15.4.1 Fasteners: Threaded rods and machine bolts are frequently used on wood transmission structures (Figure 15-14). A static-proof bolt has a washer securely fixed to the head of the bolt and is furnished with washer nuts. Variations of the static-proof bolt include shoulder eye bolts with round or curved washers welded to the eye, forged shoulder eye bolts and forged eye bolts.
MF type locknuts, used in conjunction with a regular nut or washer nut, form a solid unit which does not loosen from vibration and helps to maintain a static proof installation. The strengths and tensile stress areas of bolts conforming to ANSI C135.1 are shown in the Table 15-1.
Attach to the suspension clamps via hold down shackles
FIGURE 15-14: FASTENERS TABLE 15-1
STRENGTHS OF ANSI C135.1 MACHINE BOLTS, DOUBLE ARMING BOLTS, AND DOUBLE END BOLTS
Machine Bolt Diameter
in.
Tensile Stress Area
sq. in.
Minimum Tensile Strength
lbs.
1/2 0.142 7,800 5/8 0.226 12,400
3/4 0.334 18,350 7/8 0.462 25,400
1 0.606 33,500
Lag screws (Figure 13-5) are sometimes used in lieu of bolts when shear loads are small. A lag screw with fettered edges is driven into the wood and maintains its holding power with cone shaped threads. When lag screws are used, the moment capacity of the wood pole is reduced in the same manner as a bolt hole reduces moment capacity.
FIGURE 15-15: LAG SCREW
Anti-split bolts help prevent the propagation of checking and splitting at the end of crossarms. A three inch edge distance should be provided between the anti-split bolt and the edge of the arm.
15.4.2 Framing Fittings: The primary purpose for using grid gains is to reduce bolt hole slotting by distributing the shear load of the bolt over a large wood area. The specially shaped teeth of the grid gain press into the wood surface and offer maximum resistance to movement
Machine Bolt
Static Proof Bolt With Forged Washer
Nut
Shoulder Eye Bolt with Curved
Washer
Double Arming Bolt, Fully Threaded
Static Proof Double End Bolts with M/F
Locknuts
MF type Locknut
Static Proof Double Arming Bolts Static Proof Double End Bolts
both with and across the grain of the wood. The use of grid gains will strengthen bolt
connections and are recommended anytime a bolt must carry large shear loads. Two applications of grid gains are shown in Figure 15-16.
FIGURE 15-16: GRID GAINS
The gain plate (between a pole and a crossarm) and the reinforcing plate (on the outside of an arm) provide additional metal bearing surface for transfer of the vertical load from the crossarm to the crossarm mounting bolt. The gain plate eliminates a potential decay area between two wood contact areas. A reinforcing plate, also called a ribbed tie plate, will prevent the crossarm from splitting or checking when the nut is tightened.
When double crossarms are used to allow longer vertical spans or to increase longitudinal strength capabilities, spacer fittings Figure 15-170 are needed to separate the crossarms and to provide a point of attachment for suspension insulators. If fixed spacers are used, poles should be gained. Since the standard fixed spacing sizes are 7-1/2”, 9”, 10-1/2”, and 12”, the crossarm may be bowed +1/2 inch. The brand on the butt and face of the pole should include proper designation of the fixed spacer size. Adjustable spacers will fit a range of pole diameters. When they are used the pole need not be gained.
FIGURE 15-17: SPACER FITTING, REINFORCING PLATE AND GAIN PLATE
15.4.3 Swing Angle Brackets: Swing angle brackets are used to provide increased clearance between phase conductors and the structure to which the conductors are attached (Figure 15-18).
These brackets cab be mounted horizontally or vertically. The two primary types of angle brackets are the rod type for light loads, and angle iron type for heavier loads.
Application of Grid Gains Grid Gains
Spacer Fitting Reinforcing Plate and Gain Plate
FIGURE 15-18: SMALL ANGLE STRUCTURE WITH SWING ANGLE BRACKETS
15.4.4 Guy Attachments: The primary types of guy attachments used on wood transmission line structures include the wrap guy, guying plates, pole eye plates, guying tees, and pole bands.
Other types of guy attachments such as formed straps, angle bolt eyes, and goat hooks are used primarily on distribution lines. Guy attachments are used to attach the insulators to the structure as well as providing a means of guying the structure.
15.5 Structure Related Hardware for Concrete and Steel Structures: Much of the structure related hardware used on wood construction may be appropriate to use on steel or concrete structures. However, hardware items with grid teeth, such as grid gains or guy attachments with grid teeth, are not appropriate for use on steel or concrete structures. Likewise, lag screws and gain plates are not used on steel and concrete poles. Since steel and concrete poles do not shrink and swell with age and weather, spring washers may not be needed to keep the hardware tight over time.
In many instances, higher strength bolts are used with steel or concrete poles. Bolts such ASTM A325, Specification for High-Strength Bolts for Structural Steel Joints, may be specified instead of the ANSI C135 bolts. Table 15-2 gives the strength ratings for bolts conforming to ASTM Standard A325.
TABLE 15-2
STRENGTHS OF ASTM A325
HEAT TREATED, HIGH STRENGTH BOLTS
Machine Bolt Diameter
in.
Tensile Stress Area
sq. in.
Minimum Tensile Strength
lbs.
Minimum Yield Strength
lbs.
1/2 0.142 17,500 13,050 5/8 0.226 27,100 20,800
3/4 0.334 40,100 30,700 7/8 0.462 55,450 42,500
1 0.606 72,700 55,570
Proper selection and design of end fittings and guy attachments is necessary to obtain the necessary capacity. For example, for steel structures, it may be necessary to use reinforcing washers on the backside of a guy attachment or end fitting to prevent the nut or bolt head from pulling through the wall of the steel pole. Selection of hardware should be coordinated with the
steel pole supplier or concrete pole supplier to obtain the capacity and performance desired.
Selection of hardware should also consider proper fit with other hardware.
When using standard class concrete or steel poles, the owner should provide the pole manufacturer with the load capabilities, attachment method, and attachment location of all appurtenances. The pole manufacture should verify that the pole will not have a localized strength problem at the attachment point. Items to consider if standard class steel or concrete poles are guyed include:
• Localized buckling at the guy attachment,
• Field holes in the wrong locations,
• Unexpected torsion on the pole due to the fact that the pole is not round and the correct guy plate location does not fall on one of the pole’s flat surfaces, and
• Sliding of the slip joint under heavy conductor loads.
15.6 Corrosion of Hardware: Corrosion may be defined as the destruction of metal by a chemical or electro-chemical reaction with its environment. Certain industrial and sea coast environments accelerate the rate of corrosion. Parameters which stimulate corrosion include air (oxygen) dissolved in water, airborne acids, sulphur compounds (from cinders, coke, coal dust,) salt dissolved in water, corona, etc.
Any two dissimilar metals when placed together in the presence of an electrolyte form a simple battery. One metal becomes an anode, sacrificing itself to the other metal which becomes the cathode. One method to reduce the rate of corrosion is to select metals which are compatible with one another. Table 15-3 details the galvanic voltage of various metals commonly used for transmission line hardware. The greater the algebraic difference between the metals selected, the more rapid the rate of corrosion will be of the more electronegative metal selected.
TABLE 15-3
GALVANIC TABLE OF VARIOUS METALS Silver +.79
Copper +.34
Lead -.13
Tin -.15
Iron -.35 Chromium -.47
Zinc -.77 Aluminum -1.337
As an example, when malleable iron suspension clamps are used, aluminum liners should be furnished in order to reduce the rate of corrosion of the aluminum conductor. As another
example, the selection of staples to be used on the pole ground wire must be compatible material to the ground wire (see Drawing TM-9 in RUS Bulletins 1728F-810 and 1728F-811).
Other methods of reducing the rate of corrosion are to galvanize tin plate, paint or cover metals with corrosion inhibitors. The life of used metals can be prolonged by increasing metal
thickness.