The threaded portion of the bolt should protrude not less than 3 mm and not more than 8 mm over

the nut after it is fully tightened.

3.3 Washers

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being used in the construction of transmission line towers in India. The advantage of spring washers over flat washers is that the former, in addition to developing the full bearing area of the bolt, also serve to lock the nuts. The disadvantages, however, are that it is extremely difficult to get the correct quality of steel for spring washers, and also that they are too brittle and consequently break when the nuts are fully tightened. Furthermore, the spring washers, unlike flat washers tend to cut into and destroy the galvanising.

When spring washers are used, their thicknesses should be as recommended in IS:802 (Part II)-1978 and given in Table. 3.3

Table 3.3: Thicknesses of spring washers

(mm) ---

Bolt dia. Thickness of spring washer ---

12 2.5 16 3.5 20 4.0

---

With regard to the locking arrangement, the general practice is to lock the nuts by centre punching of the bolts or punching the threads. In special cases such as tall river-crossing towers which are subjected to unusual vibrations, the bolts are secured from slacking back by the use of

lock nuts, by spring washers, or by cross-cutting of the thread.

A minimum thickness of 3mm for washers is generally specified.

In our transmission lines, we are using spring washers under all nuts of tower. These spring washers are electro-galvanised.

3.4 Lap and butt joint

(figure 3.2 and 3.3)

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Lap splices are normally preferred for leg members as these joints are generally simpler and more economical compared to the heavier butt joints which are employed only if structural requirements warrant their use.

In lap splices, the back(heel) of the inside angle should be ground to clear the fillet of the outside angle.

3.5 Gusset plates

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In the case of suspension towers, the stresses in the web system are usually small enough to keep the use of gusset plates to the minimum. On heavier structures, however, the web stresses may be very large and it may not be possible to accommodate the number of bolts required for the leg connection in the space available on the members, thus

necessitating the use of gusset plates. Plates may also be required to reduce the secondary stresses introduced due to eccentricity to a minimum.

The bracing members should preferably meet at a common point within the width of the tower leg in order to limit the bending stresses induced in the main members due to eccentricity in the joints. To satisfy this condition, it may sometimes become necessary to use gusset plates.

3.6 Bracing to leg connections

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Typical connections of diagonals and struts to a leg member are shown in Figure 3.4.

The number of bolts required in these simple connections is derived directly from the member load and the capacity per bolt either in shear or bearing. Diagonal members which are clipped or coped for clearance purposes must be checked for capacity of the reduced net section. Note that gusset plates are not used at leg connections, but eccentricity is kept to a minimum by maintaining a clearance of 9.5mm to 16mm between members.

If the leg does not provide enough gauge lines to accommodate the required bolts in a diagonal connection, a gusset plate as shown in Figure 3.5 may be employed. The thickness of gusset plate must be sufficient to develop the required load per bolt.

Typical gusset plate connection at waist lines on the normal face for a wasp-waist tower is shown in Figure 3.6.

3.7 Connection of redundant members

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bolt connection to transfer their nominal loads. Thus, gusset plates can easily be avoided if clipping and coping are used to advantage. Typical connections, shown in Figures 3.7, 3.8 and 3.9 indicate the methods of clipping or turning members in or out to keep the number of bolts to a minimum. Figure 3.7 illustrates the use of a small plate rather than connecting five members on one bolt, as it has been found that erection of more than four thicknesses per bolt is particularly awkward.

3.8 Cross-arm connections

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The cross-arm to leg connection (Figure 3.10) must be considered as one of the most important joints on a tower since all loads originating from the conductors are transferred through the cross-arms to the tower shaft by means of these bolts. Because of its importance, a minimum of two bolts is often specified for this connection.

An example of a hanger-to-arm-angle connection on `Vee' cross-arm is shown in Figure 3.11, Both vertical and horizontal eccentricities may become excessive if the detail of this joint is not carefully worked out. Suspension towers are provided with holes at the ends of the cross-arms, as shown in Figure 3.10, for U-Bolts which receive the insulator string clamps. Strain towers, however, must be supplied with strain plates (Figure 3.12) which are not only capable of resisting the full line tension, but also shock and fatigue loads as well as wear.

3.9 step bolts and ladders

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The step bolts usually adopted are of 16mm diameter and 175mm length. They are spaced 450mm apart and extend from about 3.5 metres above the ground level to the top of the tower. The bolts are provided with two nuts on one end to fasten the bolts securely to the tower, and button heads at the other end to prevent the foot from slipping away. The step bolts should be capable of withstanding a vertical load of not less than 1.5 KN. Step bolts are provided from 3.5 m to 30 m height of the superstructure. For special structures, where the height of the superstructure exceeds 50 metres, ladders along with protection rings are provided

(in continuation of the step bolts on the longitudinal face of the tower) from 30 metres above ground level to the top of the special structure. A platform, using 6mm thick chequered plates, along with a suitable railing for access from step bolts to the ladder and from the ladder to each cross-arm, and the ground wire support is also provided.

3.10 Anti-climbing devices

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All towers are provided with anti-climbing devices at about 3.5 metres above ground level. The details of anti-climbing devices are shown in Figure 3.13.

3.11 Danger and number plates

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Provision is made on the transverse face of the tower for fixing the danger and number plates while

developing the fabrication drawing. These

accessories are generally fixed at about 4.5mm above the ground level. Fig. 3.18 and Fig.3.16 show the details of danger and number plates respectively.

The letters, figures and the conventional skull and bones of the danger plates should conform to IS:2551-1982 Specification for Danger Notice Plates and they are to be painted in signal red on the

3.12 Phase and circuit plates

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Each tension tower shall be provided with a set of phase plates. The transposition towers should have the provisions of fixing phase plates on both the transverse faces. The details of phase plate are given in Fig. 3.15.

All the double circuit towers shall be provided with circuit plate fixed near the legs. The details of circuit plates are indicated in Fig.3.17.

These plates shall also be fixed at about 4.5m above ground level.

3.13 Bird guard

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Perching of Birds on tower cross arms results in spoiling of insulator discs of suspension strings which leads to tripping of line. To overcome this problem, bird guards are fixed over suspension insulator string. The details are given at Figure No. 3.14.

Bird guards shall be used for type-I string only.

3.14 Aviation requirements :-

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