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
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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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(ATTIC* TO=*RS
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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(ATTIC* TO=*RS
*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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(ATTIC* TO=*RS
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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(ATTIC* TO=*RS
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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(ATTIC* TO=*RS
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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(ATTIC* TO=*RS
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$i#le towers
v% Self!su&&orting wide #ase towers
vi% :ued towers%
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TRANSMISSION (IN* TO=*RS
CON2I:URATIONS
2ig% . shows various configurations of self!su&&orting wide #ase towers% The main
#racing sstem ma #e of three t&es4
i% Tension sstem5 in this sstem" the diagonal mem#ers have l8r ratio high enough
to act in tension onl" the #ecome dumm when su#6ected to com&ression%
ii% Tension!com&ression sstem5 is suita#le where lateral dimensions of the tower are
not too large with res&ect to the tower loads%
iii% #raced sstem5 is suita#le onl for large towers%) The we# &atterns are so chosen that tension mem#ers are long and com&ression
mem#ers are short and the inclination of mem#ers ma #e #etween .-o to @-o %
) De&ending on the voltage rating +@@ to /-- k90 and the num#er of circuits" the
height of transmission line tower varies from " to height ;" is ke&t at ,8@ for
tangent and small angle towers" ,8/ for medium angle towers and ,8. for largeangle towers%
) The economical #ase width > is &ro&ortional to the s7uare root of the moment" and
is e$&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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TRANSMISSION (IN* TO=*RS
(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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TRANSMISSION (IN* TO=*RS
(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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TRANSMISSION (IN* TO=*RS
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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TRANSMISSION (IN* TO=*RS
=eight of tower
The weight +=0 Of tower ma #e estimated # com&arison with similar e$isting towers%
Alternativel" it ma also #e estimated with the hel& of the following formulae # Rle4= F ; kN
=here" ; is overall height of tower a#ove ground +m0
M is overturning moment at ground" due to wind" in kN!m
k constant" the value of which usuall lies #etween -%-1/ and -%-.@
=eight of conductors and ground wire
The vertical load due to conductors and ground wire shall #e #ased on the a&&ro&riate
weight s&an% A &rovision of ,%/ kN ma #e made for the weight of a lineman%
In com&uting the weight of conductor and earth wire" the weight s&an" which is ,%/
times the normal s&an or wind s&an" is used%
In #roken wire condition" @- of the weight s&an is used" accounting for ,- for the
#roken wire and /- for the s&an with un#roken wire%
2or tower and cross!arm design" the weight of maintenance crew +,%/ kN0 is used for
cross!arm design onl" an additional errection load of 1%/ kN is used%
The weight of string insulator"
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9*RTICA( (OADS DU* TO CONDUCTOR AND *ART; =IR*
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TRANSMISSION (IN* TO=*RS
(ateral loads due to wind
i. Wind load on tower structure4 =ind &ressures on towers and su&&orts shall #e
com&uted as &er IS 4 E/ ! ,LE% The wind load is then com&uted # multi&lingthe #asic wind &ressure # the e$&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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TRANSMISSION (IN* TO=*RS
(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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TRANSMISSION (IN* TO=*RS
(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$imum working tension +Tc0
of the conductor%
In case of #undle conductors" the &ull due to #roken conductor ma #e assumed to
#e e7ual to
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TRANSMISSION (IN* TO=*RS
Torsional loads4 +Mt0
Torsional moment is caused under #roken wire condition" when the #roken earth wire
or conductor wire is located at an eccentricit e with res&ect to the centre line of thetower%
The torsional moment is given as
Mt F '( e
The torsional shear &er face"
8t F mt 8
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TRANSMISSION (IN* TO=*RS
*22*CT O2 T*M'*RATUR* 9ARIATION
Temperature variations:
The tem&erature range varies for different localities under different diurnal and
seasonal conditions% The a#solute ma$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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TRANSMISSION (IN* TO=*RS
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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TRANSMISSION (IN* TO=*RS
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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TRANSMISSION (IN* TO=*RS
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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TRANSMISSION (IN* TO=*RS
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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2i ,< 2 d i f