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Need for a Specific Indian Code/Standardfor Optimization of Telecom Structures

A Brief Comparison of Wind Pressures per IS: 875 (Part-3) and TIA-222-G

Case Study by Seshendra Kumar, C. Eng (I), M.I.E., and Jason T. Kahrs, P.E.

The Indian Telecommunications Industry has the second largest telecommunication network in the world with approximately 35.96 million landlines and 652.42 million wireless subscribers.Since 2001, the Indian mobile subscriber base has increased in size by a factor of more than one-hundred. The projected growth of the Indian telecommunication industry will exceed more than1.159 billion mobile subscribers by 2013. [1]

To meet this exponential telecom growth and to keep up with advancements in wireless

technologies, telecom carriers will need to install/change their equipment on either existing ornewly built towers. Consequently, there will be a lot of scope for the structural analysis/designof telecom structures.

The typical practice for engineers to determine wind loads on telecom structures in India is to usethe generic Indian Code, IS: 875 (Part-3), “Code of Practice for Design Loads (Other thanEarthquake) for Buildings and Structures”. There are many international Codes/Standardsdeveloped specific to telecom structures such as TIA-222-G, BS-8100 etc. The purpose of this

paper is to show a need for a specific Indian Code/Standard for optimization of telecom structures based on a comparison of wind pressures determined using IS:875 (Part-3) and the International Telecom Code/Standard, TIA-222-G, “Structural Standard for Antenna

Supporting Structures and Antennas”.

The vast majority of telecommunication towers in India are self-support towers. Self-Supporttowers of 30m, 60m, and 90m heights are used in this study. The equivalent design parametersthat are used to determine the wind pressures acting at various increments of the tower height arelisted in Table 1 . The majority of the area in India consists of warm/tropic regions throughoutthe year. As such, ice loading is ignored for this study. The wind pressures are calculated bytaking the controlling load case for the structure, i.e. Basic Wind Speed without Ice. Forsimplicity, the towers are analyzed without linear and discrete appurtenances. The drag factordifference between the two codes is considered to be insignificant.

This paper was submitted for eWORLD FORUM 2011 Conference



Table 1 – Equivalent Design Parameters

Design Parameter TIA-222-G Parameter Equivalent Design Parameterin IS: 875 (Part-3)

Importance Factor/Risk Co-efficient

Importance Factor, I =1 Risk Co-efficient, k 1=1Corresponding to Mean ProbableDesign Life (MPDL) of 50yrs

Exposure Category/TerrainCategory

Exposure Category-COpen Terrain with scatteredObstructions

Terrain Category 2Open Terrain with scatteredobstructions

TopographyCategory/Topography Factor

Topographic Category, K zt = 1General Topography

Topography Factor, k 3 = 1General Topography

Tables 2 and 3 show the design parameters used for a 30m Self-Support Tower for IS: 875(Part-3) and TIA-222-G, respectively. Wind pressure graphs for 30m, 60m and 90m towers areshown in Figures 1 to 3 . As shown in Table 4 , for a 30m Self-Support Tower, the percentage

difference of the design wind pressure calculated using IS: 875 (Part-3) ranges from 29.9% to53.1% higher with respect to TIA-222-G. Similarly, as shown in Table 5 , the differencebetween the wind pressures for the 60m and 90m towers are 14.3% to 37.8% higher whendetermined using IS: 875 (Part-3). The wind pressure difference is attributed to refinedreliability, safety, and economical factors adopted by TIA.

In conclusion, the results of this study show that the wind pressures calculated based on IS: 875(Part-3) are significantly higher than that of TIA-222-G. Consequently, higher wind pressureslead to a less economical tower. Hence, it is recommended that more research is needed todevelop an Indian code/standard for Telecommunication structures considering reliability, safetyand economical factors to obtain optimized telecom structures including Self-Support, Guyed,

and Monopoles etc.

Table 2 – IS: 875 (Part-3) Tower Parameters for 30m Self-Support Tower

IS: 875 (Part-3) Parameters Units Reference

Basic Wind Speed (3 Sec gust) V b 50 m/sMean Probable Design Life of Structure (Note-

1) N 50 Years (Table-1)

Probability Factor (Risk Coefficient) k 1 1 (Table-1)Terrain Category TC 2 (5.3.2.1)

Class of Structure B (5.3.2.2)

Terrain, Height and structure Size factor k 2 1.10 (Table-2)Topography Factor (General Topography) k 3 1.00 (5.3.3.1)

Design Wind Speed at the top of Tower V z = k1*k2*k3*Vb 55.06 m/s (5.3)

Design Wind Pressure p z = 0.6*V z2 1819.0 N/m 2 (5.4)

Design Wind Force / Effective Projected Area F/(EPA) = p z 1819.0 N/m 2

2This paper was submitted for eWORLD FORUM 2011 Conference



Table 3 – TIA-222-G Tower Parameters for 30m Self-Support Tower

TIA-222-G Parameters Units Reference

Basic Wind Speed (3 sec Gust) V 50 m/sImportance Factor I 1 (Table 2-3)

Exposure Category C (2.6.5.1)Structure Class II (Table 2-1)

Wind Direction Probability Factor for Structure(Note-2) Kd 0.85 (Table 2-2)

zg (Is a factor to calculate, K z) 274 m (Table 2-4)α (Is a factor to calculate, K z) 9.5 (Table 2-4)

Velocity Pressure Coefficient Kz = 2.01(z/z g)(2/ α ) 1.26 (2.6.5.2)

Topographic Factor (For Category-1) K zt 1 (2.6.6.4)Velocity Pressure q z = 0.613K zKZt Kd V2 I 1643.53 N/m 2 (2.6.9.6)

Gust Response Factor (Note-2) Gh 0.85 (2.6.7)

Design Wind Force / Effective Projected Area F/(EPA) = q z Gh 29.28 N/m2

Note-1: Per Table-1 of IS: 875 (Part-3) Mean Probable Design Life (MPDL) for Telecommunication Towers is 100yrs. Importance Factor 1.0 is used for both codes. This value corresponds to 50 years MPDL per IS: 875 (Part-3).Note-2: Wind Direction Probability Factor (K d) & Gust Factor (G h) used in the above calculations are correspondingto the structure only.

Table 4 – Wind Pressures for 30m Self-Support Tower at each 5m interval

S. No. z (m)

IS: 875 (Part-3) TIA-222-G%

Increase in

Pressurewrt TIA-222-G

k2 Wind Pressure,per IS875-(P-3),pz (N/m 2)

Kz Wind Pressure, perTIA-222-G, q zGh (N/m 2)

1 30 1.10 1815 1.26 1397 29.9%2 25 1.08 1750 1.21 1344 30.2%3 20 1.05 1654 1.16 1283 28.9%4 15 1.02 1561 1.09 1207 29.3%5 10 0.98 1441 1.00 1109 30.0%6 5 0.98 1441 0.87 958 50.4%7 0 0.98 1441 0.85 941 53.1%




Table 5 – Percentage Differences in Wind Pressures

CaseNo.

Self-SupportTower Height(m)

Wind Pressure (N/m 2)Range in IS: 875

Wind Pressure (N/m 2)Range in TIA-222-G

% of Increase inWind PressureWRT TIA-222-G

1* 30 1851 to 1441 * 1397 to 941 * 29.9% to 53.1% * 2 60 1848 to 1297 1616 to 941 14.3% to 37.8%3 90 2018 to 1297 1761 to 941 14.6% to 37.8%

*: See Table 4 for details

Figure 1 – Wind Pressure Comparison for 30 m Self-Support Tower

0

200

400

600

800

1000

1200

1400

1600

1800

2000

3 0 2 5 2 0 1 5 1 0 5 0

D e s i g n W i n d F o r c e / ( E P A )

( N / m 2 )

Height (m)

Wind Pressure Comparision for 30m Self ‐Support Tower

IS: 875 (Part ‐3) TIA‐222 ‐G


0

200

400

600

800

1000

1200

1400

1600

1800

2000

6 0 5 5 5 0 4 5 4 0 3 5 3 0 2 5 2 0 1 5 1 0 5 0

D e s i g n W i n d F o r c e / ( E P A

) ( N / m 2 )

Height (m)


IS: 875 (Part ‐3) TIA‐222 ‐G





5

0

500

1000

1500

2000

2500

9 0 8 5 8 0 7 5 7 0 6 5 6 0 5 5 5 0 4 5 4 0 3 5 3 0 2 5 2 0 1 5 1 0 5 0

D e s i g n W

i n d F o r c e / ( E P A ) ( N / m 2 )

Height (m)


IS: 875 (Part ‐3) TIA‐222 ‐G

References:

1. Information Note to the Press (Press Release No. 42 /2010), TELECOMREGULATORY AUTHORITY OF INDIA, New Delhi, 7th September 2010

This paper was submitted for eWORLD FORUM 2011 Conference

need for a specific indian code- standard for optimization of telecom structures- seshendra kumar,...

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