1. An Overview on Transmission TowersBy
K.Divya,
Roll No D2017,
Mtech(structures)-IIIrd Sem.,
JNTUH.

2. Introduction
The transmission lines are the connecting links between generating stations and distribution systems.
Since the rate of growth being greater in the developing countries which in turn had led to the increase in the number of power stations, so consequent increase in power transmission lines and towers was observed.
This presentation deals in detail about towers and their renovation techniques for improvisation.

3. Tower components:
Transmission tower is a tall structure usually a steel lattice used to support the overhead electrical power lines.
Peak –It supports ground wire.
Cage-In between peak & tower body.
Cross arm-It supports conductors.
Bracing-To resist lateral loads.
Towerbody-Mainportion which connects cage & foundation.
Body Extension-For more clearance.
Stub-It projects tower body into the foundation.

4. Types of Towers: General classification as follows:
Self supporting -They are rigid in both longitudinal as well as transverse directions. Normally they are vertical configured.
Self supported wide base-Horizontally configured.
Flexible-They are not rigid in the transmission cable direction
Guyed type-They are grouted into the surrounding area in tension.
Classification as per IS:802(part-I)-1977 code of practice :
Tangent towers -(2° line deviation)
Small angle towers -(10° line deviation)
Medium angle towers -(30° line deviation)
Large angle towers -(60° line deviation)

5. Vertical configuration
Horizontal configuration

6. Tension tower
Guyed tower

7. Bracings:
They are provided to withstand lateral forces due to wind pressure.
Framing angle between the leg & bracing shall be not <15°.
Different patterns of bracings are as follows:
Single web system:
Double web /warren system:
Struts are designed for compression & diagonals for tension.
They are narrow base models.
Tension diagonal forms an effective support at the connection joint.
Can be applied to both large & small models.

8. Pratt system:
Portal system:
Diamond bracing system:
Multiple bracing system:
In this case shear carried by diagonal members.
For the models with Large deflections under heavy loads this pattern is preferable.
Unequal shears at top of the studs are distributed to the foundation.
Half of the Horizontal member is in tension & the other in compression.
Mainly applied to Bottom panel.
At the places where heavy river crossing this is employed.
This is similar to the warren type.
Horizontal member is designed as redundant member.
There will increase in no. of bolts, fabrications & erection cost.
But can observe reduction in weight & cost of steel.

9. Loading on towers:
Loads are applied in three directions i.e., Transverse(FX) , Vertical(FY) & Longitudinal(FZ)
1). Transverse Load - Wind on conductor, Wind on tower body & component of wire tension in transverse direction.
The determination of wind load on structure is a difficult task which involves three stages:
Analysis of meteorological data.
Simulation of wind effects in wind tunnels.
Synthesis of meteorological & wind tunnel test result.
2). Longitudinal load - Components of Unbalanced pull of the wire in the longitudinal direction.

10. 3). Vertical load – Self weight of tower & weight of wire.
The self weight of tower can be empirically calculated through Ryle’ formula ,which is a rough approximation depends on height & overturning moment.
i.e. W=kH(M)0.5 ,
where W=Weight of tower ,
H=Overall height above ground ,
M=Overturning moment at ground level due to wind,
k=Ryle’s constant (Ranges from ).
Height of the tower: The Height of the tower
can be determined by
h1=Min.Persmissible ground clearance
h2=Max.sag.
h3=Vertical spacing between conductors.
h4=Vertical clearance between groundwire and top
conductor.

11. Base width is the distance between the center of gravity of to adjacent legs of a tower on the ground
From the Ryle formula,
B=0.42√M
where B=Base width in metres,
M=Overturning Moment about the ground level(Tonne-M).

12. Types of Tower Foundation
Straight drilled shaft:
Depth .3-15m,Dia 0.5-2m.
Skin friction will resist uplift.
Used in USA, India.
Rolled drilled shaft with stem:
Similar in dimension.
Develops 2 to 3 times more resistance to uplift.
Pad & stem type footing without undercut:
Mostly preferable to Non-cohesive soils.
Presently followed in India.
No undercut provided.

13. Pad & stem type footing with undercut:
Adopted in cohesive soils.
Provides 2 to 3 times more resistance
than footing without undercut.
Pad screw anchor type foundation:
It is hybrid type.
Large uplift pressures can withstand
Used mainly for the soils whose bearing
capacity is low.
Under-reamed pile type foundation:
More than one bulb will surely increase
the uplift capacity
More than 30 m long piles can be
employed for heavy loads.

14. Grillage foundation:
Need to be maintained periodically
since corrosion can be caused.
Used mainly for cohesive soils.
Rock anchors:
Suitable in rocky areas.
They are grouted to a depth equal to 50 times that of dia. Into the rock
Raft foundation:
Used in the places where river crossings or embankments.
This footing will make rigid platform which will minimize the differential settlements.
Pyramid chimney type foundation:
It is similar to raft footing.
This has huge depths which can withstand heavy loads coming on to the tower.

15. Setting of Stub:
Excavated pits are to be concreted to a correct level as per design.
Then stubs are erected and bolted to the concrete base.
With the help of plumb bob ,alignment of the horizontal bracing are to be carried out.
Distance among the four faces of the tower & their diagonal are checked.

16. Problems involved in towers:
The strength of existing towers can be predicted by doing some failure analysis.Some problems observed are as follows:
The member elements are deteriorated due to corrosion, fatigue and overloading.
Some members have buckled due to wind stresses and also some connections have failed.
Slipping of foundations from the ground or the post members slip through the foundation concrete.
Connections may also have failed due to fatigue and dynamic loads.

17. Renovation techniques:
Some methods of renovation and strengthening are as follows:-
To install pre-tensioned guy wires at midpoint and at one quarter heights to provide additional stiffness.
To insert additional horizontal diagonal elements at every one quarter height of the tower to create diaphragms.
To assess overload factors by computing a ratio of maximum stresses versus allowable stress used in design.
Failed connections can also be strengthened by using new stiffeners and bolts at failed joints.

18. Modelling :

20. Conclusions:
The renovation of towers requires a non-linear analysis, and redesign of connections, and replacement of some elements.
Buckled elements could be replaced with new connections could also be replaced, where they have failed.
Horizontal tie members will help form diaphragms at top and middle height of towers.
The software indicates that under a certain wind speed and direction, the tower will fail: but guying and bracing the tower will help to resist lateral forces which will reduce the leg loads to acceptable levels.

21. References:
Dynamic Load Factors for Transmission Towers Due to Snapped Conductors P. Jayachandran., James F. Hannigan, Mark S. Browne and Brian M. Reynolds in ASCE Journal of Structural Engineering Division.
Response of Historic Transmission Towers – Analysis, Renovation and Strengthening Issues- Paramasiva Jayachandran, Brian Reynolds, and Mark S. Browne in ASCE Structural Engineering Division.
A. H. Peyrot, and A. M. Goulois, “Analysis of Flexible Transmission Lines,” Journal of the Structural Division, ASCE, Vol. 105, May.
11th American conference in Wind engineering - June Dynamic Effects Produced on Transmission Towers due to Line Cable Rupture - Neftali Rodriguez, Roger Morales- Israeli
Proceedings of the 9th ASCE Conference on Engineering Design and Analysis July 7-9, 2008, Israel LIFE PREDICTION OF ELECTRICAL POWER TRANSMISSION TOWERS Juan E. Salazar, Jesus A. Mendoza