transmission mono poles-serc

Advanced Course on Communication and Power Transmission Towers6-8 February 2013, CSIR-SERC, Chennai - 113, India. pp 323-371.

STEEL MONOPOLES

R. Balagopal

Scientist, Tower Testing and Research Station,CSIR-Structural Engineering Research Centre, CSIR Campus, Taramani,

Chennai 600113, INDIA.Email: [email protected]

1. INTRODUCTIONGenerally, the self supporting lattice type towers are most commonly used for power transmission. In a fast developing country faced with density of population in the urban areas, great difficulties are experienced in finding corridor (land) for new transmission lines. It is very difficult to get land for installation of conventional lattice towers for power transmission. Power utilities throughout the world are making diverse attempts to make compact lines. Compaction of a transmission line means reduction in the dimension of a line both in horizontal and vertical direction. By horizontal compaction power density over available corridors is increased by more efficient use of land and Right Of Way (ROW). Poles are suitable alternate supports to the conventional lattice towers. Steel poles have smaller plan dimension and are composed of only few pieces, compared to the lattice type towers. Poles are generally tapered and manufactured in number of pieces which can slip in to each other to form the entire pole structure. The pole circumference thickness is varied for each pole segment along the pole height to obtain a lighter structure. The pole cross section may have a rectangular, circular or regular polygonal shape of 6, 8, 12, 16, and 24 sided. Pole structures having polygonal cross sections with larger number of sides have larger flexural capacity for a specified circumference thickness. For pole structures of same flexural capacity, ones having larger base diameter to circumference thickness ratios and designed to utilize the full yield strength of the material are lighter. The pole structures are either connected by a base plate and anchor bolts to the foundation or directly embedded in to the soil or into a drilled concrete foundation. Poles with direct embedment foundations might have smaller base diameter than those with traditional base plate - anchor bolt type foundations to reduce the negative effect of extra weight, resulting from the additional pole length used for embedment.

2.0 INITIAL DESIGN CONSIDEARIONSGeometry: The basic pole structure configuration, conductor and shielding geometry insulation

324 Advanced Course on Communication and Power Transmission Towers

assembly length, swing angles, electrical clearances and shielding angles shall be determined. It is important to note that a critical loading condition may depend on the type of tubular pole structure configuration being considered.

Stress analysis: The structure designer will normally need to consider the effects of large deflections during the design. A secondary moment will develop when a structural member having an axial force deflects in a direction normal to the line of action of the force. The additional stress caused by this secondary moment is dependent on the magnitude of the axial force and the deflected shape of the member.

Clearences: Clearances from conductors to supporting structures, ground or edge of right-of-way are usually not affected significantly by deflections except perhaps where special span and line angle conditions exist.

Appearance: Line angles and unbalanced phase arrangements create load situations which will cause a pole to appear bowed. There are several methods that can be used to minimize the appearance problem. One method is to camber the pole to offset the deflection under this load so that it will appear straight and plumb. Another method is to rake the pole when setting it. The deflection at the top is determined for everyday loading. A pole can be designed to limit deflection by increasing its stiffness.

Other considerations: Pole structures should be considered flexible and relatively large deflections can result due to loads. Deflections can affect the magnitude of loads caused by an unbalanced longitudinal loading situation. The deflection of the structure and the swing of suspension insulators can significantly decrease wire tensions. Line spans, location of strain structures, structure flexibility, wire tensions and insulator lengths are some of the variables needed in the analysis to determine equivalent static loads.

Construction, finish and Transportation:

The specification should include possible limits due to equipment, access limitations, methods of hauling, assembly, erection, stringing or one circuit installed for the present with provisions for future circuits. Rigging attachment points should be provided for lifting the structure, hoisting insulators and stringing blocks, stringing, clipping in, deadending and maintenance. Special consideration should be given in the structure design for helicopter erection. The type of corrosion protection may limit structure design concepts. As an example, the diameter, length or weight of a pole section may be limited to fit available galvanizing kettle capabilities.

Steel Monopoles 325

Climbing and maintenance: The line designer should identify climbing, working and hot line maintenance provisions required. Generally, provisions should be made so that all parts of pole structures and insulator and hardware assemblies can be reached for maintenance. Detachable ladders should be fabricated in lengths which can be handled by line maintenance personnel on the structure.

Load testing : Consideration should be given to the full-scale structure testing which may be required. The necessity to perform a test may be to adequately verify the design concept, to verify connection details and to determine the level of reliability.

Stability and Fatigue analysis: Stability should be provided for the structure as a whole and for each structural element. Consideration should be given to load effects resulting from the deflected shape of the structure. Generally speaking, rigorous fatigue stress analyses of steel pole structure have proven unnecessary.

Modeling:It is important that the structure be accurately modeled for computer analyses. Finite element models should contain a sufficient number of elements to ensure that the curvature of members in the deflected position is adequately represented and the point of maximum stress is adequately defined.

3.0 ASCE/SEI 48-05 DESIGN RECOMMENDATIONSThe design stresses for members is based on ultimate strength methods using the factored loads.

Material Stress: The yield stress Fy, and the tensile stress, Fu, shall be the specified minimum values specified in ASTM standard. The modulus of Elasticity of Steel is defined as, 200 GPa.

I) Tension:

The tensile stress shall not exceed either of the following;

( ) ; ;

( ) ; .

a

b

PA

F F F

PA

F F F

gt t y

gt t u

where or

where 0 83

where Ft is the permitted tensile stress; Fy is the specified minimum yield stress; Fu is the specified minimum tensile stress; P is the axial tension force in the member; Ag is the gross cross-sectional area; and An is the net cross-sectional area.


II) COMpRESSION:The tubular members subjected to compressive forces shall be checked for general stability and local buckling. The compressive stresses shall not exceed the limiting stress values defined in the following sections.

a) Truss members with closed cross section:

For truss members with closed cross section, the actual compressive stress, fa, shall not exceed the permissible compressive stress, Fa, as determined by the following.

i) F F

KLr

CKLr

Ca yc

c

1 0 5

2

. , when

ii) FE

KLr

KLr

C CE

Fa c c y

2

2

2, ,when where

where, Fa is the permissible compressive stress; Fy is the specified minimum yield stress; E is the modulus of elasticity; L is the unbraced length; g is the radius of gyration; and K is the effective length factor. KL

r is the largest slenderness ratio of any

unbaked segment.

a) Beam Members:The limiting values of w/t and D0 /t specified in the following section may be exceeded without requiring a reduction in extreme fibre stress if local buckling stability is demonstrated by adequate experimental test.

i) Regular Polygonal Members: For formed, regular polygonal tubular members, the compressive stress P A Mc I , on the extreme fiber shall not exceed the following;

Octagonal, hexagonal, or rectangular members (bent angle 450)

F Fwt Fa y y

, when 260W

F F Fwt F

wt Fa y y y y

1 42 1 0 0 00114

1 260 351. . . ,

WW W

when

Fwt

wt Fa y

1 04 980 3512, ,

,F W

when

Steel Monopoles 327

Dodecagonal members (bent angle =30o)

F Fwt Fa y y

, when 260W

F F Fwt F

wt Fa y y y y

1 45 1 0 0 00129

1 260 374. . . ,

WW W

when

Fwt

wt Fa y

1 04 980 3742, ,

,F W

when

Hexdecagonal members (bent angle =22.5o)

F Fwt Fa y y

, when 215W

F F Fwt F

wt Fa y y y y

1 42 1 0 0 00137

1 215 412. . . ,

WW W

when

Fwt

wt Fa y

1 04 980 4122, ,

,F W

when

where, Fy is the specified minimum yield stress; Fa is the permitted compressive stress; w is the flat width of a side; t is the wall thickness; W = 2.62 for Fy or Fa in MPa; and F = 6.90 for Fa in MPa.

ii) Rectangular Members: The permissible stress specified for octagonal, hexagonal members is used. If the permissible stress value exceeds 6.9 MPa, then the equations for dodecagonal members are used.

iii) Polygonal Elliptical Members: The bend angle and flat width associated with elliptical cross section are not constant. The smallest bend angle associated with a particular flat shall be used to determine the compressive stress permitted.

iv) Round Members: For round members or polygonal members with more than

sixteen sides, the compressive stress shall not exceed the following fF

fF

a

a

b

b

1 ;

Where fa is the compressive stress due to axial load; fb is the compressive stress due to bending; Fa is the permitted compressive stress; and Fb is the permissible bending stress.

Permissible Compressive Stress: Fa

F FDt Fa yO

y

when and3800F ;


F F Dt

FDt Fa y O yO

y

0 75 950 3800 12 000. ; ,F F Fwhen

Permissible Bending Stress: Fb

F FDt Fb yO

y

when and6000F ;

F F Dt

FDt Fa y O yO

y

0 70 1600 6000 12 000. ; ,F F Fwhen

where D0 is the outside diameter of the tubular section (flat to flat outside diameter for polygonal members); t is the wall thickness; and F = 6.90 for Fy, Fb or Fa is in MPa.

III) Shear: The shear stress resulting from applied shear forces, torsional shear, or a combination of the two shall not exceed the following:

VQIb

TcJ

F F Fv v y ; .where 0 58

where, Fy is the specified minimum yield stress; Fy is the permitted shear stress; V is the shear force; Q is the moment of section about neutral axis; I is the moment of inertia; T is the torsional moment; J is the torsional moment of cross section; c is the distance from neutral axis to extreme fiber; and b equals 2 times the wall thickness.

IV) Bending: The stress resulting from bending shall not exceed either of the following; Mc

IF

McI

Ft a or

where Ft is the permitted tensile stress; Fa is the permitted compressive stress; L is the moment of inertia; M is the bending moment; and c is the distance from neutral axis to extreme fiber.

V) Combined Stresses: For a polygonal member, the combined stress at any point on the cross section shall not exceed the following;

PA

M cI

M cI

VQIt

TcJ

F Fx yx

y x

yt

2 212

3 or aa

For round members, the combined stress at any point on the cross section shall not exceed the following:

PA

M cI

M cI

VQIt

TcJ

F or Fx yx

y x

yt

2 212

3 bb

where Ft is the permitted tensile stress; Fa is the permitted compressive stress; Fb is the permitted bending stress; P is the axial force in the member; A is the cross sectional

Steel Monopoles 329

area; Mx is the bending moment about X-X axis; My is the bending moment about Y-Y axis; is the moment of inertia about X-X axis; Ix is the moment of inertia about Y-Y axis; cx is the distance from Y-Y axis to the point where stress is checked; cy is the distance from X-X axis to the point where stress is checked; V is the shear force; Q is the moment of section about neutral axis; I is the moment of inertia; T is the torsional moment of cross section; J is the torsional moment of cross section; c is the distance from neutral axis to the point where stress is checked; and t equals wall thickness.VI) Slip Joint: Slip joint in poles shall be designed to resist maximum forces and

moments in the connection. As a maximum, slip joints shall be designed to resist 50% of the moment capacity of the lower strength tube. The taper should be the same above and below the slip joint.

VII) Base Plate and Flange Plate connections: Flexural stress in the base or flange plate shall not exceed the specified minimum yield stress, of the plate material. The base and flange plate connections shall be designed to resist 50% of the moment capacity of the lowest strength tube.

VIII) Design of Anchor Bolts: The anchor bolts shall be designed to transfer the tensile, compressive, and shear loads to the concrete by adequate embedment length or by the end connection.

i) Bolts subject to tension: The bolts subject to designed to resist the sum of the tensile stresses caused by the external loads and any tensile stress resulting from prying action shall not exceed the permissible stress, Ft, as flows:

For bolts with specified proof load stress, Ft is the lowest of yield stress Fy or 0.83Fu, where Fu is the specified minimum tensile stress of the bolt. For bolts with no specified yield stress,

Ft = 0.83 Fu;

Thus TA

Fss

t ; where the stress area As is given by;

where Ts is the bolt tensile force; d is the nominal diameter of the bolt; and n is the number of threads per unit of length.ii) Shear Stress: The shear stress for anchor bolts shall be determined as follows:

VA

F Fs

v y 0 65.

where is the shear force on bolt; A d ns

4

0 9743 2., tensile stress of bolt; Fy is the

shear stress permitted; Fy is the specified minimum yield stress of bolt material; d is the nominal diameter of the bolt; and n is the number of threads per unit of length.iii) Combined Shear and Tension: For bolts subject to combined shear and tension, the

permitted axial tensile stress in conjunction with shear stress, Ft(v) shall be,


F FfFt v t

v

v( )

1

2

;

where Fv is the permitted shear stress; Ft is the tensile stress permitted; and fv is the shear stress on effective area. The combined tensile and shear stress shall be taken at the same cross section in the bolt.iv) Development Length: The minimum clear cover for concrete is specified as 76mm.

The development length for the threaded reinforcing bar used as anchor bolts shall be calculated as follows;

L ld d . . .a b gwhere Ld is the minimum development length of anchor bolt; and ld is the basic development length for the bolt shall be taken as;

For bars upto and including #11 (i.e upto 35.7 mm bar), use the larger of

lA F

fl dFd

g y

yd y

1 270 400

..

'

For

For # 14 bars (43.7 mm bars), lF

fd

y

y

2 69.

'

;

For #18 and #18J bars (56.4 mm bars), lF

fd

y

y

3 52.

'

;

where Ag is the gross area of anchor bolt; As req d( ' ) is the required tensile stress of bolt; Fy is the specified minimum yield stress of anchor bolt; fc

' is the specified compressive strength of concrete; d is the diameter of the bolt; =0.0150 for Fy and fc' in MPa and Ag in mm2, =9.67 for Fy and fc' in MPa; a =1.0 if Fy in 414 MPa or 1.2 if Fy in 517 MPa; b =0.8 for bolt spacing upto and including 152 mm, or 1.0 for bolt spacing less than 152 mm; g A As req d g( ' ) .

IX) Stress concentrations:Care must be taken to distribute loads sufficiently to protect the pole wall against local failure. Slip joints, arm to pole connections and abrupt changes in members cross section or longitudinal axis are points of susceptibility.

4. ANALYSISStress calculations for transmission structures have traditionally been based on elastic analysis. The design criteria presented in ASCE/SEI 48-05 is based on elastic stress analysis methods. Stability should be provided for the structure as a whole and for each structural element. The response of tubular transmission structures to applied loads is generally nonlinear in nature. The standard industrial practice in design is

Steel Monopoles 331

to use nonlinear finite element based computer programs. These computer programs consider the effects of large displacements and dependence of the structures stiffness on member stress levels and are capable of computing elastic stability phenomena. For pole structures the refined finite element models are necessary for the static and buckling analysis since a high level of accuracy is required at specific critical locations. For pole structures a seismic analysis produces no critical response. Analysis programs have virtually eliminated the use of linear analysis methods for most design work, However, some preliminary design and estimating work might still be accomplished by using linear analysis methods by assuming a deflected position or by using amplification factors on moments.

5. TYpICAL FABRICATION AND ERECTION DETAILS:Slip splices : Sections jointed by telescoping splices should be detailed for a nominal lap that will assure a minimum lap of 1.35 times the largest inside diameter. A commonly used practice is to specify a nominal lap 1.5 times the largest inside diameter of the female section and allow a 10% tolerance on the final assembled lap length. Supplemental locking devices are needed if relative movement of the joint is critical or if the joint might be subjected to uplift forces. In resisting uplift forces, locking devices should be designed to resist 100% of the maximum uplift load. The female section longitudinal seam welds in the splice area should be complete penetration welds for at least a length equal to the maximum lap dimension.

Circumferential welded splices:

Complete penetration welds should be used for sections joined by circumferential welds. Longitudinal welds within 75mm of circumferential welds should also be complete penetration welds.

Welded T joint connections:

Pole shaft to base plate welds, pole shaft to flange plate welds and arm to arm bracket welds are quite commonly T joint connections. Where the primary loads carried by the pole or arm are flexural, a groove weld with reinforcing fillet is recommended to satisfy the requirements for through - thickness stresses in the attachment plate.

Hole size: Typically, holes 3mm larger than the nominal bolt diameter are used except for anchor bolt holes. Anchor bolt holes in the base plate are normally 10mm oversize.


6. FOUNDATION TYpES:Steel pole structures can be installed using drilled shaft, direct embedded pile or spread footings. The soil condition will often dictate the best type to install. The base size of the structure or the available equipment may limit the foundation type.

Caisson: Caisson foundations are particularly effective in areas where the augured holes collapse or soft cohesive soils tend to slump or squeeze inward and reduce the diameter of the hole. The caisson should be provided with adjusting bolts at the top to plumb the pole into place.

Direct embedment: The bottom portion of the pole becomes the foundation member reacting against the soil. The length of the section of the pole below the ground line should be determined using a lateral resistance approach.

Reinforced concrete drilled foundation:An anchor bolt cage set in a reinforced concrete drilled foundation is a very popular foundation. The minimum foundation diameter is determined by the diameter of the bolt circle.

Pile foundation:The purpose of the pile foundation is to transfer the loads from the pole down to the denser underlying soil or rock. The skin friction of the pile is used to resist uplift. The most common piles are steel H-piles, wood pile and prestressed concrete.

Stem and pad foundation (spread):The stem and pad foundation is a basic spread footing. It is used in areas where drilled holes cave easily. Spread footings are designed to resist compression from axial loads and overturning moment from horizontal loads.

Rock anchors:Rock anchors can be designed to resist uplift, compression, horizontal shear and in some cases, bending moments.

7.0 COMpARATIVE STUDY To compare the structural behavior of lattice tower and pole structure, the static and dynamic performance of 30m and 40m Mw monopole is compared with corresponding 30m and 40m high square is considered in the present study.

Steel Monopoles 333

a) 30m and 40m high self supporting Mw monopole.

The configuration, dimensions and load application details for 30m and 40m High Mw monopole is shown in Fig. 1 and Fig. 2. The main shaft of both 30m and 40m High Mw Monopole is of twenty sided polygonal (20 sided regular polygon) in shape and made of four segments with bottom two segments of 8mm thick and the top two segments are of 6mm thick. These segments are joined by telescopic slip splice joint with minimum lap length of 1.35 times the largest inside diameter of the pole [ASCE Manual No: 72]. The diameter of the shaft is 900mm at the bottom and 150mm at the top for 40m high self supporting monopole, while the diameter of the shaft is 800mm at the bottom and 150mm at the top for 30m high self supporting monopole. The base plate is of 48mm thick welded to the bottom of the shaft. The main shaft is made of material Fe-490 with 350 MPa yield stress and base plate with yield stress of 250 MPa. The pole has been designed for a wind speed of 33 m/s and Terrain Category -2. The wind load on 40m and 30m high monopole is calculated based on IS: 875 (Part 3): 1987. NE-NASTRAN, a non-linear FE software is used for modeling both 30m and 40m high Mw monopole. In the FE model, four noded plate shell elements are used for modeling the main shaft, stiffeners and base plate. The FE Model for 30m and 40m high self supporting pole is shown in Fig. 3 and Fig. 4 respectively. The elastic plastic material property of steel was represented by an elastic-plastic bi-linear model, with the modulus of elasticity as 2E5 upto yield and 2000 MPa above yield stress. Eigenvalue analysis for both the pole structures has been carried out and the deformed FE model of both the pole structures are shown in Fig 5 and Fig 6. Both the wind load the antennae loads has been applied on the pole structures at equi-distant intervals along the height of the pole and the deflected FE model for both the pole structures is shown in Fig. 7 and Fig. 8 respectively.

500

C /S OF 20 S IDE DP OLY G ONAL P OLE

8 tk

16 tk

8 tk

16 tk

6 tk

12 tk

6 tk

4.89 kN

B AS IC P OLEC ONF IG UR AT ION

WIND LOAD +ANT E NNAE LOAD

30m MW MONOP OLE

All dimensions arein mm

7200

1100

6130

1070

6170

1060

30000

72704.80 kN

2.53 kN

2.47 kN

2.53 kN

2.54 kN

2.53 kN

2.37 kN

4.89 kN

2.48 kN

800

Fig. 1 General configuration of 30m High Mw Monopole


7600

6600

6600

7600

7600

500

1200

1000

1200

600

900

C /S OF 20 S IDE DP OLY G ONAL P OLE

8 tk

8 tk

16 tk

14 tk

6 tk

12 tk

6 tk

12 tk

6 tk

5.75 kN

4000

4000

4000

4000

4000

4000

4000

4000

4000

4000

5.62 kN

3.19 kN

3.33 kN

3.35 kN

3.31 kN

3.23 kN

3.13 kN

3.02 kN

2.91 kN

B AS IC P OLEC ONF IG UR AT ION


40m MW MONOP OLE

All dimens ions arein mm

40000

Fig. 2 General configuration of 40m High Mw Monopole

Fig. 3 FE Model (30m Mw Monopole)

Steel Monopoles 335

Fig. 4 FE Model (40m Mw Monopole)

Fig. 5 Deformed FE Model of 30m Mw Monopole (First Mode)


Fig. 6 Deformed FE Model of 40m Mw Monopole (First Mode)

Fig. 7 Deflected FE Model of 30m Mw Monopole

Steel Monopoles 337

Fig. 8 Deformed FE Model of 40m Mw Monopole

b) 30m and 40m high self supporting Mw lattice tower.

The configuration, dimensions and load application details for 30m and 40m High Mw lattice tower is shown in Fig 9 and Fig. 10 Both the lattice towers have square configuration with two slopes. The bottom and top widths of 40m Mw tower are 4.5m and 1.5 m respectively. The 30m Mw tower has base width of 3.5m and top width of 1.5m . These lattice towers have been designed for a wind speed of 33 m/s and Terrain Category -2. The wind load on 40m and 30m high lattice Mw tower is calculated based on IS: 875 (Part 3): 1987 recommendations. NE-NASTRAN, a non-linear FE software is used for modeling both 30m and 40m high lattice tower. In the FE model, two noded beam column elements are used for modeling the main leg, bracing and tie members of the tower. The undeformed FE Model of 30m and 40m high lattice tower is shown in Fig. 11 and Fig. 12 respectively. The elastic plastic material property of steel was represented by an elastic-plastic bi-linear model, with the modulus of elasticity as 2E5 upto yield and 2000 MPa above yield stress. Eigenvalue analysis for both the lattice tower has been carried out and the deformed FE model is shown in Fig. 13 and Fig. 14 respectively. Both the wind load and the antennae loads are applied at four noded point load at the top of each panel. The static analysis has been carried out and the deflected FE model for both the lattice towers are shown in Fig. 15 and Fig. 16. respectively.


2500

2500

2500

2500

2500

2500

2500

2500

2500

2500

2500

2500

LEG

ME

MB

ER

BR

AC

ING

ME

MB

ER

TIE

ME

MB

ER

130x

130x

1011

0x11

0x81

00x1

00x8

80x8

0x6

75x7

5x5

60x6

0x5

4.92 kN

4.84 kN

2.48 kN

2.56 kN

2.64 kN

2.84 kN

3.16 kN

3.24 kN

2.56 kN

3.92 kN

2.72 kN

2.64 kN

B AS IC T OWE RC ONF IG UR AT ION WIND LOAD +

ANT E NNAE LOAD

50x5

0x5

45x4

5x545

x45x

5

30m MW T OWE R

3500

1500

1500

30000


Fig. 9 General configuration of 30m High Mw square lattice tower

4500

5000

2500

2500

2500

5000

2500

2500

2500

2500

2500

2500

2500

2500

2500

40000

LEG

ME

MB

ER

BR

AC

ING

ME

MB

ER

TIE

ME

MB

ER

130x

130x

1213

0x13

0x101

10x1

10x8

100x

100x

880

x80x

675

x75x

560

x60x

5 5.12 kN

5.08 kN

2.72 kN

2.84 kN

2.96 kN

3.24 kN

3.40 kN

3.36 kN

6.84 kN

7.36 kN

2.72 kN

3.28 kN

3.08 kN

2.92 kN

B AS IC T OWE RC ONF IG UR AT ION


50x5

0x5

45x4

5x545

x45x

5

40m MW T OWE R

1500

1500


Fig. 10 General configuration of 40m High Mw square lattice tower

Steel Monopoles 339

Fig. 11 FE Model (30m Mw lattice tower)

Fig. 12 FE Model (40m Mw lattice tower)


Fig. 13 Deformed FE Model of 30m Mw lattice tower (First Mode)

Fig. 14 Deformed FE Model of 40m Mw lattice tower (First Mode)

Steel Monopoles 341

Fig. 15 Deflected FE Model of 30m Mw Monopole

Fig. 16 Deformed FE Model of 40m Mw Monopole


c) Wind and Antennae load calculation on Pole and lattice tower

a. Wind Load

The wind load on pole and lattice tower is calculated based on IS: 875 (Part 1)-1987.The following design parameters are used for calculating the wind loads.

Basic Wind Speed: 33 m/s, Terrain Category: 2, Topography factor k3=1.0,

Risk coefficient k1=1.05.

The design wind speed is calculated taking into account the terrain type, height of the structure, topography, risk level for the structure. The wind load (Fp) on the lattice tower is calculated based on solidity ratio (), the geometry of the member sections and the wind flow regime through the force co-efficient method. The wind pressure and subsequent wind load calculation on pole structure depends upon the aerodynamic effects of the pole structure. The solidity ratio () is defined as the ratio between the effective area (Ae) of the panel (areas of all members of the panel) projected on a plane normal to the wind direction and the overall area (At) of the panel. The effective area (Ae) does not include the projections of the bracing members from faces parallel to the wind direction, plan and hip bracings. The wind load (Fp) on the tower at height z is then computed using the relation,

Fp = Cf Pz Ae (1)

where Cf = force coefficient; Ae = effective frontal area at height z; Pz = 0.6Vz2 design wind pressure in N/m2 at height z and due to design wind velocity Vz in m/sec. The calculation details for design wind velocity Vz is given IS 875 (Part 3): 1987.

The additional loading effects due to wind turbulence and dynamic amplification in flexible structures such as guyed towers and pole structures is calculated using gust factor G. When the fundamental frequency of the pole structure is less than 1Hz, then dynamic loading analysis of the structure is recommended in codes of practice. The IS code recommends simplified method to calculate the peak response of wind resistance structures. F = Cf Ae Pz G (2)

The gust factor G accounts for the dynamic effects of gust on wind response towers. The values of these gust factors lies in the range of 1.5 to 2.5.The values of these gust factors changes with wind speed, decreases with height and increases with increased terrain roughness. The frequency of the pole structures is almost less than 1Hz, the wind loads on these structures is calculated based on gust factor method. The wind loads calculated based on this gust factor method is 35-25-30% higher when compared to force-co-efficient method.

Steel Monopoles 343

b. Antennae Load

Both the pole and lattice structures are subjected to same antennae loads and the deflection behavior is compared. The loading details for antennae are as follows. There are 3 nos. of GSM antennae of size 2.6m 0.3m at tower top and 3 nos. of CDMA antennae of size 1.2m 0.3m at 3m down from tower top. The wind load due to these antennae on the pole and lattice structure is calculated based on the exposed area of the antennae.

d) Simplified Numerical Model for steel pole and lattice tower

Generally, the lattice tower and pole structure represents a system with infinite degrees of freedom. In this method, this structure is discretized into a simple model with multiple degrees of freedom, thereby the continuum model is reduced to lower order discrete mass model. The entire structure is discretized into n number of nodes, with their masses lumped at these nodes. The free vibration equation of motion for an undamped mutli degree of system is represented as,

M x K x 0 (3)where [K] is the stiffness matrix, [M] is the mass matrix, {x} denotes the displacement and x denotes the acceleration vectors of different degrees of freedom. The stiffness matrix is obtained by evaluating the flexibility matrix, [F]. To obtain the flexibility matrix, a unit horizontal load was applied at the node i and the displacement at node j, is calculated, which is the fij element of the flexibility matrix [F]. The stiffness matrix [K] is obtained by inverting the flexibility matrix [F]. Equation (3) represents the eigenvalue or characteristic equation of the structural system. The natural frequencies and mode shapes are obtained by solving the eigenvalue equation.

For the numerical modeling, the entire lattice mast are discretized into finite number of elements. For the formulation of flexibility matrix , the average of combined moment of inertia of all four leg members at bottom and top of the lattice mast is considered. While formulating the mass matrix, uniform mass density and uniform combined area of all the four leg members are considered. The fundamental frequency and mode shape is obtained by solving the characteristic Eigenvalue equation.

The pole mast is descritized into finite number of elements. The cross sectional area and moment of inertia of the polygonal pole at bottom and top of the pole mast is calculated based on ASCE Manual No: 72 formulae with average thickness is considered. In the flexibility matrix formulation, the average moment of inertia at bottom and top of the pole is considered. For mass matrix formulation, average density and average cross sectional area at bottom and top of the pole is considered. The fundamental frequency and mode shape is obtained by solving the characteristic Eigen value equation.


e) Deflection Comparison.

The maximum deflection obtained from static analysis for both lattice tower and pole structures are tabulated in Table1. From the table, it can be observed that the deflection of 40m Mw monopole is 6.5 times higher than the corresponding deflection for 40m high Mw lattice tower. The deflection of 30m Mw monopole is 5 times higher than deflection for 30m high Mw lattice tower.

Table 1: Deflection Comparison of Mw Monopole and Mw lattice tower

S. No Self supporting Monopole Self Supporting Lattice TowerHeight (m) Deflection (mm) Height (m) Deflection (mm)

1 30 532 30 1032 40 1097 40 168

f) Weight Comparison.

Both the Mw tower and Mw Monopole has been designed based on working stress method. The self weight of both lattice tower and monopole has been calculated and is tabulated in Table 2.

Table 2: Weight Comparison of Mw Monopole and Mw lattice tower

S. No Self Supporting MonopoleHeight (m) Self Weight (kN) Height (m) Self Weight (kN)

1 30 35 30 282 40 48 40 38

g) Natural Frequency Comparison.

The natural frequency for both pole and lattice structures are calculated based on FE analysis and simplified numerical model and the results are tabulated in Table 3.

Table 3: Natural Frequency Comparison of Mw Monopole and Mw lattice tower

S. No Self Supporting Monopole Self Supporting Lattice TowerHeight (m) Natural Frequency (Hz) Height (m) Natural Frequency (Hz)

FE Model Simplified Model

FE Model Simplified Model

1 30 1.012 1.034 30 3.11 3.402 40 0.606 0.654 40 2.45 2.40

The monopole structures are dynamically sensitive, since the natural frequencies of these structures are close to 1Hz, when compared to lattice towers, whose frequencies are higher than 2 Hz. The natural frequency predicted from FE model and simplified model

Steel Monopoles 345

varies within 1% and hence the simplified model can be used as better approximation to estimate the natural frequency of pole and lattice structures. The deflection criteria is one of the most important aspect in communication towers. The deflection sway limit should be within 0.5 degrees for Mw towers. The deflection for pole structures exceeds this limit, but this will not cause a major problem for signal attenuation, because nowadays CDMA and GSM antenneas are used in signal communication. The self weight of monopole is 18 to 20% higher than the lattice towers. The self weight of monopoles can be further reduced by applying suitable optimization techniques. Considering all these aspects along with ease in transportation, erection, handling and reduction in land acquisition cost, these pole structures forms a suitable alternate for conventional lattice towers.

7. ANALYTICAL AND EXpERIMENTAL INVESTIGATION ON STEEL TRANSMISSION pOLE STRUCTURESAnalytical and experimental studies conducted on 400kV D/C, 0-2 degree line deviation suspension type and 132 / 220kV S/C 30o deviation self supporting mono pole structures are discussed in detail. Test results from full scale testing conducted at Tower Testing and Research Station, Chennai, India are compared with the analytical results.

7.1 400kv D/C Suspension Type pole The configuration, dimensions and load application of 400kV D/C transmission line pole is shown in Fig.17. The pole is of tapered cross section with 1850mm at bottom and 500mm at top and made in to five sections for easy transportation and erection. These sections are jointed by telescoping slip splices with minimum lap of 1.7 times the largest inside diameter. The main shaft is hex decagonal (16 sided regular polygon) in shape and made of 10mm thick sheet. The cross arms are of octagonal shape made of 6mm thickness. The cross arm ends are welded to a circular flange plate as shown in Fig.18. A separate collar of hex-decogonal in shape is used to fix the cross arms. The collar is attached to the main shaft at the required height by means of bolts. A circular cantilever bracket is welded to the collar and stiffened with plate stiffeners. The cross arm is connected to the bracket by bolts. The collar is connected to the main shaft by bolts. Transfer of load from collar to the main shaft is by friction developed by tightening the column of bolts provided in two opposite sides of the collars. The bolts are pre-tensioned to about 60% to 70% of it tensile capacity. The rotation due to broken wire loads are resisted by friction developed due to tightening of bolts and interlocking of collars with main shaft due to polygonal shape. The ground wire peaks are of octagonal shape made from 6mm sheets. The peaks are directly welded to the collars and the collars are connected to the main shaft by bolts as shown in Fig.19. The base plate is of 48mm thick ring, welded to the bottom most segment of the main shaft with provision for fixing 20nos. of 45mm dia. 12.9 grade anchor bolts. Template of 16mm thickness is


used below the base plate. The main shaft, base plate, cross arms and peaks are made of material Fe - 490 with 350MPa Yield stress and 210kN/mm2 Elastic modulus. The pole is designed for basic wind speed - 47m/sec, Security Class - 1, Terrain Category 2,Normal span of 300m, wind loads on conductor and ground wires are as per IS:802 part II Sec.2 1995, and wind loads on pole structure as per IS: 875(Part 3)-1987.

Foundation: In general, the monopole towers can not be accommodated in a regular test bed. The pole structures requires exclusively a special type of anchoring system since the extreme bolt carry maximum tension due to uplift force. A special circular foundation with rock anchors located in concentric circles to resist the uplift forces and to accommodate the pole with base plate was constructed as shown in Fig.20. The foundation bolts are tightened up to 60% of their tensile capacity. The foundation bolts likely to be subjected to maximum tension are identified prior to testing and instrumented with strain gauges. The shaft is erected using a mobile crane segment by segment (Fig.21). The cross arms and ground wire peaks are assembled to the brackets at ground levels itself and then fixed to the shaft. The verticality is ensured by using theodolites on both the axes.

Fig. 17 Configuration of 400kV D/c Pole and load point details

Steel Monopoles 347

Collar

Bracket

Fig. 18 Assembly of cross arms

Fig.19 peak with collars and brackets

Instrumented bolt Rock Anchors

Fig.20 Foundation showing rock anchors and instrumented anchor bolts

Fig. 21 Various stages of erection


Analytical and Experimental Natural Frequency of Pole

Field experiment to determine the natural frequency is conducted on the pole structure erected in the test bed. The theoretical natural frequency of the pole is determined from finite element analysis. The full pole along with cross arms, peaks etc., are modeled using plate elements in NE Nastran. In the collar locations, thickness of shaft is increased to account for the combined thickness of collar and the shaft. The theoretical frequency is found to be 1.017 and 1.0Hz in the first mode, in transverse and longitudinal directions and 4.83Hz in torsional mode. The measured experimental frequencies are 0.87Hz in first mode in both directions and 3.25Hz in torsional mode. The first few mode shapes are shown in Fig.22. To account for the additional loading effects due to wind turbulence and dynamic amplification of flexible structures like poles, gust factor G is used. When the fundamental frequency of the towers is less than 1Hz then most of the current wind loading standards recommend dynamic analysis for the structures. Certain simplified formulae were recommended in the codes to calculate the peak response of wind sensitive structures. In the present case the measured frequency of the pole structure is less than 1Hz, hence, the gust factor method given in IS:875, (Part 3) -1987 is used for determination of wind loads on the pole. The loads due to conductors, earth wires and insulators are determined as per IS: 802 (Part 1/Sec 1):1995.

Fig. 22 The first three eigenmodes of monopole tower

Finite Element (FE) ModelingPlate shell elements are used for modeling the main shaft, cross arms, peaks, stiffeners and base plate. This element typically resists membrane shear and bending forces. The non-linear analysis capability of NE Nastran, accounting for the geometric and material

Steel Monopoles 349

non linearity, is used to analyse the model and to obtain the pre-ultimate behaviour. The elastic- plastic material property of steel is represented by a bi-linear model, having modulus of elasticity up to a yield stress equal to 2105 MPa and 2000 MPa beyond yield stress. The incremental load and predictor-corrector iteration under each load increment is used in the non linear range. The load is applied in 30 to 40 steps until the limit point is reached in the load deformation behaviour. Loads are defined to simulate the field condition environment for which the structure is designed. The boundary conditions are specified for each degree of freedom of each grounded node. Collars, slip splices and flange plates in cross arms are not modeled separately. Instead, the thickness of the shaft at collar and slip splice locations are increased to account for the combined thicknesses. Both linear and non-linear static analysis has been carried out. The body wind loads are applied as distributed discrete nodal loads along the full height of the structure. The conductor and ground wire loads are applied as nodal loads at the appropriate locations. The finite element model is shown in Fig.23.

Fig. 23 Finite element model: various components

Comparison of Test Vs Analytical Resultsa) Security conditionIn the present study, the pole is tested for single conductor broken at a time with 75% wind condition. These tests are conducted on right side ground wire peak, top, middle conductors and on left bottom conductor. During bottom and middle conductor broken condition tests, rotation of collar by about 40mm, 56mm respectively were noticed


(Fig.24) in the direction of loading while increasing the longitudinal load from 20% to 40%. No increase in rotation was noticed during further loading. The deformed FE model is shown in Fig.25. The test and analysis foundation bolt forces and shaft stresses in right ground wire broken case are given in Fig.26 and 27. The analysis deflections are compared with test in Fig.28. During top conductor broken test, the collar along with cross arm assembly has slipped and started rotating like a free body with increase in longitudinal load as shown in Fig.29. It was noticed that the friction developed at the interface of collar and main shaft due to tightening of bolts and interlocking forces between the folded sides of the main shaft and collar was not sufficient to resist the longitudinal (twisting) load even though number of bolts provided are same in all collars. The rotation may be due to reduction in contact area (only 50%) between shaft and collar due to reduced diameter of main shaft at top cross arm level when compared to other cross arm levels. The reduced widths of polygonal shaft sides or faces are also one of the reasons for not developing the interlocking forces necessary for resisting the twisting force. In order to avoid the free body rotation, through bolts passing from one side of the shaft to diametrically opposite side is provided as shown in Fig.30.

Fig. 24 Rotation of collar

Fig. 25 Deformed FE models in right ground wire and middle conductor broken condition

Steel Monopoles 351

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400 450 500 550 600

BOLT FORCE IN kN

PE

RC

EN

TAG

E O

F LO

AD

1A

1T3T 2T 2A

Fig. 26 Foundation bolt force in right ground wire broken condition

0

10

20

30

40

50

60

70

80

90

100

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250STRESS IN N/mm2

PE

RC

EN

TAG

E O

F LO

AD

1T0T

2T0A5T5A

6T4T

3T7T

8T9T1A

9A 6A

Fig. 27 Shaft stresses in right ground wire broken


0

10

20

30

40

50

60

70

80

90

100

0 250 500 750 1000 1250 1500DEFLECTION IN mm

PE

RC

EN

TA

GE

OF

LO

AD

TTTALTLA

TT Transverse Test : TA Transverse Analytical

LT Longitudinal Test : LA Longitudinal Analytical

Fig. 28 Pole deflection in right ground wire broken test

Fig. 29 Rotation of top cross arm

Steel Monopoles 353

Fig. 30 Through bolts arresting rotation

b) Reliability conditionIn this case, the pole is subjected to transverse and vertical loads only. The loads resulting from wind on conductors, insulators etc. are applied at the cross arm tips. The wind load on pole shaft between the cross arms are lumped at collar levels in both test and analysis. The wind load on shaft below bottom cross arm level is lumped in to a single point. The forces measured in the foundation bolt during testing and analysis is shown in Fig.31. Stresses at the bottom segment of the shaft in FE analysis are compared with the test results in Fig.32. The deformation of the pole measured in test using theodolite and FE analysis are shown in Fig.33. Deformed FE model and stress pattern in the bottom segment of the pole are shown in Fig.34.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800 900 1000 1100BOLT FORCE IN kN

PER

CEN

TAG

E O

F LO

AD

1T2T

1A

2A

T TESTA ANALYTICAL

Fig. 31 Foundation bolt force in reliability test


0

10

20

30

40

50

60

70

80

90

100

-350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350

STRESS in N/mm

PE

RC

EN

TAG

E O

F LO

AD

1T

0A

0T

8A8

2 2A 3T

7T

5T4T 6T

1A

A - ANALYTICAL T - TEST

5A

Fig. 32 Shaft stresses in Reliability test

0

10

20

30

40

50

60

70

80

90

100

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000

DEFLECTION IN MM

PE

RC

EN

TA

GE

OF

LO

AD

ANALYTICALTEST

Fig. 33 Pole deflection in reliability test

Steel Monopoles 355

Fig. 34 Deformed FE model and shaft stresses in N/mm2 at bottom in reliability test

7.2 132/220KV S/C 300 Deviation poleThe pole is of tapered cross section with a diameter of 1200mm at bottom and 450mm at top and is made in to five sections. The main shaft is hex decagonal (16 sided regular polygon) in shape and made of different thicknesses varying from 12mm to 6mm. The pole is without cross arms and peaks. The conductors are connected to the tension insulators on either side of the pole. Jumpers are used to connect the conductors. Supports are used for hanging the jumper insulators. The pole during erection and testing is shown in Fig.35. The foundation bolts and bottom portion of shaft were instrumented with strain gauges. Fig.36 shows the deformation of pole in FE analysis and during testing in Reliability and Dead end load cases. The measured stresses in shaft and foundation bolt forces are shown in Fig.37 and 38.

Fig. 35 Various stages of erection, rigging of 132/220kV 30O dev. Mono pole in Test pad


Fig. 36 Instrumented foundation bolts and shaft, buckled stiffener and Deformed pole dur-ing test and FE analysis

0

10

20

30

40

50

60

70

80

90

100

-350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350

6T

5T

6AT 3A

2A2 T

3 T

1T1A4T

4A

A - ANALYTICAL T - TEST

Fig. 37 Shaft stresses in dead end condition

Steel Monopoles 357

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400 450 500 550 600 650

2A 2T

A ANALYTICALT TEST

Fig. 38 Foundation bolt force in dead end condition

The mono pole structures are successfully tested for all load cases. The non linear FE model shaft stresses, deformations and foundation bolt forces are compared with the test results. The measured test deflection at top of shaft is more by about 55% to 80% to that of analysis deflection in reliability load condition. The measured force in the extreme foundation bolt is 10% more than the analytical force. The measured stresses at bottom of shaft are almost the same as that of analysis. Rotation of cross arms is noticed during broken conductor loads. This rotation is less at bottom conductor level when compared to middle conductor level.

8.0 EXpERIMENTAL INVESTIGATION ON OCTAGONAL LIGHTING pOLE WITH ACCESS HOLETubular steel poles are laterally flexible and relatively large deflection occurs due to lateral loads. Street lighting columns are usually made from thin walled tubes of circular or octagonal in cross section. These lighting columns will have an access hole in the region of high bending moment. The problem under investigation is the bending strength of thin walled octagonal tapered street lighting columns with access hole in the region of critical moment location. The lighting columns are of 8m and 12m high with uniform thickness of 4mm. The primary loads on the lighting columns are mainly from wind loads. The wind load acting on these columns causes large bending moment in the bottom locations and the situation is further complicated by the presence of the access


hole. The critical case is when the wind acts a direction such that the access hole lies in the most highly compressed region of the octagonal cross section. Strength and stability assessment of these lighting columns becomes important, whenever they are subjected to extreme wind loads.

Octagonal poles are usually designed within the elastic limit of the material. Under extreme wind load cases, the stress limits are allowed to exceed the elastic limit and a moderate amount of plastic yielding is allowed. Due to the presence of access hole, the bending moment capacity is not only reduced due to the reduced cross section, but also by local plate buckling in the region of hole edges. Stress concentration effect also intensify the effect of local buckling. Because of the interaction between yielding and local buckling, the local buckling stress in the region of access hole is usually higher. Full scale tests on the lighting pole has been conducted, loaded upto 100% design load and further increase upto local failure and the corresponding deflections and strain responses are correlated in this paper.

M. Dicleli has concluded that the pole structures with polygonal shape have larger flexural capacity for a specified circumferential thickness. The number of sides in a polygon is determined by the specified circumferential thickness of the pole cross section and its diameter. The location of maximum stress along the height of the pole is a function of load distribution and the relative orientation of forces acting on the pole. S. A. Baban. et. al) conducted pure bending test on 12 specimens of both circular and octagonal cross sections both with and without the access hole at the position of extreme fibre compression. The results gave a quantitative indication of the weakening effect of the access hole and have formed an essential basis for the development of design formulae later. G.H.Little) developed empirical formulae for generating design curves for the bending strength of thin walled non tapered steel tubes, both circular and octagonal in cross section, along with the allowance for the presence of access hole in the compression zone based on the test results conducted by S. A. Baban et. al.

Present StudyIn the present study, analytical and experimental studies conducted on 12m street lighting without access hole and 8m high octagonal with and without access hole is discussed in detail. Test results from full scale testing conducted at Tower Testing and Research Station, SERC, Chennai are compared with the analytical results.

The configuration, dimensions and load application details for 12m octagonal pole is shown in Fig.39. The main shaft is octagonal (8 sided regular polygon) in shape and made of 4mm thick throughout the height. The diameter of the shaft is 200mm at bottom and 100mm at top. The base plate is of 16mm thick welded to the bottom of the shaft. In the access hole location the shaft is reinforced with 16mm square bars around the periphery. The main shaft is made of Fe-490 material with 350MPa Yield stress and base plate with yield stress of 210MPa.

Steel Monopoles 359

Fig.39. 12m Octagonal Pole basic configuration and load point details

Fig.40 8m Octagonal Pole basic configuration and load point details

The configuration, dimensions and load application details for 8m octagonal pole is shown in Fig.40. The main shaft is octagonal (8 sided regular polygon) in shape and made of 4mm thick throughout the height. The diameter of the shaft is 150mm at bottom and 75mm at top. The base plate is of 20mm thick welded to the bottom of the shaft. The location of access hole is 0.5m from bottom of the shaft. The main shaft is made of material Fe-490 with 350 MPa Yield stress and base plate with yield stress 210MPa.

The design parameters are common for both the poles. They are as follows: Basic Wind Velocity 47 m/s; Terrain Category 2; Wind load on Pole Structure

as per IS:875 (Part 3):1987.

Preliminary AnalysisThe selection of structural configuration of the street lighting column such as the pole dimension, number of sides of the polygon, material yield stress and plate thickness


is left to the design engineer. Street lighting columns with regular octagonal cross section will not be subjected to lateral buckling or lateral torsional buckling, because of their symmetry. The European standard EN 40-3-3 specifies the requirements for the verification of the design of the lighting columns by limit state principles, where the effects of factored load are compared with the relevant resistance of the structure. Of the two limit states, the ultimate limit state corresponds to the load carrying capacity of the lighting column, whereas the serviceability limit state corresponds to the deflection the lighting column.

For closed regular octagonal cross section as shown in Fig.41, the bending strength of the section shall be calculated from the equation,

Mux Mf Z

uyy p

m

g1

310in Nm (1)

wherefy is the characteristic strength of the material (in N/mm2); f1 is the value obtained

from the curve appropriate to the cross section with value of R t f E Ey ; ; is the modulus of elasticity of the material = 2105 N/mm2; R is the mean radius of the cross section (in mm); t is the wall thickness (in mm); gm is the partial material factor (1.15); Zp is the plastic modulus of the closed regular cross section (in mm3); where Z R tp 4 32 2. for octagonal cross section.

Fig. 41 Cross sectional details of 12m, 8m poles

Fig. 42 Access hole detail for 12m, 8m poles

Steel Monopoles 361

Similarly, the bending strength of the reinforced openings in regular cross section as shown in Fig.42. is calculated from the equation,

Mf Z

Mf Z

uxy pnr

muy

y pyr

m

g

g

63

6310 10

and in Nm (2)

where f6 is a factor for reinforcement provided given by

2

2 0 32

2

2

t t E

t t E RLfw

w y

.

.

Zpnr is the plastic modulus of the section including the effect of door reinforcement

about plastic neutral axis n-n (in mm3) = 2 2 2902R t

BBx xcos sin cos

q

q q

with B A Rt m mx c ex x .Zpyr is the plastic modulus of the section including the effect of door reinforcement

about plastic neutral axis y-y (in mm3).

= 2 12R t By cos sinq q with B A Rt m my c ey y .q is the half of the angle of the opening in degrees.m m m mex ey x y, , , are the distance from the centroid of the door reinforcement to the

corresponding x-x and y-y axes respectively.

tw is the thickness of the reinforcement at the side of the door opening in (mm); is effective length of opening inn(mm); Ac is effective cross sectional area of door reinforcement in (mm2).

Little el. al. has developed empirical formulae to find the maximum sustainable moment (Mmax) for a non tapered steel tube for an octagonal cross section with or without the access hole. The Moment can be found directly from the tabulated values[1] or it can be calculated form the equation given below,

M M M M M Mp cr cr max max maxh (3)where Mmax is the maximum sustainable moment.

Mp is the Plastic Moment from tabulated values[1]; Mcr is the moment at buckling assuming elastic behaviour. M zp cr e for closed octagonal sections and

M zp cr e / for sections with access hole, where q 1 0 213. ;Sin D

t cr ; is the

extreme fibre compressive bending stress at elastic buckling; D is the Diameter of the inscribed circle of octagonal section.

q is the access hole size; t is the wall thickness; h is the Ayrton Perry factor given in (1).

ze is the elastic section modulus.


Finite Element (FE) ModelingPlate shell elements are used to model the shaft, stiffeners and base plate. This element typically resists membrane shear and bending forces. The non-linear analysis capability of NE-NASTRAN, accounting for geometric and material non-linearity, is used to analyse the model and to obtain the pre-yielding behavior. The elastic-plastic material property of steel is represented by a bi-linear model, having modulus of elasticity up to yield stress equal to 2E5MPa and 2000MPa beyond yield stress. The incremental load and predictor-corrector iteration under each load increment is used in the nonlinear range. The load is applied in 30 to 40 steps until the limit point is reached in the load deformation behavior. Loads are defined to simulate the field condition environment for which the structure is designed. The boundary conditions are specified for each degree of freedom for each node. Both linear and non-linear static analysis has been carried out. The body wind loads are applied as discrete nodal loads along the height of the structure. The finite element model of both the poles is shown in Fig. 43.

Fig. 43 Finite Element Model of 12m and 8m Pole

Comparison of Test Vs Analytical Results

The testing was performed in vertical position simulating the field condition. All load measuring sensors and other instruments are located in such a manner loss of accuracy

Steel Monopoles 363

as a result of rigging is minimum. A digital control system with recently calibrated instruments was used for simultaneous smooth load application with precise servo controlled hydraulic actuators. The pole is instrumented on salient locations as shown in Fig.44 using electrical resistant foil type strain gauge.

Fig. 44 Location of Strain gauges for 12m and 8m Pole

For 12m street lighting pole with doors in closed condition, the load was incremented step by step and reached up to 100% load in both serviceability and ultimate limit state conditions and kept constant for 5minutes. The pole test was declared successful since the mast withstood the 100% load for 5minutes duration with the deflection of 269mm and 425mm. The load was increased beyond 100% and reached up to 3 times above the required load. At this point of load, prying deformation was observed in the base plate indicating large deformation at constant load and hence concluded as failure. The deformed finite element model and tested pole is shown in Fig.45. The test and FE analysis shaft stresses is shown in Fig.46. It was observed up to 100% of design load, the actual stress and theoretical stresses are very close and within 10% and deviates when load reached 300% indicating large inelastic strains. The analysis deflections are compared with the test as shown in Fig.47. The deflections are direct response of the applied load which also indicated large deflections at 300% of load. Non linearity in deflections are observed beyond 50% of design load indicating the need for non - linear


analysis of monopole structures. The bending strength comparison was made with the codal provision and Littles equation along with the test and analytical results as shown in Fig.48. The bending strength of the monopoles are conservatively estimated by European Code EN-40-3-3. The full scale experimental bending strength is marginally lower than previous researchers (Littles) and un-conservative with analytical values.

Fig. 45 Deformed Pole in FE analysis and Test (8m)

0

25

50

75

100

125

150

175

200

225

250

275

300

-450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450

PER

CEN

TAG

E O

F LO

AD

SHAFT STRESS in N/mm2

1T1A 2T2A

3A

3T4A4T

5T 5A

T- TESTA- ANALYTICAL

Fig. 46 Shaft Stresses of 12m Pole

Steel Monopoles 365

Fig. 47 Deflection of 12m Pole

Fig. 48 Comparison of Bending Strength for 12m Pole

For 8m street lighting pole with access hole in closed condition, the load was incremented step by step and reached up to 100% load in both serviceability and ultimate limit state conditions and kept constant for 5minutes. The pole test was declared successful since the mast withstood the 100% load for 5minutes duration with the deflection of 200mm and 336mm constant in both the tests. The load was increased beyond 100% and reached up to 2 times more than the required loads. The deflection at this stage of loading was recorded as 488 mm and then off loaded. Then the test was continued with the access hole in open condition and the loads was incremented in steps of 25% up to 325% of load. At this stage of loading, local buckling was observed in the shaft above the access hole as shown in Fig.49 and concluded as failure. The deformed finite element model and tested pole is shown


in Fig.50. The test and FE analysis shaft stresses is shown in Fig.51. The deflection and strain responses are similar to the previous experiments indicating non-linear behavior of the mast. The analysis deflections are compared with the test as shown in Fig.52. The bending comparison was made with the codal provision and Littles equation along with the test and analytical results as shown in Fig.53. The behavior was similar to the previous experiments but the bending strength estimated by previous researcher (Littles) was very un-conservative and much lower than European Code when compared with full scale testing.

Fig. 49 Local buckling of plate near access hole in analysis and test

Remarks:The non-linear finite element stresses, deformations are compared with the test results. The measured deformation at top is more compared to that of analytical deflection. The measured shaft stresses are same as analysis stresses. For 8m Pole with the access hole in open condition, the measured shaft stresses and deformations are comparable with the analytical stresses.

From the comparison with the codal values for 12m Pole, the EN 40-3-3 is always on the conservative side when compared with the analytical and experimental values. The lower experimental bending strength is due to the distress in the base plate. For 8m Pole, the experimental and analytical bending strength are almost equal and higher the strength according to codal regulations. The Littles experimental values are highly un-conservative because the specimens tested by the author were non-tapered and behavior of the pole as a whole structure is different from the component specimens.

Steel Monopoles 367

Fig. 50 Deformed FE Model and deflected pole during test

Fig. 51 Shaft Stresses of 8m Pole.


Fig. 52 Deflection of 8m Pole

Fig. 53 Comparison of bending strength for 8m Pole.

Steel Monopoles 369

AppENDIX: pROpERTIES OF VARIOUS TUBULAR SECTIONS


9. ACKNOWLEDGEMENTSThe author is thankful to the Director, CSIR-Structural Engineering Research Centre, Chennai, India for the kind support and encouragement.

Steel Monopoles 371

10.0 REFERENCES1. Design of Steel Transmission Pole Structures, ASCE/SEI 48-05, American Society

of Civil Engineers.

2. Design of Steel Transmission Pole Structures, ASCE Manuals and Reports on Engineering Practice No.72, American Society of Civil Engineers.

3. Test report on 400kV D/C PAtype Suspension Pole Structure, Report no.CNP671 41, 2006.

4. N. Prasad Rao, S.J.Mohan and N. Lakshmanan (2004), A Semi Empirical Approach for Estimating Displacements and Fundamental Frequency of Transmission Line Towers, Int. Jnl. of Structural Stability and Dynamics, Vol.4, No.2, June 2004, pp181-195.

5. IS 875 (Part 3) 1987, Code of Practice for Design Loads (other than Earthquakes) for Buildings and Structures, Part 3 Wind Loads., Bureau of Indian Standards., New Delhi.

6. IS 802 (Part 1/ Sec1):1995, Use of Structural Steel in Over Head Transmission Line Towers, Code of Practice Part 1 Materials, Loads and Permissible Stresses., Bureau of Indian Standards., New Delhi.

7. Guidelines for Electrical Transmission Line Structural Loading, ASCE Manuals and Reports on Engineering Practice No.74, American Society of Civil Engineers, 1991.

8. Test report on 132/220kV S/C 300 Dev. Mono Pole, Report no.CNP6046 41, 2006.

9. Dicleli, M. (1997), Computer-Aided Optimum Design of Steel Tubular Telescopic Pole Structures, Computers and Structures, Vol.62, No.6, pp. 961-973.

10. Baban. S. A., Little, G. H., (1984), Tests on Bending Strength of circular and octagonal steel tubes, including the effect of a hole, The Structural Engineer, Vol.62B, No.3, pp.45 -52.

11. Little, G. H. (1984), Design Curves for the bending strength of circular and octagonal steel tubes, including the effect of a hole, The Structural Engineer, Vol. 62B, No.3, pp. 53 -68.

12. EN 40-3-3 (2003), European Standard for Lighting Columns Part 3-3: Design Verification Verification by calculation.

Transmission Towers-SERC 331Transmission Towers-SERC 332Transmission Towers-SERC 333Transmission Towers-SERC 334Transmission Towers-SERC 335Transmission Towers-SERC 336Transmission Towers-SERC 337Transmission Towers-SERC 338Transmission Towers-SERC 339Transmission Towers-SERC 340Transmission Towers-SERC 341Transmission Towers-SERC 342Transmission Towers-SERC 343Transmission Towers-SERC 344Transmission Towers-SERC 345Transmission Towers-SERC 346Transmission Towers-SERC 347Transmission Towers-SERC 348Transmission Towers-SERC 349Transmission Towers-SERC 350Transmission Towers-SERC 351Transmission Towers-SERC 352Transmission Towers-SERC 353Transmission Towers-SERC 354Transmission Towers-SERC 355Transmission Towers-SERC 356Transmission Towers-SERC 357Transmission Towers-SERC 358Transmission Towers-SERC 359Transmission Towers-SERC 360Transmission Towers-SERC 361Transmission Towers-SERC 362Transmission Towers-SERC 363Transmission Towers-SERC 364Transmission Towers-SERC 365Transmission Towers-SERC 366Transmission Towers-SERC 367Transmission Towers-SERC 368Transmission Towers-SERC 369Transmission Towers-SERC 370Transmission Towers-SERC 371Transmission Towers-SERC 372Transmission Towers-SERC 373Transmission Towers-SERC 374Transmission Towers-SERC 375Transmission Towers-SERC 376Transmission Towers-SERC 377Transmission Towers-SERC 378Transmission Towers-SERC 379

transmission mono poles-serc

Documents

pole structures

pole height

pole segment

pole circumference thickness

entire pole structure

additional pole length

pole cross section

power transmission towers6