6.Design of Steel Towers

8/17/2019 6.Design of Steel Towers

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Design of Steel

StructuresDesign of Towers and Masts


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INTRODUCTION

A tower or mast is a tall skeleton structure with a relativel small cross!section" which

has a large ratio #etween height and ma$imum width%

A tower is a freel standing self su&&orting structure fi$ed to the #ase or foundationwhile a mast is tall structure" &inned to the #ase of foundation and #raced with gus

etc%

A''(ICATION ) US*

i% *lectric &ower transmission +,- to ./ m high0

ii% Microwave transmission for communication

iii% Radio transmission +short and medium wave wireless0

iv% Television transmission +,-- m to 1-- m0

v% Satellite rece&tion

vi% Air traffic control

vii% 2lood light stand +,/ to /- m0

viii% Meteorological measurements

i$% Derrick and crawler cranes

$% Oil drilling masts%

$i% Over head tanks%


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INTRODUCTION

C(ASSI2ICATION

De&ending u&on the si3e and t&e of loading" towers are grou&ed into two heads4

+a0 Towers with large vertical loads5 +such as those of over head water tanks" oil

tanks" meteorological towers etc%0 have their sides made u& of vertical or inclined

trusses%

+#0 Towers with mainl hori3ontal wind loads5 su#6ected &redominantl to wind loads

categori3ed as4

i% Self!su&&orting towers or 2ree standing towers or (attice towers

2ree standing towers" known as lattice towers" are generall s7uare in &lan and

are su&&orted # four legs" fi$ed to the #ase% These towers act as vertical

cantilever trusses" su#6ected to wind and8or seismic loads% 2ree standing towers

are commonl used for T% 9% microwave transmission" &ower transmission" flood

light holding etc%

ii% :ued towers or Masts

gued towers are hinged to the #ase" and are su&&orted # gu wires attached to

it at various levels" to transmit the wind forces to the ground% Due to this reason"

gued tower of the same height is much lighter than a self!su&&orting tower%

;owever" it re7uires much larger s&ace in &lan" to accommodate the &lacement

of gu ro&es%


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2ig% , T&ical free standing towers

2ig% < :ued tower or Mast


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(ATTIC* TO=*RS

CON2I:URATIONS AND >RACIN: S?ST*MS

The self su&&orting towers" su#6ected &redominantl to wind loads" are called

lattice towers.

Such towers are s7uare or rectangular in &lan% The width # of the side face at the

#ase ma var #etween ,8@ to ,8,< of the height of tower%

The to& width of towers is ke&t #etween ,%/ to 1 m or more" de&ending u&on the

re7uirement%

Some common configurations with #racing sstems are listed as4

i% Single diagonal #racings +2ig #racing +2ig


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(ATTIC* TO=*RS

CON2I:URATIONS AND >RACIN: S?ST*MS

iv% #racing +2ig


7/412ig% 1 (attice tower configurations with #racing


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(ATTIC* TO=*RS

(OADS ACTIN: ON TO=*RS

2ollowing are the various t&es of loads acting on a lattice tower4

a0 :ravit loads +W g0

i% =eight of mem#ers

ii% =eight of &latforms" railings" ladders" lifts etc%

iii% =eight of antenna" instruments" a&&liances etc%

iv% =eights of gussets and secondar #racings

v% (ive loads

#0 (ateral loads

i% =ind load

ii% Seismic loads

c0 *rection loads

) The gravit loads are almost fi$ed" since these are de&endent on the structural

design" Seismic load is also not critical as mass of the structure is not ver heav

and it is more near the ground% ;owever" ma$imum wind &ressure is the chief

criterion for the design of lattice towers%


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(ATTIC* TO=*RS

CA(CU(ATIONS 2OR =IND (OAD +IS E/ 'art III0

The designed wind s&eed V z +m8s0 is given #

93 F

=here" 9> 4 #asic wind s&eed in at ,- m height

4 &ro#a#ilit factor

4 terrain" height and structure si3e factor

4 to&ogra&h factor" the value of which varies from , to ,%.The designed wind &ressure pz +N8m


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2OR TO=*RS COM'OS*D O2 2(AT SID*D M*M>*RS

2OR SUAR* TO=*RS COM'OS*D O2

ROUND M*M>*RS

2OR TRIAN:U(AR TO=*RS COM'OS*D O2

ROUND M*M>*RS


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(ATTIC* TO=*RS

Tower A&&urtenances4

The wind loading on tower a&&urtenances" such as ladders" conduits" lights"

elevators etc% shall #e calculated using a&&ro&riate net &ressure coefficients forthese elements%

Tower mountings4

Usuall" towers have mountings such as antenna dishes etc% on these mountings

can #e com&uted # suita#l selecting &ressure coefficient% The values of C f for some

limited sha&es are given as4

9A(U*S O2 2ORC* CO*22ICI*NT Cf 2OR SO(ID S;A'*S O2 MOUNTIN:


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ANA(?SIS AND D*SI:N

The wind loads" acting at &anels &oints have two effects

i% ;ori3ontal shear effect due to lateral load

ii% 9ertical force due to moments due to lateral load

) The lateral load due to wind is resisted mainl # the we# mem#ers while the

gravit loads and the vertical force due to wind moments are resisted # chords or

leg mem#ers%

) At an level under consideration" let =g #e the gravit load and Mw #e the moment

due to lateral loads% Then force 2l due to lateral loads is given #

2or a s7uare #ase tower4

2or a triangular #ase tower4

2or a multi J &ost tower4) Similarl" if K is the inclination of the tower leg with the a$is of the tower" the

force due to gravit loads is given #

" where" N F no% of legs in tower


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(ATTIC* TO=*RS

ANA(?SIS AND D*SI:N

;ence the total force 2 in the leg is given #

2F

The lateral load +i%e% wind shear0 is resisted # the we# mem#er in tension at the

section%

The leg mem#ers are designed as com&ression mem#ers while the we# mem#ers

as tension mem#ers%

The width of #ase is taken e7ual to ,8 to ,8,< of the height" while the

inclination or &itch of the sides is ke&t #etween ,8,@ to ,8.-%

2ig% . Ma$imum vertical force in &osts


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*AM'(*

A @- m high microwave lattice tower is to #e #uilt near Agra where the terrain at thesite is nearl level ground with terrain of categor


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SO(UTION

Selection of tower configurations

ee& >8; ratio as ,8"

;ence" >ase width > F @-8 F E%/

ee& to& ,< m &ortion &erfectl straight +vertical0" and remaining &ortion inclined%

(et us kee& @ &anels in this to& height of ,< m so that length of leg mem#er in this

&ortion F < m%

Inclination of #ase legs F tan !, +E%/!1%/08+


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SO(UTION

Selection of tower configurations

The inclination of diagonals at various heights will #e as under4

Segment I 4 K@- F K. F tan!,


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SO(UTION

Computation of wind loads

>asic wind s&eed F .E m8s +for Agra0" k, F ,%-E5 k1 F ,%- +&lain

ground0% The structure is of class C" and terrain is of categor


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SO(UTION

Computation of wind loads

The average frontal area for various segments are as under4

AI F 1%/ Q ,< F .< m


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(ATTIC* TO=*RS

SO(UTION

The lateral loads at various segments will #e as under4

Segment I 4 2(I F A Cf &3 F +.< $ -%


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TRANSMISSION (IN* TO=*RS

Transmission line towers are used for su&&orting the e$tra high voltage +*;A0

electric transmission lines% Due to ver heav currents these transmission lines

should #e carried at a higher level from the ground level%

2ollowing are various t&es of structures which su&&ort the electric &ower

transmission lines4

a0 Structures made of tim#er

i% =ood &oles

ii% =ood ;!&oles

#0 Structures made Of concrete

i% R%C%C% &oles

ii% 're!stressed concrete &oles

c0 Structures made of structural steel

i% Round or I!section steel &oles

ii% 2a#ricated steel &oles

iii% 2le$i#le towers

iv% Semi!fle$i#le towers

v% Self!su&&orting wide #ase towers

vi% :ued towers%


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CON2I:URATIONS

2ig% . shows various configurations of self!su&&orting wide #ase towers% The main

#racing sstem ma #e of three t&es4

i% Tension sstem5 in this sstem" the diagonal mem#ers have l8r ratio high enough

to act in tension onl" the #ecome dumm when su#6ected to com&ression%

ii% Tension!com&ression sstem5 is suita#le where lateral dimensions of the tower are

not too large with res&ect to the tower loads%

iii% #raced sstem5 is suita#le onl for large towers%) The we# &atterns are so chosen that tension mem#ers are long and com&ression

mem#ers are short and the inclination of mem#ers ma #e #etween .-o to @-o %

) De&ending on the voltage rating +@@ to /-- k90 and the num#er of circuits" the

height of transmission line tower varies from " to height ;" is ke&t at ,8@ for

tangent and small angle towers" ,8/ for medium angle towers and ,8. for largeangle towers%

) The economical #ase width > is &ro&ortional to the s7uare root of the moment" and

is e$&ressed #

> F where" M is over turning moment

is const% and varies from -%-L to -%,@


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2ig% @ Self!su&&orting wide #ase towers


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(OADS ON TO=*R

The transmission line towers are su#6ected to the following loads4

a0 9ertical loads

i% =eight of tower structure

ii% =eight of insulator strings and fittings

iii% =eight of &ower conductors

iv%=eight of ground wire

v% =eight of ice coatings +if an0

vi% =eight of maintenance crew +line man0 with tools +,%/ kN0

#0 (ateral or hori3ontal loads

i% =ind +or seismic0 load on conductors

ii% =ind +or seismic0 load on ground wire

iii% =ind +or seismic0 load on insulator string

iv% =ind +or seismic0 load on tower structure

v% Transverse com&onents of tensions in conductors and earth wire


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(OADS ON TO=*R

c0 (ongitudinal loads +'0

i% Un#alanced &ull due to a #roken conductor

ii% Un#alanced &ull due to #roken ground wire

iii% Seismic load on wires

iv% Seismic load on tower structure

v% (oad due to tem&erature variation

d0 Torsional +Mt0

i% Due earth wire #roken

ii% Due to conductor #roken

Conditions of designDesign is done under two conditions4

iii% Normal condition

iv% >roken wire Condition5 A #roken wire condition occurs when a wire +(e! conductor

wire or earth wire0 #reaks from one line" giving rise to an un#alanced longitudinal

force%


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As &er IS 4 -< +'art ,0" the following #roken wire conditions ma #e assumed in

design%


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D*SI:N S'AN

The following terminolog is used for various t&es of s&an4

i% Normal s&an4 It is the centre to centre distance #etween towers%

ii% =ind s&an4 The wind s&an +or wind load s&an0 is the sum of the two half s&ans

ad6acent to the su&&ort under consideration%

iii% =eight s&an4 the weight s&an +or vertical load s&an0 is the hori3ontal distance

#etween the lowest &oints of the conductor" on the two s&ans ad6acent to the

tower%

2ig% E 9arious t&es of s&ans


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=eight of tower

The weight +=0 Of tower ma #e estimated # com&arison with similar e$isting towers%

Alternativel" it ma also #e estimated with the hel& of the following formulae # Rle4= F ; kN

=here" ; is overall height of tower a#ove ground +m0

M is overturning moment at ground" due to wind" in kN!m

k constant" the value of which usuall lies #etween -%-1/ and -%-.@

=eight of conductors and ground wire

The vertical load due to conductors and ground wire shall #e #ased on the a&&ro&riate

weight s&an% A &rovision of ,%/ kN ma #e made for the weight of a lineman%

In com&uting the weight of conductor and earth wire" the weight s&an" which is ,%/

times the normal s&an or wind s&an" is used%

In #roken wire condition" @- of the weight s&an is used" accounting for ,- for the

#roken wire and /- for the s&an with un#roken wire%

2or tower and cross!arm design" the weight of maintenance crew +,%/ kN0 is used for

cross!arm design onl" an additional errection load of 1%/ kN is used%

The weight of string insulator"


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9*RTICA( (OADS DU* TO CONDUCTOR AND *ART; =IR*


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(ateral loads due to wind

i. Wind load on tower structure4 =ind &ressures on towers and su&&orts shall #e

com&uted as &er IS 4 E/ ! ,LE% The wind load is then com&uted # multi&lingthe #asic wind &ressure # the e$&osed &ro6ected area" using a&&ro&riate solidit

ratio and wind force coefficient% In case of lattice steel and other com&ound

structures" the wind &ressure on the leeward side mem#ers ma #e taken as one!

half the &ressure on wind ward side mem#ers% The wind &ressure intensit on

towers varies from ,%/ to


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(ateral loads due deviation K

In addition to the lateral load due to wind" lateral +or hori3ontal0 load is also induced

due to deviation in the line wires% Thus if T is the tension in the wire" the lateral loaddue to deviation in the direction will #e e7ual to


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(ongitudinal loads4

(ongitudinal loads are mainl caused due to #roken wire conditions" and these

loads have much more effect on the design of the tower than an other load% The un#alanced &ull due to #roken conductor" in case of su&&orts with sus&ension

strings" ma #e assumed e7ual to /- &er cent of the ma$imum working tension +Tc0

of the conductor%

In case of #undle conductors" the &ull due to #roken conductor ma #e assumed to

#e e7ual to


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Torsional loads4 +Mt0

Torsional moment is caused under #roken wire condition" when the #roken earth wire

or conductor wire is located at an eccentricit e with res&ect to the centre line of thetower%

The torsional moment is given as

Mt F '( e

The torsional shear &er face"

8t F mt 8


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*22*CT O2 T*M'*RATUR* 9ARIATION

Temperature variations:

The tem&erature range varies for different localities under different diurnal and

seasonal conditions% The a#solute ma$imum and minimum tem&eratures" which ma

#e e$&ected in different localities in the countr are indicated on the ma&s of India

in 2ig L ) ,-" res&ectivel%

These ma #e used for assessing the tem&erature stresses on conductors and ground

wires% The a#solute ma$imum tem&erature values given in figure shall #e increased

# a#out ,Eo C to allow for the suns radiation" heating effect of current" etc%" in theconductor%

Effect of temperature on cable tension

A conductor hangs freel #etween two su&&orts +towers0 at the ends% It is therefore

su#6ected to tension T% The value of ca#le tension T de&ends u&on the tem&erature%

The tension in conductor #ecomes +i%e% Tma$%0 when the atmos&heric tem&erature t isminimum +tmin 0% This ma$imum tem&erature should not e$ceed the allowa#le

tension in the conductor%

Similarl" the tension in the conductor #ecomes minimum +i%e% Tmin0 when the

atmos&heric tem&erature t is +i%e% tma$0% ;ence the resulting tensile stress in the

conductor is tem&erature de&endent%


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2ig% L Ma& showing highest ma$imum tem&erature 2ig% ,- Ma& showing lowest minimum tem&erature


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ANA(?SIS AND D*SI:N

Analysis of tower

A transmission line tower is a three!dimensional cantilever truss% Its analsis as a

s&ace frame is highl tedious% ;owever" a ma6orit of the forces acts onl at its to&

end% The conventional &rocedure is to anal3e it # resolving the tower in &lanar

frames% 2ig ,, shows various situations of hori3ontal load '

2ig% ,, (ongitudinal load on tower


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ANA(?SIS AND D*SI:N

Design of members

The mem#ers of the tower are either tension mem#ers or com&ression mem#ers%

Since the mem#ers are slender" secondar stresses are ignored% The design of towers

are done as &er recommendations contained in IS 4 -< ! ,LEE% Some of the salient

recommendations are given here%

i. actors of safety: In accordance with Rule E@+,0 +a0 of Indian *lectricit Rules

,L/@" the factor of safet +n0 in the design of structural mem#ers of steel

transmission line towers ma #e assumed as


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ANA(?SIS AND D*SI:N

ii. Allowable stress: The allowa#le stresses given here are #ased on

recommendation contained in IS 4 -< ! ,LEE" using the factors of safet +n0s&ecified a#ove" for steel in general +having ield stress f 0 and for steel to IS 4


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'*RMISSI>(* STR*SS f ac IN COM'R*SSION AS '*R IS 4 -<


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ANA(?SIS AND D*SI:N

Slenderness ratio

IS 4 -< s&ecifies the following limiting values of l8r ratio where ( is the actual length ofthe mem#er" #etween the centres of end connections%

!ermissible stresses in bolts:

The 6oints of tower are made # using #olts" to facilitate eas an 7uick installation% Thefollowing are the &ermissi#le stresses4

,% 'ermissi#le tensile stress on root of thread 4 ,L.8n N8mm<


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2OUNDATION 2OR TO=*RS

The sta#ilit of a tower de&ends #oth on the strength as well as sta#ilit of

foundations%

The foundation for a tower is designed for the following forces8momenlsa0 Downward load on the leg

#0 U&lift load on leg

c0 ;ori3ontal thrust

d0 Over turning moments

) :enerall% the load acting on the to& of a footing is inclined" and this Inclined load

can resolved into vertical and hori3ontal +or lateral0 com&onents%

) The lateral and longitudinal loads" acting at a great height cause large overturning

moments% which are to #e resisted # the foundation with a minimum factor of

safet of three%

2OUNDATION 2OR TO=*RS


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Documents

6.Design of Steel Towers