III to IV the increase is about 28% and from zone IV to Zone V the increase

in displacement is about 35%. The tower with Y bracing is having most displacement and the

tower with K bracing is having the least on the compared models.

• In zone V as the height increases from 30m to 40m there is an increase in stress of about 30%

whereas from 40m to 50m there is an increase of about 25% and when the height changes from

30m to 50m there is an increase of about 45% in stress.


• It can be observed that in response spectrum analysis the stress for tower with XX bracing is more

at the change point leg than that of the bottom leg member this is due to the absence of horizontal

bracing along the tower.

• From the above analysis it can be concluded that the wind is the predominate factor in the tower

modelling than the seismic forces but the seismic effect cannot be fully neglected as observed from

the results.

• All the models have been checked for Indian standards. The tower model of 50m with XB bracing

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

and seismic zones. In those models either change of property can be done or combination of

bracings can be used and redesigned.

• The vertical members are more prominent in taking the loads of the tower than the horizontal and

diagonal member, the member supporting the antenna structures at higher elevation are likely to

have large influence on the behavior of the tower.

ACKNOWLEDGEMENT

I would like to thank Almighty and my family. I express my sincere thanks and regards to Dr. B

Shivakumaraswamy, Professor and Head of Civil Engineering Department, Dr. A.I.T, Bengaluru, for his

constant support and encouragement throughout my work. I would like to extend my heartfelt gratitude to

all the staff members, Department of Civil Engineering, Dr. A.I.T, Bengaluru, for their direct or indirect

support during the work.

REFERENCES

[1] A. Jesumi, and M.G. Rajendran (2013): “Optimal Bracing System for Steel Towers”, International Journal of

Engineering Research and Applications, Volume 3, Issue 2, April 2013, ISSN: 2248-9622.

[2] Amiri G. and Boostan A., (2002): “Dynamic Response of Antenna-Supporting Structures”, 4th Structural Specialty

Conference of the Canadian Society for Civil Engineering, pp.1-9.

[3] G. Ghodrati Amiri, M.A. Barkhordari, and S. R. Massah (2004): “Seismic Behavior of 4-Legged Self-Supporting

Telecommunication Towers”, 13th World Conference on Earthquake Engineering Vancouver, B.C., Canada,

August 2004.

[4] G. Ghodrati Amiri and S. R. Massah (2007): “Seismic Response of 4-Legged Self-Supporting Telecommunication

Towers”, International Journal of Engineering, Volume 20, No. 2, August 2007.

[5] Nitin Bhosale, Prabhat Kumar and Pandey.A.D (2012): “Influence of Host Structure Characteristics on Response

of Rooftop Telecommunication Towers”, International Journal of Civil and Structural Engineering Volume 2, No

3, February 2012, ISSN 0976 – 4399.

[6] Patil Vidya M. and Lande Abhijeet C. (2012): “Structural Response of Lattice Steel Masts for Seismic Loading”,

IOSR Journal of Mechanical and Civil Engineering, September 2012, ISSN: 2278-1684.

[7] Richa Bhatt, A.D.Pandey and Vipul Prakash (2013): “Influence of Modeling in the Response of Steel Lattice

Mobile Tower under Wind Loading”, International Journal of Scientific Engineering and Technology, Volume 2

Issue 3, April 2013, ISSN: 2277-1581.

[8] Siddesha.H (2010): “Wind Analysis of Microwave Antenna Towers”, International Journal of Applied Engineering

Research, Dindigul, Volume 1, No 3, 2010, ISSN 0976-4259.


[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 Transmission Line

Towers, Part 1: Materials, Loads and Permissible Stresses. Bureau of Indian Standards, New Delhi.

[11] IS: 875 (part 3):1987, Indian Code of Practice for Design Loads (other than Earthquake) for Buildings and

Structures, Part 3: Wind Loads. Bureau of Indian Standards, New Delhi.

[12] IS: 1893 (part 1): 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures, Part 1: General

Provisions and Buildings. Bureau of Indian Standards, New Delhi.

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ANALYSIS OF TELECOMMUNICATION TOWER SUBJECTED TO