The necessity of a clear bid specification for the purchase of standard class concrete poles is very important to the bid evaluation process and the acquisition of structurally adequate poles. The specification should contain sufficient requirements and information so that all bids can be evaluated equally and so that the manufacturer clearly understands what is expected of the manufacturer.

While this standard class concrete pole specification does not prohibit the application to poles which are guyed, which are subjected to unbalanced lateral loads, or which have deflection or other special limitations, the owner must be prudent in this type of application.

It is recognized that, with the proper understanding and usage of some computerized structural analysis and transmission line design programs, it is possible to select a standard class concrete pole which might otherwise be beyond the scope of this specification. The owner must be sure that combined bending and buckling analysis is performed, that cracking strength and zero tension strength is evaluated, and that deflections are properly modeled.

The owner should recognize when the design of a concrete pole may be more prudently accomplished using the Guide Specification for Spun, Prestressed Concrete Pole and Concrete Pole Structures, RUS Bulletin 1724E-206, which requires the actual loading conditions to be specified. In using Bulletin 1724E-206, the manufacturer assumes full responsibility in designing and manufacturing a structurally adequate pole.

In some cases, utilities prefer to specify certain spun concrete poles to be designed according to a standardized loading criteria, much like the standard classifications for wood poles.

In utilizing standard class spun concrete poles, a complete structural analysis is still required for all structures. All appropriate loading criteria are considered in the analysis. Once the required concrete pole strength is determined, a standard class spun concrete pole which meets the actual loading conditions can be selected. A design example is shown in Appendix C.

This specification was developed to establish a standard classification system and to assist the owner in procuring a standard class concrete pole which is properly designed for the intended loading criteria.

This guide specification attempts to eliminate ambiguity in specifying and purchasing standard class concrete poles. Since it has become a widespread practice in the industry to design and manufacture poles which are based on the wood pole classification system of the American National Standards Institute (ANSI 05.1), the concrete pole classifications developed in this specification generally follow the wood pole classification system. However, to avoid confusion with the wood pole classifications, the concrete pole classifications have a unique naming system.

"Wood pole equivalent" is a term that may be defined in a number of ways. For purposes of this commentary, the term "wood pole equivalent" is defined as a standard class prestressed concrete pole which is equated by required ultimate loading to an ANSI 05.1 standard class wood pole. The equation is made by a ratio of the overload factors applicable for each pole type and loading criteria.

The design and purchase of concrete poles as an equivalent to wood poles can be vague even with clear instructions. As such, the owner should be sure that the equivalency is properly determined. Once the equivalency is determined, the owner should specify the standard class concrete pole based on the classifications detailed in Section 4.1.2. In doing this, the manufacturer will not be involved in the equivalency process and the ambiguity should be eliminated.

The wood pole equivalency is based on the required ultimate moment capacity of the pole at the groundline based on embedment depths shown in ANSI 05.1. In obtaining a suitable equivalency, the owner must consider factors other than the equivalent groundline moment. For example, the differences in material and section properties of the wood pole versus the concrete pole will result in differences in buckling analysis, pole deflections, secondary moments, applied wind forces, and so forth.

It is impossible to completely equate the concrete pole and wood pole at all points along the pole. The owner must be certain that the concrete pole selected by equivalency methods will have a strength sufficient for the actual application.

The equivalency factor (Eq.F) is defined as the ratio of the concrete pole overload factor to the wood pole overload factor for a given loading condition.

For example, for NESC Grade B district wind loading, the concrete pole overload factor is 2.5 and the wood pole overload factor is 4.0. Thus, the equivalency factor will be 2.5/4.0 = 0.625.

The equivalency factor is a useful concept to understand as the owner requires a wood pole equivalent under various loading conditions and overload factors. Several examples of equivalencies are listed in the following sections.

For the NESC Grade B district loadings, the NESC allows for an overload factor of 2.5 to be applied to a transverse wind load on a concrete pole while it requires an overload factor of 4.0 to be applied to a transverse wind load on a wood pole. As such, the ultimate strength requirement for the concrete pole will be less than the ultimate strength of the wood pole for the district wind loading conditions.

For example, the owner designs a transmission line for wood poles based on NESC district wind loading conditions. The owner wishes to purchase a concrete pole which is equivalent to a Class 1 wood pole. Based on ANSI 05.1, the Class 1 wood pole groundline strength is derived by applying a horizontal ultimate load of 4,500 pounds at 2' from the pole tip based on a simple cantilever. Since the owner had classed the wood pole based on an NESC overload factor of 4.0, the owner wishes to select a concrete pole meeting the same NESC district wind loading conditions. To do this, the owner will multiply the required tip loading of 4,500 pounds by 2.5/4.0, which equals 2,812 pounds. The 2.5/4.0 ratio (or 0.625 Eq.F) adjusts for the difference between wood and concrete overload factors for NESC district loads. The owner will then select a standard class concrete pole which has an ultimate moment capacity based on the horizontal tip loading of at least 2,812 pounds. From Section 4.1.2, the owner selects a class C-02.8 pole, which has a tip loading of 2,825 pounds.

Based on the method shown in this example, Table A-1 (at the end of this section) is a tabulation of wood pole equivalencies based on the NESC Grade B district wind loading.

For the NESC Grade B extreme wind loadings, this specification requires an overload factor of 1.1 to be applied to a transverse extreme wind load on a concrete pole. RUS recommends an overload factor of 1.5 to be applied to a transverse extreme wind load on a wood pole. As such, the ultimate strength requirement for the concrete pole will be less than the ultimate strength of the wood pole for the NESC extreme wind loading conditions.

For example, the owner designs a transmission line for wood poles based on NESC extreme wind loading conditions. The owner wishes to purchase a concrete pole which is equivalent to a Class 1 wood pole. Based on ANSI 05.1, the Class 1 wood pole groundline strength is derived by applying a horizontal ultimate load of 4,500 pounds at 2' from the pole tip based on a simple cantilever. Since the owner had classed the wood pole based on an NESC extreme wind overload factor of 1.5, the owner wishes to select a concrete pole meeting the same extreme wind loading conditions. To do this, the owner will multiply the required tip loading of 4,500 pounds by 1.1/1.5, which equals 3,300 pounds. The 1.1/1.5 ratio (or 0.733 Eq.F) adjusts for the difference between wood and concrete extreme wind overload factors. The owner will then select a standard class concrete pole which has an ultimate moment capacity based on the horizontal tip loading of at least 3,300 pounds. From Section 4.1.2, the owner selects a class C-03.4 pole, which has a tip loading of 3,375 pounds.

Based on the method shown in this example, Table A-2 at the end of this section is a tabulation of wood pole equivalencies based on the NESC Grade B extreme wind loading.

Occasionally, the owner may wish to order a concrete pole which has the same ultimate strength as a specified wood pole class. One common application of this is when the owner designs a transmission line using wood pole properties, but utilizing concrete pole overload factors. In this case, the owner has accounted for the difference in wood versus concrete overload factors during the design of the project.

For example, the owner designs a transmission line for wood poles based on NESC district wind loading conditions. However, knowing that concrete poles will be utilized, the owner uses the NESC district wind load overload factor of 2.5 (applicable to concrete poles) in the calculations. The owner selects a wood pole Class 1 at a specific location. Thus, the owner wishes to purchase a concrete pole which is equivalent in ultimate strength to a Class 1 wood pole. Based on ANSI 05.1, the Class 1 wood pole groundline strength is derived by applying a horizontal ultimate load of 4,500 pounds at 2' from the pole tip based on a simple cantilever. Therefore, the owner will require a concrete pole with an ultimate moment capacity based on the same 4,500 pound tip loading. From Section 4.1.2, the owner selects a Class C-04.7 concrete pole, which has a tip loading of 4,700 pounds.

Based on the method shown in the this example, Table A-3 at the end of this section is a tabulation of wood pole equivalencies based on the ultimate-to-ultimate strength comparison, or 1.0 equivalency factor.

Using the wood pole equivalency methods described, the owner can develop equivalency tables for other ratios of wood versus concrete overload factors.

For example, the design of a heavy angle or deadend transmission line structure may be predominantly controlled by the conductor tension rather than wind on the wire. The NESC specifies an overload factor of 1.65 to be applied to concrete pole wire tensions and an overload factor of 2.0 to be applied to wood pole wire tensions. The resulting equivalency ratio would be 1.65/2.0, or an equivalency factor of 0.825. From a review of the tables at the end of this section, Table A-2 has an equivalency factor of 0.733 while Table A-3 has an equivalency factor of 1.0. Rather than developing a new equivalency table for this special application, the owner might prefer to select a concrete pole equivalency based on the Table A-3 which has an equivalency factor greater than the required factor of 0.825.

• Calculate the induced design moment in the pole from applied loads with overload factors.
  
• Determine the equivalent tip load for this moment.
  
• Select the pole from the standard class pole from Table 2 of the Specifications

Loads (Section 4.1)

The primary loads for concrete poles are weather and erection loads. Common handling loads are determined by the manufacturer and included in the manufacturer’s design. Weather, construction and maintenance loads need to be determined by the owner in order to select the proper standard class pole.

Overload factors for NESC light, medium, and heavy loading districts should be at least equal to those given in the applicable edition of NESC for Grade B construction. Overload factors for extreme ice and extreme wind should be at least 1.1.

In addition to using the NESC district loading requirements, the ASCE publication, "Guidelines for Transmission Line Structure Loading" can be used to provide owners with procedures for the selection of design loads and load factors related to climate, accidents, construction and maintenance.

Once the design loadings have been determined, a design of the structure should be performed by the owner’s engineer or structural designer. It is recommended that a nonlinear structural analysis computer program be utilized to consider the loadings, secondary moments (p-delta effect), and effects of foundation rotations and deflections. As a minimum, an approximate method for determining the ultimate moment capacity should be utilized, such as the methods given in RUS Bulletin 1724E-200, Design Manual for High Voltage Transmission Lines.

Once the structural analysis has been completed, the owner’s engineer or structural designer may select a standard class concrete pole, which has the ultimate moment capacity greater than the design loading requirements. Consideration should be given for strength requirements at all points along the pole, not just at the groundline.

Prior to selecting a standard class concrete pole, the owner should determine the effect of the secondary moments due to the vertical loadings, including the effect of the pole weight, during the transmission line design process.

Whenever there is a transverse or longitudinal load, the pole will deflect in the direction of the load. As a result, the vertical load is no longer in its original position. The vertical load moves over as the pole deflects, causing additional moments in the pole. Also, the pole weight can place significant secondary moment loads in the pole. The additional stress caused by this secondary moment is dependent on the magnitude of the vertical load and deflected shape of the pole. Many pole designs, particularly tall poles, have to be calculated for the position of equilibrium of forces in the fully displaced position. The solution typically takes many iterations. A full nonlinear analysis will consider the change in orientation of the loads relative to the displaced positions of the structural members.

As a minimum, an approximate method for determining the effect of the secondary moments should be utilized, such as the method given in the RUS Bulletin 1724E-200.

Pole Tip Strength (Section 4.1.2)

This specification sets minimum ultimate moment capacity requirements near the pole tip for each standard pole classification. The similar ANSI 05.1 requirement is generally overlooked, misunderstood or not considered by manufacturers and others who seek to standardize pole sizes based on the wood pole classification.

Upon a careful study of the ANSI 05.1 wood pole specification, one should understand that the horizontal loading applied at 2' from the pole tip is for the purpose of determining a required groundline ultimate moment capacity for any length pole of the given class. However, the minimum required wood pole tip size is specified apart from the horizontal loading requirement

For example, according to ANSI O5.1, a Class 1 wood pole must have a circumference of 27" at the tip. When applied to the Douglas Fir or Southern Yellow Pine poles with a fiber stress of 8,000 psi, the resulting tip strength is calculated as 41.5 ft-kips for the Class 1 wood pole.

Because the conductors and shield wire supports are typically located on crossarms away from the pole axis, significant moments can be generated in the pole near the tip. The moments are greatly increased whenever a braced pole top assembly is utilized. These moments are not accounted for by applying the horizontal ultimate loading alone. Therefore, in the design of transmission poles, it is critical that a minimum ultimate moment capacity be specified near the pole tip. In the absence of a minimum tip strength requirement, a concrete pole tip strength can theoretically be negligible.

The minimum pole tip strength required by this specification should be suitable for most transmission line applications. However, the owner must be sure that the tip strength is properly evaluated, especially when working with wood pole equivalencies and braced structures.

Point of Fixity (Section 4.1.2)

Point of fixity for this specification is defined as the location on the pole where maximum moment occurs. Maximum moment is calculated by the pole designer using the loadings provided by the owner and multiplying those loadings by the appropriate moment arms. The existing soil and backfill has to be able to support the pole with these bending moments applied. The location of this point of fixity could be at or below the groundline. The exact location is theoretical and depends on the soil condition and backfill used to support the pole.

For the standard class pole, the point of fixity should remain at the same location on the pole, regardless of the embedment depth the owner may specify for a given application. Otherwise, the required pole strength could vary as the location of the point of fixity varies. Within the scope of this standard class pole specification, the point of fixity is arbitrarily considered to

be located at a distance from the pole butt, which is equal to 7% of the pole length. This value seems to work quite well over a range of pole lengths and is approximately the same value as a point of fixity located at 1/3 of the distance below the groundline based on an embedment depth of 10% of the pole length + 2'.

The reinforcing steel required at the point of fixity is required to continue to the pole butt. However, due to the loss of prestressing steel strength near the pole butt, the ultimate moment capacity near the pole butt will be reduced.

Tip Loading (Section 4.1.2)

The tip loading is used to develop a required ultimate moment capacity diagram at any point along the pole from 2' below the pole tip down to the point-of-fixity. This ultimate moment capacity is determined by multiplying the tip load by the moment arm based on a simple cantilever. As a result, the required ultimate moment diagram is linear in shape. This same method may be utilized in structural analysis and automated transmission line design computer programs to develop an array of ultimate moment requirements for standard concrete pole sizes.

Pole Deflection (Section 4.1.3 and 4.1.4)

Although significant horizontal pole deflection limitations are considered to be beyond the scope of this standard class concrete pole specification, some allowances can be made for these effects. They should be considered during the analysis of the actual loading conditions applied to the concrete pole. Typically, this type of analysis should be accomplished by nonlinear structural analysis techniques. Since the electrical clearances must be assured in the operation of transmission lines, deflections must remain within an acceptable range.

This specification limits the allowable pole deflection to 15% of the pole height above the point of fixity when the tip load specified in Section 4.1.2 is applied under a horizontal testing procedure under short term loading conditions. Long term loading will cause continued deflection due to the plastic deformation of the concrete.

The owner should recognize that the actual pole deflection for an application will be less than the specified deflection limit of 15% of the pole height. With the standard class pole, all of the loading is applied near the pole tip. In a typical transmission line application, the actual horizontal loading will be some distance from the pole tip. As such, the actual deflection at the conductor under short term ultimate loading conditions can be expected to be less than 10% of the height above ground.

This specification also limits the allowable pole deflection to 5% of the pole height above the point of fixity when 40% of the tip load specified in Section 4.1.2 is applied under a horizontal testing procedure under long term loading conditions. This 40% loading approximates the unfactored NESC loading conditions as is discussed in the commentary on cracking strength.

The NESC requires that electrical clearances be maintained under a wind loading of 6 psf. It is expected that the deflection of a standard class pole under this 6 psf loading condition will be less than 3% of the height above ground.

For situations where the owner wishes to know the deflection for a standard class pole, the owner should use a suitable structural analysis computer program in which the actual design loading conditions and concrete pole properties are input into the program. Another option would be to ask the pole manufacturer to provide the analysis.

If the owner has special deflection limitations, it is recommended that RUS Bulletin 1724E-206, Guide Specification for Spun, Prestressed Concrete Pole and Concrete Pole Structures, be utilized instead of this specification. In doing so, there will be little doubt as to what the actual pole deflections will be under all loading conditions.

Cracking Strength (Section 4.1.5)

Cracking strength is defined as the point at which the concrete just begins to separate due to exceeding the tensile strength of the concrete on the tension face of the pole.

To minimize the potential for corrosion of the reinforcing steel, it is desirable to avoid cracking under the unfactored NESC district loading conditions, or any other service loads specified by the owner. Under this standard class concrete pole specification, these service loads are not specified, but are considered to be 40% of the specified ultimate loads.

For concrete poles designed within the limits of this specification, the predominant pole loading will be transverse wind loads. The service load is determined based on the ratio of the transverse overload factor. The NESC overload factor applied to district wind loads is 2.5. As such, the service load will be equal to the ultimate load divided by 2.5, or 40% of the ultimate load.

For typical concrete pole designs, initial cracking occurs at about 40-55 percent of the ultimate strength of the pole. Therefore, the requirement for the cracking strength to be at least 40% of the required ultimate strength should not cause the pole to be stronger than when considering ultimate strength alone.

Since it may be theoretically possible to have a cracking strength at less than 40% of the ultimate strength, it is not desirable to do so. By requiring the cracking strength to exceed 40% of the required ultimate strength, the owner is assured of an adequate cracking strength for the standard class concrete pole.

The zero tension strength is defined as the moment at which a crack that was previously created by exceeding the cracking moment strength will open again. Under this condition, an applied moment will not cause any tensile stress in the concrete.

It is important to avoid open cracks in situations of significant unbalanced lateral loading and in extremely corrosive environments in order to protect the steel reinforcing. Typical structures with permanent unbalanced lateral loads are unguyed angle and unguyed deadend structures. While the design of these structure types is generally outside the recommended scope of this specification, this specification does require a minimum zero tension strength for all pole classes.

It has been demonstrated that the zero tension strength will typically be 70% to 85% of the cracking strength. With a minimum cracking strength of 40% of ultimate, 70% of this value would be equal to 28% of ultimate. Thus, it is natural for all spun concrete poles to have a zero tension strength of at least 28% of ultimate. As such, this specification requires the standard class concrete pole to have a zero tension strength exceeding 28% of the required ultimate strength.

For situations where the owner wishes to select a standard class pole based on a minimum zero tension strength, this specification may be utilized. A typical situation where the owner may wish to do this is when the owner uses a transmission line design computer program in which zero tension strength values are input for each pole type.

The owner should recognize that the zero tension strength for most concrete poles is greater than the minimum required strength of 28% of the ultimate strength. In fact, the zero tension strength can be as high as 50% of the ultimate strength. Thus, for concrete pole applications which must be designed for the zero tension strength requirements, such as unguyed or unbalanced lateral loadings, it is quite possible for the owner to obtain the concrete pole at a significantly lesser cost by submitting the actual loading conditions to the manufacturer using RUS Bulletin 1724E-206.

Although significant foundation rotation and deflection criteria are considered to be beyond the scope of this standard class concrete pole specification, some allowances can be made for these effects. They should be considered during the owner’s analysis of the actual loading conditions to apply to the concrete pole. Typically, this type of analysis is accomplished by nonlinear structural analysis techniques.

Once the structural analysis has been completed (including foundation rotations and deflections, p-delta effect, etc.), the owner may select a standard class concrete pole which has the ultimate moment capacity greater than the design loading requirements.

It is recommended that RUS Bulletin 1724E-206 be utilized whenever the longitudinal loads may result in a significant unbalanced lateral loading condition. In this case, the design of the structure based on zero tension strength is emphasized. (Refer to the Commentary regarding Zero Tension Strength.)

Because concrete poles are flexible structures, there may be a reduction in induced moments in a pole under some types of longitudinal loads due to the restraining effect of the overhead ground wires. Traditionally, static longitudinal loads are specified due to the complexity of calculating the influence of structure flexibility.

It is generally beyond the scope of this standard class concrete pole specification to consider guy wires in the design of the structure. It is recommended that RUS Bulletin 1724E-206 be utilized instead.

However, a typical situation where the owner may wish to use this specification for guyed poles is when the owner uses a transmission line design computer program, or other structural analysis program, in which minimum strength values are input for each pole type and the program is capable of combined bending and buckling analysis of guyed concrete poles.

It may also be possible to specify certain types of guyed concrete poles based on wood pole analysis techniques as detailed in RUS Bulletin 1724E-200. Once the wood pole class is determined, a standard class concrete pole could be selected based on an equivalency. In this case, it is recommended that the 1:1 equivalency ratio be utilized. It is generally agreed that a concrete pole has a greater buckling strength than an equivalently classed wood pole; therefore, the selected concrete pole class should be adequate for a situation in which a wood pole would normally be specified. The owner should use caution in using this equivalency method and its usage should be prudently influenced by the owner’s experience in similar applications where actual design loadings were utilized under similar guying conditions.

Any time a concrete pole structure is guyed, the guy type, size, modulus of elasticity and guy slope or angle has to be determined by the owner and properly modeled in the analysis of the concrete structure. As is required by RUS Bulletin 1724E-206, the load in the guy wire should be limited to 65 percent of its ASTM rated breaking strength under actual ultimate loading conditions. The concrete pole and guy wire(s) must be designed as a system.

The guy modulus of elasticity can increase from a minimum value at the time of manufacture, to a maximum value, which results from periodic stretching and relaxing during the load cycles. Ranges from 19,000 ksi to 28,000 ksi have been stated. The ASCE steel pole specification (ASCE Manual 72) has suggested a guy wire modulus of elasticity of 23,000 ksi be used by the engineer whenever it is not specified.

The location of the groundline for the standard class pole should be specified on the owner’s drawings.

While the strength of the standard class pole is not effected by the groundline location, the proper placement of climbing devices, ground wire clips, cant hole, vent hole, name plate, and so forth, depends upon the location of the groundline.

In addition, the ultimate moment capacity at the groundline is to be noted on the manufacturer’s drawings (see Section 7.2.5) and stamped on the pole name plate (see Section 4.12.1).

Air Entrainment in Spun Concrete Poles (Section 4.2)

Air entrainment in spun concrete poles is similar to air entrainment in normal concrete except the fabrication processes of pumping, vibrating, and spinning causes a large percentage of the entrained air to migrate out of the concrete. The general effects of air entrainment are to increase workability, decrease density (unit weight), decrease strength, reduce bleeding and segregation, and increase durability. For a spun concrete pole, the spinning process creates a very dense concrete and counteracts the air entrainment effects. Since pumping occurs prior to the pole being spun, the air entrainment effects are present during the fabrication of spun poles. The percentage of air entrained in a spun concrete pole after it is spun is unknown. However, it is believed poles that have concrete containing an air entrainment agent will have a higher void ratio than those without this agent. The owner has to be aware that as the percentage of air entrainment increases the concrete strength decreases.

Grounding (Section 4.5)

All internal reinforcing should be bonded electrically to the external pole ground wire. This will keep the external ground and internal reinforcing potential voltage differences lower during lightning events. There have been reports of step lugs and other materials embedded in the concrete, near or in contact with the reinforcing, being dislodged as a result of lightning. Spliced poles should have reinforcing on each side of the splice bonded electrically to the external pole ground wire. This should lower potential voltage differences of embedded material between each pole section.

An option is available in the specification for full scale testing of poles. For a manufacturer, which has been designing and fabricating concrete poles with the same processes for a good number of years, the need for testing of a concrete pole is questionable. Pole testing may be appropriate in cases where there are unusual requirements, new fabrication techniques or when new suppliers are used to validate their design.

In order to properly evaluate bids, the specification requires certain information to be supplied with the bid. This information may be supplied on the preliminary drawings from the Bidder. Using the forms in Attachment C will allow quick review of the information and simultaneous comparison of all bidders' information.