Koi Koi Grace
by on March 8, 2019
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This section covers the structural and material properties of various elements that comprise a typical transmission structure. The three basic construction materials for transmission structures are wood, steel and concrete, although fiber glass (composite) poles and cross arms are being increasingly used for both transmission and distribution lines. Composite cross arms are also becoming popular for distribution under-build circuits on transmission poles given the ease of installation and structural strength.

Structure costs usually account for 30% to 40% of the total cost of a transmission line. Therefore, selecting an optimum structure and material becomes an important part of a cost-effective design. A structure study is usually performed to determine the most suitable structure configuration and material based on cost, construction and maintenance considerations, along with electric and magnetic field effects. Indirect considerations include material availability, environmental issues and other local constraints.

Key factors to consider when evaluating structure configuration:

• Depending on voltage and other factors, a horizontal phase configuration often gives the lowest structure cost.

• If ROW costs are high or if the width of ROW is limited, a vertical configuration may give a lower line cost.

• Horizontal configurations, however, may require more tree clearing.

• Vertical configurations may have a narrower ROW but structures are generally taller and may have aesthetic objections from public. Also, foundations may be costlier.

• From electric and magnetic field perspectives, a vertical configuration will have lower field strengths at the edge of ROW than horizontal configurations. Delta circuits will have lowest single-circuit field strengths.

• If H-Frames are considered as an option, both single and double X-braced configurations must be evaluated as they sustain

 

Key factors to consider when evaluating material choice:

• Pole type structures (wood, concrete, composite or steel) are generally used for voltages less than 230 kV; steel poles and lattice towers are preferred for higher voltages.

• For relatively smaller spans and lower voltages, wood structures are economical.

• In areas subject to severe climactic loads or on lines with bundled conductors, wood and concrete are uneconomical; steel structures provide a cost-effective option.

• If large, unbalanced longitudinal loads are specified as part of the loading scheme, guyed pole systems or lattice towers are better suited for this application.

• For voltages under 230 kV, wood H-Frames will usually give the lowest initial installed cost when larger level spans (up to 1000 ft or 300 m) are used.

 

The choice of a particular structure material often depends on the design spans of the line. Figure 3.1 shows the relationship between common transmission structure materials and design spans. It must be noted that the word ‘poles’ includes both single poles as well as multiple-pole systems such as H-Frames.

Figures 3.2, 3.3 and 3.4 show the classifications of structure types based on materials, function and support configuration. The advantages and disadvantages of various materials and structure types are shown in Table 3.1.

 1-) Wood

Wood poles used for transmission purposes are generally made of Douglas Fir or Southern Yellow Pine material with a designated fiber (bending) strength or Modulus of Rupture (MOR) of 8000 psi (55.2 MPa). They are directly embedded into the ground to a specified depth. In single poles, design is governed by bending at ground line and setting depth needed to resist lateral overturning forces. For wood cross arms, bending stress is generally limited to 7400 psi (51 MPa). Modulus of Elasticity ‘E’ usually varies

 

from 1,800 ksi for Southern Yellow Pine to 1,920 ksi for Douglas Fir (12.40 GPa to 13.23 GPa).

Wood poles are used in single-pole, two-pole (H-Frames) and three-pole (angles and deadends) configurations, in both guyed and unguyed applications. Common transmission voltage range is 69 kV to 230 kV although they have been also used in 345 kV systems.

 

Structurally, ANSI O5.1 (2015) categorizes wood poles into various classes in terms of a single lateral load applied 2 ft. (61 cm) below the top of the pole (See Table A2.1, Appendix 2). These standards cover 15 pole classes and lengths up to 125 ft. (38.1 m). Class 2 and higher (H-1, H-2 etc.) wood poles with tapered circular cross section are currently being used for small (30 ft. or 9.14 m) to large heights (100 ft. or 30.5 m). Wood pole dimensions based on class and height are also shown in Table A2.4 of Appendix 2. Note that the table refers to Douglas Fir and Southern

 

Yellow Pine poles. The diameters at pole top and 6 ft. (1.83 m) from the bottom are shown and diameters at GL (ground line) can be interpolated from these values.

The NESC (2012), in addition to specifying design loadings, also requires all utility structures to satisfy other strength-related norms. RUS Bulletins 200 (2015) and 700 (2011) also stipulate guidelines for wood pole preservative treatment and coatings. Since wood is a bio-degradable material, it degrades and deteriorates with time. Chemical treatment of poles with preservative coatings will increase durability; treated wood poles typically last up to 40 years. From a structural perspective, strength factors are normally used in design to account for this variation and decrease in wood strength. Special care is taken with protective coatings of the embedded or buried portion of the wood poles since soil contains corrosive elements which eat away the pole material. Woodpeckers are also known to attack wood poles by punching holes and many utilities are known to specify a protective nylon mesh wrap around transmission poles. In warmer climates, termites are also a problem.

For guyed wood poles, buckling under vertical loads must be checked as well as need for special bearing pads below pole butt to distribute large axial loads over the soil layer.

The performance of wood poles as a structural material is also dependent on various other issues such as moisture content, knots, decay, time in service etc. For more information on biological characteristics of wood, the reader is referred to ANSI O5.1, textbooks on wood structures, AITC Standard 109 (2007) and RUS Bulletins 700, 701 and 702 (2011).

 

1-a ) Laminated Wood

Laminated wood poles are manufactured using ¾ in. (19 mm) to 1 in. (25.4 mm) thick laminations of Douglas Fir or Southern Pine, glued using a high strength adhesive and cured in controlled atmospheric conditions. The pole can be tapered in both the transverse and longitudinal directions but usually the taper is provided in one direction only. All poles have a rectangular cross section. All design and preservation standards that apply to regular wood systems also apply to laminated wood; however, ANSI O5.2 (2012), RUS Bulletin 701 (2011) and AITC Standards 109, 110 and 111 (2007, 2001 and 2005) etc. are also additionally specified. Engineered laminated (E-Lam) wood poles are currently available in size from 30 ft. to 135 ft. (9.1 m to 41.1 m) and in various classes (Laminated Wood Systems, 2012). E-Lam poles have been successfully installed on transmission and distribution lines in single pole, H-Frame, tangent, angle, deadend, guyed and unguyed configurations.

Foundations for E-Lam poles can be direct-embedded with aggregate backfill, expansive foam or concrete. In soft soils and soils with low ground water table, a corrugated steel culvert with standard backfill is used. Foundation reinforcing systems include steel bearing angles affixed to the E-Lam pole in the embedded portion to provide added lateral resistance.

 

2-) Steel

Tapered steel poles with various cross sections (round, 8-sided or 12-sided) are currently being used for moderate to large heights 50 ft. to 150 ft. (15 m to 45.7 m).

 

The poles are generally made of ASTM A572 Grade 65 (galvanized) or ASTM A871 Grade 65 (weathering) steel material with a stress rating of 65,000 psi (448.2 MPa). They are generally attached to concrete pier foundations via a base plate or directly embedded into the ground to a specified depth. As with wood poles, design is governed by bending at ground line and setting depth needed to resist lateral overturning forces. Since steel is not a biodegradable material and statistical strength variation is much less than wood, no strength reduction is required. Therefore, a strength factor of 1.00 is used. For multi-sided poles, local buckling must be checked in terms of the applicable flat width-to-thickness ratios. For round poles, local buckling is referred to the diameter-to-thickness ratio D/t, where ‘D’ is the average diameter of the pole shaft.

The length of steel pole shafts is generally limited by several parameters such as handling size at the manufacturing facility, length of the galvanizing tank, length of the flat-bed truck required for transport and other local constraints including weight limits on highway bridges. The average piece length is about 50 ft. to 60 ft. (15.24 m to 18.3 m). Taller poles are assembled by piecing together pole sections of various lengths either by means of splices (circumferential welds or overlap) or flange plates. The overlap length in case of a slip joint is generally about 1.5 times the bottom diameter of the upper piece. If flanges are used, these flange plates are welded to the pole segment and bolted using high strength structural bolts.

An important component of a steel pole is the weld connecting the pole shaft to the flange or base plate. For strength and structural integrity, these welds are generally recommended to be full penetration welds. Specifications for steel poles and structures are given in RUS Bulletins 204 (2008), 224 (2007) and ASCE Standard 48-11 (2011). Tables A3.1 (a and b), A3.2 and A3.3 of Appendix 3 shows various steel types and fasteners used in USA and elsewhere. Table A3.4 depicts the shapes used for transmission structures along with their geometric properties useful for design checks.

Tangent poles are directly embedded into the ground with a butt plate or shoe to help transfer axial load over a larger area and thereby minimize soil stress. Large angle and deadend poles are generally attached to concrete foundations by means of base plates and anchor bolts. Typical arrangements are shown in Chapter 4. The design basically involves verifying if the depth of embedment (setting depth) is adequate for resisting the ground line moments, and shears, in case of self-supported single poles. For concrete piers, soil data is needed to check pier ground line rotations and deflections, resistance to lateral loads and skin (side) friction resistance for resisting uplift and compressive loads.

Anchor bolts transfer tensile, compressive and shear loads from the structure to the concrete shaft. Threaded re-bars are the most common type of anchor bolt material. Table A3.13 of Appendix 3 gives material data for various anchor bolt steels. Bolt diameters range from 5/8’’ (16 mm) to 1¾’’ (44.5 mm); for larger piers, #18J rebar (2¼’’ or 57.2 mm diameter) meeting ASTM A615 Grade 75 is used. Current standard for anchor bolts is unified under ASTM F1554, with three grades, namely, 36, 55 and 105. The tensile strength of these threaded rods range from 60 ksi to 125 ksi (414 MPa to 862 MPa). Common diameters specified for transmission-level poles are 1½’’ to 2¼’’ (38 mm to 57.2 mm). The minimum embedment (development) length of anchor bolts into concrete is discussed in Section 3.5.1.2.

 2-) Wood equivalent steel poles

Manufacturers offer steel poles which are structurally ‘equivalent’ to wood poles – and referring to a given equivalency ratio – to enable designers specify quick replacements for damaged wood poles. These WES (wood-equivalent-steel) poles are available in both round and 12-sided shapes as well as light and heavy duty applications. However, caution should be exercised in using and specifying such ‘equivalent’ poles since it is impossible to equate the steel pole and wood pole at all points along the length. Also, the differences in material and section properties will result in differences in buckling capacity, deflections, secondary moments etc. (RUS Bulletin 214, 2009).

ANSI O5.1 (2015) defines wood pole classes with reference to tip load applied at 2 ft from pole tip. Steel pole equivalencies with ANSI wood poles are based on the “Equivalency Factor’’ (EF) which is defined as:

 

The WES poles originally developed by steel pole manufacturers are based on the ratio of overload factors for wood and steel used in the older versions of NESC. For Rule 250B loading, for example, the overload factors are 2.50 (wood) and 4.0 (steel). The equivalency factor is therefore 2.50/4.0 = 0.625. The associated strength factor is 1.0 for both wood and steel.

 

Standard class steel poles by RUS

The classification of standard steel poles based on the Equivalency Factor is shown in Table A3.5. These classes are defined per RUS Bulletin 214 in terms of a single lateral load applied 2 ft. (61 cm) below the tip of the pole. This strength requirement of RUS steel poles also includes a specified moment capacity that must be available 5 ft. (1.52 m) from the top. Additionally, RUS assumes that the point of fixity is located at a distance of 7% of pole length measured from the bottom; the pole must develop ultimate moment capacity at this location. Tables A3.6 to A3.12 give various design data for these standard steel poles. These poles, when specified, reduce lead times involved in bidding, design, drawing preparation and ordering of material. The reader must keep in mind that the dimensions of standard class steel poles vary slightly from manufacturer to manufacturer; values shown in the above tables are typical to the referenced fabricator.

 

Example 3.1 Determine the RUS Standard Class designation for a Class H1 wood pole for (a) Transverse wind load and (b) Extreme wind load. 

 

Solution: Class H1 wood pole is defined as a pole rated for a horizontal lateral load of 5,400 lbs. (24.03 kN) applied at 2 ft from pole tip. (a) Equivalency Factor for transverse wind = 0.65 Required horizontal load capacity of steel pole = 0.65 * 5,400 = 3,510 lbs. (15.62 kN) Referring to Table A3.5, the pole class that is closest to this load is S-03.5 (3,510 lbs.). (b) Equivalency Factor for extreme wind = 0.75 Required horizontal load capacity of steel pole = 0.75 * 5,400 = 4,050 lbs. (18.02 kN) Referring to Table A3.5, the pole class that is closest to this load is S-04.2 (4,160 lbs.).

It must be noted that absolute point-to-point wood-to-steel equivalency does not exist and the engineer is cautioned to exercise sound judgment while determining equivalency. The reader is also referred to RUS Bulletin 214 and ASCE 48-11 which explain the issues and limitations related to various equivalencies in detail. ANSI O5.1 also limits the maximum wood pole class to H6; however, several manufacturers developed WES poles even for higher classes of H7 to H10. For higher voltages and heavier loads, standard class poles are often not adequate; in such cases, steel poles are custom-designed with larger diameters and thicknesses (See Example in Appendix 1).

 3-) Concrete

Spun, prestressed, high strength concrete poles are currently used as transmission poles for heights ranging from 50 ft. to 120 ft. (15.2 m to 36.6 m). Square sections are typically used for distribution lines and street lighting poles; circular sections are preferred for transmission structures. The nominal taper of the concrete pole shall not exceed 0.216 in/ft. (1.8 cm/m). Pole dimensions vary from manufacturer to manufacturer, but typical size ranges are as follows:

Top outside diameter 7 in. to 16 in. (17.8 cm to 40.6 cm)

Bottom outside diameter 18 in. to 42 in. (45.7 cm to 106.7 cm)

These hollow poles are usually directly embedded into the ground to a specified depth. For single poles, design is governed by bending at ground line and setting depth needed to resist lateral overturning forces (bending and shear) without cracking of concrete or excessive bearing pressure on soil below the pole due to pole weight. Specifications for concrete poles and structures are given in RUS Bulletins 206 (2008), 216 (2009), 226 (2007) and ASCE Manual 123 (2012).

Concrete poles are also classified into various classes in terms of a single lateral load applied 2 ft. (61 cm) below the top of the pole. Table A4.1 of Appendix 4 shows various standard concrete pole classes per RUS Bulletin 216 as well as data on common prestressing steels (Table A4.2). RUS strength requirements for concrete poles include

 

a minimum ultimate moment capacity at 5 ft. (1.52 m) from the pole top to ensure adequate bending strength at locations of high stress. Additionally, RUS assumes that the point of fixity is located at a distance of 7% of pole length measured from the bottom; the pole must develop ultimate moment capacity at this location.

Commercial manufacturer catalogs such as StressCrete (2009) also provide important information on the geometry and sizes of spun concrete poles. Special situations demand custom-designed poles of larger heights and diameters.

The suggested minimum concrete cover for steel is ¾ in. (19 mm). The 28-day compressive strength of concrete shall not be less than f c = 8500 psi (58.6 MPa). The range of concrete strengths employed in transmission pole manufacture is 8500 psi to 12000 psi (58.6 to 82.7 MPa).

The Modulus of Elasticity (in psi) of concrete as computed from ACI 318 (2014) is 57000 f c for 3 ksi (20.7 MPa) ≤ f c ≤ 12 ksi (82.8 MPa). Since cracking is a design issue, two moduli are generally defined: cracked and un-cracked. Nominal values often used are 6000 ksi (41.4 GPa) for un-cracked and 2000 ksi (13.78 GPa) for cracked concrete. The modulus of rupture fr is defined as 7.5  f c (in psi).

Prestressing steel strands are generally 3/8 in. to ½ in. dia. high-strength galvanized wires, Grade 250 ksi to Grade 270 ksi (1722 MPa to 1860 MPa), with tensile capacity ranging from 20 kips (89 kN) to 41 kips (182 kN). For other information, the reader is referred to ASCE-PCI Guides for Concrete Poles 257 and 412 (1987 and 1997). Confinement is provided by spiral wire sizes ranging from No. 5 to No. 11 (1 4 in. to 3 8 in. dia.) and minimum spacing between spirals is 1 in. (25.4 mm). Spacing should not exceed 4 in. (10.2 cm) under any circumstances. Closer spacing may be required at the pole tip and butt segments where large radial stresses occur during load transfer from strands to concrete.

 4-) Lattice Towers

Lattice Transmission Towers with steel angle members are commonly used as line support structures for heights ranging from 50 ft. to 300 ft. (15.2 m to 91.4 m) and for spans up to 3,500 ft. (1,067 m) and more. These towers can be self-supporting or guyed, usually with square bases. The towers are generally made of ASTM A36 or A572 Grade 50 steel members, connected by bolts (via gusset plate or direct member-to-member) or rivets. Angle sizes and geometrical properties are listed in AISC Manuals, ASD (1989) or LRFD (1995) or the latest combined version of the steel manual (2013).

The types of structural steel generally used on lattice towers and bolts used in tower joints are shown in Tables A3.1a and A3.2 in Appendix 3. The commonly used fastener specifications for latticed steel towers are ASTM A394 for bolts and A563 for nuts (See Appendix 13). The value of “E’’, the Modulus of Elasticity of steel, is taken as 29,000 ksi (200 GPa) for all steels. All tower bolts come with a washer, nut and a lock nut.

In contrast to wood or steel pole design, lattice tower design is governed by individual member behavior, often involving buckling (compression) and yielding (tension) or connection failures. Member slenderness ratios play an important role in lattice tower analyses along with amount of restraint offered at member ends which depends on the number of bolts (or rivets) used in the joint. Given the 3-dimensional nature of a lattice tower, structural stability is a critical issue checked during computer modeling.

 5-) Composite

Although fiberglass composite poles are becoming increasingly popular for a variety of reasons, one product that is now increasingly used are the composite cross arms in both transmission and distribution structures. Composite cross arms are widely used on distribution structures, both in tangent as well as deadend applications. The components are also integrated into PLS-POLETM library for ready use.

Composite materials in general are non-isotropic and their elastic properties vary based on the direction and orientation of the constituent fibers with reference to applied loads. They are also dependent on type of epoxy bonding materials used in construction. This requires that the non-isotropic nature be considered in structural analysis. To facilitate easier analysis, some simplifications are made. One such simplification is the use of “bulk’’ material properties which represent the global response of the structure to a given loading. These bulk properties are determined through testing and theoretical calculations.

Appendix A14 contains information on composite poles as well as material properties useful for computer modeling. Care must be used in adopting values for computer models.

 

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