Concrete Poles

Guide for the Design and Use of CONCRETE POLES

Prepared by the Concrete Pole Task Committee of the Committee on Electrical Transmission Structures of the Structural Division of the American Society of Civil Engineers April 1987 Published by the American Society of Civil Engineers 345 East 47th Street New York, New York 10017-2398

PREFACE

A Task Committee of the Committee on Electrical Transmission Structures was formed in 1984 to prepare a concrete pole design and use guide. The Task Committee has produced this Guide which brings together in one document, as much information as time and the collective knowledge of the Task Committee permits. No claim is made that this document is complete as it stands. Through future use, additional thoughts and ideas will be identified that should be included. Hopefully this will be a living, working document that will be updated as additional knowledge becomes available. The potential exists for the proliferation of Design Guides and Standards written under the auspices of various organizations. There are already documents relating to concrete poles that have been published by IEEE, ASTM and PCI and all of them refer to ACI-318. Now comes ASCE with its document. Such a proliferation soon becomes both confusing and counterproductive if there is no coordinating force. This Task Committee was chosen carefully to include people that were not only knowledgeable in the field of concrete poles, but who were also active in IEEE, ASTM, PCI and AC1. Indeed, not all of the Task Committee members are members of ASCE. It is the hope of the Task Committee that this Guide will be jointly endorsed by all of these organizations as a focal point for information on concrete poles. The intent is not to usurp the prerogatives and responsibilities of the other organizations, but for this committee to serve as a coordinating group to insure that other documents do not become overlapping and contradictory. The Task Committee recognizes that there are areas in which information is lacking or incomplete. There is certainly work that needs to be done under the auspices of ASTM. We hope to be able to work with that committee to develop the necessary techniques and knowledge to be able to write testing standards associated with the manufacture of concrete poles. The committee also recognizes the need for research into some areas in which there is an abysmal lack of knowledge. It is hoped that somewhere in the industry, this research can be funded and undertaken with the results being available for the good of the industry. Users of this Design and Use Guide are encouraged to ask questions or send comments and information that should affect the content of the Guide. Since neither the chairman nor the committee as a whole intend to abandon the project, comments and questions may be addressed

to the chairman for consideration in future meetings. Anyone with a strong interest in becoming a committee member should contact the chairman. Respectfully Submitted.

Concrete Pole Task Committee Steven Bull Dennis Mize William Ford Tarun Naik Fouad Fouad Robert Roane Tim Hardy Thomas Rodgers, Jr. Samuel Hogg Vincent Schuster Michael McCafferty Jerry Tang William Mickley William Howard. Chairman Committee on Electrical Transmission Structures William M. Howard Ronald E. Randle John D. Mozer Gene M. Wilhoite Anthony M. DiGioia, Jr., Chairman

CONCRETE POLE DESIGN AND USE GUIDE William M. Howard Committee Chairman

INTRODUCTION

This guide presents the generally accepted procedures for the design, fabrication, inspection, testing and installation of concrete poles. It addresses poles which are either spun cast or statically cast and which are prestressed, partially prestressed or conventionally reinforced. The primary emphasis is on spun, prestressed poles which are widely recognized as the ultimate in light weight and durability. Most prestressed poles are of the pretensioned variety and, therefore, post tensioned poles receive little attention in this guide. Also, although many uses for concrete poles are recognized, the guide is heavily weighted toward electric utility uses. Other new types of concrete poles, such as fiber reinforced poles, will be developed in the future and must be addressed by later updates of this guide. Many portions, but certainly not all, of ACI-318 and ACI-318R are applicable to concrete poles and various references to ACI— 318 will be made. It is intended that the definitions and notations used in this guide are consistent with those used in ACI-318. (See Appendix B of this Guide for Notations used herein.) This guide is performance oriented. It presents certaip theories and methods that are generally recognized as good practice, but allows for innovative and unique circumstances to be fully acceptable upon presentation of sufficient test data to demonstrate that proper performance can be achieved. The fundamental premise is that where strength, durability and aesthetics can be equalled or improved upon through new methods, nothing should stand in the way of implementing such methods. This philosophy is consistent with Commentary on ACI 318-83 in which paragraph 18.4.3 states, "This section provides a mechanism whereby development of new products, materials, and techniques in prestressed concrete construction need not be inhibited by limits on stress which represented the most advanced requirements at the time the code provisions were adopted". * President, Power Line Systems, Inc., 6701 Seybold Road, Madison, WI 53719

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CONCRETE POLES DESIGN 1.0 INITIAL DESIGN CONSIDERATIONS This section is written especially for the user. It specifically details the information which users should include in their specification to allow the structure designers to properly and efficiently accomplish their tasks. 1.1 General The structure design requires consideration of many aspects including loading, fabrication techniques, method of shipment, construction and maintenance methods, terrain, types of foundations, corrosion, structural and electrical geometry and clearances, local restrictions and codes. The user is to select the necessary structure design loading criteria. Structure loading may use /d^ta furnished in the ANSI C2 "National Electrical Safety Code" (NESC)'-' the ASCE "Guidelines for Transmission Line Structural Loading" , AASHTO "Standard Specifications fofcStructural Supports for Highway Signs, Luminaires and Traffic Signals" , Electronic Industries Association (EIA) Standards or independent selections based on known local environmental conditions (such as high winds or heavy ice conditions). When using the ASCE "Guidelines for Transmission Line Structural Loading" an exclusion limit is required for pole strengths. Each manufacturer should conduct a full scale testing program to develop its own values for the exclusion limit. (The exclusion limit is simply the percentage of poles that fail at less than nominal design strength.) In the absence of adequate test data, an exclusion limit of 35 shall be used. 1.2 Load Expression It is recommended that loading conditions be expressed as load trees, using an orthogonal coordinate system as shown in Figure 1-1 on the next page. Conductor and shield wire loads should be shown at the conductor and shield attachment points. The weight of the hardware and insulators should be included in these loads. Wind on structure should be expressed in psf (pounds per square foot). Loads should be ultimate including all safety and overload factors. 1.3 Determination of Performance Requirements Poles are designed by the ultimate strength method, to resist the largest factored load. It is the user's responsibility to determine if the word "resist" means to resist the maximum loads without permanent, unacceptable deformation (damage) to the pole, or if it means to resist the loads without failure (collapse) of the pole, recognizing that it requires a stronger pole to resist damage than to resist collapse. In the case of a damaged pole, the steel will have been stretched beyond its elastic limit and/or some concrete will have spalled off the pole. The pole will be permanently deformed, will no longer perform as it was designed to, and will need to be replaced; but it is still maintaining

4 CONCRETE POLES DESIGN the conductors in such a configuration that the line remains energized. A pole which has collapsed is one which has reached such a state that the line can no longer carry power. 1.4 Determination of "Normal Everyday" (Frequent Condition) Loads For unguyed angle or deadend pole structures, it is desirable to consider deflections under "normal everyday" loads. A pole with large deflections under such conditions is undesirable. User should specify what loads are to be considered "normal everyday". 1.5 Longitudinal Loading Because of the possibility of catastrophic cascading failure, the most important loading condition to be evaluated for any transmission line is that caused by the simultaneous loss of tension on all condutors. For pole type self-supporting structures, the deflection of the structure itself, will provide a significant tension reduction in the wires. The length of suspension insulator strings can also greatly influence the structure loading under unbalanced longitudinal loading conditions since the decrease in tension caused by the swing of long insulator strings can be significant. Both of these factors should be included in the unbalanced loading condition as long as proper consideration is given to any impact loading imposed on the structure. For longitudinal loading calculations, spans used should approximate actual line spans. A longitudinal analysis is particularly essential when comparing alternate designs and materials because it is necessary to be sure that the alternates being considered are, indeed, equivalents. For example, a lattice tower, being a much more rigid structure than a pole structure, must be designed significantly stronger in order to provide the same degree of protection against cascading failures. The combination of flexibility, mass and mode of failure that are inherent in concrete poles make them more resistant to cascading failures than are structures made of other materials. Under individual broken conductor conditions, restraint will be offered to the structures by the intact wires. Calculations should properly reflect the structure deflection and insulator swing, and the resulting change in wire spans and tensions. Proper evaluation of the effects of broken conductors requires the use of sophisticated computer programs. From such an analysis, an equivalent static load can be established for the design and testing of the structure. If testing of the structure does not confirm the expected deflections, additional evaluations should be made. 1.6 Geometry The basic pole structure configuration, conductor and shielding geometry (i.e., horizontal, vertical, delta, single poles, H-frames, etc.), insulation assembly length, swing angles, electrical clearances

CONCRETE POLES DESIGN 5 and shielding angle should be made clear to the structure designer. However, the structure designer should be allowed as much latitude as possible to determine the design details of the structure. 1.7 Foundations Consider the type of foundation, foundation rotational allowance and soil parameters (e.g. evaluate bearing and uplift criteria and strength of both natural soil and backfill). When specifying the maximum value for foundation rotation and deflection for all load cases, the user should establish the performance requirements for the combined pole and foundation installation. In determining this value, the user may consider aesthetics, phase-tostructure clearances, phase-to-ground clearances, structure to obstruction clearances or even the ability to replumb a structure. The specifying of a rotation and deflection for each load case is a refinement in analysis and design which allows the user to match types and probability of loads with foundation response. For instance, under rarely occurring conditions such as a 50-year extreme wind load, one might allow more foundation deflection and rotation than under more common loads with the expectation that the cost of occasionally straightening a structure will be less than the cost of stronger, more expensive foundations. In the case where foundation rotation-deflection is specified, the manufacturer should include such effects in the calculations of final deflected pole stresses. The rotation and deflections, when specified, should be for the respective loads with overload factors. 1.8 Design Restrictions Examples of design restrictions are length, weight, deflection or other limitations imposed due to local codes or conditions. 1.9 Deflection 1.9.1 General Structures must be analyzed for deflection to insure that they have adequate strength. The large deflections frequently observed in pole structures under horizontal loads cause additional stress due to the vertical loads being applied while the pole is in the final deflected position. The stress analysis for this is covered in Section 2.0. 1.9.2 Clearances Clearances from conductors to supporting structures, ground, or edge of right-of-way are usually not affected significantly by pole deflections except, perhaps, on special long span or line angle conditions. The user must be aware of this possibility and must compensate for reduced clearances where they can occur. Clearances to the structure itself may be maintained by specifying certain combinations of conductor

6 CONCRETE POLES DESIGN down drop and line angle at the structure and the required clearance. This clearance should be maintained to the deflected structure under the specified loading condition. 1.9.3 Appearance Deflections can play an important part in the appearance of a structure. At line angles or where all vertical conductors are on one side of a pole structure, the constant load in one direction will cause the structure to bow and, if the pole was originally set vertically, it may appear to be near failure. There are several methods that can be used to compensate for this. One method is to rake the pole when setting it. The deflection at the top of the pole is determined for the everyday loading and the pole is tilted this predetermined amount so that, under the everyday loading, the top of the pole is vertical. In this case, the pole will be curved, but because the top portion is vertical, the curvature is unlikely to be noticeable. Designing the structure to limit deflection is a possibility, but this can be expensive because of the extra heavy pole that will be required. Precambered poles are another possibility. It should be recognized, however, that the predictability of results in precambering concrete poles is poor, at best, and few manufacturers are prepared to precamber at all. Finally, guys may be used to limit deflections. 1.10 Transportation and Erection The design should consider equipment or access limitations and loads caused by methods of loading, unloading, hauling, assembly, erection and stringing (including longitudinal load due to line snagging in traveller). It should be kept in mind that the largest stress level a concrete pole may see in its lifetime can occur by lifting it clear of the ground while it is in a horizontal position, as is common in loading and unloading. Indeed, the induced stresses can be so great that it may sometimes be necessary to require the use of multiple point picks to avoid damaging the poles. Experience suggests that transportation and erection loads generally should not be controlling among the various construction loads. Transportation loads can be controlled by using adequate support under the poles (i.e. do not allow long overhangs or unsupported lengths). Erection with single point picks is not a problem as long as much of the weight of the pole is supported on the ground until the pole is in an upright position. Since poles and structures are normally erected by lifting at a point well above the center of gravity, the pole butts remain on the ground until the pole is erect and excessive bending loads during erection are thus avoided. It is the manufacturer's responsibility to clearly indicate on the

CONCRETE POLES DESIGN 7 erection drawings, any restrictions to be observed by the contractor in the handling, transportation and erection processes. However, both users and manufacturers should realize that restrictions add to the cost of installation and should be kept to a minimum. For example, it may be desirable to additionally-reinforce those guyed poles which would otherwise require little prestressing steel to handle the service loads, so that the pole can be handled in a normal manner during construction, because the cost of the extra steel will likely be less than the cost of unusual handling procedures during construction. Poles most likely to be susceptible to damage during transportation and erection are poles designed for light loading conditions, guyed poles, unusually long poles, poles with substantial weights in attached accessories and poles that must be lifted at or near their center of gravity. Unless the poles have been designed to withstand a single point pick at the center of gravity after complete assembly (including a 1.5 overload factor), special handling instructions should be clearly indicated on the erection drawings. In general, then, it is usually the lifting of the entire pole weight while the pole is in the horizontal position that is the controlling handling condition. This load is caused by the weight of the pole itself (plus the weight of any items that may be attached to the pole). To allow for shock loads that may occur while the pole is being lifted, an overload factor of 1.5 should be appled to the dead weight of the pole and attached accessories. It is also necessary for the user to specify whether the pole is to withstand a single point pick or whether multiple point picks can be required by the manufacturer. 1.11 Attached Items User is responsible for informing the manufacturer what accessories are to be mounted on the poles as well as the weight of those accessories so that the poles may be properly designed. 1.12 Guying It is important to define as many knowns as possible, such as restrictions, right-of-way limitations, use of particular guy wire or anchor types, guy angles, quantity of guys, placement tolerances and terrain considerations. The structure designer should be allowed as much latitude as possible in determining the details of the guying scheme to be used. 1.13 Climbing and Maintenance Identify climbing, working and hot line maintenance provisions required. The primary means of climbing concrete poles is with the same removable ladder system used to climb steel poles. This system is available from all pole manufacturers. Many other options are available if the user prefers. The particular method to be used will need to be discussed with the individual manufacturers since not all producers are prepared to offer all options.

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CONCRETE POLES DESIGN Grounding Pole grounding can best be accomplished by utilizing one or more of the prestressing strands as the electrical path to ground. In addition, a separate ground wire may be attached to the exterior surface of the pole or it may be placed in the cavity in the center of the pole. In either case, it should be bonded to the prestressing steel to avoid lightning damage to the pole. User should specify the desired method of 1.14

grounding. 1.15 Other Considerations Any other special conditions that may affect the design should be considered (e.g. reverse wind on bisector guyed light angle structure may control design or environmental conditions may suggest special concrete mixes). Finally, it should be remembered that, like wood poles, concrete poles lend themselves to use under standardized design conditions using a strength/length classification system. In fact, concrete poles can be designed so as to meet the same loading conditions as the wood pole heights and classes. As more users and designers begin to treat concrete poles conceptually like wood poles for design purposes, the costs of both design and manufacturing will decrease substantially. 2.0 - DESIGN 2.1 General For each loading condition considered, it is necessary to analyze the effects of the loads on the structure to determine the tensions, compressions, moments, shears and torsions that the structure must resist at its different locations and the resultant deflections. The reason for using reinforced concrete as a construction material is to take advantage of the best attributes of both concrete and steel. Concrete is relatively inexpensive, excellent in compressive strength and, when properly made, is relatively unaffected by the environment. The primary disadvantage is its low tensile strength. Steel, on the other hand, is excellent in tension but it is more expensive than concrete and is also readily attacked by the environment. Thus the objectives are to use as little steel as possible, to place it in the tension zones of the member and to use the concrete to protect the steel from the elements. In some ordinary reinforced applications, steel may, on occasion, be used to resist compression. 2.2 Design Theory 2.2.1 General As outlined in paragraph 1.3, concrete poles are designed by the ultimate strength method wherein the applied service loads are multiplied by overload factors and the pole is designed to resist the

CONCRETE POLES DESIGN 9 largest factored load. A pole Section should also be designed -so that normal everyday (frequent condition) unfactored loads will not cause the concrete to. go into tension. (See paragraph 1;.4) 2.2.2 Bending The most common loading conditions for poles result in the pole being called upon to resist bending moments. When the bending moments are large enough, the concrete on the outside curvature of the pole will go into tension and, perhaps, crack. Tangent poles (the most common case) designed according to NESC light, medium or heavy loading are unlikely to ever crack under service loads. The 2.5 overload factor used in these cases to determine the required ultimate strength, means that the service load is 40% of the ultimate load. Concrete in a prestressed concrete pole normally does not go into tension until the load is around 40% to 50% of the ultimate load. Thus the service load is about equal to or less than the load which causes the concrete to go into tension. Where very low overload factors are used (such as are common in the 1.0 to 1.1 range for high winds), the poles will crack under the unfactored loads. However, since loads of such a great magnitude are applied to the pole seldom, if ever, opening of cracks under such loads will not occur often enough to be detrimental to the long term durability of the poles. Indeed, for tangent structures which have been properly designed for ultimate strength under factored loads, it is difficult to imagine any set of circumstances where a pole would be in a cracked condition even 0.017. of its life. Unguyed angle or dead-end poles do, however, require careful attention to insure that they are not in a cracked state under "normal everyday" (frequent condition) loads. The detailed methodology for determining the bending strength of a reinforced concrete section is well documented in various text books on Reinforced Concrete. However the fundamental assumptions bear repeating here: 2.2.2.1 The section must satisfy the basic test of static equilibrium (i.e. the tension loads and the compression loads must be equal; and the summation of the internal moments about the neutral axis must be equal to the external moment applied to the section). 2.2.2.2 Strains for both concrete and steel shall be assumed to be directly proportional to the distance from the neutral axis. 2.2.2.3 Tensile strength of concrete shall be neglected in flexural calculations except for the express purpose of determifling when the first cracks are expected to appear, (i.e. determining the cracking moment). This is done to account for the fact that once the pole has cracked (and poles are expected to crack), the concrete no longer has any tensile strength. Some have suggested that poles might be designed, handled and used in such a manner that they never crack. Such an approach is impractical, unnecessarily restrictive and, ultimately, it

10 CONCRETE POLES DESIGN cannot be guaranteed that the pole did not crack anyway. 2.2.2.4 The stress/strain relationships must be determined for the specific materials used. A balanced design is one in which the yield strain of the steel and the limit strain of the concrete are reached simultaneously. A balanced design produces the most efficient section. 2.2.2.5 When designing to allow damage but resist collapse, the concept of balanced design is not valid since some of the steel may be intentionally allowed to exceed its elastic limit. Except in a rare case of a highly under reinforced section, the failure will occur in the concrete, and the steel will not rupture. This is due to the steel going into a plastic state, thereby picking up an ever increasing load; while the neutral axis moves toward the compression side of the section, which must balance the increasing steel load on a decreasing concrete area, until the concrete strain reaches the point where the concrete ruptures. 2.2.3 Column Loading Buckling is seldom a limiting factor in the design of concrete poles. However, when unusually large vertical loads are encountered (e.g. large guyed loads or guys with short guy leads) it is necessary to check for a buckling condition, particularly on taller poles. 2.2.4 Shear Shear is seldom a consideration in concrete pole design. For normal direct burial conditions, soil strengths dictate that the pole must be buried deeply enough to preclude shear problems. Normal burial depths will equal or exceed 10% of the pole length plus 2 feet and poles with such burial depths need not even be checked for shear. The critical conditions that bear checking occur when very large moments are applied near either end of the pole. For example, poles set into solid rock or buried into a concrete foundation socket, may not be buried very deeply, in which case, it is necessary to check for shear to ensure that the pole does not split lengthwise along the neutral axis due to exceeding the concrete shear stress limits. 2.2.5 Torsion Good theory for the design of concrete poles to resist torsional loads does not exist. Furthermore, the combined effect of the stresses occurring in a prestressed concrete pole which is subjected to simultaneous bending, column loading, prestress loading and torsional loading is so complex as to defy reasonable mathematical modeling. Only after extensive research will it be possible to develop mathematical formulas and prove them out to the point where they can be used with confidence. In the meantime, little can be done to assure proper performance under torsional loads other than to test a pole for those conditions that suggest the liklihood of significant torsional loads being applied.

CONCRETE POLES DESIGN 11 2.3 Concrete Properties 2.3.1 Stress/Strain Relationships Curves showing the relationship between stress and strain for concrete vary widely depending primarily upon the strength of the concrete. For normal strength concrete, the curves are distinctly nonlinear and allowable strain is usually limited to 0.003 inches/inch. However as the strength of the concrete is increased to the ultra-high strength level, the curves become very linear all of the way to rupture, which may occur at strains considerably less than 0.003 inches/inch. For those manufacturers who prefer not to perform the necessary testing to develop their own curves, the provisions of ACI 318 provide a satisfactory basis for design parameters for concrete in the ordinary strength ranges. For higher strength concretes, ACI provisions may or may not provide acceptable results. According to ACI 318-83 paragraph 10.2.6, "Relationship between concrete compressive stress distribution and concrete strain may be assumed to be rectangular, trapezoidal, parabolic, or any other shape that results in prediction of strength in substantial agreement with the results of comprehensive tests". Manufacturers are, therefore, expected to conduct "comprehensive tests" to develop their own stress/strain curves for any concrete with strengths beyond the applicability of ACI provisions. 2.3.2 Concrete Compressive Strengths - f The specified compressive strength of the concrete (f ) is determined by the manufacturer based on a number of considerations (see discussion under 3.0 Fabrication) but should not be less than 5000 psi and preferrably 7000 psi or more. Although concrete compressive strengths are conventionally determined at 28 days, it is not required that strengths be measured at that time, and the manufacturer should be allowed to specify strengths at later times to utilize the continuing growth in concrete strength which occurs over time. The use of longer times should, however, be clearly indicated at the time of bidding and on the drawings so that a pole is not fully loaded before the time that the concrete reaches its specified compressive strength. 2.4 Reinforcing Steel 2.4.1 Stress/Strain Relationships Stress/Strain curves for steel do not vary as much as they do for concrete. These curves are provided to the pole manufacturers by the steel suppliers and from the curves can be determined Modulus of Elasticity (E ), Yield Stress (f ), and Ultimate Stress (f ). For purposes of determining the strength of the section at the moment of collapse, ACI 318-83 paragraph 10.2.4 states that for nonprestressed reinforcing steel, the stress in the reinforcement that is below yield stress level shall be taken as E times steel strain. For s

12 CONCRETE POLES DESIGN strains greater than that corresponding to f , stress in reinforcement shall be considered independent of strain and equal to f . Since prestressing sttand behaves differently than reinforcing steely the PCI Design Handbook suggests the following formulas for the stress/strain relationships of the prestressing steel:

When using a combination of prestressed and non-prestressed steel in a member, the provisions of ACI 318 shall apply. 2.4.2 Longitudinal Reinforcement The primary purpose of longitudinal reinforcement is to resist the tension forces in the pole caused by bending moments applied to the pole and, in the case of prestressing steel, to impart prestressing loads into the concrete. The steel must be properly held in place during the placement, consolidating and curing of the concrete, so that proper concrete cover and steel to steel clearance is achieved. The methods used should be left to the manufacturer who may be called upon by the user to demonstrate the adequacy of its methods. Longitudinal reinforcement is normally placed uniformly throughout a symmetrical cross section. It is possible to obtain some degree of increased strength about one bending axis, even though the cross section has a symmetrical shape, by placing the steel as far as possible from the axis about which the bending occurs. Such a technique is rare, however, because the additional strength which can be generated about a particular axis is not large, and handling problems may be encountered due to the resultant weakness about the weaker axis. 2.4.3 Circumferential Reinforcement In order to control longitudinal cracking from several potential sources and to improve the shear and torsional strength of the pole, circumferential reinforcing is required throughout the full length of the pole. Theories to allow for good design practice are not well developed, particularly for prestressed pole sections. However, drawing upon common practice that generally provides satisfactory results, the ratio of the volume of circumferential steel to the volume of the concrete shall not be less than 0.1%. The spacing between the circumferential reinforcements shall not be greater than 4 inches or the radius of the pole. Because of prestressing loads near the ends of poles and possible shear or torsion loads, additional circumferential steel may be required. A spacing greater than 4 inches may be allowed if the manufac-

CONCRETE POLES DESIGN 13 turer presents evidence of satisfactory performance and user agrees. 2.5 Concrete Cover Over Steel In addition to structural requirements, the purpose of concrete cover over steel is to protect the steel from corrosion. The thickness of cover required, may vary according to the degree of corrosiveness of the environment in which the pole will be used as well as the quality of the concrete and its ability to protect against the hostile environment. More cover provides greater protection only to the point where the steel cannot be attacked. Excess cover adds nothing to the durability of the pole but does add unnecessary weight and cost. For static cast, ordinary reinforced concrete the provisions of ACI 318 apply for determining cover requirements. In the case of static cast, prestressed concrete poles, ACI.g318-83 and the PCI "Guide Specification for Prestressed Concrete Poles" both call for 1 inch of cover. This appears to be consistent with other generally accepted practices and provides satisfactory results in most cases. A review of specifications for spun concrete poles from widely differing parts of the world where they have many years of experience shows that required cover varies between 13mm (approximately 1/2 inch) to 19mm (approximately 3/4 inch). As an average for the standard practices both domestic and abroad, it is recommended that design cover be 3/4 inch over the primary steel with 5/8 inch being allowed over the spiral reinforcement. Lesser covers should be allowed if the manufacturer can demonstrate through tests that its concrete is of extremely low porosity so as to protect the steel and develop structural strength with less cover and that the steel can be placed with sufficient accuracy to provide adequate cover under reasonable fabrication deviations. 2.6 Concrete/Steel Bond Due to the fact that large moments are seldom applied near the ends of poles, the analysis of the development of the bond between concrete and steel is largely ignored. In circumstances where there are large moments near the ends of poles (e.g. a davit arm at the top of a pole, a joint connecting parts of a multi-piece pole or a pole set shallow into a rock hole) it is necessary to examine the bond development. It is also important to consider bond development in the event that some of the steel is cut by drilling holes in a part of the pole in which the steel is highly stressed. ACI 318 covers bond development. In addition to normal bond development, end anchorages of various descriptions can be used. It is possible that, because of high prestress forces, longitudinal cracks may develop at the ends of the pole. If this occurs, it may be necessary to increase the concrete cover or the amount of spiral reinforcement or both.

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CONCRETE POLES DESIGN Prestress Loads 2.7.1 Steel ACI 318 provides guidance as to allowable tensile stress in the prestressing tendons both at the time of application of the jacking force and immediately after prestress transfer. At jacking it allows 0.94 f but not greater than 0.85 f or maximum value recommended by the manufacturer. Immediately after pres'tress transfer the maximum allowable stresses are 0.82 f but not greater than 0.74 f . 2.7.2 Concrete ACI 318-83 paragraph 18.4.1(a) states that stresses in the concrete immediately after prestress transfer (before time-dependent prestress losses) shall not exceed 0.60 f . where f. is the compressive strength of concrete at time of initial prestress. However paragraph 18.4.3 states that the permissible stresses in the concrete may be exceeded if shown by test or analysis that performance will not be impaired. 2.7.3 Loss of Prestress In the Commentary on ACI 318-83 paragraph 18.6.1 several references are cited which indicate how "reasonably accurate estimates of prestress losses can be easily calculated". It also points out that the accuracy of the calculations have little effect on the ultimate strength of the member. It does, however, have some effect on the cracking load and the deflections of the member. Loss of prestress requires calculations that consider anchorage seating, elastic shortening of the concrete, creep of the concrete, shrinkage of the concrete and relaxation of the tendons. 2.8 Direct Burial Considerations Because no special treatment is required for the portion of the pole that is buried, the poles can be buried any convenient depth. The rule—of-thumb, which many engineers use as a left-over from their wood pole experience, is to bury the pole 10% of its length plus 2 feet. For lower strength concrete poles this may provide satisfactory results. However, since concrete poles are, in general, much stronger than wood poles, it follows that stronger (and presumably deeper) foundations would be in order. It is also necessary to determine whether it is more cost effective to use a conservative foundation design or to plan to straighten an occasional leaning pole. There is a tendency (which should be avoided) to penalize the cost of a concrete pole line in comparison to a wood pole line by using more stringent foundation criteria for concrete poles while using the old rule-of-thumb criteria for the wood poles. As far as the integrity of the pole is concerned, any type of backfill is satisfactory. Many people use either native soil or crushed rock backfill. Some use concrete backfill but it is doubtful that the results are any different than with a well compacted granular backfill

CONCRETE POLES DESIGN 15 since either backfill is likely to be considerably stiffer than the surrounding natural soil. Concrete backfill has the advantage that it need not be compacted, but that advantage is likely to be more than offset by the disadvantage of having to temporarily support the structure while the concrete sets. When designing the pole, there are two items to be considered in relation to the foundation. The most common is to realize that the maximum moment in the pole occurs below ground and not at the ground line. Since most poles are tapered and their strength continues to increase below ground line, it appears quite safe and common to ignore the additional below ground moment and design the pole based on the moment at the ground line. The other consideration comes up only rarely. If a pole is set in an unusually shallow manner (e.g. in a rock excavation or in a barrelled hole) the shear forces developed along the longitudinal neutral axis need to be considered to avoid having the pole split longitudinally at the butt. In the case of spun poles which have thin walls and a large void, consideration should be given to the magnitude of the down load and the ability of the soil to keep the pole from being forced further into the ground. In general, unguyed single poles do not need to have the bottoms of the poles plugged. Guyed poles either need the bottoms plugged or may need large bearing plates placed under the butt to resist the down load. For unguyed H-frames, uplift shoes that are commonly used may provide enough down load capability as well, to avoid the need for plugging the pole bottom. When uplift shoes are not used, plugging or bearing plates may be necessary in poor soil conditions. 2.9 Guyed Structures To properly analyze a guyed structure, certain assumptions must be made regarding guy tensions and pole deflections. In the absence of clear directives to the contrary, it should be assumed that the axis of the pole will be straight under normal, everyday loads. This means that once the conductors are sagged, the guys will be adjusted so that the pole top is returned to the position in which it was originally set (regardless of whether or not the pole was raked when it was set). For design purposes it will be assumed that there is no moment in the pole under a no wind condition at the specified temperature (60 degrees Farenheit if no other temperature is specified). 2.10 Grounding It is apparent that concrete poles are sufficiently good conductors that current will travel through the pole on its way to the ground. Therefore the question is not whether to use the pole as a ground, but how to best protect the pole, the operating system and people. The reasonable choices are to use the steel in the pole as the exclusive path to ground or to place a separate ground wire down either the interior of a hollow pole or the exterior of any pole to carry some of the current to ground. If a separate wire is used, it should be

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CONCRETE POLES DESIGN bonded by any of several available alternatives to the steel within the pole. Bonding of hardware to concrete poles may or may not be necessary. The two primary reasons for bonding hardware on wood structures is to prevent pole fires and to control radio noise. Obviously pole fires are not a concern with concrete poles and, since the hardware does not loosen on concrete poles as it does on wood poles, radio noise is not a problem. Some are concerned about damage to the pole if lightning should travel through unbonded hardware and seek a path to ground through the pole. Although there are recorded instances of small areas of concrete being knocked loose due to lightning travelling this route, the damage has always been minor, repairable and extremely rare. Most users apparently find that the cost of bonding far outweighs any possible savings in cost of repairing damage. 2.11 Bolted Connections Most hardware is bolted to concrete poles with galvanized through bolts.Good practice dictates that the bolts not overload the concrete and that they be properly tightened. Also, low strength machine bolts should be used. Bolts such as ANSI C135.1 or AS1M A307 are the types commonly used in power line construction. Designing for use of lower strength bolts helps to insure that the bolt loads do not exceed the concrete bearing strength, and, since the low strength bolts are commonly available, lost bolts will be replaced with bolts of the correct strength. Recognizing that, in certain cases, higher strength bolts may be required to carry the loads, the designer should check bolt to concrete bearing loads. Sleeving of holes may be necessary as a means of reducing concrete bearing stress. To spread the concentrated loads under the head of the bolt and under the nut, a square curved washer or other similar plate should be placed between the head or nut and the pole. For A 307 bolts over 1 inch in diameter or A 325 bolts over 3/4 inch in diameter, use either two 1/4 inch thick washers or a single 3/8 inch washer. Use of cast washers is not recommended. The turn-of-the-nut method for tightening bolts is superior to torquing bolts and nuts, particularly when they are galvanized. In most cases the bolt will be properly tightened if the nut is first tightened snugly (snugly is defined as the degree of tightness caused by the first impacting of an impact wrench) and then the nut receives an additional turn depending on bolt length as follows: Short bolts (length less than 4 times the diameter) - 1/3 turn; Medium length bolts (length between 4 and 8 diameters) - 1/2 turn; and long bolts (length greater than 8 diameters) - 3/4 turn. Except near the ends of a spun pole that does not have the end plugged, the strength of the pole is sufficient to withstand any reasonable degree of bolt tightness. If a hollow spun pole shows signs of cracking longitudinally when the bolts are tightened, a decision can be made to tighten the bolts less or to use a steel sleeve in the hole or to plug the end of the pole if that is where the cracks are occurring.

CONCRETE POLES DESIGN 17 It is recognized that low strength bolts are not usually pretensioned. However, this recommended tightening procedure will both keep the bolts tight and protect the pole from damage by over tightening. For shear connections in which the bolt will bear against the side of the through hole, the maximum bolt bearing load will be determined by multiplying the diameter of the bolt times the wall thickness times f . In the absence of confirming tests, it is assumed that the bolt to concrete interface carries all of the load and none of it is carried through friction. For solid poles (or hollow poles with very thick walls), a maximum effective wall thickness for calculating the bearing load is 3 inches. 2.12 Climbing Attachments The primary means of climbing concrete poles is with the same removable ladder system used to climb steel poles. This system is available from all producers. Many other options are available if the user prefers. Per paragraph 1.13, the particular method to be used should be discussed with the individual producers if it is other than normal ladders since not all producers are prepared to offer all options. It is recommended that every individual part of the climbing system where a lineman could conceivably place his foot should be able to withstand a static load of 750 pounds without permanent deformation. In addition, any part of the climbing system which is considered to be a safety attachment point should be able to withstand without breaking, a load of 500 pounds dropped 18 inches. 2.13 Inserts Inserts should be made of materials which will not deteriorate in the environment in which they are placed. Care should be taken to insure that the materials in the concrete, the insert and the bolt do not react unfavorably with each other. The anchorage of the inserts in the concrete should be such that they do not pull loose under the design load or any unusual loads that could conceivably be applied. Preferrably they are designed and anchored in such a fashion that the bolts will break before the inserts pull loose. It is necessary to insure that bolts do not bottom out in the insert. This may require coordination between user and/or one or more suppliers. 2.14 Pole Splices There are occasions in which it is desirable to connect two or more pole parts together into a single pole. This is accomplished with some form of a splice. Many different versions are available but they all have one thing in common that needs to be addressed in the design. Since large moments are generated at the mating ends of the pole sections, it

18 CONCRETE POLES DESIGN is necessary to insure that the reinforcing steel and the connection apparatus are properly anchored as a part of the pole (see discussion in section 2.6 Concrete/Steel Bond). Since the connections are made of steel, reference to ASCE Steel Pole Design Guide for design and fabrication practices is recommended. 2.15 Pole Identification Data All poles (including each piece of two piece poles) will have certain data indicated on a data plate or cast into the pole itself. At a minimum, data to be shown will include: Manufacturer's name. Weight of pole (or weight of pole section). Ultimate design moment (at ground line except for the top section of a two piece pole where ultimate design moment will be that at the connection). Length of pole (or length of pole section). Date of manufacture. Identification number (to allow manufacturer to match a specific pole with the manufacturing data records). 2.16 Attachments and Accessories An almost unlimited variety of attachments and accessories are appropriate for use with concrete poles. The design of steel attachments, accessories and guys should follow applicable provisions of the ASCE Steel Pole Design Guide. Pieces made of wood, fiberglass, aluminum or other materials should be designed to meet established standards for those materials as appropriate to the intended end use. 3.0 FABRICATION 3.1 General Since one of the primary reasons for using concrete poles is to achieve a long, maintenance free life as a support structure, it follows that the concrete and other materials should reflect the use of the finest available materials and workmanship. The design and manufacturing techniques should make use of the latest and best thinking in terms of producing durable and high strength concrete. Not only does the emphasis on high strength produce lighter poles, the various techniques and procedures that produce high strength concrete also make for more durable concrete. The particular mix to be used is at the discretion of the manufacturer and should be considered as proprietary information. The manufacturer is responsible to the purchaser to demonstrate that finished

CONCRETE POLES DESIGN 19 concrete is being provided that meets the strength, durability and aesthetic requirements of the specifications. Only materials that are certified for specified properties shall be used. Certification of all materials shall be checked and in-house laboratory tests shall be performed on concrete ingredients before material is used. Traceability of material tests and certifications shall be maintained a minimum of 15 years after fabrication has been completed. 3.2 Concrete 3.2.1 Cement Portland cement shall conform to the requirements of ASTM C150 or shall be portland blast-furnace slag cement or portland-pozzolan cement conforming to the requirements of ASTM C595. The provisions of ACI 318 address situations where sulfate resistant concrete is desirable. The use of Type II or Type V cements are sometimes specified. It is important to recognize that sulfate resistance is obtained in ways other than use of the two special types of cement. A low C.,A content in the cement is required. Type II is specified at less than 87. while Type V is specified at less than 57.. Cement with up to 10% of C^A can be used where the w/c ratio is 0.40 or less. Many Type I cements meet these requirements. Also, the use of flyash can make Type I cements more sulfate resistant than the special types. The user should specify the type of environment in which the pole is to be used and allow the manufacturer to determine the best mixes to be used. 3.2.2 Aggregates The aggregates shall conform to ASTM C33 or C330 except that the requirements for grading shall not apply. The manufacturer will establish the gradation requirements for aggregates used in its own concrete, based on testing and experience. However, the maximum size aggregate shall be 3/4 of the clear spacing between reinforcing steel and the surface of the pole or between individual bars or wires. Certain aggregates have undesirable reactions with alkali compounds. Tests and requirements to insure that aggregates are not alkali reactive are covered by ASTM C227, C289 and C295. 3.2.3 Water Mixing water shall be free of oils, organic matter and other substances in amounts that may be harmful to concrete or reinforcement. It shall not contain chloride ions in excess of 500 PPM or sulfate ions in excess of 1000 PPM. In general, water from normal drinking supply will meet the requirements necessary to produce quality concrete.

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CONCRETE POLES DESIGN 3.2.4 Admixtures Chemical admixtures shall conform to ASTM C494. Air-entraining agents shall conform to ASTM C260 and fly-ash or other pozzolanic admixtures shall be in accordance with ASTM C618. Admixtures shall not contain chloride ions in quantities that will cause the total water soluble chloride ion content of the concrete to exceed 0.06% of the weight of the cement. Other additives have been and will continue to be developed which are desirable to use for various reasons such as combatting chloride attack or to color the concrete or to increase the strength and durability of the pole. Use of such additives should be permitted as long as the manufacturer submits satisfactory evidence to indicate that proper testing has been done to insure adequate performance in the environment in which the pole is to be used. 3.3 Reinforcing Steel 3.3.1 Prestress Steel Uncoated 7 wire, stress relieved (including low relaxation) strand will be in accordance with ASTM A416. Uncoated, stress relieved wire will conform with ASTM A421. For uncoated high strength steel bar the provisions of ASTM A722 will apply. Both galvanized and epoxy coated strands are manufactured but experience is limited and it is likely that for properly manufactured concrete poles, little, if any, benefit would accrue from the use of coated strand in pole applications. 3.3.2 Reinforcing Bars Deformed billet steel will be according to ASTM A615. Deformed axle steel will comply with ASTM A617 Deformed low alloy steel will meet the provisions of ASTM A706. 3.3.3 Spiral Wire Cold drawn steel for the spiral wire will meet the provisions of ASTM A82. Deformed steel wire shall meet the provisions of ASTM A496

CONCRETE POLES DESIGN21 3.3.4 Welding Welding of prestress strand is not permitted except at exposed ends and only after the pretension has been released. Mild steel reinforcing may be welded only near the ends of the pole. Circumferential steel may be welded as long as sufficient strength remains after welding to meet design requirements. Where welds are to carry structural loads, they must meet the provisions of AWS Dl.l and develop suitable strength. 3.4 Accessories Many accessories are available to be cast into or attached to concrete poles. Materials should meet the provisions of the following specifications: Structural steel - ASTM A36, A572, A588, A633GrE. Bolts and nuts - ANSI C135.1 or ASTM A307, A325 Welding - AWS Dl.l and D1.4 Malleable iron - ASTM A47 Zinc Alloy AC41A - ASTM B240 Plastic - ASTM D2133 Stainless steel - ASTM A666 PVC conduit - ASTM D2729 Alluminum alloy 355 - ASTM B26 Almag - ASTM B108 Hot dipped galvanizing - ASTM A123, A153 and A385. It shall meet the provisions of A143 for the prevention of embrittlement. No double dips will be allowed. Zinc-rich coating - MIL-P-2135, self curing, one component, sacrificial. 3.5 Bolt Holes And Block-Outs At the manufacturer's option, bolt holes and or block-outs may be either cast, drilled or otherwise cut into the pole. Cutting of the steel in the pole is acceptable as long as the manufacturer warrants that the remaining strength in the pole meets or exceeds the design requirements. When steel is cut, it is not necessary to provide any

22 CONCRETE POLES DESIGN particular protection from corrosion (except in the severe case where the pole will be placed in or immediately adjacent to salt water) since the probability of a detrimental level of corrosion occuring inside the holes is very small. 3.6 Finishing The manufacturer's basic responsibility is to provide poles that meet or exceed the design strength requirements, that have a pleasing and workmanlike appearance and that have smooth, dense and hard surfaces that will not deteriorate in the elements. Patching will be acceptable provided that the structural adequacy and the appearance of the product are not impaired. Many other custom services are available at a price. Items in this category include but are not limited to such things as plugging either or both ends of a hollow pole, providing a rain cap for the pole, creating a special textured finish for the pole, installing hardware items on the poles in the factory, painting the pole, etc. 3.7 Fabrication Tolerances Following is a list of tolerances that manufacturers usually meet in the normal course of business. Stricter tolerances can usually be met if that should be necessary, but tighter tolerances have a cost. Length - Plus 12 inches and minus 6 inches. Cross Section - Plus or minus 5% with a minimum 1/4 inch. Wall Thickness - Plus 20% and minus 10% with a minimum of 1/4 inch. Note that the wall thickness requirements are normally determined for some critical section such as the groundline. Other areas of the pole may not require as much thickness. Therefore, greater minus tolerances are acceptable in some areas of the pole where calculations and/or tests indicate that the pole will perform satisfactorily. Weight - Plus 20% and minus 10% except that, with the approval of the purchaser, poles heavier than 20% over the estimated weight may be used. (Caution: Be certain that poles are marked with actual or greater than actual weights to avoid accidents during construction.) Sweep - 1/4 inch per 10 feet of length. Bolt Holes - Plus or minus 1/8 inch for holes within a bolting group and plus or minus 1 inch for the centerline of the group from the end of the pole. Bolt hole diameters will be 1/8 inch greater than the bolt diameter. Blockouts - Plus or minus 1 inch. End Squareness - Plus or minus 1/2 inch per foot of diameter.

CONCRETE POLES DESIGN 23 Reinforcement Placement - Plus or minus 1/4 inch for individual pieces and plus or minus 1/8 inch for the centroid of a group. Spacing of individual circumferential reinforcements may vary plus or minus 25% as long as the total required quantity per foot is maintained. 3.8 Quality Control 3.8.1 General The best assurance of a quality product is a consistent, thorough testing program. It begins with testing the raw materials, continues through the manufacturing process and finally includes tests on the finished product. Mill certifications, test data and manufacturing data should be filed and saved for a period of 15 years or longer. 3.8.2 Raw Materials 3.8.2.1 Cement With each new load of cement, the mill certifications should be checked to insure that the cement is not only within the ASTM standards, but that the new load is similar to previous loads. Variations in the cement, even within the ASTM tolerances, can produce differing end results in the finished concrete. 3.8.2.2 Aggregate Daily, the aggregate should be checked for moisture content (ASTM C566) and a seive analysis should be run (ASTM C136). Weekly, Specific Gravity (ASTM C127) and Absorption (ASTM C128) tests should be performed. 3.8.2.3 Reinforcement Mill certifications for the reinforcing steel should be checked even though it is seldom that any problems are found. 3.8.3 Concrete 3.8.3.1 Wet Samples Two primary tests are run on wet concrete. One of these is Air Content (ASTM C231). For static cast poles this test should be run on a daily basis. Since for spun poles most of the air is spun out anyway, the test is of lesser importance and can probably be run on a weekly basis as a means of keeping track of the uniformity of the concrete The other test for wet concrete is Unit Weight (ASTM C138). This test should be run daily.

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CONCRETE POLES DESIGN 3.8.3.2 Cured Samples ASTM C39, C172 and C192 as well as ACI 318 outline most of the requirements for taking, curing and testing concrete samples. (Note: Pad capping of samples will be acceptable if the manufacturer presents satisfactory data correlating the results with standard ASTM results.) These methods are very adequate for statically cast concrete, but need some modifications for spun concrete. In order to be most representative of the concrete in a spun pole, the test samples must be spun and cured similarly to the pole itself (i.e. spun with the same G forces, for the same time and cured for the same times at the same temperatures). The manufacturer should be prepared to demonstrate through full scale testing, that the strength of the spun concrete samples are representative of the strength of the concrete in the pole. If the manufacturer wishes to take advantage of the higher strength of spun concrete in the pole, but still wishes to use static cast samples as the primary manufacturing control, he may choose to statistically correlate the static samples to pole strengths through full scale testing. Thereafter, the static samples may be the primary control, even though the sample test results are less than both the design strength and the actual concrete strength in the pole. The validation process must be repeated at least every six months and upon the request of the user. The manufacturer may use the results of static cast samples directly, without any correlation, but design strength may not exceed the test results achieved according to the ASTM specifications. In summary, it is recommended that the foregoing ASTM and ACI specifications be followed with the exception that the samples should be spun. Tests should be run either daily or for each 25 cubic yards of concrete, whichever occurs more often. Each test should consist of 4 cylinders. One is tested at the time of application of the prestress, one at 7 days and one at the age at which f is determined. The remaining sample is a spare in the event there is a^roblem with one of the tests, or it can be saved for long term strength and durability investigations . 3.8.3.3 Meeting the Requirements of f There is variability in the strength of both the concrete and the concrete samples from batch to batch. In order to insure that the concrete in the pole is almost always at least as strong as the design strength (f ) it is necessary to manufacture concrete at an average strength Chat is greater than the desired design strength. In general, those manufacturers whose concrete has less variability can manufacture to a lower average strength than can those with greater variability. Determining the specific answers is a statistical problem which is covered well in ACI 318. It should also be noted that in the Commentary for ACI 318, it states that if the standard deviation is determined using cement from only one source, the data is valid only for cement from that source.

CONCRETE POLES DESIGN 25 It should be pointed out that although concrete strengths are usually determined and specified at 28 days, there is no hard and fast rule that this particular age must be adhered to. Any other reasonable time such as 56 days or 90 days may be specified by the manufacturer but, per ACI 318, the test age shall be indicated in the design drawings or specifications. 3.8.3.4 Use of Core Tests In the case of spun poles which have thin walls and large amounts of steel, it is not usually possible to take a core sample that meets the ASTM requirements for overall size and dimensional ratios. Therefore the use of core samples to determine concrete strength in spun poles is inappropriate. 3.8.3.5 Requirements for Tensioning Steel Most prestressing steel is tensioned with hydraulic rams and the tension in the steel can be directly related to the hydraulic pressure in the ram. Under the provisions of ACI 318 and PCI MNL 116 the rams must be calibrated with a direct measurement of the tendon elongation and any differences in excess of 57. must be ascertained and corrected. 3.9 Inspection The purpose of the manufacturer's pole inspections is to insure that the pole that is delivered to the construction forces has been properly fabricated and shipped. The inspection is largely visual, although a Schmidt Rebound Hammer can be utilized to give a rough idea as to the uniformity of the strength of the concrete within a pole or among a group of poles. It should not be expected to provide information as to the absolute strength of the concrete. A complete visual inspection would include: Check the appearance of the surfaces of the pole for soundness of the concrete and possible spalling of the concrete as well as the color. Minor honey-combing, surface spalling and mold seam-line bleeding is normally acceptable if the structural strength is not impaired. Check the straightness of the pole. Be sure that the holes are properly located. Insure that all items that are supposed to be attached to the pole are indeed there and in good condition. Note the existence of cracks, if any, and determine the significance of such cracks. The most common question to arise during inspections is the significance of different types of cracks. It should be pointed out that

26 CONCRETE POLES DESIGN not all cracks are detrimental to the product and, indeed, poles are expected to crack under certain conditions. Hairline cracks, although they may be quite visible during times when the pole has been wet and is surface dry, will probably not cause a problem with long term durability. It is not likely that oxygen or moisture will enter hairline cracks to cause degradation of either the concrete or the steel. If a crack is opened wide enough to accept an ordinary sheet of paper (approximately 8 mils), it should be sealed to keep moisture out. Wide cracks are unacceptable except within one or two feet of the bottom of the pole which will be buried. Cracks within one or two feet of the ends of poles may occur during the detensioning process. Unless they are open cracks, they will not cause structural problems. Those cracks that are buried will never be a problem. If there is concern about sufficient moisture penetrating cracks near the top of the pole to cause freeze/thaw damage, those cracks can be waterproofed. Structurally, they are not a problem unless a very large moment is to be applied to the end of the pole. Longitudinal cracks (other than hairline cracks) are generally undesirable. Circumferential cracks that do not close generally indicate that the steel has been stretched beyond its elastic limit. If that is determined to be the case, the pole will no longer perform the job for which it was intended and should not be used. 4.0 LOAD TESTING 4.1 General The ultimate check on the adequacy of the entire design and manufacturing process is the full scale test. Poles may be tested in either a horizontal or an upright position. If only the pole is being tested, a horizontal test is entirely satisfactory and easier than an upright test. In instances where the pole is being tested as a part of an entire structure, it is likely that the entire assembled structure will need to be tested in the vertical position. A pole structure test should be considered a guide to good structural design practice. The contract documents shall designate the organization that is responsible for the structural design specifications set forth in the contract. Overall responsibility for the structure testing should lie with one person representing this organization. This person should be totally familiar with the structure's design and approve the proposed procedure for structure testing. Also, this person should be present at all times during the testing sequence and approve each decision made during the process. The single person having these responsibilities shall be called the Responsible Test Engineer. In a traditional proof test, the test set up is made to conform to the design conditions (i.e. only static loads are applied), the structure has level, well-designed foundations and the restraints at the load

CONCRETE POLES DESIGN 27 points are the same as in the design model. This kind of test will verify the adequecy of the main components of the structure and their connections to withstand the static design loads specified for that structure as an individual entity under controlled conditions. Proof tests provide insight into actual stress distribution of unique configurations, fit-up verification, performance of the structure in a deflected position and other benefits. The test cannot confirm how the structure will react in the transmission line where the loads may be more dynamic, the foundations may be less than ideal and there is some restraint from intact wires at the load points. Paragraphs 4.2 through 4.14 present guidelines based on performing a proof test using a test frame that has facilities to install a single structure in an upright position, to load and monitor pulling lines in the vertical, transverse and longitudinal directions and to measure deflections. Guidelines for a horizontal test are presented in Paragraph 4.15. 4.2 Foundations It is unlikely that soil conditions at the test site will match those at the installation site. Fortunately, if a few precautions are taken, it will make very little difference to the test results. 4.2.1 Single Pole Structures The primary consideration in designing and installing a single pole foundation is to be able to control the ground line rotation so as not to exceed the allowable design rotation. For test purposes, the actual amount of rotation makes very little difference within a wide range except under very heavy vertical loads where secondary moments can be significant. 4.2.2 H-Frame Structures Normally for an H-Frame, the critical point in the structure is at the top of the cross brace. The magnitude of the ground line rotation has very little effect on the structure at the top of the cross brace. It is important, however, that the uplift and down-thrust be adequately contained so that the structure does not suffer premature failure due to unanticipated loads as a result of twisting the structure. 4.3 Material The test structure should be made of materials that are representative of the materials that will be used in the production structures. Mill test reports and other test results should be available for each important member in the test structure. All test structure material should conform to the minimum requirements of the material specified in design.

28 4.4

CONCRETE POLES DESIGN Fabrication Fabrication of the prototype structure for testing shall be done in the same manner and to the same tolerances and quality control as will be done for the production structures. 4.5 Stress Determination Stress determination methods, primarily strain gauging, may be used to monitor the loads in individual components during testing. 4.6 Assembly and Erection The test structure should be assembled in accordance with the manufacturer's recommendations. It may be desirable to specify detailed methods or sequences for the test structure to prove the acceptability of proposed field erection methods. Pick-up points designed into the structure should be used during erection as part of the test procedure. The completed structure should be set within the tolerances permitted in the construction specification. After the structure has been assembled, erected and rigged for testing the user or his designated representative should review the testing arrangement for compliance with the contract documents. Safety guys or other safety features may be loosely attached to the test structure and used to minimize consequential damage to the structure or to the testing equipment in the event of a premature failure, especially if an overload test to failure is specified. 4.7 Test Loads The loads to be applied to the test structure shall be the loads specified for design and should include all appropriate overload factors. Wind-on-structure loads are normally applied in a test as concentrated loads at selected points on the structure in a pattern to make a practical simulation of the in-service uniform loading. The magnitudes and points of application of all design loads should be developed by the structure designer and approved by the user before the test. 4.8 Load Application Load lines shall be attached to the load points on the test structure in a manner that simulates the in-service load application as much as possible. The attachment hardware for the test shall have the same degrees of movement as the in-service hardware. V-type insulator strings shall be loaded at the point where the insulator strings intersect. If the insulators for the structures in service are to be a style that will not support compression, it is recommended that wire rope be use for simulated insulators in the test. If compressed or cantilever insulators are planned for the structures, members that will simulate those conditions should be used. As the test structure deflects under load, load lines may change

CONCRETE POLES DESIGN 29 their direction of pull. Adjustments must be made in the applied loads so that the vertical, transverse and longitudinal vectors at the load point in the deflected shape are the loads specified in the structure loading schedule. Test rigging should be designed with an adequate safety factor for the specified test loads. 4.9 Loading Procedure The number and sequence of load cases tested shall be specified by the structure designer and approved by the user. It is recommended to test first those load cases having the least influence on the results of successive tests. Secondly, the sequence should simplify the operations necessary to carry out the test program. Loads are normally incremented to 50, 75, 90 and 100 percent of the maximum specified load and to the load at which the concrete first cracks (usually in the range of 50 to 60 percent). If the test facility does not have the capability for continous recording of loads, an additional increment to 95 percent may be added. After each increment is applied there shall be a hold to allow time for reading deflections and to permit the engineers observing the test to check for signs of structural distress. The maximum load for each load case shall be held for five minutes. Loads should be removed between load cases except that in some noncritical situations, with the permission of the Responsible Test Engineer, the load may be adjusted as required for the next load case. Unloading shall be controlled to avoid overstressing any members. 4.10 Load Measurement All applied loads shall be measured as close to the point of application to the test structure as possible. Loads shall be measured through a suitable arrangement of strain devices or by predetermined dead weights. The effects of pulley friction should be minimized. Load measurement by measuring the load in a single part of a multi-part block and tackle arrangement should be avoided. Strain devices shall be used in accordance with manufacturer's recommendations and calibrated prior to, and after the conclusion of the testing sequence. 4.11 Deflections Structure deflections under load shall be measured and recorded. Points to be monitored shall be selected to verify the deflections predicted by the design analysis. Deflection readings shall be made for the before-load and load-off conditions as well as at all intermediate holds during loading. All deflections shall be referenced to common base readings, such as the initial plumb positions, taken before any test loads are applied. Upon release of test loads after a critical load case test, a structure will normally not return fully to its undeflected starting position. The testing specifications should state how much deviation is

30 CONCRETE POLES DESIGN acceptable. 4.12 Failures Following the provisions of Paragraph 1.3, the decision will have already been made as to whether failure occurs when there is a permanently deformed structure or when the structure collapses. If a premature structural failure occurs, the cause of the failure mechanism shall be determined and corrected. Failed and damaged members shall be replaced. The load case that caused the failure shall be repeated. Load cases previously completed need not be repeated. After the structure has successfully withstood all load cases, and assuming that the structure was not tested to destruction, the structure shall be dismantled and all members inspected. 4.13 Disposition of Test Structure The test specification should state what use, if any, may be made of the test structure after the test is completed. Undamaged components are usually accepted for use in the line. If an overload test to failure has been performed, caution should be exercised in accepting the parts that appear to be undamaged since they may have been overloaded. 4.14 Report The testing agency shall furnish a test report in the number of copies required by the job specifications. The report should include: a. The designation and description of the structure tested. b. The name of the utility that will use the structure. c. The name of the organization that specifed the loading and test arrangement of the structure. d. The name of the Responsible Test Engineer. e. The name of the fabricator. f. A brief description and the location of the test facility. g. The names and affiliations of the test witnesses. h. The dates of testing each load case. i. Design and detail drawings of the structure including any changes made during the testing program. j. A rigging diagram with detail of the point of attachment to the structure. k. Calibration records of the load measuring devices.

CONCRETE POLES DESIGN 31 1. A loading diagram for each load case tested. m. A tabulation of deflections for each load case tested. n. In the case of a failure: Photographs of the failure. Loads at the time of failure. A brief description of the failure. The remedial actions taken. The physical dimensions of the failed members. Test coupon reports of failed members, if required. o. Photographs of the overall testing arrangement and rigging. p. Air temperature, wind speed and direction, any precipitation and other pertinent meteorological data. q. Mill test reports. r. Additional information specified by the Purchaser. 4.15 Horizontal Testing 4.15.1 General Horizontal testing is primarily used to test free standing single pole structures. A majority of the previous paragraphs of this section apply also to horizontal testing. A full scale nondestructive horizontal test should verify the structural integrity of the pole to withstand the maximum design stresses. All critical points along the pole shaft should be tested to maximum design load. 4.15.2 Test Arrangement The structure is normally placed in a horizontal position as shown in Figure 4.1 or 4.2. One or more locations along the shaft will be selected as the load pulling points. The purpose of the load pull(s) will be to duplicate maximum design stress at all critical points in the pole shaft based on the cross sectional geometry of the shaft and yield strength of the materials. (Critical points are those points on the shaft with the highest stress.) The design moment for the shaft will be less than the test moment. Additional bending moment is needed to account for axial, shear and torsional stresses that cannot be applied due to the test configuration. 4.15.3 Equipment Used in the Test The load(s) are pulled at predetermined point(s) along the shaft by crane(s) or other suitable pulling structures. Loads shall be determined with calibrated load cell(s) located in the pulling line. A transit should be set up away from the test structure and used to make the deflection measurements.

32

CONCRETE POLES DESIGN 4.15.4 Test Procedure for Pole Test - Vertical Pull (Fig. 4-1) 4.15.4.1 Dead Load Pickup Before the test begins, the actual weight of the structure should be known. When in a horizontal position, the dead weight of the structure will cause a bending moment in the pole shaft. The procedure should consist of picking the structure up at the pull point(s) to determine loads while the other end just rests on the compression pad. The calculated reaction(s) at the pull point(s) should correspond fairly closely to the actual load cell reading in order for the remainder of the test to be considered accurate. 4.15.4.2 Design Load Test With the structure in a horizontal position and the dead load pickup completed, loading should continue to engage the hold down strap. Incremental loads should then be pulled, as indicated in the test requirements, with deflection readings being taken at predetermined points along the structure and the uplift and compression points. Each incremental load will be held for the required time before proceeding to the next load increment. After testing the structure, it should be unloaded to "Dead Load Pickup" so that final deflection readings can be taken. A final inspection will be made on the shaft for any damage. 4.15.5 Test Procedure for Pole Test - Horizontal Pull (Fig. 4-2) The pole is placed between the reaction blocks and locked in place. One or more wheeled support devices shall be used to support the weight of the free end of the pole. An initial load of at least 10% of the maximum test load should be applied to "set" the pole into the blocking. When the "setting" load is removed, the zero position is then established from which to measure subsequent deflections. It is very important for obtaining accurate results, that the wheeled support device operate with a minimum of friction. Ideally the set-up includes steel wheels with bearings or steel rollers, either of which roll on steel plate. All of the rolling surfaces must be kept free of debris. 5.0 Assembly and Erection 5.1 General As a point of reference, spun, prestressed concrete poles are handled during construction very similarly to wood poles. As with poles of other types, they can be damaged or broken if they are abused, but they will withstand much more abuse than steel poles and roughly the equivalent abuse of wood poles. One advantage of concrete poles is that if they are damaged during construction, it is usually obvious, whereas there is the possibility of cracking a wood pole and never knowing that it is cracked. Be sure to check construction drawings for any special handling instructions.

34 5.2

CONCRETE POLES DESIGN Handling One of the most critical handling phases for any pole is lifting it clear of all supports while it is in the horizontal position because the moment generated by its own weight may be significant. Since concrete poles tend to be heavier than other types, more attention must be paid to the manner in which they are lifted. Some poles are designed to be lifted with a single point pick at the center of gravity and some require multiple point picks. It is the manufacturer's responsibility to provide the user with lifting instructions for their particular poles and it is the user's responsibility to insure that those instructions are relayed to the construction forces. 5.3 Hauling Common sense is important in determining good hauling practices. A particular set-up that may be highly acceptable for hauling over a smooth paved highway may be entirely inappropriate for hauling the same load over a plowed and frozen field. In general, no more than 1/3 of the length of the pole should be unsupported and, if the terrain conditions indicate that the pole will be handled roughly, the unsupported length should be less than that. In those instances where hauling equipment cannot be driven adjacent to the setting location, it may be necessary to drag the pole along the ground. Concrete poles will withstand this abuse as well as wood poles. If hardware is already attached to the pole, it will be necessary to secure the pole in such a manner as to keep it from rolling around its longitudinal axis as it is dragged. As is expected with the dragging of any pole, common sense is required to avoid damage to the pole. The construction forces are responsible for the proper handling of poles and if they do not have any handling instructions or if the instructions are unclear, they are responsible for contacting the user for the necessary information. 5.4 Framing Concrete poles are generally framed like wood poles, (i.e. with the use of through bolts) but they will be easier to frame than wood poles because the holes can be more accurately drilled. Bolts should be tightened according to the assembly drawings but in the absence of any tightening instructions, reference to paragraph 2.11 of this guide and some common sense will work well. In most cases, the bolts will generally break before any damage is done to the poles. Near the ends of the pole, however, it is possible to tighten the bolts to the point where longitudinal cracks develop. If this occurs, loosen the bolts slightly but be sure they are still snug. Again, normal construction techniques such as raising the pole with a single choker at the erection pick point will present no problems. The primary caution is that if the pole has to be moved and the entire pole is lifted clear of the ground, the same procedures used in

CONCRETE POLES DESIGN 35 unloading must be followed again. 5.5 Field Drilling Most concrete poles will be sent from the factory with the necessary holes already in place. Occasionally, however, it will be necessary to drill one or more holes in the field. This can be easily accomplished with a rotary hammer drill, a carbide tipped bit of the appropriate size and a cutting torch. First determine which of the following two types of poles is to be drilled and then follow the appropriate set of instructions . 5.5.1 Full Length Reinforcing Steel Some manufacturers determine the amount of steel required by the ground line design moment capacity and carry that quantity of tendons throughout the entire length of the pole even though less steel could be used in the upper parts of the pole. Since holes are normally drilled in the upper parts of a pole where there is a considerable excess of steel, it is permissible to cut limited numbers of strands in the drilling process. CAUTION - DO NOT DRILL HOLES NEAR THE GROUND LINE FOR POLES USED IN SINGLE POLE TANGENT APPLICATIONS. DO NOT ERILL NEAR THE LOWER END OF THE TOP SECTION OF A TWO PIECE POLE AND DO NOT DRILL NEAR A CROSSBRACE ATTACHMENT IN H-FRAME CONSTRUCTION. These are the areas for which the steel requirements were determined and cutting the steel in these areas may weaken the pole below its design requirement. If there is any question as to the advisability of cutting tendons, contact the pole manufacturer for guidance. By referring to the manufacturer's drawings, it may be possible to find areas where drilling can occur without cutting prestressing steel. Once it has been determined that it is permissible to drill the pole, mark the location and drill with a rotary hammer drill and a carbide tipped bit. If steel is struck, stop drilling and burn the steel with the cutting torch. Then continue drilling. For best accuracy, mark the pole on both sides and drill both sides toward the middle. Mold marks, which are usually visible on the pole, make handy reference points from which to locate the hole on the opposite face of the pole. 5.5.2 Drop Out Reinforcing Steel As the need for steel decreases toward the top of the pole, some manufacturers stop a portion of the steel by dropping the tendons out through the side wall of the pole or they may install additional steel in critical areas by the use of post tensioned strand. In these methods, there is not the excess of steel near the pole tops and the steel should not be cut. This does not preclude drilling these poles. It means, however, that care should be used to insure that steel is not cut. Since there is less steel in pole tops of this type, there is more space between the tendons and it is easier to miss the tendons during the drilling process but cutting a strand means that the pole may be weakened below its design strength. The actual drilling of these poles is accomplished in the same

36 CONCRETE POLES DESIGN manner as for the previous poles. A cutting torch will still be necessary because even though the tendons are to be avoided, there is still a high probability of having to cut through the spiral steel. 5.5.3 Circumferential Steel Cutting of circumferential steel is difficult to avoid, but is acceptable at any time unless the pole is to be subjected to severe torsional loads. 5.6 Field Cutting There will be occasions in which it is desirable to shorten a pole in the field. This can be accomplished without damage to the pole by cutting with a small, hand held concrete saw and an abrasive cut off blade. The blade will cut both the concrete and the steel. For hollow spun poles, carefully mark a straight line around the circumference and saw along the mark. 5.7 Erection Concrete poles are erected in the same manner as other poles. Assuming that the poles were properly placed before they were framed, a single point pick with a choker is usually permissible. The choker should be placed well above the center of gravity unless the drawings indicate that the pole can be single point picked at the center of gravity. This means that as the pole is raised from the horizontal, much of the weight stays on the ground until the pole is nearly in the vertical position. Once it reaches the vertical position, it will not be damaged by lifting its full weight with a single point pick. Because the surface of a concrete pole is smooth and hard, safe operations require use of the same choker techniques as for steel poles. IMPROPER USE OF CHOKERS CAN RESULT IN THE POLE SLIPPING AND CAUSING INJURY OR PROPERTY DAMAGE. Chokers must be tight around the pole. If the chokers are slippery, they may be padded with a sticky material. A positive stop against sliding can be provided by attaching the choker below a solid piece of hardware (Note that a ladder clip does NOT qualify as solid hardware). Guyed poles, whether or not they are raked, should be initially set in what ever positions they will be under normal every-day loads. This means that regardless of what ever bending and flexing occurs during construction and long term use, once the conductor installation is complete and the guys are adjusted under normal everyday loads, the top of the pole should be in the same location as it was originally set. 5.8 Climbing Concrete poles are climbed in the same manner as steel poles. Just as most steel poles are climbed with the standard climbing ladders, all of the manufacturers provide attachments to concrete poles to accomodate the same ladders. Other climbing arrangements are also available and may have been selected by the user.

CONCRETE POLES DESIGN 37 5.9 Field Inspections Questions about cracks in concrete poles are frequent. It should be realized that although some types of cracks may be detrimental, concrete poles are expected to crack under certain conditions. Circumferential cracks that do not close when the pole is either properly supported on the ground or is erected, indicate a pole in which the steel has been stretched beyond its elastic limit and it should be rejected. Circumferential cracks may open during construction or during severe service conditions but they usually all close once the severe loads are removed, and the pole has not been harmed as long as they do close. Due to the process of releasing the tension on the steel in prestressed poles, circumferential cracks may develop within a few inches of either end of the pole. Those at the bottom end may be ignored. Those near the top should be weatherproofed with epoxy or other coatings, if they are not tightly closed. Longitudinal cracks are less common. At either end, they may have been caused by the application of prestress loads. If longer longitudinal cracks occur near the bottom of the pole, they have likely been caused by stacking the poles. Longer longitudinal cracks near the top may be caused by over tightening of the through bolts. As long as the cracks are only hairline cracks, as opposed to open cracks, they are not detrimental to the long life of the pole. Any open cracks should be investigated for the cause and a determination should be made as to the structural adequacy of the pole. If it is decided that the pole is to remain in service, the cracks should be filled and sealed from the weather to prevent further degradation of the pole. 6.0 Quality Assurance 6.1 General Quality assurance is the responsibility of the user. At the time of bidding, user should specify the degree of perfection he desires in design, fabrication, structure testing and field construction. The extent of the quality assurance program may vary based on initial investigations, the user's experience, the manufacturer's experience and past performance, and the degree of reliability required for the specific job. The following guidelines may serve in preparing specifications which include a quality assurance program. 6.2 Design and Drawings The quality assurance specification should indicate the degree of involvment by user, and the procedure for review of the design concept, detailed calculations, stress analyses and the manufacturer's drawings. Stress analyses of the main structure and all of its component parts,

38 CONCRETE POLES DESIGN including all attachments and connections, should be considered. The fabricator's drawings need checking to ensure they contain proper and sufficient information for fabrication and erection in accordance with the requirements of the user's specification. (Refer to Section 2.0 Design.) 6.3 Fabrication 6.3.1 Materials The specification should include the requirement for review and agreement on the manufacturer's material specifications, his sources of supply, material identification, storage, traceability procedures and acceptance of certified mill test reports. (Refer to Sections 3.2 Concrete and 3.3 Reinforcing Steel.) 6.3.2 Material Preparation The user may specify that either he or his agent inspect the manufacturer's equipment and process facility to ascertain that the procedures are satisfactory, the tolerances are within specified limits and the existing quality control program is satisfactory. (Refer to Section 3.8 Testing.) 6.3.3 Nondestructive Testing The specification should indicate the requirements for acceptance of the type and procedure of all nondestructive testing and inspection programs employed during each step in the fabrication process. The user may specify that the manufacturer furnish copies of testing and inspection reports. The user may also perform independent random sample testing to verify results of manufacturer's testing. (Refer to Section 3.9 Inspection.) 6.3.4 Tolerances It is necessary that acceptable fabrication tolerances be specified and agreed upon by the purchaser and manufacturer. Good fabrication quality is an important factor in minimizing field construction and performance problems. (Refer to Section 3.7 Fabrication Tolerances.) 6.3.5 Surface Coatings Where painting or other coloring is required, the system, procedures and methods of application should be acceptable to both the user and the manufacturer. Also the system should be suitable for both the product and its intended exposure. If galvanizing of accessories is required, the procedure and facilities should be agreed upon by the user and the manufacturer. After galvanizing, nondestructive testing may be specified to ensure that there have been no adverse changes to the finished product.

CONCRETE POLES DESIGN 39 When metallizing is required, the procedures and facilities should be in accordance with coating supplier's recommendations and acceptable to both user and manufacturer. 6.3.6 Shipping Prior to the start of fabrication, the user should review the fabricator's methods and procedures for packaging and shipping. When receiving materials, all product should be inspected for shipping damage prior to accepting delivery. If damage is apparent, the user should immediately notify the delivering carrier. If the shipments are FOB destination, making the manufacturer responsible for correcting damages, the user should notify the manufacturer of any damage and cooperate with him in filing damage claims with the carrier. User is also responsible for checking to see that all materials listed on the accompanying packing lists are accounted for. Where a discrepancy exists, both the carrier and the manufacturer should be notified. 6.3.7 Quality Control A review should be made and agreement reached on all quality control programs, organizational setups and procedures. It is necessary that rejection criteria be established and agreed upon prior to the start of any fabrication. (Refer to Section 3.8 Quality Control.) 6.4 Structure Testing Structure tests may be specified. The specification should indicate the position of the structure in the test, the test procedures, methods of load application, the load for each loading condition, and who is to be the Responsible Test Engineer. Agreement is necessary on all testing equipment and metering devices used for calibration. All post-testing inspection, nondestructive testing and evaluation procedures should be acceptable to the user. The report of the structure testing should determine the acceptability of the structure as specified. 6.5 Field Construction The user should review proposed construction quality control programs and procedures to determine that all phases of field construction will comply with the requirements as specified in the user's specifications and the manufacturer's designs and drawings; and to assure that adequate records are being maintained during construction such that there will be sufficient data provided to accept the completed work.

Appendix A 1)

2)

3)

4)

5)

6) 7) 8)

9)

10)

11)

12)

BIBLIOGRAPHY ACI Committee 318, "Building Code Requirements for Reinforced Concrete (ACI 318-83)", American Concrete Institute, Detroit, 1983, 111 pp. ACI Committee 318R, "Commentary on Building Code Requirements for Reinforced Concrete (ACI 318R-83)", American Concrete Institute, Detroit, 1983, 155 pp. National Electrical Safety Code, 1987 Edition, American National Standards Institute ANSI C2, Institute of Electrical and Electronic Engineers, Inc., New York, NY. Guidelines for Transmission Line Structural Loading, Committee on Electrical Transmission Structures, American Society of Civil Engineers, New York, 1984, 166 pp. Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, AASHTO Subcommittee on Bridges and Structures, 1986. EIA-RS-222-C, Electronic Industries Association Standard, March 1960. PCI Design Handbook, Precast and Prestressed Concrete, Third Edition. Prestressed Concrete Institute, Chicago, 1985. PCI Committee on Prestressed Concrete Poles, "Guide Specification for Prestressed Concrete Poles", PCI Journal, V. 27. No. 3, MayJune 1982, pp. 18-29. PCI Committee on Prestressed Concrete Poles, "Guide for Design of Prestressed Concrete Poles", PCI Journal, V. 28, No. 3, May-June 1983. pp. 22-87. Task Committee on Steel Transmission Poles, "Design of Steel Transmission Pole Structures", Committee on Analysis and Design of Structures, ASCE Structural Division, 1978. Manual for Quality Control for Plants and Production of Precast and Prestressed Concrete Products, MNL-116-85, Prestressed Concrete Institute, Chicago, 1985. "State of the Art - Prestressed Concrete Poles", PCI Journal, Vol. 29, No. 5, Sept-Oct 1984. 41

Appendix C DEFINITIONS CASTING METHODS Precast Member - A member which is cast in some location other than the location in which it is to be used. All poles are likely to be precast. Spun Cast Member - A member cast in a mold that spins during the consolidation phase. The resulting centrifugal force causes the pole to be hollow and the concrete to be highly consolidated. Since this force is very large, dry (low water/cement ratio) concrete can be consolidated in this manner, usually with some of the water spinning out to reduce the water/cement ratio even further. Because spun concrete has a lower than normal water/cement ratio and a higher than normal density it is much stronger and more durable than static cast concrete. The end result is that the member can be lighter because less concrete is required when it is stronger. The concrete is much more impermeable and, therefore, more durable. Static Cast Member - A member which is cast in a mold that does not move during the casting and consolidating of the concrete (except for the possibility of vibrating the mold as an aid in consolidating the concrete). LOADINGS Maximum (Ultimate) Design Load - The load that the pole is designed to resist. This load is the maximum service load multiplied by some overload factor. The user must select not only the load and the load factor, but also must determine whether the pole is to resist the maximum design load without permanent unacceptable deformation (damage) or without failure (collapse). A stronger pole is required to resist without permanent deformation than without collapse. Maximum Service Load - The maximum load that the pole is ever expectedto encounter (exclusive of overload factors). This load may be used for checking deflections and clearances. Normal Everyday (Frequent Condition) Load - A load that a pole may be expected to encounter on a frequent basis. User should specify the normal everyday load. MOMENTS ultimate Moment - Depending on the user's choice as to whether the pole must resist permanent deformation or collapse, this is the moment at which the chosen one of these events occurs. The moment capacity at each section must be equal to or greater than the ultimate moment produced in the section by the maximum design loads. 45

46

CONCRETE POLES DESIGN Damage Moment - The maximum allowable moment at a section without creating permanent, unacceptable deformation in the section. Note that under one of the possible assumptions in the previous definition, damage moment and ultimate moment may be the same. Cracking Moment - The moment at a section when the concrete first cracks. Although this moment is of little significance from a design and use standpoint, it is useful in helping to determine the overall accuracy of the design and manufacturing processes. During testing of a completed pole, the concrete should not crack earlier than the anticipated cracking moment. No Concrete Tension Moment - The maximum moment a section can withstand without allowing the concrete to go into tension. The magnitude of the pretensioning forces is the primary controlling factor affecting this moment. The No Concrete Tension moment capacity of any section must be equal to or greater than the moment caused at that section by the normal everyday loads. REINFORCEMENT Ordinary Reinforced Concrete - Concrete in which the reinforcing steel(normallymildsteelrebar) is simply placed in its designed location. It is not used to impart a compressive force across the concrete section. Partially Prestressed Concrete - Concrete in which some of the reinforcing steel is conventionally placed and some of it is stretched in such a manner as to impart a compressive stress in the concrete. This results in a member in which the concrete has some compressive force under a no-load condition, but not as much as a fully prestressed member, with the end result that cracks will appear at a larger moment than in an ordinary reinforced member but at a smaller moment than in a fully prestressed member. Prestressed Concrete - Concrete in which all of the primary reinforcing steel is used to impart a compressive stress to the concrete before the member is subjected to the loads it was designed to handle. Thus, under bending loads, a much larger load must be applied to the member before the internal compressive stress in the concrete is overcome and the concrete finally goes into tension and cracks. Since larger loads are required to open cracks, the cracks are open less often (if at all) during the life of the member and the member does not deteriorate due to the elements. Further, when the loads are removed from the member, the cracks close tightly due to the tension in the steel. Prestressing steel is special, very high strength steel (either strand or wire) which is stretched before it is bonded to the concrete. After it is bonded to the concrete, it is the spring action of the steel that squeezes the concrete and causes its initial compressive stress (prestress).

DEFINITION

47

TENSIONING Post Tensioned Member - A prestress member in which the concrete is poured and cured without tensioning the steel. Usually ducts are cast into the concrete to keep the steel from bonding to the concrete or to provide a space for placement of the steel after the concrete is cured. In this method, the steel is initially stretched against the cured concrete itself rather than against the molds or bulkheads. The advantage of post tensioning is that bulkheads or heavy stressing molds are not required. The disadvantage is that it requires more work in manufacturing. Pretensioned Member - A member in which the prestressing steel is stretched against bulkheads or the mold while the concrete is cured and forms its bond with the steel. When the steel is cut loose from the end supports, the bond between the concrete and steel allows the steel to impart the prestressing load to the concrete.