3.0 Basic details of design of steel lattice antenna towers

3.1 Self supporting/self standing structures

Steel lattice towers have provided an economic solution to the communication industry over many

years. Most early towers were generally square in plan and constructed of bolted angle sections. The

design of such structures evolved rapidly with the advent of electricity transmission lines whose

towers were designed for maximum efficiency (i.e. Lowest weight of tower). Generally, each type of

electricity transmission line tower was subjected to full scale test to destruction for ensuring the

calculated design capacity and avoiding of immature mistakes in steel fabrication detailing, etc.

Typically lattice towers vary in face width from top to bottom and depending on the form of the

structure, different bracing patterns are adopted appropriate to the loading to be carried. Heights of

steel tower vary from 10m - 200m with the taller structure using built-up angle members for their legs

in the bottom panels.

In terms of the number used, self standing lattice towers are used more extensively than guyed masts

up to a height of about 150m. Above 150m height, the number used rapidly decline. It may be due to

the increase of cost more rapidly with it's height. The main advantage of self standing lattice

structures lie in their good torsional rigidity and in the elimination of stays (guy wires) with the related

savings in the area of the site.

Due to above advantages and cost effectiveness, the self standing steel lattice towers are the

common choice in the mobile communication networks. The height range of 10-80 meters is generally

used in above networks. While the triangular towers are used for lightly loaded structures, square

type heavy duty towers are adopted in heavily loaded or the locations where extreme weather

conditions exist. (i.e. Hill tops, Coastal areas, etc.)

3.2 Steel lattice type guyed mast or cable stayed masts

The design and construction of guyed lattice masts is based on and has developed from classical

beam and bridges, so that bracings, joints and other items follow the established practice with only

minor modifications to suit particular conditions and for attaching feeders and antennas.

However, unlike self supporting structures, as guyed masts consist of steel cables/ wire ropes as its

main structural element, non-linear analysis of the structure should be done for deciding its behavior

and dynamic response under its design wind loading. Although the guy mast towers having more

advantage on initial cost of construction, etc. the facts such as the larger land requirement,

comparatively lower torsional rigidity and high cost of maintenance is making the choice of self

standing tower more feasible in the range of 10-100 meters heights. However, for the towers with

more than 100m heights, the guyed masts always tend to be the most economical solution.

But, as the scope of this research is limited to only self supported steel lattice antenna towers, details

about steel lattice type guy masts will be not discussed further.

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Chapter 03 - Basic details for design of antenna towers

3.3 Design codes of practices - Brief history and versions currently in practice

• An American Trades body, the Electronics Industries Association (EIA) produced its own

standard EIA-222, for steel antenna towers and antenna supporting structures. The first version

of this was published in 1964 and the current version is ANSI/TIA-222-G (2005).

• The German code, DIN4131, for steel radio towers and masts was first published in 1969 and

has been regularly updated. The current version was published in 1991.

• United Kingdom produced a standard for loading of lattice towers, BS8100 Parti, in 1986.

Currently there are four (04) separate parts as follows,

1. BS8100, Par t i - Code of practice for loading, (1986)

2. BS8100, Part 2 - Guide to the background and use of part 1 (above) (1986)

3. BS8100, Part 3 - Code of practice for strength assessment of members of lattice

towers and masts (1999)

4. BS8100, Part 4 - Code of practice for loading of guyed masts (1995)

• The Canadian Standards Association (CSA-S37) published their first standard in 1965. The

current version was published in 2001.

• The Australian Standards first produced a Code AS3995, for design of steel lattice towers and

masts in 1991. The latest revision was issued in 1994.

• Euro code , 1993-1997 - (Part 3 - Design of steel structures, towers, mast and chimneys)

Other than above specially issued codes for steel lattice towers and masts, following codes of

practice are also still popular among the antenna tower designers. All of those codes are either code

of practice for electricity transmission line structures or wind loadings on structures.

• CP3 : Chapter V : Part2 : 1972 - Code of basic data for the design of buildings, published by

British standards institution. (This standard is obsolete).

• ASCE 10-97, Design of lattice steel transmission structures - a standard published by the

American Society of Civil Engineers in 1991. This was to develop a version of the " Guide for

design of steel Transmission towers, Manuals and reports on engineering practice - No 52",

first published in 1971 by the same above institution.

• IS 802 (Part1/Sec1), Code of practice for use of structural steel in overhead transmission line

towers published by The Bureau of Indian standards in 1995. While the above IS 802 deals

with materials, loads and stresses in overhead transmission line towers, The IS 875 (Part3) -

Indian code of practice for wind loads on building and structures, (first published in 1964) deals

about the wind loads. The most recent revision of IS 875 was issued in 1987.

• Although Sri Lanka has no code of practice of its own that related with towers or wind loads,

The Ministry of Local Government, Housing and Construction has prepared and published a

design manual called " Design of Buildings for High winds - Sri Lanka" in 1980. This document

contains some useful design data such as wind speed distribution within the island,

recommendations for design wind speed for different category of structures, density of air, etc.

• Similar codes may be available in other counties like China, South Africa, Poland, Russia, etc.

but there is no evidence about use of those codes for any antenna tower designs in Sri Lanka.

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Chapter 03 - Basic details for design of antenna towers

3.4 Common computer software

Although, there are many general structural engineering computer software available in practice,

none of them provide the in-built capacity of designing structural members according to accepted

antenna tower design codes. On other hand, there are several other computer software programs,

which are for antenna tower and related structure designs. However, if we are going to use firstly

explained type of software (i.e - STAAD Pro, SAP2000, etc.), then we should be careful about the

applying of all required FOS's and other design data ( for example, all nodes are to be pin jointed,

etc.) to the computer model. After the process of structural analysis, the results can be sorted,

grouped and use for designing each of members/member groups according to the respective

engineering code of practice. The designing of members has to be done using a separately

developed computer program or spreadsheet program (i.e - Excel, Lotus 123, etc.).

On the other hand, if we use latter explained type of software, then it will have more flexibility on both

structural analysis as well as member design processes, resulting in more economized structure with

comparatively in short designing period and less possibility of mistakes. Such computer programs

usually have useful in-built data such as different types of panels, antenna details, cables, etc too.

Therefore, such computer software can makes the antenna tower designing process more efficient

and accurate.

Some of popular antenna tower design computer software which are currently in practice are listed

below,

Item Name of software Supporting code of practices

1 MS TOWER BS8100, EIA22-G, EIA222-F, AS3995, etc.

2 TOWER EIA222-G, EIA222-F, CSA S37-01

3 RISA TOWER EIA222-G

4 I TOWER BS8100, EIA22-G, EIA222-F, Eurocode (ENV 1993-3-1:1997)

3 STAAD.TOWER EIA222-G, EIA222-F

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