1

� या� या� या� यापक प�रचालन मसौदापक प�रचालन मसौदापक प�रचालन मसौदापक प�रचालन मसौदा

हमारा सदंभ� हमारा सदंभ� हमारा सदंभ� हमारा सदंभ� : सीईड� सीईड� सीईड� सीईड� 7777/ट�ट�ट�ट�-3333 & 4444 16-08080808-2012201220122012

त कत कत कत कनीक! सिमित नीक! सिमित नीक! सिमित नीक! सिमित : सरंचनागत इंजीिनय�रंग और सरंचनागत खंडसरंचनागत इंजीिनय�रंग और सरंचनागत खंडसरंचनागत इंजीिनय�रंग और सरंचनागत खंडसरंचनागत इंजीिनय�रंग और सरंचनागत खंड (वषय सिमित (वषय सिमित (वषय सिमित (वषय सिमित

सीईड� सीईड� सीईड� सीईड� 7777

+ा, त कता� :

1 िस(वल इंजीिनयर� (वभाग प�रष- के /िच रखने वाले सद0 य

2 सीईड� 7, सीईड� 7:1 एंव सीईड� 7:2 के सभी सद0 य

3 /िच रखने वाले अ4 य िनकाय

महोदयमहोदयमहोदयमहोदय(य6य6य6य6),,,,

िन7 निल8खत मानक6 के मसौदे सलं9 न ह::

+लेख स;ं या+लेख स;ं या+लेख स;ं या+लेख स;ं या श=षकश=षकश=षकश=षक

सीईड� 7(7850) िशरोप�र +ेषण लाईन टावर6 म@ सरंचना इ0 पात उपयोग क! र�ित सBंहता

भाग 1 सामCी, भार और अनमुत 8,र तबल

अनभुाग 1 सामCी और भार का भारतीय मानक मसौदा

[आईएस 802 (भाग 1/ अनभुाग 1 का पाँचवा पनुर�Hण)]

(आईसीएस 91.080.10)

सीईड� 7(7851) िशरोप�र +ेषण लाईन टावर6 म@ सरंचना इ0 पात उपयोग क! र�ित सBंहता

भाग 1 सामCी, भार और अनमुत 8,र तबल

अनभुाग 2 अनमुत 8,र तबल का भारतीय मानक मसौदा

[आईएस 802(भाग 1/ अनभुाग 2 का चौथा पनुर�Hण)]

(आईसीएस 91.080.10)

कृपया इन मानक6 के मसौदो का अवलोकन कर@ और अपनी स7 मितयॉ यह बताते हए भेजे Bक यBद ुये मानक6 के /प म@ +कािशत हो तो इन पर अमल करने म@ आपके � यवसाय अथवा कारोबार म@ L या

कBठनाइया आ सकती ह: ।

स7 मितया भेजने क! अिंतम ितिथ 31313131-10101010-2012201220122012.

2

स7 मित यBद कोई हो तो कृपया अधोह0 ताHर� को उपरिल8खत पते पर सलं9 न फोमQट म@ भेज@ या

ईमेल कर द@ ।

यBद कोई स7 मित +ा, त नह�ं होती है अथवा स7 मित म@ केवल भाषा स7 ब4 धी SुBट हई तो उपरोL तु

+लेख को यथावत अिंतम /प Bदया जाएगा । यBद स87 म त तकनीक! +कृित क! हई तो (वषय सिमित के ुअT यH के परामश� से अथवा उनक! इU छा पर आगे क! काय�वाह� के िलए (वषय सिमित को भेजे जाने के

बाद +लेख को अिंतम /प दे Bदया जाएगा ।

यह +लेख भारतीय मानक W यरूो क! वबैसाइट पर भी है ।

ध4 यवाद ।

भवद�यभवद�यभवद�यभवद�य

Sd/-

( ड� के अCवाल ड� के अCवाल ड� के अCवाल ड� के अCवाल )

वYैािनक वYैािनक वYैािनक वYैािनक ‘एफएफएफएफ’ एंव +मखुएंव +मखुएंव +मखुएंव +मखु (िस(वल इंजीिनयर�िस(वल इंजीिनयर�िस(वल इंजीिनयर�िस(वल इंजीिनयर�) email : ced@bis.org.in

सलं9 नसलं9 नसलं9 नसलं9 न : उप�र ल उप�र ल उप�र ल उप�र ल8खत 8खत 8खत 8खत

3

DRAFTS IN WIDE CIRCULATION

DOCUMENT DESPATCH ADVICE

Reference Date

CED 7/T- 3 & T-4 16-08-2012 TECHNICAL COMMITTEE: Structural Engineering and Structural Sections Sect ional Committee, CED 7 ADDRESSED TO :

1. Interested Members of Civil Engineering Division Council, CEDC 2. All Members of CED 7, CED 7:1 & CED 7:2 3. All others interested

Dear Sir (s),

Please find enclosed the following draft standards: Doc No. Title i) CED 7(7850) Draft Indian Standard on U se of Structural Steel in Overhead

Transmission Line Towers – Code of Practice Part 1 Materials, Loads and Design Strengths Section 1 Materials and Loads [Fifth Revision of IS 802

(Part 1/Sec 1)]; ICS 91.080.10 ii) CED 7(7851) Draft Indian Standard on U se of Structural Steel in Overhead

Transmission Line Towers – Code of Practice Part 1 Materials, Loads and Design Strengths Section 2 Design Strengths [Fifth Revision of IS 8 02

(Part 1/Sec 2)]; ICS 91.080.10

Kindly examine the draft standards and forward your views stating any difficulties which you are likely to experience in your business or profession, if these are finally adopted as Indian Standard to National Standard. Last Date for comments: 31 10 2012.

4

Comments, if any, may please be made in the format as given overleaf and mailed to the undersigned at the above address.

In case no comments are received or comments received are of editorial nature, you will kindly permit us to presume your approval for the above draft standards as finalized. However, in case comments of technical in nature are received then these may be finalized either in consultation with the Chairman, Sectional Committee or referred to the Sectional Committee for further necessary action if so desired by the Chairman, Sectional Committee.

The drafts are also hosted on BIS website www.bis.org.in . Thanking you, Yours faithfully,

Sd/-

(D.K. Agrawal) Sc ‘F’ & Head (Civil En gg.) e-mail : ced@bis.org.in Encl: As above

5

FORMAT FOR SENDING COMMENTS ON BIS DOCUMENTS

(Please use A4 size sheet of paper only and type within fields indicated. Comments on each clause/subclause/table/fig etc. be started on a fresh box. Information in column 3 should include reasons for the comments and suggestions for modified working of the clauses when the existing text is found not acceptable. Adherence to this format facilitates Secretariat’s work)

NAME OF THE COMMENTATOR/ORGANIZATION:

Doc. Number and Title: CED 7 (7851)

USE OF STRUCTURAL STEEL IN OVERHEAD TRANSMISSION L INE TOWERS — CODE OF PRACTICE PART 1 MATERIALS, LOADS AND DESIGN STRENGTHS Section 2 Design Strengths [4th Revision of IS 802 (Part 1/Sec 2)]

Sl No.

(1)

Clause/Subclause/ Para No.

(2)

Comments/suggestions

(3)

6

For BIS Use only Doc: CED 7(7851) August 2012

(Not to be reproduced without the permission of BIS or used as an Indian Standard) ______________________________________________________________________Structural Engineering and Structural Last date for comments: Sections Sectional Committee, CED 7 31 October 2012 ______________________________________________________________________

Draft Indian Standard

USE OF STRUCTURAL STEEL IN OVERHEAD

TRANSMISSION LINE TOWERS — CODE OF PRACTICE

PART 1 MATERIAL, LOADS AND DESIGN STRENGTHS

Section 2 Design Strengths

[Fourth Revision of IS 802 (Part 1/Sec 2)]

ICS 91.080.10 FOREWORD (Formal clauses to be added later) Transmission towers are tall structures, usually steel lattice towers, used to support overhead power lines. Transmission line towers are key infrastructural components. The standards under IS 802 series have been prepared with a view to establish uniform practices for design, fabrication, inspection and testing of overhead transmission line towers. This standard is a part of a series of standards thus formulated and covers requirements in regard to material, loads and design strengths apart from other relevant design provisions. The other parts in the series are:

Part 2 Fabrication, galvanizing, inspection and packing Part 3 Testing Part 4 Requirements for latticed switchyard structures (under preparation) Part 5 Requirements for tall river crossing towers (under preparation) This standard (Part 1) was first published in 1967 and subsequently revised in 1973, 1977 and 1995. The standard in its third revision was split in two sections, namely Section 1 Materials and loads, and Section 2 Permissible stresses. Some of the major modifications made in this standard (Section 2) in this revision are:

a) Title and scope has been modified to cover design strength and other design parameters in place of permissible stresses.

b) Permissible stresses in structural members have been defined separately for angle and hollow sections.

b) Requirements on permissible stresses in bolts have been modified.

7

c) Requirements on net effective area for angle section in tension has been modified. d) Requirements on stitch bolts have been incorporated. e) Provision for welded connections have been included. f) Annex C on examples of determination of slenderness ratios has been modified.

Design provisions or other items not covered in this standard shall generally be in accordance with IS 800:2007 ‘General Construction in Steel – Code of Practice’. While preparing this code, practices prevailing in the country in this field have been kept in view.

8

For BIS Use only Doc: CED 7(7851) August 2012

(Not to be reproduced without the permission of BIS or used as an Indian Standard) ______________________________________________________________________Structural Engineering and Structural Last date for comments: Sections Sectional Committee, CED 7 31 October 2012 ______________________________________________________________________

Draft Indian Standard

USE OF STRUCTURAL STEEL IN OVERHEAD TRANSMISSION LINE TOWERS — CODE

OF PRACTICE

PART 1 MATERIAL, LOADS AND DESIGN STRENGTHS Section 2 Design Strengths

[Fourth Revision of IS 802 (Part 1/Sec 2)]

ICS 91.080.10 1 SCOPE 1.1. This standard (Part 1/Sec 2) stipulates the design strengths and other design parameters to be adopted in the design of self-supporting steel lattice towers (using angles/circular hollow sections) for overhead transmission lines. 1.1.1. Materials, type of towers, loading and broken wire conditions are covered in Section 1 of this standard. 1.1.2. Provisions on fabrication and testing of transmission line towers have been covered in Part 2 and Part 3 respectively of the standard. 1.1.3. Provisions for loads and design strengths for latticed switch yard structures are covered in IS 802 (Part 4) (under preparation) 1.1.4 Provisions for loads and design strengths for tall river crossing towers shall be covered in a separate standard (under preparation).

NOTE : 1 While formulating the provisions of this standard it has been assumed that the structural connections are through bolts. 2 For critical connections, welded joints can be adopted subject to approval of purchaser/end user.

1.2. This standard does not cover guyed towers.

9

2 REFERENCES The Indian Standard listed below contains provisions, which through reference in this text, constitute provisions of this standard. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent edition of the standard indicated below. IS No. Title

800: 2007 Code of practice for general construction in steel (third revision)

802 (Part 4) (under preparation)

Code of practice for use of structural steel in overhead transmission line towers and substation structures: Part 4 Requirements for latticed switchyard structures

12427: 2001 Fasteners – Threaded steel fasteners – Hexagon head transmission tower bolts – Specification (first revision)

3 STATUTORY REQUIREMENTS 3.1 Statutory requirement as laid down in the 'Indian Electricity Rules, 1956 or by any other statutory body applicable to such structures as covered in this standard shall be satisfied. 3.2 In addition to compliance with local and provincial byelaws, fire and safety laws and civil aviation requirements applicable to such structures as specif ied by purchaser/end user shall be complied with. 4 CONDUCTOR TENSION 4.1 The conductor tension at everyday temperature and without external load should not exceed the following percentage of the ultimate strength of the conductor:

Initial unloaded tension : 35 percent Final unloaded tension : 25 percent

provided that the ultimate tension under everyday temperature and full wind or minimum temperature and two-thirds wind pressure does not exceed 70 percent of the ultimate tensile strength of the cable.

Note - For 400 kV and higher voltage lines, the final unloaded tension of conductors at everyday temperature shall not exceed 22 percent of the ultimate tensile strength of conductors and 20 percent of the ultimate tensile strength of groundwire.

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5 DESIGN STRESSES 5.1 Axial Stresses in Tension for Angle Sections

The estimated tensile stresses on the net effective sectional areas (see 9) in various members shall not exceed minimum guaranteed yield stress of the material. However in case the angle section is connected by one leg only, the estimated tensile stress on the net effective sectional area shall not exceed Fy, where Fy, is the minimum guaranteed yield stress of the material. 5.2 Axial Stresses in Compression for Angle Section s 5.2.1 The estimated compressive stresses in various members shall not exceed the values given by the formulae in 5.2.2. 5.2.2 The allowable unit stress Fa, in MPa on the gross cross sectional area of the axially loaded compression members shall be:

a) yc

a FC

rkLF

−=

2/

2

11 when, KL/r ≤ Cc

and,

b) 2

2

)/( rkL

EπFα

= when, cCKL/r >

where

Cc = yFE /2π

Fy = minimum guaranteed yield stress of the material, MPa E = Modulus of elasticity of steel, that is 2 x 105 MPa, KL/r = largest effective slenderness ratio of any unbraced segment of the member, L = unbraced length of the compression member (see 6.1.1) in cm, and r = appropriate radius of gyration in cm.

5.2.2.1 The formulae given in 5.2.2 are applicable provided the largest width thickness ratio b/t is not more than the limiting value given by:

(b/t)lim = 210 / yF

where b = distance from edge of fillet to the extreme fibre in mm, and t = thickness of flange in mm.

5.2.2.2 Where the width thickness ratio exceeds the limits given in 5.2.2.1, the formulae given in 5.2.2 shall be used substituting for Fy, the value Fcr given by:

a) ycr Ftb

tbF

−=

lim)/(

)/(677.0677.1 , when yFtbtb /378/)/( lim ≤≤

11

and

b) 2)/(

65550

tbFcr = , when

yFE

b 378>

NOTE — The maximum permissible value of b/t for any type of steel shall not exceed 25.

5.2.2.3 The redundant members shall be checked for 2.5 percent of axial load carried by the member it supports. 5.3 Axial stresses for Circular Hollow Sections

Design stresses of circular hollow sections shall be as stipulated in Annex A. 5.4 Stresses for Bolt Design Ultimate stresses in bolts conforming to IS 12427 shall not exceed the value given in Table 1. 5.4.1 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 strengths.

Table 1 Ultimate Stresses for Bolt Design, MPa

(Clause 5.4)

Sl No.

Nature of Stress Stress for Bolts of Property Class

Remarks

5.6 5.8 8.8

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

i) Tensile Stress N/mm² 500 500 800

ii) Shear: Shear stress on gross area

of bolts 310 322 515 For gross area of bolts

shall be determined in accordance with 10.4. For bolts in double shear the area to be assumed shall be twice the area defined

iii) Bearing: a) Bearing stress on gross

diameter of bolts 620 620 960

b) Bearing stress for mild steel material (Fy = 250 N/mm² & Fu = 410 N/mm²)

500 500 500

c) Bearing stress for high tensile steel material

(Fy = 350 N/mm² & Fu = 490N/mm²)

700 700 700

The bolt area in bearing shall be determined in accordance with 10.5.

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Table 1 (concluded)

Sl No.

Nature of Stress Stress for Bolts of Property Class

Remarks

5.6 5.8 8.8

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

d) Bearing stress to be considered as minimum of 2 times yield stress (2Fy) and 1.5 times ultimate tensile stress (1.5Fu) for steel having yield stress (Fy) & ultimate tensile stress (Fu) other than above.

iv) Tension:

Axial tensile stress 280 380 580 The stress area As is given by

2974.0

4

−=n

dAs

π

where d = Nominal diameter of bolt and n = Number of thread per unit of length.

6 SLENDERNESS RATIOS 6.1 The slenderness ratios of compression and redundant members shall be determined as follows:

Type of Members Values of KL/r

a) Compression Members

i) Leg sections or joint members bolted in both faces at connections for 0 < L/r < 120

L/r

ii) Members with concentric loading at both ends of the unsupported panel for 0 < L/r < 120

L/r

iii) Member with concentric loading at one end and normal framing eccentricity at the other end of the unsupported panel for 0 < L/r < 120

30 + 0.75 L/r

iv) Member with normal framing eccentricities at both ends of the unsupported panel for 0 < L/r < 120

60 + 0.50 L/r

v) Member unrestrained against rotation at both ends of the unsupported panel for 120 < L/r < 200

L/r

vi) Member partially restrained against rotation at one end of the unsupported panel for 120 < L/r < 225

28.6 + 0.762 L/r

vii) Member partially restrained against rotation at both ends of the unsupported panel for 120 < L/r < 250

46.2 + 0.615 L/r

b) Redundant Members

i) For 0 < L/r < 250 L/r

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NOTE — The values of KL/r corresponding to (a) (vi) and (a) (vii), the following evaluation is suggested: i) The restrained member must be connected to the restraining member with at least two bolts. ii) The restraining member must have a stiffness factor I/L in the stress plane (I = Moment of

inertia and L = Length) that equals or exceeds the sum of the stiffness factors in the stress plane of the restrained members that are connected to it.

iii) 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 end the centre line of the framing leg.

6.1.1 In calculating the slenderness ratio of the members, the length L should be the distance between the intersections of the centre of gravity lines at each end of the member. 6.2 Examples showing the application of the procedure given in 6.1 and 6.1.1 and method of determining the slenderness ratio of legs and bracings with or without secondary members are given in Annex B.

NOTE — Where test and/or analysis demonstrates that any other type of bracing pattern is technically suitable, the same can be adopted.

6.3 The limiting values KL/r shall be as follows:

i) Leg members, ground wire peak member and

lower members of the cross arms in compression 120

ii) Other members carrying computed stresses 200

iii) Redundant members and those carrying nominal stresses

250

6.4 Slenderness ratio L/r of a member carrying axial tension only, shall not exceed 400.

7 MINIMUM THICKNESS 7.1 Minimum thickness of galvanized and painted tower members shall be as follows: Minimum Thickness, mm

Galvanized Painted

i) Leg members, ground wire peak member and lower members of cross arms in compression

5 6

ii) Other members 4 5

7.2 Gusset plates shall be designed to resist the shear, direct and flexural stresses acting on the weakest or critical 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

14

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. 8 NET SECTIONAL AREA FOR TENSION MEMBER 8.1 The net sectional area shall be the least area which is to be obtained by deducting from the gross sectional area, the area of all holes cut by any straight, diagonal or zig-zag line across the member. In determining the total area of the holes to be deducted from gross sectional area, the full area of the first hole shall be counted, plus a fraction part X, of each succeeding hole cut by the line of holes under consideration. The value of X shall be determined from the formula:

gd

P² X

41 −=

where, P = longitudinal spacing (stagger), that is the distance between two successive

holes in the line of holes under consideration; g = transverse spacing (gauge), that is the distance between the same two

consecutive holes as for P; and d = diameter of holes.

For holes in opposite legs of angles, the value of g should be the sum of the gauges from the back of the angle less the thickness of the angle. 9 NET EFFECTIVE AREA FOR ANGLE SECTION IN TENSION 9.1 In the case of single angle connected through one leg, the net effective section of the angle shall be taken as:

A1 + A2k where

A1 = effective sectional area of the connected leg. A2 = the gross cross-sectional area of the unconnected leg, and

)3(

3

21

1

AA

Ak

+=

where lug angles are used, the effective sectional area of the whole of the angle member shall be considered. 9.2 In the case of pair of angles back to back in tension connected by one leg of each angle to the same side of gusset, the net effective area shall be taken as:

A1 + A2k where

A1 and A2 are as defined in 9.1, and

)5(

5

21

1

AA

Ak

+=

15

NOTE — The area of the leg of an angle shall be taken as the product of the thickness and the length from the outer corner minus half the thickness, and the area of the leg of a tee as the product of the thickness and the depth minus the thickness of the table.

10 BOLTING 10.1 Minimum Diameter of Bolts The diameter of bolts shall not be less than 12 mm. 10.2 Preferred Sizes of Bolts Bolts used for design of transmission line towers shall be of diameter 12, 16, 20 and 24 mm. 10.3 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. 10.4 Gross Area of Bolt For the purpose of calculating the shear stress, the gross area of bolts shall be taken as the nominal area of the bolt. 10.5 The bolt area for bearing shall be taken as d x t where d is the nominal diameter of the bolt, and t the thickness of the thinner of the parts jointed. 10.6 The net area of a bolt in tension shall be taken as the area at the root of the thread. 10.7 Holes for Bolting The diameter of the hole drilled/punched shall not be more than the nominal diameter of the bolt plus 1.5 mm for up to 20 mm diameter bolts and 2.0 mm for 24 mm diameter bolts. 10.8 Stitch Bolts Stitch shall be spaced so that the governing slenderness ratio between bolts for any component of the built-up member does not exceed the requirements in 10.8.1, 10.8.2 and 10.8.3. 10.8.1 The angles connected together back-to-back (in contact) or separated back-to-back by a distance not exceeding the aggregate thickness of the connected parts shall be provided with stitch bolt at a pitch not exceeding 1 000 mm. The slenderness ratio of individual component between adjacent stitch bolts shall not be more than 75 percent that of the two members together. 10.8.2 Where the angles are back to back but not connected as per 10.8.1, each angle shall be designed as a single angle connected through one leg only in accordance with 9.1.

16

10.8.3 When two tees are placed back to back but are not connected as per 10.8.1, each tee shall be designed as a single tee connected to one side of a gusset only in accordance with 9.2.

11 FRAMING 11.1 The angle between any two members common to a joint of a trussed frame shall preferably be greater than 20° and never less than 15° due to uncertainty of stress distribution between two closely spaced members. 12 WELDING For critical connections welded joints can be adopted subject to approval of purchaser / end user. Special care should be taken to assure proper galvanizing and to avoid acid bleeding at pockets in structural assemblies.

17

ANNEX A

(Clause 5.3)

DESIGN STRESSES � CIRCULAR HOLLOW SECTIONS A-1 Cross Section Classification The role of cross section classification is to identify the extent to which the resistance and rotation capacity of cross sections is limited by its local buckling resistance. The classification of a cross-section depends on the width to thickness ratio of the parts subject to compression. The limiting width-to-thickness ratios for compression parts shall be obtained from Table A.1.

Table A.1 Limiting width-to-thickness ratios for co mpression members

Class of Section Sl

No. Circular hollow Sections, including welded tube subjected to

Ratio

Class 1 Plastic

Class 2 Compact

Class 3 Semi-compact

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

i) Moment D/t 42�² 52�² 146�²

ii) Axial Compression D/t Not applicable 88�²

NOTES: 1 Elements which exceed semi-compact limits are to be taken as of slender cross-section (Class 4). 2 � = (250 /fy)

1/2.

A-2 Axial Resistance in Tension A-2.1 The design value of the tension force NEd at each cross section shall satisfy:

0.1.

≤Rdt

Ed

N

N

A-2.2 For sections with holes the design tension resistance Nt,,Rd should be taken as the smaller of:

a) The design plastic resistance of the gross cross-section.

0.

M

yRdpl

AfN

γ=

b) The design ultimate resistance of the net cross-section at holes for fasteners

2.

9.0

M

ynetRdu

fAN

γ=

18

where, Anet = Net Area of the section fu = Ultimate tensile strength of the material. γM0 = Partial safety factor (see Table B.2) γM2 = Partial safety factor (see Table B.2)

A-3 Axial Resistance in Compression A-3.1 The design value of compression force NEd at each cross section shall satisfy

0.1.

≤RdC

Ed

N

N

A-3.2 The design resistance of the cross section for uniform compression NC, Rd should be determined as follows:

MO

yRdc

AfN

γ=. for Class 1, 2 and 3 cross sections, and

0

.M

yeffRdc

fAN

γ= for Class 4 cross sections

where, Aeff = Effective Area

5.0

250

/

88

×=

yftDA

D = Outer Diameter of circular hollow sections A-4 Member Buckling Resistance in Compression

1.

M

yRdb

AfN

γχ

= for Class 1, 2 and 3 cross sections

1.

M

yeffRdb

fAN

γχ

= for Class 4 sections

22

1

λφφχ

−+= but χ ≤ 1.0

where,

γM1 = Partial safety factor (see Table B.2)

( )[ ]22.015.0 λλαφ +−+=

α = is the imperfection factor & non-dimensional slenderness, α = 0.49 for cold finished, α = 0.34 for hot finished

cr

y

N

Af=λ for Class 1, 2 and 3 cross sections

19

cr

yeff

N

fA=λ for Class 4 cross sections

A-4.1 Elastic Critical force and non-dimensional slenderness for flexural buckling

( ) 2

2

2

2

/ L

EI

rL

EANcr

ππ ==

where, E = Young’s Modulus of Member A = Gross Area of the member I = Moment of Inertia of the member r = Radius of gyration L = Length of the member

Table B.2. Partial safety factors γM for Towers

Sl No.

Definition Partial Safety Factor

(1) (2) (3)

i) γM0 1.00 ii) γM1 1.00 iii) γM2 1.25

A-5 Connections shall be designed in accordance with IS 800.

20

ANNEX B (Clause 6.2)

EXAMPLES OF DETERMINATION OF SLENDERNESS RATIOS

B-0 Example of determining the effective length of compression members of towers based on the provision given in 6.1 are given below. B-1 LEG MEMBER USING SYMMETRICAL BRACING

Method of Loading/ Rigidity of joints

Slenderness Ratio

Concentric Loading

vvr

L from 0 to 120

=

vvr

L

r

kL

No restraint at Ends

vvr

L from 120 to 200

=

vvr

L

r

kL

B-2 LEG MEMBER USING STAGGERED BRACING (Nos. of st aggered parts 4 and more)

Method of Loading/ Rigidity of joints

Slenderness Ratio

Concentric Loading (see Note below) xxr

L2.1 or

yyr

L2.1 or

vvr

l from 0

to 120

=

vvr

L

r

kL

No restraint at Ends (see Note below)

xxr

L2.1 or

yyr

L2.1 or

vvr

l from

120 to 200

=

vvr

L

r

kL

Note � If total number of staggered parts are 4 or more as shown in figure

21

B-3 LEG MEMBER USING STAGGERED BRACING (Nos. of st aggered parts up to 3)

Method of Loading/ Rigidity of joints

Slenderness Ratio

Concentric Loading

xxr

L or

yyr

L or

vvr

l from 0

to 120

=

r

L

r

kL

No restraint at Ends

xxr

L or

yyr

L or

vvr

l from

120 to 200

=

r

L

r

kL

Note � If total number of staggered parts up to 3 as shown in figure

B-4 EFFECT OF END CONNECTIONS ON MEMBER CAPACITY

Method of Loading/ Rigidity of Joints

Slenderness Ratio

Tension system with compression strut (eccentricity in critical axis)

VVr

L from 0 to 120

+=r

L

r

kL5.060

Bracing Requirements ( Single Angle Members ):

Single bolt connection, no restraint at ends VVr

L from 120 to 200

=r

L

r

kL

Multiple bolt connection partial restraint at both ends

VVr

L from 120 to 250

+=r

L

r

kL615.02.46

22

B-5 CONCENTRIC LOADING TWO ANGLE MEMBERS

Method of Loading/ Rigidity of Joints

Slenderness Ratio

Tension system strut compression concentric loading xxr

L or

yyr

L from 0 to 120

=r

L

r

kL

Bracing Requirements (Two Angle Members):

Single bolt connection, no restraint at ends xxr

L or

yyr

L from 120 to 200

=r

L

r

kL

Multiple bolt connection partial restraint at ends xxr

L or

yyr

L from 120 to 250

+=r

L

r

kL615.02.46

B-6 HORIZONTAL MEMBER OF K-BRACING-TWO ANGLE MEMBER

Method of Loading/ Rigidity of Joints

Slenderness Ratio

Tension-compression system with compression strut:

Multiple bolts connection partial restraint at ends and intermediate

yyr

L5.0 or

xxr

L from 120 to 250

+=r

L

r

kL615.02.46

Bracing Requirements (Two Angle Members):

Concentric load at ends, eccentric loading at intermediate in both directions

yyr

L5.0 or

xxr

L from 0 to 120

+=r

L

r

kL75.030

23

Concentric loading at ends and intermediate

yyr

L5.0 or

xxr

L from 0 to 120

=r

L

r

kL

B-7 EFFECT OF SUBDIVIDED PANELS FOR THE HORIZONTAL MEMBER AND END CONNECTIONS ON MEMBER CAPACITY Method of

Loading/ Rigidity of Joints

Slenderness Ratio

Tension system with compression strut:

vvr

L5.0 or

xxr

L from 0 to 120

+=r

L

r

kL50.060

Bracing Requirements: Single bolt connection, no restraint at ends for intermediate:

vvr

L5.0 or

xxr

L from 120 to 200

=r

L

r

kL

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

vvr

L5.0 from 120 to 225

+=r

L

r

kL762.06.28

Partial restraint at both ends

xxr

L from 120 to 250

+=r

L

r

kL*615.02.46

Multiple bolt connection Partial restraint at ends and intermediate

vvr

L5.0 or

xxr

L from 120 to

250

+=r

L

r

kL615.02.46

24

B-8 CONCENTRIC LOADING TWO ANGLE MEMBER, SUBDIVIDED PANELS OF A HORIZONTAL MEMBER Method of

Loading/ Rigidity of Panel

Slenderness Ratio

Tension system with compression strut: Concentric loading

yyr

L5.0 or

xxr

L from 0 to 120

=r

L

r

kL

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

yyr

L5.0 or

xxr

L from 120 to 200

=r

L

r

kL

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

yyr

L5.0 from 120 to 220

+=r

L

r

kL*762.06.28

Partial restraint at both ends

xxr

L from 120 to 250

+=r

L

r

kL*615.02.46

Multiple bolt connection Partial restraint at ends and intermediate

yyr

L*5.0 or

xxr

L from 120 to 250

+=r

L

r

kL615.02.46

25

B-9 X - BRACINGS WITH AND WITHOUT SECONDARY MEMBER S

Slenderness Ratio Critical of

AB/rvv

AC/rvv or CB/rvv or *AB/rxx or *AB/ryy or *AD/rvv

AC/rvv or CB/rvv or *AD/rvv

AD/rvv or *AF/rxx or DC/rvv or *AE/rvv or CB/rvv or *AB/rxx or *AB/ryy or EF/rvv

AD/rvv or *AF/rxx or DC/rvv or *AE/rvv or CB/rvv or AC/rxx or *AC/rxx or EF/rvv

AD/rvv or *AF/rxx or DC/rvv or CB/rvv or *AE/rvv or EF/rvv

AE/rvv or *AF/rxx or ED/rvv or *AE/rvv or DC/rvv or CB/rvv

* Application for tension compression system only i.e. tensile stresses in one bracing must be at least

equal to 75 percent of the compressive stress in the other bracing.

# The corner stay should be designed to provide lateral support adequately.

26

B-10 K-BRACINGS WITH AND WITHOUT SECONDARY MEMBERS

Slenderness Ratio Critical of

AB/rvv

AC/rvv or CB/rvv or AB/rxx or AB/ryy

AC/rvv or CB/rvv

AD/rvv or DC/rvv or CB/rvv or AB/rxx or AB/ryy

AD/rvv or DC/rvv or CB/rvv or AC/rxx or AC/ryy

AD/rvv or DC/rvv or CB/rvv

AE/rvv or ED/rvv or DC/rvv or CB/rvv

# The corner stay should be designed to provide lateral support adequately.