Transmission Towers

Transmission Towers*Department of Civil Engineering, SJCE

Department of Civil Engineering, SJCE

IntroductionTypes of Transmission TowersDesign Philosophies: Peak Wind Approach Mean Wind ApproachGoverning loads for Microwave TowersAnalysis & DesignCase Study-32m height Microwave towerConclusionsReferences

*Department of Civil Engineering, SJCE


Department of Civil Engineering, SJCE *


Department of Civil Engineering, SJCE *Masts of the Rugby VLF transmitter in EnglandSteel lattice tower


Department of Civil Engineering, SJCE *First modern TV Tower in Stuttgart Sapporo TV tower, at Odori park


Microwave antenna towers are tall latticed steel structure widely used in the arena of telecommunication. These towers are subjected to serve wind environment and are vulnerable to failure.

It is the designers responsibility to minimize the cost and o satisfy all functional requirements.. However, IS:800-1984, IS:802-1992, IS:875-1987(PART 3) are used for the design purpose.*Department of Civil Engineering, SJCE


There are two distinct approaches for designing the structural subjected to wind excitation :-

Peak Wind Approach Mean Wind Approach



In this approach wind speed averaged over a short duration of few seconds (normally 3 seconds) is measured at every hours of day at a standard height of 10m.

The yearly maximum wind recorded over a number of consecutive years are considered for computing extreme value, that is the peak wind for various return periods. These are also known as basic wind speed. The design wind speed is obtained my multiplying basic wind speed by different factors such as Risk factor, Terrain height and size factor and topography factor. The design wind pressure is obtained from multiplication of shape factor and square design wind speed.

The peak wind approach is referred as Force Coefficient Method in IS:875 (PART 3) 1987



In mean wind approach wind speeds averaged over a larger duration varying from 10minutes to one hour are measured at every hour of the day, throughout the year. Out of these, the maximum speed is the mean wind of the year. The yearly maximum mean winds recorded over number of consecutive years are considered for computing extreme mean wind speed for stipulated return period which is termed as reference wind speed.

The basic pressure is multiplied with gust factor and shape factor to arrive at design pressure. The equivalent static design pressure is expected to produce the effect identical to the one produced by actual turbulent wind for appropriate probability level.

The mean wind approach is referred as Gus Effectiveness Method in IS:875(PART 3) 1987.



The basic aspects associated with the evaluation of wind effect on tower are discussed here. Design of microwave tower is predominantly governed by the wind forces acting on the tower and antenna attached to them. The effect of dead load due to the weight of antennas, tower and other attachments are comparatively small. *Department of Civil Engineering, SJCE


Calculation for design wind speed:

The value of basic wind speed for structure is modified to obtain the design wind speed by using different modification factors and computed as:

Where, K1:Risk coefficient K2:Terrain Category, height and structure size factor K3:Topography factor


Vz=Vb*k1*k2*k3


The design wind pressure at any height above the mean ground level shall be obtained by the following relationship:-

where ,Pz=design wind pressure at height z in N/m2Vz=design wind speed at height z in m/s*Department of Civil Engineering, SJCE

Pz=0.6Vz2


Along wind load on a structure at any height z in

Where,Fy - along wind load on structure at any height z corresponding to Area AeCf - Drag coefficient which depends upon solidity ratioAe - effective frontal area considered for the structure at height zPz - Design pressure at Height zG-Gust factor, recommended by IS:875(Part 3)-1987

Gf - Peak factor defined as the ratio of the expected peak value to the root mean value o f fluctuating load componentr - Roughness factor B - Background factor


G=1+gf *r* [B(1+)2+SE/]

Fy=Cf*Ae*Pz*G


Se/ - Measure of resonant component of the fluctuating wind load

S - Size reduction factor

E - Measure of available energy in wind stream at the natural frequency

- Damping Co efficient to be accounted only forHeight > 75 m in Category 4 & Height > 25 m in Category 3.



Wind load on microwave towers is calculated by using the forced coefficients specified in code. This force coefficient Cf takes into account of solidity, shape, etc.,

Where,

Fy - along wind load on structure at any height z corresponding to Area AeCf - Force coefficient for the structureAe - Effective frontal area of the structurePd Design wind pressure - in calculation of Pd,

the coefficient K2 which is used in force coefficient method is different from the one which is used in mean wind approach.


Fy=Cf x Ae x Pd


Latticed tower depends on flow regime, solidity ratio, shape of structural members, member size and distance between windward and leewards face.

When wind blows into corner to tower, the maximum load may be taken as 1.2 times the load for wind blowing against a face, irrespective of solidity ratio of the panel. For the case of triangular latticed tower the values of force coefficient will be about 0.8 times the value given for square one.

For sub critical flow (Vd

The wind load on parabolic antenna reflectors are calculated by using the force coefficients Cfa, Cfs shown in Table 1.

Where,Ac= /4*d2 , d=diameter of antennaThe portion of the total wind load (In Percentage) shared by different faces of the tower is shown in Table 2.Department of Civil Engineering, SJCE *

Fa=Cfa * Ac * P

Fs=Cfs * Ac * P


For the purpose of analysis, the tower is idealized as 3D truss elements. Joints are considered as pin joints having three degrees of freedom i.e., 3 translations only. Members of tower are connected to each other by bolting. Tower joints are semi-rigid in nature and modeling as pin joints having 3 degree of freedom provides results close to realistic behavior. In 3d truss analysis, member local stiffness matrices are of order 6 X 6. Member system matrix from local to global coordinate system is transformed with the help of usual transformation rule:

S, Kg are the member stiffness matrices in global and local coordinates respectively; T is transformation matrix and superscript T denotes transpose.

The structural stiffness matrix corresponding to all joint degrees of freedom is obtained by assembling the member stiffness matrices in global coordinates using the code number approach. The material behaviour is assumed to be linearly elastic.


S = TT * Kg * T


From the static analysis of tower, the member of axial forces for individual member are computed and after the analysis, tower members are grouped according to their (leg, side bracing, plan bracing) and panel position for design. Now maximum compressive and maximum tensile forces are sorted out for every group of members. These forces are then used to design the members as per IS Codal provisions.

Secondary bracings are provided so that slenderness ratio for compression members may be under limit as given table:Department of Civil Engineering, SJCE *



Type of memberMaximum L/r ratio permitted Leg 150 Main Bracing 200 Secondary Barcing 250


Analysis and design of 32 meter height microwave tower is presented here. Four antennas are mounted at top level of tower each having 1.0m dia. Fig. shows the configuration of example tower with all dimensional details. The tower is designed to support 4 antennas of 1m dia each at its top level. The tower is assumed to be suited in terrain type-II which is a terrain with well scattered obstructions having height generally 1.5m to 10m.

The basic wind speed is assumed as 39m/sec corresponding to wind zone-II. A 100 year return period of wind is assumed and a factor of 1.06 is taken for risk factor, topography is assumed similar to that of plane groundDepartment of Civil Engineering, SJCE *


A methodology is described in detail for design of antennatowers. The following conclusions are drawn from the numericalexample presented:

Design of leg members of tower is governed by the diagonal wind case.Design of main bracing is governed by load case corresponding to the wind blowing normal to the face of the tower.Design of bracing member to top panel is governed by minimum thickness criteria.The weight of secondary bracings is of the order of 25% of the total weight of tower.Base width of tower in the range of 1/5th of height results in minimum weight tower.



Gere, J.M. and Weaver, W.Jr., Analysis of framed Structure , D. Van Nostrand Co., Princeton, New Jersey, 1986.

Meek, J.L., Matrix structural Analysis, Mc.Graw Hill book company.

Document No.SMBD7(2005) P3,Draft Indian Standard Code of Practice for self supporting steel microwaves towers, Part I:Loads and permissible stresses, Indian Standard Institution, New Delhi 1981.

Wind Resistant Design of Microwave Towers - Sunil Saraswat, Mahesh Chandra and Rajesh Deoliya Wind Effects on Structures Allied Publishers, 1997-By S.K.Agarwal & Lakshmy.PDepartment of Civil Engineering, SJCE *


IS:802 (Part 1):1992, IS Code of Practice for use of structural steel in overhead transmission line tower, Part 1:Load permissible stresses, Indian Standard Institutions, New Delhi, 1978, Bureau of Indian Standard Publication.

IS:875 (Part 3) -1987, Code of practice for design loads[Other than earthquake] for building and structure, Bureau of Indian Standard Publication.

IS:800-1984, Code of Practice for design of steel structure, Bureau of Indian Standard Publication.

IS:4091-1979, Code of Practice for Design & Construction of Foundations for Transmission Line Towers & Poles (1st Revision) First Reprint September 1992-Bureau of Indian Standards, Manak Bhavan, 9Bahadur Shah Zafar Marg, New Delhi-110002.



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