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Reference Date CED 7/T- 65 18.06.2010

TECHNICAL COMMITTEE: Structural Engineering & Structural Sections Sectional Committee, CED 7 ________________________________________________________________ ADDRESSED TO: ALL MEMBERS OF CED 7 & CED 7:1 Dear Sir,

Please find enclosed the following documents:

Last date for comments: 17.07.2010 . Comments, if any, may please be made in the format as enclosed

herewith and mailed to the undersigned at the above address or sent over email. The documents are also hosted on BIS website www.bis.org.in. Thanking you, Yours faithfully, (J. Roy Chowdhury) Scientist-E (Civil Engg.) e-mail: jayanta@bis.org.in

Encl: as above

Doc No. Title

CED 7(7759)P Use of Steel in Communication Towers – Code of Practice

Preliminary Draft

CODE OF PRACTICE FOR SELF SUPPORTING MICROWAVE TOWERS USING STRUCTURAL STEEL

PART1: MATERIALS, LOADS AND PERMISSIBLE STRESSES

FORWARD (Formal clauses to be added later) With a view to establish uniform practice for design, fabrication, erection, testing and inspection of steel towers meant for communication purposes, the Bureau of Indian Standard is bringing out this code covering materials, loads and permissible stresses for self supporting microwave towers. Provision for fabrication, erection inspection and testing of these towers are covered in IS 800: 2007 and IS 802(part III): 1978. Provisions of this code are also applicable for towers meant for supporting flood lights etc. In the design of these towers, which are comparatively light structure and also that the wind pressure is the main criterion of their design, it was felt that simultaneous occurrence of earth quake and maximum wind pressure are unlikely to take place. Specific provisions of earthquake forces have therefore, not been specified in this standard. However in particular region where earthquakes are experienced frequently, earthquake forces may be considered in the design of these towers in accordance with IS 1893. As the code envisage for the present fabrication of these towers, by means of bolted connections only, the provisions of this code relates mainly to the structural steel conforming to IS 226 : 1975, grade Fe 410-0 of IS 1977 : 1975 and IS 961 : 1975. The permissible stresses not covered at present in this part may generally be adopted in accordance with IS 800. Numerical values of loads and stresses and formulae for calculating permissible stresses etc. have been given both in SI and metric units. It is proposed to change over to SI units completely in the near future.

In the preparation of this code, valuable assistance has been derived from the following publications:

1. DIN 4131 – Antennen tragwerke aus stahl; Berechnung und Ausfunrung 2. IS 875 (part 3) : 1987, Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures. Part 3 Wind loads (Second Revision). 3. Guide for Design of Steel Transmission Towers issued by American Society of Civil Engineers, New York, 1971. 4. ANSI/TIA–222–G : 2006, TIA STANDARD, Structural Standard for Antenna Supporting Structures and Antennas.

1 SCOPE This standard stipulates the various design considerations to be adopted in the design of self supporting steel lattice towers for telecommunication purposes, including components of antenna fixtures forming an integral part of the supporting structure. This part of the code covers materials, loads, combination of loads, permissible stresses and some design provisions.

NOTE – While formulating the provision of this code, it has been assumed that the tower consists of flat sided members and the structural connections are through bolts only.

Compliance with this code does not relieve any one from the responsibility of observing local and provisional building bylaws, fire and safety laws and civil aviation requirements pertaining to such structures. 2 TERMINOLOGY 2.1 Antenna Antenna means a conductor for radiating or receiving radio/ electromagnetic waves exclusive of the connecting wires between its main portion and the apparatus associated with it, and shall include any reflector, director and screen designed to produce a specific radiation pattern. 2.2 Antenna Assembly Antenna assembly means any structure comprising of designed and fabricated as an integral part of an antenna or an antenna array. 2.3 Antenna Supporting Structure A structure that support antennas or antenna arrays. 2.4 Return Period Return period is the mean interval between recurrences of a climatic event of defined magnitude. The inverse of return period gives the probability of exceeding the event in one year. 2.5 Safety The ability of a system not to cause human injuries or loss of life. It relates, in this code, mainly to protection of workers during construction and maintenance operations.

3 STATUTORY REQUIREMENTS Compliance with this standard does not relive any user from the responsibility of observing the local and provincial building bylaws, fire and safety laws and other civil aviation requirements applicable to such structures. 4 SYMBOLS Ae = effective frontal area Ae = effective sectional area Ag = gross cross – sectional area Ago = gross area of the outstanding leg An = net area of the total cross - section Anc = net area of the connected leg AtgAtn = minimum gross and net area in tension from the bolt hole to the toe of the angle, end bolt line, perpendicular to the line of force respectively (see Fig.4b) AvgAvn = minimum gross and net area in shear along line parallel to external force respectively (see Fig.4a) B = width of the plate Bs = shear leg width Cf = force coefficient or drag coefficient D = diameter of cylinder Dh = diameter of bolt hole (bolt dia + 2.0 mm) Є = yield stress ratio (250/fy)0.5 F = Force normal to the surface fcc = Euler buckling stress = π2E/(KL/R)2 fcd = Design compressive stress fu = characteristic ultimate tensile stress fy = characteristic yield stress g = gauge length between the bolt holes (Fig.3)

h = number of bolt holes in the critical section hx = height of development of a velocity profile at a distance x down wind from a change in terrain category. I = subscript for summation of all the inclined legs KL = effective length KL/r = effective slenderness ratio k1 = probability factor (risk coefficient) k2 = terrain, height and structure size factor k3 = topography factor Lc = length of P = factored applied axial force Pd = design wind pressure Pd = Design axial compressive strength Ps = staggered – pitch length between the bolt holes (Fig.3) Pz = design wind pressure in N/m2at height R = radius of gyration T = thickness of the leg T = Factored tension Td = design strength under axial tension Tdb = design strength of bolt under axial tension: Block strength at end connection Tdg = yielding strength of grass section under axial tension Tdn = rapture strength of net section under axial tension Vd = regional basic wind speed Vz = design wind velocity in at height W = Outstand leg width

X = stress reduction factor ratio Α = imperfection factor α = explained in the context β = expression explained in the context γm1 = partial safety factor against ultimate stress = 1.25 γm0 = partial safety factor against yielding stress and buckling = 1.10 λe = equivalent slenderness ratio λ = non - dimensional effective slenderness ratio 5 MATERIALS 5.1 Structural Steel The tower members including plate-forms shall be of structural steel conforming to any of the grade, as appropriate, of IS 2062. Medium and high strength structural steels with known properties conforming to other national and international standards may also be used subject to the approval of the purchaser. 5.2 Bolts Bolts for tower connections shall conform to IS 12427 or property class 4.6 or 5.6 conforming to IS 6639. High strength bolts, if used (only with structural steel of IS 8500) shall conform to property class 8.8 of IS 3757. Foundation bolts shall conform to IS 5624. 5.3 Nuts Nuts shall conform to IS 1363 (Part 3). The mechanical properties shall conform to property class 4 or 5 as the case may be as specified in IS 1367 (Part 6) except that the proof stress for nuts of property class 5 shall be as given in IS 12427. Nuts to be used with high strength bolts shall conform to IS 6623. 5.4 Washers Washers shall conform to IS 2016. Heavy washers shall conform to IS 6610. Spring washers shall conform to type B of IS 3063. Washers to be used with high strength bolts and nuts shall conform to IS 6649.

5.5 Galvanization Structural members of the towers, plain and heavy washers shall be galvanized in accordance with the provision of IS 4759. Threaded fasteners shall be hot dip galvanized to conform to the requirements of IS 1367 : (Part 13). Spring washers shall be hot dip galvanized as per service Grade 4 of IS 4759 or electro galvanized as per service Grade 3 of IS 1573 as specified by the purchaser. 5.6 Other Materials Other materials used in the construction of the tower shall conform to appropriate Indian Standards wherever available. 6 LOADS 6.1 General Loads in general shall be according to the relevant provisions of IS:875 unless otherwise specified herein under. 6.2 Types of Loads The loading for the design of towers shall be made up as follows and shall be considered acting as specified below:

a) Dead load of the system b) Loads resulting from attachment of supplementary c) Live loads d) Snow load, if applicable e) Wind loads f) Seismic load g) Any other loads as specified (wind on ice).

6.3 Dead Load of the system The dead weight of the structure, antenna supporting fixtures, platforms, ladders etc. Loads resulting from the attachments/appurtenances; the weight of the antennas, antenna mounts, power/signal cables, conduits, lighting equipments, climbing devices, sign boards, anti-climbing devices etc. 6.4 Live loads On ladders and plateforms, etc. a uniformly distributed live load of 2 000 N/sqm (200 kg/sqm) shall be assumed. This also applies for the surfaces of bases with aerials. On the individual parts of the ladders, plateforms etc. shall be designed for a lifting (single) load of 3 000 N (300 kg) at the most unfavourable places if this is more unfevourable than the above mentioned load. A horizontal load of 500 N/sqm (50 kg/sqm) acting inward or outward on the railing shall also be assumed.

6.5 Snow Loads Unless the authentic data for snow loads in the zone, where the tower is to be erected, are available and if the snow load is to be taken into account, a uniform 30 mm. Icing on structural members, cables and aerials may be assumed or as specified by the authorities concerned. 6.6 Wind Loads The total wind load F on tower surfaces shall be calculated as follows and shall be assumed as acting horizontally:

F = Cf.P.Ac Where, Cf = force coefficient/ drag coefficient Pd = design wind pressure Ac = effective frontal area.

NOTE – While calculating the surface area of tower members facing the wind direction an increase of 10% shall be made on account of gusset plates, sign boards etc.

The force coefficient/drag coefficient and dynamic wind pressure shall be considered as per IS : 875 (Part 3). However the force coefficient for elements involved in tower design and overall force coefficient for lattice type of towers are extracted from IS : 875 (Part 3) and give for reference. 6.6.1 Force coefficients for wires and cables shall be as given in Table 1 according to the diameter (D), the design wind speed (Vd) and the surface roughness.

Table 1 Force Coefficients for Wires and Cables (I / D = 100) (Clause 6.6)

Force Coefficient, Cf for

Sl No.

Flow Regime

Smooth Surface

Moderately Smooth Wire

(Galvanized orPainted)

Fine Stranded Cables

Thick Stranded Cables

(1) (2)

(3) (4) (5) (6)

i) D/Vd < 0.6 m2/s

- - 1.2 1.3

ii) D/Vd > 0.6 m2/s

- - 0.9 1.1

iii) D/Vd < 0.6 m2/s

1.2 1.2 - -

iv) D/Vd < 6 m2/s

0.5 0.7 - -

6.6.1.1 Single Frames − Force coefficients for a single frame having either a) all flat sided members, or b) all circular members in which all the members of the frame have either, 1) DVd less than 6 m2/s, or 2) DVd greater than 6 m2/s; shall be as given in Table 2 according to the type of the member, the diameter (D), the design wind speed (Vd ) and the solidity ratio (Φ).

Table 2 Force Coefficients for Single Frames (Clause 6.6.1)

Force Coefficient, Cf for

Circular Sections

Sl No.

Solidity Ratio Φ

Flat-sided Members

Subcritical flow (DVd < 0.6 m2/s)

Supercritical flow (DVd . 0.6 m2/s)

(1) (2) (3) (4) (5)

i) 0.1 1.9 1.2 0.7 ii) 0.2 1.0 1.2 0.8 iii) 0.3 1.7 1.2 0.8 iv) 0.4 1.7 1.1 0.8 v) 0.5 1.6 1.1 0.8 vi) 0.75 1.6 1.5 1.4 vii) 1.00 2.0 2.0 2.0 NOTE – Linear interpolation between values is permitted. Force coefficients for a single frame not complying with the above requirements shall be calculated as follows:

)(sup γγ −+= lCC erff flatfsub

flatfsub

sub

circsub CAA

lCA

A)( γ−+

Where =fsuperC force coefficient for the supercritical circular members as given in Table 2 =fsubC force coefficient for the subcritical circular members as given in Table 2 =fflatC force coefficient for the flat sided members as given in Table 2 =circsubA effective area of subcritical circular members

=flatA effective area of flat-sided members =ubsA +circsubA flatA

=γ eflowcalSupercriti

inflametheofAreaA

a

6.6.2.1 Multiple frame buildings This section applies to structures having two or more parallel frames where the windward frames may have a shielding effect upon the frames to leeward side. The windward frame and any unshield parts of other frames shall be calculated in accordance with ‘single frame’, but the wind load on the parts of frames that are sheltered should be multiplied by a shielding factor which is dependent upon the solidity ratio of the windward frame, the types of the members comprising the frame and the spacing ratio of the frames. The values of the shielding factors are given in Table 3.

Table 3 Shielding Factor Η for Multiple Frames (Clause 6.6.2)

Sl No.

Effective Solidity Ratio β

Frame Spacing Ratio

< 0.5 1.0 2.0 4.0 > 8.0

(1) (2) (3) (4) (5) (6) (7)

i) 0 1.0 1.0 1.0 1.0 1.0 ii) 0.1 0.9 1.0 1.0 1.0 1.0

iii) 0.2 0.8 0.9 1.0 1.0 1.0

iv) 0.3 0.7 0.8 1.0 1.0 1.0

v) 0.4 0.6 0.7 1.0 1.0 1.0

vi) 0.5 0.5 0.6 0.9 1.0 1.0

vii) 0.7 0.3 0.6 0.8 0.9 1.0

viii) 1.0 0.3 0.6 0.6 0.8 1.0 Linear interpolation between values is permitted

Where there are more than two frames of similar geometry and spacing, the wind load on the third and subsequent frames should be taken as equal to that on the second frame. The loads on the various frames shall be added to obtain total load on the structure. The frame spacing ratio is equal to the distance, centre to centre of the frames, beams or girders divided by the least overall dimension of the frame, beam or girder measured at right angles to the direction of the wind. For triangular framed structures or rectangular framed structures diagonal to the wind, the spacing ratio should be calculated from the mean distance between the frames in the direction of the wind. Effective solidity ratio, β = Φ for flat sided members. Β is to be obtained from Fig 1 for members of circular cross sectiona.

SOLIDITY RATIO, Φ

FIG. 1 EFFECTIVE SOLIDITY RATIO, β FOR ROUND MEMBERS 6.6.3 Lattice Towers − Force coefficient for lattice towers of square or equilateral triangle section with flat sided members for wind blowing against any face shall be as given in Table 4. For square lattice towers with flat sided members the maximum load, which occurs when the wind blows into a corner shall be taken as 1.2 times the load for the wind blowing against a face. For equilateral triangle lattice towers with flat sided members, the load may be assumed to be constant for any inclination of wind to a face. Force coefficients for lattice towers of square section with circular members, all in the same flow regime, may be as given in Table 5. Force coefficients for lattice towers of equilateral triangle section with circular members all in the same flow regime may be as given in Table 6.

Table 4 Overall Force Coefficient for Towers Composed of Flat Sided Members

(Clause 6.6.3)

Sl No.

Solidity Ratio Φ

Force Coefficient for

Square Towers Equilateral Triangular Towers

(1) (2) (3) (4) i) 0.1 3.8 3.1

ii) 0.2 3.3 2.7 iii) 0.3 2.8 2.3 iv) 0.4 2.3 1.9 v) 0.5 2.1 1.5

Table 5 Overall Force Coefficient for Square Towers Composed of Rounded Members

(Clause 6.6.3)

Table 6 Overall Force Coefficient for Equilateral Triangular Towers

Composed of Rounded Members (Clause 6.6.3)

Sl No.

Solidity Ratio of

Front Face

Force Coefficient for

Φ Subcritical flow ( 6.0<dDV m2/s)

Supercritical flow ( 6.0>dDV m2/s)

On to face On to corner On to face On to corner (1) (2) (3) (4) (5) (6)

i) 0.05 2.4 2.5 1.1 1.2

ii) 0.1 2.2 2.3 1.2 1.3

iii) 0.2 1.9 2.1 1.3 1.6

iv) 0.3 1.7 1.9 1.4 1.6

v) 0.4 1.6 1.9 1.4 1.6

vi) 0.5 1.4 1.9 1.4 1.6

Sl No.

Solidity Ratio of Front Face

Force Coefficient for

Φ Subcritical flow ( 6.0<dDV m2/s)

Supercritical flow ( 6.0>dDV m2/s)

All wind directions

All wind directions

(1) (2) (3)

(4)

i) 0.05 1.8 0.8

ii) 0.1 1.7 0.8

iii) 0.2 1.6 1.1

iv) 0.3 1.5 1.1

v) 0.4 1.5 1.1

vi) 0.5 1.4 1.2

6.6.4 Tower Appurtenances − The wind loading on tower appurtenances, such as ladders, conduits, lights, elevators, etc, shall be calculated using appropriate not pressure coefficients for these elements. Allowance may be made for shielding effect from other elements.

6.7 Design Wind Pressure The design wind pressure at any height above mean ground level shall be obtained by the following relationship between wind pressure and wind velocity: Where zp = design wind pressure in N/m2 at height z, and zV = design wind velocity in m/s at height z.

NOTE − The coefficient 0.6 (in SI units) in the above formula depends on a number of factors and mainly on the atmospheric pressure and air temperature. The value chosen corresponds to the average appropriate Indian atmospheric conditions.

6.8 Basic Wind Speed Basic wind speed map of India, as applicable to 10 m height above mean ground level for different zones of the country is given in Fig. 2. The basic wind speed is based on peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to mean heights above ground level in an open terrain (Caregory 2). Basic wind speeds presented in Fig 2 have been worked out for 50 years return period. Basic wind speed for some important cities/towns is also given in Annex A. 6.9 Design Wind Speed (Vz) The basic wind speed (Vb) for any site shall be obtained from Fig 2 and shall be modified to include the followinf effects to get design wind spped at any height (Vz) for the chosen structure:

i) Risk level; ii) Terrain roughness, height and size of structure; and iii) Local topography.

It can be mathematically expressed as:

321bz kkkVV = Where =zV design wind speed at any height z in m/s; =1k probability factor (risk coefficient); =2k terrain, height and structure size factor; and =3k topography factor as explained in the following paragraph. NOTE – Design wind speed up to 10 m height from mean ground shall be considered constant.

2zz Vp 6.0=

6.9.1 Risk coefficient factor (k1 Factor) Fig. 2 gives the basic wind speeds for terrain Category 2 as applicable at 10 m above ground level based on 50 years mean return period. The suggested life period to be assumed in design and the corresponding k1 factors for different class of structures for the purpose of design is given in Table 7. In the design of all structures, a regional basic wind speed having a mean return period of 50 years shall be used except as specified in the note of Table 7. 6.9.2 Terrain, height and structure size factor ( k2 Factor) 6.9.2.1 Terrain – Selection of terrain categories shall be made with due regard to the effect of obstructions which constitute the ground surface roughness. The terrain Category used in the design of a structure may vary depending on the direction of wind under consideration. Wherever sufficient meteorological information is available about the nature of wind direction, the orientation of any building or structure may be suitably planned. Terrain in which a specific structure stands shall be assessed as being one of the following terrain categories:

a) Category 1 : Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the structure is less than 1.5m.

NOTE – This category includes open sea-coasts and flat treeless plains.

b) Category 2 : Open terrain with well scattered obstructions having heights generally between 1.5 to 10 m.

NOTE – This is the criterion for measurement of regional basic wind speeds and includes airfields, open parklands and undeveloped sparsely built-up outskirts of towns and suburbs. Open land adjacent to sea coast may also be classified as Category 2 due to roughness of large sea waves at high winds.

c) Category 3 : Terrain with numerous closely spaced obstructions having the size of building –structures up to 10m in height with or without a few isolated tall structures.

NOTES 1 This category includes well wooded areas, and shrubs, towns and industrial areas full or partially developed. 2 It is likely that the next higher category than this will not exist in most design situations and that selection of a more severe category will be deliberate. 3 Particular attention must be given to performance of obstructions in areas affected by fully developed tropical cyclones. Vegetation which is likely to be blown down or defoliated cannot be relied upon to maintain Category 3 conditions. Where such situation may exist, either an intermediate category with velocity multipliers midway between the values for a Category 2 and 3 given in Table 2, or Category 2 should be selected having due regard to local conditions.

Fig 2 Basic Wind Speed Map of India

Table 7 Risk Coefficients for Different Classes of Structures in Different Wind Zones

(Clause 6.9.1)

Sl No.

Class of Structure Mean Probable

Design Life of Structure

in Years

k1 Factor for Basic Wind Speed of m/s

33 39 44 47 50 55

(1) (2) (3) (4) (5) (6) (7) (8) (9)

i) All general building and structures

50 1.0 1.0 1.0 1.0 1.0 1.0

ii) Important buildings and structures such as hospitals,communication buildings/towers, power plant structures.

100 1.05 1.06 1.07 1.07 1.08 1.08

NOTES: 1 The factor is based on statistical concepts which take account of the degree of reliability required and period of time in years during which these will be exposure to wind, that is, life of the structure. Whatever wind speed is adopted for design purposes, there is always a probability (however small) that it may be exceeded in a storm of exceptional violence; the greater the period of years over which these will be exposed to the wind, the greater is the probability. Higher return periods ranging from 100 to 1000 years (implying lower risk level) in association with greater periods of exposure may have to be selected for exceptionally important structures, such as, nuclear power reactors and satellite communication towers. Equation given below may be used in such cases to estimate k1 factors for different periods of exposure and chosen probability of exceedance (risk level). The probability level of 0.63 is normally considered sufficient for design of buildings and structures against wind effects and the values of k1 corresponding to this risk level are given above.

2 ( )( )

BA

PlN

lBA

xk

4

11

x

Nnn

50,0.62

PN,1 +

⎥⎦

⎤⎢⎣

⎡

⎭⎬⎫

⎩⎨⎧ −−

−==

Where N = mean propable design life of structure in years; NP = risk level in N consecutive years (probability that the design wind speed is exceeded at least once in N successive years), nominal value = 0.63; PN,X = extreme wind speed for a given value of N and NP ; and

63.0,50X = extreme wind speed for N = 50 years and NP = 0.63. A and B are coefficients having the following values for different basic wind speed zones:

Zone A B 33 m/s 83.2 9.2 39 m/s 84.0 14.0 44 m/s 88.0 18.0 47 m/s 88.0 20.5 50 m/s 88.8 22.8 55 m/s 90.8 27.3

d) Category 4 : Terrain with numerous large high closely spaced obstructions.

NOTE – This category includes large city centres, generally with obstructions above 25m and well developed industrial complexes.

6.9.2.2 Variation of wind speed with height for different sizes of structures in different terrains ( factor) – Table 8 gives multiplying factors (k2) by which the basic wind speed given in Fig. 2 shall be multiplied to obtain the wind speed at different heights, in each terrain category for different sizes of structures. The buildings / structures are classified into the following three different classes depending upon their size:

a) Class A : Structures and/or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) less than 20m.

b) Class B : Structures and / or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) between 20m and 50m. c) Class C : Structures and / or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) greater than 50m.

Table 8 k2 Factors to Obtain Design Wind Speed Variation With Height in Different Terrains for Different Classes of Building/Structures

(Clause 5.3.2.2)

Sl No.

Height Terrain Category 1

Terrain Category 2

Terrain Category 3

Terrain Category 4

m Class

Class

Class

Class

A B C A B C A B C A B C (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

i) 10 1.05 1.03 0.99 1.00 0.98 0.93 0.91 0.88 0.82 0.80 0.76 0.67 ii) 15 1.09 1.07 1.03 1.05 1.02 0.97 0.97 0.94 0.87 0.80 0.76 0.67 iii) 20 1.12 1.10 1.06 1.07 1.05 1.00 1.01 0.98 0.91 0.80 0.76 0.67 iv) 30 1.15 1.13 1.09 1.12 1.10 1.04 1.06 1.03 0.96 0.97 0.93 0.83 v) 50 1.20 1.18 1.14 1.17 1.15 1.10 1.12 1.09 1.02 1.10 1.05 0.95

vi) 100 1.26 1.24 1.20 1.24 1.22 1.17 1.20 1.17 1.10 1.20 1.15 1.05 vii) 150 1.30 1.28 1.24 1.28 1.25 1.21 1.24 1.21 1.15 1.24 1.20 1.10 viii) 200 1.32 1.30 1.26 1.30 1.28 1.24 1.27 1.24 1.18 1.27 1.22 1.13 ix) 250 1.34 1.32 1.28 1.32 1.31 1.26 1.29 1.26 1.20 1.28 1.24 1.16 x) 300 1.35 1.34 1.30 1.34 1.32 1.28 1.31 1.28 1.22 1.30 1.26 1.17

xi) 350 1.37 1.35 1.31 1.36 1.34 1.29 1.32 1.30 1.24 1.31 1.27 1.19 xii) 400 1.38 1.36 1.32 1.37 1.35 1.30 1.34 1.31 1.25 1.32 1.28 1.20 xiii) 450 1.39 1.37 1.33 1.38 1.36 1.31 1.35 1.32 1.26 1.33 1.29 1.21 xiv) 500 1.40 1.38 1.34 1.39 1.37 1.32 1.36 1.33 1.28 1.34 1.30 1.22

6.9.2.3 Terrain categories in relation to the direction of wind – The terrain category used in the design of a structure may vary depending on the direction of wind under consideration. Where sufficient meteorological in information is available, the basic wind speed may be varied for specific wind direction. 6.9.2.4 Changes in terrain categories – The velocity profile for a given terrain category does not develop to full height immediately with the commencement of that terrain category but develop gradually to height (hx) which increases with the fetch or upwind distance (x). The relation between the developed height (hx) and the fetch (x) for wind flow over each of the four terrain categories may be taken as given in Table 9. For structures of heights greater than the developed height (hx) in Table 9, the velocity profile may be determined in accordance with the following:

i) The less or least rough terrain, or ii) The method described in Appendix B of IS: 875 (Part 3).

Table 9 Fetch and Developed Height Relationship (Clause 6.9.2.4)

Developed Height, hx

m

Sl No.

Fetch

Terrain Category 1

Terrain Category 2

Terrain Category 3

Terrain Category 4

(1) (2) (3) (4) (5) (6)

i) 0.2 12 20 35 60 ii) 0.5 20 30 35 95 iii) 1 25 45 80 130 iv) 2 35 65 110 190 v) 5 60 100 170 300 vi) 10 80 140 250 450 vii) 20 120 200 350 500 viii) 50 180 300 400 500

6.9.2.5 Topography (k3 Factor) – The basic wind speed Vb given in Fig 1 takes account of the general level of site above sea level. This does not allow for local topographic features such as hills, valleys, cliffs, escarpments, or ridges which can significantly affect wind speed in their vicinity. The effect of topography is to accelerate wind near the summits of hills or crests of cliffs, escarpments or ridges and decelerate the wind in valleys or near the foot of cliffs, step, escarpments, or ridges. The effect of topography will be significant at a site when the upwind slope (θ) is greater than about 3 degree and below that, the value of k3 may be taken to be equal to 1.0. The value of k3 is confined in the range of 1.0 to 1.36 for slopes

greater than 3 degree. The method of evaluating the value of k3 for values greater than 1.0 is given in Annex G of IS: 875 (Part 3). It may be noted that the value of k3 varies with height above ground level, at a maximum near the ground, and reducing to 1.0 at higher levels. 6.9.2.6 Dynamic Pressure of Wind, P – On the basis of measured maximum wind velocities for different parts of the country including wind of short durations as in squall, the country has been divided into three zones of low, medium and heavy wind pressures. The dynamic wind pressure, P shall be decided by the appropriate authority having regard to meteorological data. In the absence of any authentic meteorological data, the dynamic wind pressure shall be assumed as given in section 7 and 8 of IS:875 (part 3).

NOTES The wind pressure given shall be assumed as acting horizontally on the face of the tower. For square towers, the maximum load occurs when wind blows diagonally to the face on tower. For square towers the wind load shall thus be increased by factor (1.414) and shall be assumed as acting along a diagonal. Where two antennae are attached to a structure at the same level and are aligned in the direction of wind so that one may partially shielded the other the total wind load on the antennas so aligned shall be considered equal to 1.5 times the wind load for one antenna. In case of icing, the wind pressure on the additional face area shall be reduced to 75 percent of the dynamic wind pressure. In the case of latticed tower, the force coefficient Cf shall be applied to the solidity ratio φ altered by icing. The wind load on ladders, power conductors or conduits, antennae, transmission lines, service struts etc. or those members occurring in only one face of the structure, shall be obtained by considering the unit basic wind pressure with appropriate force coefficient as acting, normal to one time, the exposed projected area, including the specified ice covering, if any, of such attachment. The seismic load on structure shall assumed in accordance with relevant clauses of IS:1893. Other loads due to handling, erection, temperature variation, settlement of foundation etc. shall also be taken into account wherever necessary. The design stresses in each part of the structure shall be the most unfavourable values obtained from the combination due to various loads as given below:

Sl No.

Condition of Load

Normal Zone Snow Zone

(1) (2) (3) (4)

i) Storm (wind) 1.2DL+LL+1.5WL+ML 1.2DL+LL+1.5WLI+DLI+ML ii) Seismic 1.2DL+LL+EL+ML 1.2DL+DLI+EL+ML

7 DESIGN OF TENSION MEMBERS 7.1 Tension Members Tension members are linear members in which axial forces act to cause elongation (stretch). Such members can sustain loads upto the ultimate load, at which stage they may fail by rupture at a critical section. However, if the gross area of the member yields over a major portion of its length before the rupture load is reached, the member may become non-functional due to excessive elongation. Plates and other rolled sections in tension may also fail by block shear of end bolted regions (Fig. 4A and 4B). The factored design tension T, in the members shall satisfy the following requirement:

T <Td where Td = design strength of the member The design strength of a member under axial tension , Td, is the lowest of the design strength due to yielding of gross section, Tdg; the rupture strength of critical section, Tdn; and block shear Tdb; given below respectively. 7.2 Design Strength Due to Yielding of Gross Section The design strength of members under axial tension, Tdg; as governed by yielding of gross section, is given by

Tdg = Ag. fy / γm0

where fy = yield stress of the material, Ag = gross area of cross-section, a γm0 = partial safety factor for failure in tension by yielding = 1.10. 7.3 Design Strength Due to Rupture of Critical Section 7.3.1 Plates The design strength in tension of a plate, Tdn, as governed by rupture of net cross-sectional area, An , at the holes is given by

Tdg = 0.9 An. fu / γm1

where

γm1 = partial safety factor for failure at ultimate stress (see Table 5), .fu = ultimate stress of the material, An = net effective area of the member given by,

An = tg

pndb

i ⎥⎦

⎤⎢⎣

⎡+− ∑

i

sih 4

2

where b, t = width and thickness of the plate, respectively. dh = diameter of the bolt hole (2 mm in addition to the diameter of the hole, in case the directly punched holes) g = gauge length between the bolt holes, as shown in Fig.3, ps = staggered pitch length between line of bolt holes, as shown in Fig.3, n = number of bolt holes in the critical section, and i = subscript for summation of all the inclined legs. 7.3.2 Threaded Rods The design strength of threaded rods in tension, Tdn, as governed by rupture is given by

Tdn = 0.9 An fu / γm1

where An = net root area at the threaded section. 7.3.3 Single Angles The rupture strength of an angle connected through one leg is affected by shear lag. The design strength, Tdn, as governed by rupture at net section is given by:

Tdn = 0.9 Anc fu / γm1 + β Ago fy /γm0

where

β = 1.4 – 0.076 (w/t) (fy/fu) (bs/Lc ) ≤ (fuγm0/fyγm1) ≥ 0.7

where w = outstand leg width, bs = shear lag width, as shown in Fig. 6, and Lc = length of the end connection, that is the distance between the outermost bolts in the end joint measured along the load direction or length of the weld along the load direction.

FIG. 3 ANGLE WITH SINGLE LEG CONNECTION For preliminary sizing, the rupture strength of net section may be approximately taken as:

Tdn = α An fu /γm1 where α = 0.6 for one or two bolts, 0.7 for three bolts and 0.8 for four or more bolts along the length in the end connection or equivalent weld length, An = net area of the total cross-section, Anc = net area of the connected leg, Ago = gross area of the outstanding leg, and t = thickness of the leg. 6.3.4 Other Section The rupture strength, Tdn, of the double angles, channels, I-sections and other rolled steel sections, connected by one or more elements to an end gusset is also governed by shear lag effects. The design tensile strength of such sections as governed by tearing of net section may also be calculated using equation in 6.3.3, where β is calculated based on the shear lag distance, bs, taken from the farthest edge of the outstanding leg to the nearest bolt/weld line in the connected leg of the cross-section.

6.4 Design Strength Due to Block Shear The strength as governed by block shear at an end connection of plates and angles is calculated as given below: 6.4.1 Bolted Connections The block shear strength, db of connection shall be taken as the smaller of,

⎥⎥⎦

⎤

⎢⎢⎣

⎡+=

m1

utn

m0

yvgdb γγ

fAfAT

9.03

⎥⎥⎦

⎤

⎢⎢⎣

⎡+=

m0

ytg

m1

udb γγ

fAfAT vn

39.0

where Avg, Avn = minimum gross and net area in shear along bolt line parallel to external force, respectively (1-2 and 3-4 as shown in Fig. 4A and 1-2 as shown in Fig. 4B), Atg, Atn = minimum gross and net area in tension from the bolt hole to the toe of the angle, end bolt line, perpendicular to the line of force, respectively (2-3 as shown in Fig. 4B), and fu, fy = ultimate and yield stress of the material, respectively.

FIG. 4 BLOCK SHEAR FAILURE

7 DESIGN OF COMPRESSION MEMBERS 7.1 Design Strength Common hot rolled and built-up steel members used for carrying axial compression, usually fail by flexural buckling. The buckling strength of these members is affected by residual stresses, initial bow and accidental eccentricities of load. To account for all these factors, the strength of members subjected to axial compression is defined by various buckling classes. The design compressive strength Pd, of a member is given by:

P < Pd where Pd = Ae fcd where Ae = effective sectional area as defined in (Effective Sectional Area) fcd = design compressive stress, obtained as per below The design compressive stress, of axially loaded compression members shall be calculated using the following equation:

[ ] m0ym0ym0y

cd γγχλϕφ

γ//

/5.022

fff

f ≤=−+

=

where � = 0.5 [1 + α(λ – 0.2) + λ2] λ = non-dimensional effective slenderness ratio = ccy ff / = ( ) ErKLf 22/ πy fcc = Euler buckling stress = π2E / (KL/r2) where KL/r = effective slenderness ratio or ratio of effective length, KL to appropriate radius of gyration, r α = imperfection factor = 0.49 χ = stress reduction factor (Table 10) for different buckling class, slenderness ratio and yield stress.

= ( )[ ]5.022

1λϕφ −+

λmo = partial safety factor for material strength. Calculated values of design compressive stress, fcd for different slenderness ratios are given in Table 11.

Table 10 Stress Reduction Factor, χ for Column Buckling (Angle Section) ____________________________________________________________________________________________________________

Yield stress, fy (MPa)

KL/r ↓

200 210 220 230 240 250 260 280 300 320 340 360 380 400 420 450 480 510 540 10 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

20 0.999 0.997 0.994 0.992 0.990 0.987 0.985 0.981 0.976 0.972 0.968 0.964 0.961 0.957 0.953 0.948 0.943 0.938 0.933

30 0.948 0.944 0.941 0.937 0.933 0.930 0.926 0.920 0.913 0.907 0.901 0.895 0.889 0.883 0.877 0.869 0.861 0.853 0.845

40 0.896 0.891 0.885 0.880 0.875 0.870 0.866 0.856 0.847 0.838 0.829 0.820 0.812 0.803 0.795 0.783 0.771 0.760 0.748

50 0.841 0.834 0.827 0.821 0.814 0.807 0.801 0.788 0.776 0.763 0.752 0.740 0.729 0.717 0.706 0.690 0.675 0.660 0.645

60 0.783 0.774 0.765 0.757 0.748 0.740 0.732 0.716 0.700 0.685 0.670 0.656 0.642 0.628 0.615 0.596 0.578 0.561 0.544

70 0.722 0.711 0.700 0.690 0.680 0.670 0.660 0.641 0.623 0.605 0.588 0.572 0.557 0.542 0.528 0.508 0.489 0.471 0.454

80 0.659 0.646 0.634 0.622 0.611 0.600 0.589 0.568 0.548 0.529 0.512 0.495 0.479 0.464 0.450 0.430 0.412 0.395 0.379

90 0.596 0.583 0.569 0.557 0.544 0.533 0.521 0.499 0.479 0.460 0.443 0.426 0.411 0.397 0.383 0.365 0.348 0.332 0.318

100 0.536 0.522 0.508 0.495 0.483 0.471 0.459 0.438 0.418 0.400 0.384 0.368 0.354 0.341 0.328 0.311 0.296 0.282 0.269

110 0.480 0.466 0.453 0.440 0.428 0.416 0.405 0.385 0.366 0.349 0.333 0.319 0.306 0.294 0.283 0.268 0.254 0.242 0.230

120 0.430 0.416 0.403 0.391 0.379 0.368 0.358 0.339 0.321 0.306 0.291 0.278 0.267 0.256 0.246 0.232 0.220 0.209 0.199

130 0.385 0.372 0.360 0.348 0.337 0.327 0.317 0.299 0.283 0.269 0.256 0.244 0.234 0.224 0.215 0.203 0.192 0.182 0.173

140 0.346 0.333 0.322 0.311 0.301 0.291 0.282 0.266 0.251 0.238 0.227 0.216 0.206 0.197 0.189 0.178 0.168 0.160 0.152

150 0.311 0.300 0.289 0.279 0.269 0.261 0.252 0.237 0.224 0.212 0.202 0.192 0.183 0.175 0.168 0.158 0.149 0.141 0.134

160 0.281 0.270 0.260 0.251 0.242 0.234 0.227 0.213 0.201 0.190 0.180 0.172 0.164 0.156 0.150 0.141 0.133 0.126 0.120

170 0.255 0.245 0.236 0.227 0.219 0.212 0.205 0.192 0.181 0.171 0.162 0.154 0.147 0.140 0.134 0.126 0.119 0.113 0.107

180 0.232 0.223 0.214 0.206 0.199 0.192 0.186 0.174 0.164 0.155 0.147 0.139 0.133 0.127 0.121 0.114 0.107 0.102 0.096

190 0.212 0.203 0.195 0.188 0.181 0.175 0.169 0.158 0.149 0.140 0.133 0.126 0.120 0.115 0.110 0.103 0.097 0.092 0.087

200 0.194 0.186 0.179 0.172 0.166 0.160 0.154 0.144 0.136 0.128 0.121 0.115 0.110 0.105 0.100 0.094 0.089 0.084 0.079

210 0.178 0.171 0.164 0.158 0.152 0.146 0.141 0.132 0.124 0.117 0.111 0.105 0.100 0.096 0.092 0.086 0.081 0.076 0.072

220 0.164 0.157 0.151 0.145 0.140 0.135 0.130 0.122 0.114 0.108 0.102 0.097 0.092 0.088 0.084 0.079 0.074 0.070 0.066

230 0.152 0.145 0.140 0.134 0.129 0.124 0.120 0.112 0.105 0.099 0.094 0.089 0.085 0.081 0.077 0.073 0.068 0.065 0.061

240 0.141 0.135 0.129 0.124 0.120 0.115 0.111 0.104 0.098 0.092 0.087 0.082 0.078 0.075 0.071 0.067 0.063 0.060 0.056

250 0.131 0.125 0.120 0.115 0.111 0.107 0.103 0.096 0.090 0.085 0.081 0.076 0.073 0.069 0.066 0.062 0.058 0.055 0.052

_____________________________________________________________________________________________________________

Table 11 Design Compressive Stress fcd (Angle Section) ___________________________________________________________________________________________________________

Yield stress, fy, (MPa)

KL/r ↓

200 210 220 230 240 250 260 280 300 320 340 360 380 400 420 450 480 510 540 10 182 191 200 209 218 227 236 255 273 291 309 327 345 364 382 409 436 464 491

20 182 190 199 207 216 224 233 250 266 283 299 316 332 348 364 388 412 435 458

30 172 180 188 196 204 211 219 234 249 264 278 293 307 321 335 355 376 395 415

40 163 170 177 184 191 198 205 218 231 244 256 268 280 292 304 320 337 352 367

50 153 159 165 172 178 183 189 201 212 222 232 242 252 261 270 282 295 306 317

60 142 148 153 158 163 168 173 182 191 199 207 215 222 228 235 244 252 260 267

70 131 136 140 144 148 152 156 163 170 176 182 187 192 197 202 208 213 218 223

80 120 123 127 130 133 136 139 145 149 154 158 162 165 169 172 176 180 183 186

90 108 111 114 116 119 121 123 127 131 134 137 140 142 144 146 149 152 154 156

100 97.5 100 102 104 105 107 109 112 114 116 119 120 122 124 125 127 129 131 132

110 87.3 89.0 90.5 92.0 93.3 94.6 95.7 97.9 100 102 103 104 106 107 108 110 111 112 113

120 78.2 79.4 80.6 81.7 82.7 83.7 84.6 86.2 87.6 88.9 90.1 91.1 92.1 93.0 93.8 94.9 95.9 96.8 97.6

130 70.0 71.0 71.9 72.8 73.5 74.3 75.0 76.2 77.3 78.3 79.2 80.0 80.7 81.4 82.0 82.9 83.6 84.3 84.9

140 62.9 63.6 64.4 65.0 65.6 66.2 66.7 67.7 68.6 69.3 70.0 70.7 71.2 71.8 72.3 72.9 73.5 74.1 74.6

150 56.6 57.2 57.8 58.3 58.8 59.2 59.7 60.4 61.1 61.7 62.3 62.8 63.3 63.7 64.1 64.6 65.1 65.5 65.9

160 51.1 51.6 52.1 52.5 52.9 53.3 53.6 54.2 54.8 55.3 55.7 56.1 56.5 56.9 57.2 57.6 58.0 58.4 58.7

170 46.4 46.8 47.1 47.5 47.8 48.1 48.4 48.9 49.3 49.8 50.1 50.5 50.8 51.1 51.3 51.7 52.0 52.3 52.6

180 42.2 42.5 42.8 43.1 43.4 43.6 43.9 44.3 44.7 45.0 45.3 45.6 45.8 46.1 46.3 46.6 46.9 47.1 47.3

190 38.5 38.8 39.0 39.3 39.5 39.7 39.9 40.3 40.6 40.9 41.1 41.4 41.6 41.8 42.0 42.2 42.5 42.7 42.9

200 35.3 35.5 35.7 35.9 36.1 36.3 36.5 36.8 37.0 37.3 37.5 37.7 37.9 38.1 38.2 38.4 38.6 38.8 39.0

210 32.4 32.6 32.8 33.0 33.1 33.3 33.4 33.7 33.9 34.1 34.3 34.5 34.7 34.8 34.9 35.1 35.3 35.4 35.6

220 29.9 30.1 30.2 30.4 30.5 30.6 30.8 31.0 31.2 31.4 31.5 31.7 31.8 31.9 32.1 32.2 32.4 32.5 32.6

230 27.6 27.8 27.9 28.0 28.2 28.3 28.4 28.6 28.8 28.9 29.1 29.2 29.3 29.4 29.5 29.7 29.8 29.9 30.0

240 25.6 25.7 25.9 26.0 26.1 26.2 26.3 26.4 26.6 26.7 26.9 27.0 27.1 27.2 27.3 27.4 27.5 27.6 27.7

250 23.8 23.9 24.0 24.1 24.2 24.3 24.4 24.5 24.7 24.8 24.9 25.0 25.1 25.2 25.3 25.4 25.5 25.6 25.7

_____________________________________________________________________________________________________________

7.2 Angle Struts 7.2.1 Single Angle Struts The compression in single angles may be transferred either concentrically to its centriod through end gusset or eccentrically by connecting one of its legs to a gusset or adjacent member. 7.2.1.1 Concentric loading − When a single angle is concentrically loaded in compression, the design strength may be evaluated using compressive strength design Pd. 7.2.1.2 Loaded through one leg − The flexural torsional buckling strength of single angle loaded in compression through one of its legs may be evaluated using the equivalent slenderness ratio, as given below:

23

221 φvve λλλ kkk ++=

where k1, k2, k3 = constants depending upon the end condition, as given in Table 12.

=

250

2επε

λ⎟⎟⎠

⎞⎜⎜⎝

⎛

= vvvv

rI

and ( )

ε

ϕεπ

λ

250

2/221 tbb +

=

where I = centre to centre length of the supporting member rvv = radius of gyration about the minor axi b1, b2 = width of the two legs of the angle t = thickness of the leg ε = yield stress ratio ( 250/fy)0.5

Table 12 Constants k1, k2 , and k3 (Clause 7.5.1.2)

Sl

No.

(1)

No. of bolts at each end connection

(2)

Gusset/Connecting Member Fixity 1)

(3)

k1

(4)

k2

(5)

k3

(6)

i) Fixed 0.20 0.35 20

≥ 2 Hinged 0.70 0.60 5

ii) Fixed 0.75 0.35 20 1 Hinged 1.25 0.50 60

1) Stiffness of in-plane rotational restraint provided by the gusset/connecting member. For partial restraint, the λe can be interpolated between the λe results for fixed and hinged cases.

The redundant members shall be checked individually for 2.5 percent of axial load carried by the member to which it supports. 7.3 Stresses in Bolts Ultimate stresses in bolts confirming to property of class 5.6 of IS 12427shall not exceed the value given in Table 13. For bolts confirming to IS 3757, permissible stresses and other provisions governing the use of high strength bolts shall be made to IS 4000. Where the material of bolt and the structural member are of different grades, the bearing strength of the joint shall be governed by the lower of the two.

Table 13 Ultimate Stresses in Bolts

(Clause 7.3) Sl

No. Nature of Stress Permissible Stress for

Bolts of Property Class 5.6

Remarks

(1) (2) (3) (4) i) Shear:

Shear stress on gross area of bolt

310 For gross area of bolts (see ). For bolts in double shear the area to be assumed shall be twice the area defined.

ii) Bearing: Bearing stress on gross diameter of bolts

620 For bolt area in bearing

iii) Tension: Axial tensile stress

250 ---

7.4 SLENDERNESS RATIOS 7.4.1 The slenderness ratio of compression and redundant members shall be determined as given in Table 14.

Table 14 Slenderness Ratios (Clause 7.4.1)

Sl No.

Type of member Value (KL/r)

(1) (2) (3)

i) Leg sections or members bolted on either ends in both flanges

L/r

ii) Member with concentric loading at both ends of the unsupported panel with value of L/r up to and including 120

L/r

iii) Member with concentric loading at one end and normal eccentricities at the other end of the unsupported panel with values of L/r up to and including 120

30 + 0.75 L/r

iv) Members with normal framing eccentricities at both ends of the unsupported panel for value of L/r up and including 120.

60 + 0.50 L/r

v) members unrestrained against rotation at both ends of the unsupported panel for value of L/r from 120 to 200

L/r

vi) Members partially restrained against rotation at one end of the unsupported panel for values of L/r over 120 up to and including 225

28.6 + 0.762 L/r

vii) Members partially restrained against rotation at both ends of the unsupported panel for value of L/r over120 up to and including 250

46.2 + 0.615 L/r

NOTE − The values of KL/r corresponding to (vi) and (vii) of Table 14, the following evaluation is suggested.

a) The restrained member must be connected to the restraining member by at least two bolts. b) The restraining member must have a stiffness factor I/L in the stress plane (I = Moment of Inertia, L= Length) that equals or exceeds the sum of the stiffness factors in the stress plane of the restrained members that are connected. c) Angle members connected by one leg should have the holes located as close to the outstanding leg as feasible. Normal framing eccentricities at load transfer connection imply that connection holes are located between the heel of the angle and the centre line of the framing leg.

Where test and/or analysis demonstrates that any other type of bracing pattern is found technically suitable, the same may be adopted. Gusset plates shall be designed to resist the shear, direct and flexural stresses acting on the weakest cross section. Re-entrant cuts shall be avoided as far as practical. Minimum thickness of gusset shall be 2 mm more than lattice it connects only in case when the lattice is directly connected on the gusset outside the leg member. In no case the gusset shall be less than 5 mm in thickness. A single bolt connection shall not be considered as offering restraint against rotation. A multiple bolt connection properly detailed to minimize eccentricities shall be considered to offer partial restraint if connection is to a member having adequate flexural strength to resist rotation of the joint. Points of intermediate support shall not be considered as offering restraint to rotation unless they meet the criteria outlined under the slenderness ratio. Where prior test experience has demonstrated that specific connection provide greater restraint than assumed above, the KL/r values specified in Table 15 may be modified accordingly. In the design of members, the longer L shall be the centre to centre of intersection at each end of the member. Example showing the application of the procedure contained in Table 14 and methods of determining the slenderness ratio of leg and bracing members are given in Appendix A. The limiting value of L/r shall be as follows:

a) Leg members and main bracing members = 150 b) Members carrying computed stresses = 200 c) Redundant members and those carrying nominal stresses = 250

7.4.2 The slenderness ratio of members carrying axial tension only shall not exceed 375. 8 MINIMUM THICKNESS Minimum thickness of galvanized and painted tower members shall be as follows:

Minimum Thickness in mm

Members Type

Galvanized Painted

a) Leg members and main bracing members

5 6

b) Other members

4 5

9 BOLTING 9.1 Minimum Diameter of Bolts The diameter of the bolt shall not be less than 12 mm. Preferred size of bolts used for the erection of microwave tower shall be of diameter 12, 16 and 20. 9.2 Area of Bolt For purpose of calculating the shear stress the gross area of bolt shall be taken as the nominal area of the bolt. The bolt area for bearing shall be taken as d.t, where d is the diameter of the bolt and t is the thickness of the thinner of the parts joined. The net area of the bolt in tension shall be taken as the area at the root of the thread. 9.3 Holes for Bolting The diameter of the hole drilled or punched shall not be more than the nominal diameter of the bolt plus 1.5 mm. 9.4 The length of bolts shall be such that the threaded portion does not lie in the plane of contact of members. The projected portion of the bolt beyond the nut shall be between 3 to 8 mm. 10 CONNECTIONS The angle between any two members common to a joint of a trussed from shall be preferably greater than 20°and never less than 20° due to uncertainty of stress distribution between two closely spaced members.

ANNEX A (Clause 8.1.2)

EXAMPLE OF DETERMINATION OF SLENDERNESS RATIOS

A-1 Example of determining the effective length of compression members of towers based on the provision given in Table 15 are given below. A-2 LEG MEMBER USING SYMMETRICAL BRACING

Method of Loading/Rigidity of Joint Slenderness Ratio

Concentric loading vvrL from 0 to 120,

vvrL

rKL

=

No restraints at ends vvrL from 0 to 120,

vvrL

rKL

=

A-3 LEG MEMBER USING STAGGERED BRACING

Method of Loading/ Rigidity of Joint

Slenderness Ratio

Concentric loading

xxrL or

yyrL or

zzrL67.0 from 0 to 120,

rL

rKL

=

No restraints at ends

xxrL or

yyrL or

zzrL67.0 from 0 to 120,

rL

rKL

=

A-4 EFFECT OF END CONNECTIONS ON MEMBER CAPACITY Method of Loading/

Rigidity of Joint Slenderness Ratio

Tension system with compression strut (eccentricity in critical axis)

vvrL from 0 to 120,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 5.060

Bracing requirements (Single angle members) Single bolt connection, no restraint at ends

vvrL from 120 to 200,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Multiple bolt connection, partial restraint at both ends.

vvrL from 120 to 250,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.61546.2

A-5 CONCENTRIC LOADING TWO ANGLE MEMBERS Method of Loading/

Rigidity of Joint Slenderness Ratio

Tension system with compression struts - concentric loading

xxrL or

yyrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Bracing requirements (Two angle member) Single bolt connection, no restraint at ends

xxrL or

yyrL

from 120 to 200,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Multiple bolt connection, partial restraint at both ends.

xxrL or

yyrL

from 120 to 250,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.61546.2

A-6 BRACING TWO ANGLE MEMBER Method of Loading/

Rigidity of Joint Slenderness Ratio

Tension- compression system with compression strut

yy

0.5rL or

xxrL

from 120 to 250,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.61546.2

Bracing requirements (Two angle member) Concentric load at ends, eccentric loading at intermediate in both directions

yy0.5

rL or

xxrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.7530

Concentric loading at intermediate

yy0.5

rL or

xxrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

A-7 DETERMINATION OF DIAGONAL BRACING Method of Loading/

Rigidity of Joint Slenderness Ratio

Tension- compression system (member carrying equal and opposite stresses). Eccentricity in critical axis

vv0.5

rL or

xx0.75

rL

from 0 to 120,

Single bolt connection, no restraint at ends. Multiple bolt connection and concentric loading

vv0.5

rL or

xx0.75

rL

from 120 to 200,

A-8 EFFECT OF SUBDIVIDED PANELS AND END CONNECTIONS ON MEMBER CAPACITY

Method of Loading/ Rigidity of Joint

Slenderness Ratio

Tension system with compression strut: Eccentricity in critical axis.

vvrL0.5 or

xxrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.5060

Bracing requirements: Single bolt connection, no restraint at ends or intermediate. .

vv0.5

rL or

xxrL

from 120 to 200,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Multiple bolt connection at ends. Single bolt connection at intermediate joint: Partial restraint at one end, no restraint at intermediate Partial restraint at both ends

vvrL0.5 from 120 to 225,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.76228.6

xxrL from 120 to 250

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.615264 .

Multiple bolt connection: Partial restraint at ends and intermediate.

vv0.5

rL or

xxrL

from 120 to 250,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.61546.2

A-9 CONCENTRIC LOADINGTWO ANGLE MEMBER, SUBDIVIDED PANEL Method of Loading/ Rigidity of Joint

Slenderness Ratio

Tension system with compression strut: Concentric loading

yyrL0.5 or

xxrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Bracing requirements: Single bolt connection, no restraint at ends and intermediate.

yy0.5

rL or

xxrL

from 0 to 120,

⎟⎠⎞

⎜⎝⎛ =

rL

rKL

Multiple bolt connection at ends. Single bolt connection at intermediate joint. Partial restraint at one end, no restraint at intermediate.

yyrL0.5

from 120 to 200,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.762628.

Partial restraint at both ends. xxr

L from 120 to 250,

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.61546.2

Multiple bolt

connection, partial restraints at ends and intermediate.

yy0.5

rL or

xxrL

from 120 to 250

⎟⎠⎞

⎜⎝⎛ +=

rL

rKL 0.615246.

A-10 X-BRACINGS WITH AND WITHOUT SECONDARY MEMBERS Slenderness Ratio Critical of

vvrAB /

vvrAC / or vvrCB / or

xxrAB /* or yyrAB /* or

vvrAD /*

vvrAC / or vvrCB / or vvrAD /*

Slenderness Ratio Critical of

vvrAD / or xxrAF /* or

vvrDC / or vvrAE /* or

vvrCB / or xxrAB /* or

yyrAB /*

vvrAD / or xxrAF /* or

vvrDC / or vvrAE /* or

vvrCB / or xxrAC / or

yyrAC /*

vvrAD / or xxrAF /* or

vvrDC / or vvrCB / or

vvrAE /*

vvrAE / or xxrAF /* or

vvrED / or vvrAE /* or

vvrDC / or vvrCB /

A-11 K-BRACINGS WITH AND WITHOUT SECONDARY MEMBERS Slenderness Ratio Critical of

vvrAB /

vvrAC / or vvrCB / or xxrAB / or

yyrAB /

vvrAC / or vvrCB /

Slenderness Ratio Critical of

vvrAD / or vvrDC / or vvrCB / or

xxrAB / or yyrAB /

vvrAD / or vvrDC / or vvrCB / or

xxrAC / or yyrAC /

vvrAD / or vvrDC / or vvrCB /

vvrAE / or

vvrED / or

vvrDC / or

vvrCB /