Attach-3_NABE_Handbook_9th.pdf

smaller land area. However, the cost of the tower in-creases as the anchor distance decreases. The approxi-mate relationship of cost to anchor distance for a repre-sentative 1,200 ft television broadcast tower is shown in Figure 2.2-1.

It is often possible to position a guyed tower on an irregularly shaped plot or to obtain long term lease agreements or easements for guy paths and anchor locations in order to minimize the tower cost without obtaining large, rectangular land areas.

Self-Supporting Towers Self-supporting towers may be either square or trian-gular in cross section. While it is usually more econom-ical to use a triangular cross section, there are situations

Figure 2.2-1. Effects of anchor distance on cost of a 1,200 foot guyed TV broadcast tower using ANSI/TIA/EIA Standard 222- F-1996.

2.2

DESIGN, ERECTION, AND MAINTENANCE

OF ANTENNA STRUCTURES

JOHN WINDLE, P.E.

CONSULTING STRUCTURAL ENGINEER, WEST CHESTER, PA

THOMAS J. HOENNINGER, P.E.

STAINLESS, INC., NORTH WALES, PA

TOWER LIGHTING SECTION BY:

LEWIS WETZEL

HUGHEY & PHILLIPS, INC., WENHAM, MA

INTRODUCTION

The purpose of this chapter is to provide broadcast engineers and managers information concerning the design, erection, and maintenance of antenna struc-tures. While fundamental principles of the design and behavior of these structures will be discussed, this chapter is not intended to enable readers to design and build their own tower, but provides instead a basic understanding of these unique structures to facilitate planning, modifying, and maintaining broadcast facil-ities.

TOWER CHARACTERISTICS

All towers may be classified in one of the two basic groups, guyed or self-supporting. As their names im-ply, guyed towers depend on cables extending from the tower to anchors located some distances from the tower base for their structural integrity, while self-supporting towers rely solely on their own construction as a cantilevered space truss.

With only a few exceptions, the cost of the actual tower structure and foundations is considerably less for a guyed tower than for one that is self-supporting.

The advantage of the self-supporting tower is relatively small land area required. Therefore, the choice between guyed or self-supporting depends to a large degree on the availability and cost of real estate.

A self-supporting tower requires a nearly square plot of land with equal sides that are 8% to 20% of the tower’s height, provided local zoning rules do not require tower height radius of land.

The amount of land required for a guyed tower depends on the distance between the tower base and the guy anchors. This distance is preferably between 70% and 80% of the height, which would require a rectangular plot having sides equal to 125% and 145% of the height.

Because of the great flexibility in guyed tower de-sign, it is possible to reduce the anchor distance to as little as 35% of height, thereby requiring a much

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 201

SECTION 2: BROADCAST TOWERS AND SYSTEMS

where a square cross section is a better choice. The principle structural elements are the legs, the web brac-ing in each face, and if required for stability, horizontal diaphragm bracing. The legs are usually sloped (ta-pered) to provide adequate strength and stability as the height increases. The degree of slope is an option of the designer to suit the equipment supported, the required rigidity, and the available land area. The slope is sometimes varied within a tower to maintain a desir-able balance between the costs of leg members and bracing, or to reduce the foundation loads. Frequently, the legs in the top section of the tower will be parallel to simplify the mounting of equipment (see Figure 2.2-2).

There are several different configurations of bracing members for the individual truss panels. The choice is influenced by the width of the panel, the magnitude of the wind and ice loads imposed, the location of equipment and required stability. Continuity in trans-ferring the applied loads through the structure without significant eccentricity is essential regardless of the configuration used.

Guyed Towers Guyed towers are almost always of triangular cross section although there are a few unique conditions for microwave and panel type FM and TV antenna sup-ports where a square cross section is advantageous.

The principle structural elements are the legs, the web bracing in each face, and the guy support systems (see Figure 2.2-3). Except for sections at the tower base and locations where the width changes, the legs are parallel. The width of the tower is usually constant throughout the height of the tower with the exception of sections supporting antennas requiring a specific width of support structure. The base section is often tapered to a single point to provide a pivot support to eliminate large bending and torsional moments.

Theoretically, there are an infinite number of ar-rangements of guy cables to support a tower. The most common arrangement is three cables spaced at 120° with one attached to each leg, as shown in Figure 2.2-4(a). This is the minimum number of cables that can be used. When the tower supports equipment, which imposes large twisting moments (torque), it is necessary to provide six cables at a level to maintain torsional stability. If the torque is localized, the guys at this location may be attached to triangular frames as shown in Figure 2.2-4(b). If the torque occurs through-out the height, it may be desirable to double-guy the tower at every level as shown in Figure 2.2-4(c).

The number of guy cable levels required to support the tower is dependent on a number of factors including the height of the tower, width, location of equipment and the environmental loading conditions. Because the tower is an axially compressed column, its strength is a function of its slenderness. While design codes permit

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 202

Figure 2.2-2. Elevation view typical self supporting tower.

Figure 2.2-3. Elevation view typical guyed tower.

DESIGN, ERECTION, AND MAINTENANCE OF ANTENNA STRUCTURES

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 203 slenderness ratios resulting in triangular towers having a span-to-width ratio as great as 49, it is usually eco-nomical to limit the ratio to a maximum of 30. While there is no upper limit to the number of guy levels imposed by any code, a practical limit for economical design is ten.

The position of equipment on the tower is an im-portant factor in determining the location of guy levels.

Preferably, guy attachments should not be located within the apertures of side mounted TV and FM broadcast antennas. Equipment producing large local-ized wind loads, such as microwave antennas or clus-ters of two-way radio cabinets and antennas, should not be positioned near the center of a span between guys.

If the tower will be subjected to ice loading, it is desirable to reduce the number of guy levels to mini-mize loads imposed on the tower by ice accumulation on the guy cables.

The number of anchors in each guy direction is dependent on several factors including the number of guy levels, the soil conditions, topography, and obstacles. As a general guideline, it is desirable to limit the number of guy levels attached to a single anchor to five. However, there is nothing absolute about this number, and other conditions may dictate using an anchor for a greater number. There are some soil conditions where it may be economical to provide two or more smaller anchors, while another instance the use of one large anchor might be desirable. If minimizing the area within which the tower would fall in the event of collapse is a consideration, a minimum of two anchors should be used in each direction. Where the elevations of the anchors differ from the tower base, it is desirable to vary the distance of the anchors from the tower base to maintain nearly equal initial tensions in the guy cables. Anchors higher than the tower base should be moved toward the tower; and anchors that are lower, away from the tower. The designer should specify the amount of movement.

Materials Nearly all broadcast towers are made from steel because it provides the most economical structure. The selection of the grade and shape of steel is obviously an important design consideration.

Steel used for towers commonly has low carbon content with yield strengths in the range of 36,000 to 60,000 psi. These materials have good ductility and are suitable for welding. Some towers have been built using higher grade materials with yield strengths up to 100,000 psi, but the savings in weight are more than offset by higher base prices and increased fabrication costs. Regardless of the grade of material, the steel’s mechanical and chemical properties should be certified by the producing mill to ensure that it conforms to the design requirements.

The shape of the material as well as its size and strength affects the tower’s load carrying capacity. The shape also has a significant effect on the magnitude of loads produced by wind. Design standards permit a reduced wind load on round members as little as 57% of the wind load for flat or angular members of the same width. For this reason, solid round bars, round structural tubes and pipe are often used. This advantage in wind load is offset somewhat by increased fabrica-tion costs, due to the necessity of welding plates to connect the various members.

There is no one grade or shape of materials that is best. The choice depends to a large degree on the preference of the designer and the type of fabricating facilities available.

A factor equally as important as the selection of the grade and shape of the structural steel is the design of the connections. For shop welded connections, the

Figure 2.2-4. Typical guy arrangements.

loaded. They are advantageous for connecting closely spaced insulators where the length of dead end grips is unacceptable.

• Wedge Type Socketsare available for guys up to 1-1/4 in. diameter and develop 100% of their strength. They are most advantageous for guys larger than those for which dead end grips are available.

• A servingis a connection made by rolling the indi-vidual wires of a strand back on the strand itself.

This method has for the most part been replaced with dead end grips, but it is advantageous for small guys with closely spaced insulators.

Insulators Insulators in radio frequency applications must with-stand mechanical and electrical stress in a varied, changing exterior environment. Selection of insulators should be made with these factors in mind. Insulators primarily designed for 60 Hz applications are unsuit-able, particularly at high RF powers.

The most common insulating material is a wet pro-cess porcelain, which has excellent compressive strength and good insulating capabilities for frequen-cies up to 2 MHz. Synthetic materials are also used.

Some other types of insulators are as follows:

• Base insulatorsfor AM towers are made from porce-lain with appropriate steel end plates or ferrous/non-ferrous castings. For guyed towers, a rocking ar-rangement is provided in the form of a convex plate and pin at the top, or a pivot pin at the bottom of the assembly to hold the tower in place and relieve the porcelain from bending loads which could cause cracking. For self-supporting towers, the insulators are bolted between the tower leg and base pier, and are designed to sustain both uplift and download while keeping the porcelain in compression.

• Sectionalizing insulatorsare sometimes required to isolate sections of a guyed tower. Where a compres-sion load only is applied, a guyed tower base with minor modifications can be used. If a tension load is anticipated, a push-pull insulator similar to the type used for self-supporting towers is required. Under no circumstances should the porcelain be put in tension.

• Guy insulatorsare available for primary insulation and for break-up purposes. Primary insulation (insu-lators next to the tower) should be selected to with-stand the full voltage appearing at the guy attachment point. This is to ensure that sufficient insulation re-mains if all thebreak-up insulators in theguy lineflash over. Break-up insulators, used to reduce reradiation, are selected to withstand the transmitter induced volt-age and static voltage. Break-up insulators are usually low voltage types, sometimes protected from flash-over and subsequent power arc by a static dissipation device. Guy insulators are available in many styles classified as either compression or tension types.

Finishes Steel is susceptible to deterioration from atmo-spheric corrosion. To prevent deterioration, the tower members and hardware must be given a protective

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 204 compatibility of the base and filler metals and required preheat temperatures must be considered. The proce-dures used must be qualified and the welders certified to use them. Inspection procedures should be compati-ble with the weld design.

Bolts for field assembly may be of various types.

Usually those for the main load carrying members are high strength. If positive resistance to slippage of the connections is required, they should be designed as friction connections.

Guys The most common material for tower guys is galva-nized strand. This material has excellent strength and durability. Its structural elongation due to seating of the individual wires in the strand is small and can be almost entirely eliminated by pre-stressing the strand to 50% of its breaking strength at the factory. This should be performed for guys on tall towers with fac-tory connected end fittings.

For guys on AM towers, and those close to FM and TV antenna apertures, a nonconductive material is sometimes desirable. Two such materials that have been used are Kevlar rope and fiberglass rods. When using these materials, careful attention must be given to protection against corona effects, fatigue, and deteri-oration due to exposure to ultraviolet light. Also, their elongation characteristics under load must be evalu-ated. They require delicate handling at all times.

Just as for the tower structure, the connections for the guys are as important as the guy material itself.

Some of the most common connections are as follows:

• Socketsof forged or cast steel attached with molten zinc or epoxy resins develop the full strength of the guy. They are normally installed at the factory and proof loaded to 50% of the guy breaking strength.

This type of fitting is most common for the larger guys used on tall towers.

• Dead end gripsare preformed spiral wire loops in the shape of large hairpins. The two legs of the hairpin are wrapped around the guy with its closed end forming an eye. These grips are used for guys up to 1 in. in diameter and usually develop their full strength. They are easily installed in the field, but the ends must be completely snapped into place and a protective device installed to prevent ice from sliding down the guy and loosening the grip.

• Clipsused to clamp the ends of guys (when properly applied and tightened) develop 90% of the guy’s strength for sizes up to and including 7/8 in. and 80% for larger sizes. To install them it is necessary to bend the strand back on itself to form a loop;

thus, the use of clips on large cables is difficult. The saddle of U-bolt type clips must be installed on the load side and not the dead end side, which provides another potential error in their installation.

• Swaged sleevesdevelop between 85% to 100% of a guy’s strength depending on the size of the guy and equipment used to squeeze the sleeve. These fittings are usually installed at the factory and can be proof also requires that any continuous ladder more than 20 ft in height be equipped with a safety device. This device consists of a continuous rail, either rigid or cable, running up the center of the ladder. A clamping device attached to the climber’s safety belt rides along this rail. As long as the climber is in a normal position, the clamp slides freely; if the person begins to fall, a cam actuated mechanism freezes the clamp to the rail and prevents the person from falling.

Elevators For tall towers supporting multiple antennas, it is often desirable to install an elevator. Most tower eleva-tors are of the power, cable driven type with a capacity of 500 to 750 lbs. and a speed between 80 and 100 ft per minute. They consist of a drive mechanism, car, guide rails, hoist cable with supporting sheaves, ten-sion weights, electronic controls and a two-way com-munications system.

Considerable attention must be given to elevator safety features. These should include limit switches to prevent travel beyond the upper and lower landings on the tower, an automatic brake on the driving mecha-nism that is activated by an interruption in power, a mechanism to automatically clamp the car to the rails in the event of a broken hoist cable and interlocks to prevent operation with the car gate open. It is advisable to determine the applicable state or municipal govern-ment regulations that may apply and whether permits, tests and inspections are required before the tower and elevator system are designed.

The added wind and dead loads from an elevator system are substantial and must be considered in the tower design. Also, careful attention must be given to the positioning of the ladder, RF transmission lines and electrical conduits in relationship to the elevator.

The ladder must be positioned so it is accessible from the elevator car and can be used for an emergency descent. While the elevator hoist cables can be re-strained in guides on the return side, they are free to move about under wind load on the lifting side.

Therefore, the conduits and transmission lines must be protected from hoist cables striking and damaging them. If a side mounted TV or FM antenna produces a high RF field within the hoistway, protection must be provided to prevent arcing between the hoist cables, the tower structure, and other appurtenances.

Transmission Line Bridges To allow for the different thermal expansion and contraction characteristics of broadcast towers and transmitter buildings, it is necessary to keep the tower separated from the transmitter building. Unless the transmission line is placed underground, it is necessary to provide a structural support for it at a height compat-ible with the transmitter location in the building. The top of the support can be covered with steel grating or plate to protect the line from falling ice. The details of this structure can become quite involved for sites with multiple antennas, uneven terrain and roadways, or obstacles between the tower and building.

coating. This coating is usually zinc, which has excel-lent resistance to corrosion, and, because it is higher in the electrochemical series of the periodic table of elements, it provides cathodic protection to exposed steel surfaces adjacent to it. Even though the zinc coating may be scraped or otherwise damaged, it con-tinues to inhibit corrosion of these exposed areas, and rust will not develop beneath adjacent zinc coats.

There are several methods for applying the zinc in-cludinghotdipgalvanizing,flamespraying,electroplat-ing and painting. All must be applied to clean surfaces.

• Hot dip galvanizingconsists of dipping the steel into a bath of molten zinc. A metallurgical bond develops between the steel and the zinc, which adheres to it.

When galvanizing tubular members, it is necessary to provide holes in both ends to ensure that the inside surfaces are coated. Careful attention must be given to the type of base and weld metals used, as well as to the welding and forming procedures used in fabrication, to safeguard against possible embrittle-ment of the steel when galvanized. When properly applied, this process provides the most durable coating.

• Flame sprayingconsists of spraying molten zinc at high pressure onto the steel surfaces. The bond in this process is mechanical rather than metallurgical.

The coating produced is more porous and has less resistance to abrasion than the hotdip galvanized coating. It cannot be used for the inside of hollow sections or other cavities where access is difficult.

• Electroplating, while suitable for small objects, does not produce a coating thick enough to withstand a hostile environment. This method is not recom-mended for tower parts or hardware.

• Zinc rich paintconsists of extremely finely divided zinc in an inorganic or organic vehicle. It is not a metal coating method, but rather a painting proce-dure. Its resistance to abrasion and durability are less than hotdip galvanizing. This procedure is, however, useful for maintenance.

Ice Prevention Coatings are available to reduce the adherence of water to surfaces and subsequently the formation of ice on them. However, no reliable means exists to completely remove the risk of severe ice accretion.

Access Facilities A tower must have some access facilities in order to maintain it and the equipment the tower supports.

For small towers, the bracing members of the tower itself often serve as steps, or step bolts are attached to one leg or face.

Ladders For taller broadcast towers, a fixed ladder inside the tower is desirable. The Occupational Safety and Health Administration (OSHA) standards for these ladders require a minimum clear width between side rails of 16 in. and a maximum rung spacing of 12 in. OSHA

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Stairways The lower landing for a tall, guyed broadcast tower with an elevator is often 30 ft or more above ground level. A stairway may be desirable to permit easier access to the landing. This structure can be combined with the transmission line support bridge, or it may be completely separate. It may also be desirable to install a small capacity boom above the lower landing to lift radio cabinets or other equipment onto the landing.

ANTENNA STRUCTURE REGISTRATION

REQUIREMENTS

On November 30, 1995, the FCC adopted rules to streamline the Commission’s antenna structure clear-ance process by requiring owners to register their an-tenna structures with the Commission by June 1998.

The registration generally applies to structures more than 60.96 m (200 ft) above ground or located near an airport. Detailed information concerning registration is available from the FCC’s Forms Distribution Center at 1-800-418-FORM or through the Internet at http:// www.fcc.gov/wtb/antstruc.html.

Upon registering an antenna structure with the Com-mission, an owner will receive a Registration (FCC Form 854R) containing a Registration Number which uniquely identifies the structure. The owner must pro-vide each tenant licensee and permittee with a copy of the Registration. In turn, licensees and permittees must reference this registration number on all subse-quent Commission filings.

Marking and Lighting Antenna Structures to Meet FAA/FCC Requirements

Unless otherwise specified by the Commission, each new or altered antenna structure to be registered on or after July 1, 1996, must conform to the FAA’s marking and lighting recommendations set forth on the structure’s FAA determination of “No Hazard,” as referenced in the following FAA Advisory Circulars:

AC 70/7460-1H,Obstruction Marking and Lighting and AC 150/5345-43D,Specification for Obstruction Lighting Equipment.These documents contain FAA recommendations for marking and lighting structures which pose a potential hazard to air navigation. The Commission considers these specifications, standards and general requirements stated in these documents to be mandatory. The Advisory Circulars listed above may be obtained from the U.S. Department of Trans-portation, Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, Maryland 20785. Fax requests for these publications may be sent to (301) 386-5394.

CERTIFICATION PROCEDURE FOR FAA

APPROVED LIGHTING EQUIPMENT

To assure that all lighting equipment manufactured to meet the specifications in the FAA Advisory Circulars, a certification program was established. On January 1, 1990, ETL Testing Laboratories, Inc., began adminis-tering the certification program for airport and obstruc-tion lighting equipment. Lighting equipment approved by the FAA prior to January 1, 1990, continued its approved status without additional product qualifica-tion testing for a period of five years through a grand-father clause, provided sufficient documentation exists to support the certified status.

The FAA’s Advisory Circular 150/5345-53,Ap-proved Airport Equipmentlists the certified lighting equipment that ETL has verified as meeting the appli-cable FAA specifications. When building or modifying an antenna structure that involves the use of obstruction lighting equipment, it would be wise to check the FAA approved lighting equipment list or require a certificate of compliance from the manufacturer.

FCC Lighting Requirements During construction of an antenna structure for which obstruction lighting is required, a temporary light must be installed at the uppermost point of the structure. This light shall be similar in type and inten-sity to the permanent light that is required for that level. In addition, as the height of the structure exceeds each level at which permanent obstruction lights will be required, another level of temporary lighting must be installed. If practical, the permanent obstruction lights may be installed and operated at each required level as construction progresses.

Inspection of Antenna Structure Lights and Associated Control Equipment

The owner of any antenna structure which is regis-tered with the Commission and has been assigned light-ing specifications:

(a) (1) Shall make an observation of the antenna struc-ture’s lights at least once each 24 hours either visually or by observing an automatic properly maintained indicator designed to register any fail-ure of such lights, to insure that all such lights are functioning properly as required: or alternatively.

(2) Shall provide and properly maintain an auto-matic alarm system designed to detect any failure of such lights and to provide indication of such failure to the owner.

(b) Shall inspect at intervals not to exceed three months all automatic or mechanical control de-vices, indicators, and alarm systems associated with the antenna structure lighting to insure that such apparatus is functioning properly.

Notification of Extinguishment or Improper Functioning of Lights

The owner of each antenna structure which is regis-tered with the Commission and has been assigned light-ing specifications:

(a) Shall report immediately by telephone or fax to the nearest Flight Service Station or office of the

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 206 when the northern sky illuminance reaching a vertical surface falls below a level of 60 footcandles (645.8 lux) but should occur before it drops below 35 footcandles (376.7 lux). These lights should switch from their twi-light to night mode of operation when the level falls below 5 footcandles (53.8 lux) but should occur before reaching a level of 2 footcandles (21.5 lux). The control device should reverse intensity changes when going from night to twilight to day within these same levels.

Options for Marking and Lighting Antenna Structures

Red obstruction lights are used to increase conspicu-ity during nighttime. Painting is required for daytime and twilight marking. The red obstruction lighting sys-tem is composed of flashing omnidirectional beacons (L-864) and steady burning lights (L-810). When the system is comprised of more than one level of flashing beacons, the beacons should flash simultaneously.

Medium Intensity Flashing White Lighting System Medium intensity flashing white (L-865) obstruction lights provide conspicuity both day and night and elim-inates the need to paint the structure. The medium intensity flashing white light system is composed of flashing omnidirectional lights. These systems are used on structures up to 500 ft (153 m).

Dual Red and White Flashing Medium Intensity Lighting System

The dual lighting system includes red flashing bea-con or beacons (L-864) and steady burning red lights (L-810) for nighttime and medium intensity flashing white lights (L-865) for daytime and twilight use.

These systems may be comprised of a red flashing beacon (L-864) and a medium intensity flashing bea-con (L-865) or a single beacon which incorporates both the red and white flashing beacons (L-864/ L-865). These systems are used on structures up to 500 ft (153 m) and eliminate the need to paint the structure.

High Intensity Flashing White Lighting System Lighting with high intensity flashing white obstruc-tion lights (L-856) provides the highest degree of con-spicuity both day and night. When this system is oper-ated 24 hours a day, other methods of marking and lighting may be omitted. These systems are used on structures from 500 ft. (153 m) to the maximum height allowed by the FAA.

Dual Red and White Flashing High Intensity Lighting System

The dual lighting system includes flashing red lights (L-864) and steady burning red lights (L-810) for nighttime and high intensity flashing white lights (L-856) for daytime and twilight use. When using this system, marking the structure with paint is eliminated.

These systems are used on structures from 500 ft (153 m) to the maximum height allowed by the FAA.

FAA any observed or otherwise known extinguish-ment or improper functioning of any top steady burning light or any flashing obstruction light, re-gardless of its position on the antenna structure, not corrected within 30 minutes. Such reports shall set forth the condition of the light or lights, the circumstances which caused the failure, the proba-ble date for restoration of service, the FCC Antenna Structure Registration Number, the height of the structure (AGL and AMSL), and the name, title, address, and telephone number of the person mak-ing the report. Further notification by telephone or fax shall be given immediately upon resumption of normal operation of the light or lights.

(b) An extinguishment or improper functioning of a steady burning side intermediate light or lights, shall be corrected as soon as possible, but notifica-tion to the FAA of such extinguishment or im-proper functioning is not required.

Recording of Structure Light Inspections in the Owner Record

The owner of each antenna structure which is regis-tered with the Commission and has been assigned light-ing specifications must maintain a record of any observed or otherwise known extinguishment or im-proper functioning of a structure light and include the following information for each such event:

(a) The nature of such extinguishment or improper functioning.

(b) The date and time the extinguishment or improper operation was observed or otherwise noted.

(c) The date and time of FAA notification, if appli-cable.

(d) The date, time and nature of adjustments, repairs or replacements made.

Time When Lights Should be Exhibited Red obstruction lights may be operated by a control device adjusted so the lights will be turned on when the northern sky illuminance reaching a vertical surface falls below a level of 60 footcandles (645.8 lux) but before reaching a level of 35 footcandles (376.7 lux).

The control device should not turn the lights off until the northern sky illuminance rises to a level of at least 60 footcandles (645.8 lux). The lights may also remain on continuously.

Medium intensity white and dual obstruction lights should be operated by a control devise so the lights will switch from their day/twilight to night mode of operation when the northern sky illuminance reaching a vertical surface falls below a level of 5 footcandles (53.8 lux) but before reaching a level of 2 footcandles lux). The control device should reverse intensity changes when going from night to day/twilight mode.

High intensity white and dual obstruction lights should be operated by a control device so the lights will switch from their day to twilight mode of operation

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Other Electrical Circuits to Consider in Antenna Structure Planning

During the planning and design stages for an antenna structure, there are other electrical circuits that are necessary or should be considered to make its operation more efficient and provide a source for future income.

Most tall antenna structures require deicing circuits with their associated control devices. An ac utility circuit can provide access to 120 volt power at selected elevations on the structure and will reduce costly main-tenance time. If the structure’s height justifies an eleva-tor, control circuits for the elevator are necessary.

Circuits to provide power and multi-pair cables to various platforms are needed for rental communication system customers.

DESIGN STANDARDS

The vast majority of towers in the United States have been designed in accordance with the EIA-222,Struc-tural Standards for Steel Antenna Towers and Antenna Supporting Structures.This standard has been used since 1959 when it replaced the Radio Electronic Tele-vision Manufacturers Association (RETMA) Standard TR-116. The current revision “F” of EIA-222 was issued in 1996. The standard has been approved by the American National Standards Institute and carries the designation ANSI/TIA/EIA-222-F-1996.

This standard is intended to provide minimum crite-ria for specifying and designing steel antenna towers and antenna supporting structures. Unlike general specifications and building codes, it is applicable only to antenna tower and supporting structures. As such it contains criteria specific to these structures that are not readily available elsewhere. Therefore, it is always advisable to specify that your tower must conform to this standard.

Appendix A: Purchase Checklistof this standard is provided to alert the purchaser to the most common area where site-specific data may be required to supple-ment the minimum criteria of the standard.

Statutory Most municipal and state governments have statu-tory codes regulating the design of structures. Many of these are patterned after or include one of several model codes. The most common of these are:

• Building Officials and Code Administrator Interna-tional (BOCA) BasicBuilding Code.

• International Conference of Building Officials (ICBO) Uniform Building Code.

• Southern Building Code Congress (SBC)Standard Building Code.

These codes cover all types of structures and are directed primarily toward conventional types of build-ings. As such, they do not contain all the criteria neces-sary to design broadcast towers. For example, none of them includes a recommended safety factor for guy cables.

The industry standard ANSI/TIA/EIA-222-F-1996 is compatible with these codes. In fact, its use for calculating and applying wind loads is required by the BOCA Basic Building Code and, as an approved American National Standard, is permitted by the ICBO Uniform Building Code.

Since it is necessary to comply with the applicable statutory requirements, it is important to determine what these requirements are and include them in the purchase specifications for the tower.

Loads, Analysis & Safety Factors In addition to a tower’s own dead weight and the dead weight of the appurtenances and equipment it supports, the tower must withstand the forces of nature, wind, ice, temperature changes, and earthquakes.

Wind Load Wind produces a principal load on tower structures.

For design purposes it is represented as a horizontal static force.

Wind load is specified in terms of a basic wind speed at 10 m (33 ft) above ground level. The ANSI/ TIA/EIA-222-F-1996 standard provides a tabulation of recommended minimum values for this speed for each county in the United States. This standard also gives specific procedures and factors for calculating wind loads considers the following:

• Wind pressure is proportional to the square of wind speed.

• Wind speed and, consequently, wind pressure vary with respect to the height above ground.

• The effects of gusts of brief duration, which exceed the fastest mile basic wind, speed.

• The effects of the configuration, size, proportions, shape and orientation with respect to the wind direc-tion of the structural components of the tower and its appurtenances.

An example of wind load calculation for a typical broadcast tower is given in Figure 2.2-5.

Since the wind may act from any direction it is necessary to apply the calculated wind loads in any horizontal direction to determine the maximum stresses produced in the structure. For a triangular tower, three directions must be considered; while for a square tower, two are sufficient (see Figure 2.2-6).

In addition to this direct load in the direction of the wind (drag), there may also be a component of load perpendicular to the wind direction (lift). These lift components are calculated in a manner similar to that for drag forces using different shape coefficients that vary with respect to the angle of attack between the member’s geometric axis and the wind direction. They are most significant for wind acting on guy cables, microwave antennas and rectangular waveguides.

Ice Loads Ice accumulations have two effects on a tower. The weight of the ice acts directly on the structure in the same manner as the dead weight. The ice accumulation

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Figure 2.2-5. Wind force calculations for latticed towers using ANSI/TIA/EIA-222-F-1996.

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 210 tic deformations from stress changes. The effects are greatest for those cables having the flattest angle with the ground. While the stresses produced are consider-ably less than those produced by wind and ice loads, they should be considered in the design of guyed tower.

Seismic Loads Loads due to earthquakes are considered to act hori-zontally and are dependent on the mass and stiffness of the tower. They are usually less than those produced by wind but are distributed in a different manner.

Procedures for calculating these loads are given in some of the design standards including ASCE 7, but not in ANSI/TIA/EIA-222-F-1996. While a tower properly designed for wind loads is usually adequate for seismic forces, they cannot be neglected in areas with frequent and intense earthquake occurrences.

Structural Models & Analysis A self-supporting tower may be described structur-ally as a cantilevered space frame or truss. Although it may have many different members, it is a relatively simple structure, and the determination of the forces in the individual members due to the applied static loads is easily done using fundamental principles of structural mechanics. The potential modes of failure are buckling of individual leg or bracing members under compressive loads, and shear or tension failures of the connections.

A guyed tower is a much more complex structure than a self-supporting tower. Whereas there is only one basic path through a self-supporting tower for the loads to be transferred to the ground, there are several for a guyed tower. The distribution of the loads among these paths is dependent upon the relative stiffness of guy systems and the tower shaft.

Each span of the tower has a stiffness with respect to relative deflections from axial and shear forces and bending and torsional moments. This stiffness is a also increases the area exposed to the wind and conse-quently the load produced by the wind. This increase is substantial on small components such as guy cables, tension rods, ladders, small diameter transmission lines and reflector screens for antennas. It is also possible for the ice accumulation to alter the aerodynamic shape of members, thereby requiring the use of a different coefficient in calculating the wind load. An example is a set of closely spaced parallel coaxial lines. Without ice each would be considered a round cylindrical mem-ber. With accumulated ice, they would present a large flat area to the wind requiring a different coefficient.

Ice produces an entirely different stress distribution in a tower than wind, so it is not reasonable to merely increase the design wind load to provide for ice accu-mulation. It is also a misconception that ice will break up and blow off the tower, and therefore, ice and wind need not be considered simultaneously. The ANSI/ TIA/EIA-222-F-1996 Standard states that unless other-wise specified by the purchaser, 75% of the wind load shall be applied in combination with the ice load. This is equivalent to reducing the basic wind speed by a factor of 0.866. In areas where ice is likely to form, it is advisable to specify a basic wind speed to act concurrently with the ice load as well as the basic wind speed without ice), (70 mph with ice and 80 mph without ice).

While the ANSI/TIA/EIA-222-F-1996 Standard emphasizes the need to consider ice loads, it does not provide specific recommendations for the magnitude of the accumulation. This responsibility is left with the purchaser.

Temperature Changes Changes in temperature have no significant load producing effects on self-supporting towers, but they can on guyed towers. Because of their differences in length, the guy cables expand and contract different amounts than the tower itself and thereby require elas-

Figure 2.2-6. Wind directions to be considered.

beyond safe limits indicating a potential failure condi-tion. Considering the usual fundamental periods of tall guyed towers, it appears that towers taller than 1,200 to 1,300 ft. should be investigated dynamically as well as statically.

There are two other phenomena related to the dy-namics of wind that are important in guyed tower design. These are aeolian vibrations andgalloping, both of which involve periodic loading.

Aeolian vibrations are low amplitude, high fre-quency movements, which occur in the tower guy cables due to a phenomenon known as vortex shedding.

If they are not suppressed through the use of dampers, they can result in destruction of the filaments in the tower lights at the least, or fatigue failure of guy cable and collapse of the tower at the worst. Dampers attached at one or both ends of the guy cables have proven effective in controlling these vibrations and should be considered for all tall, guyed towers.

Galloping is a condition of instability involving large amplitude, low frequency movements. It is caused by the perpetual amplifications of periodic loads due to the motion of the body itself. The most dramatic and well-known example of galloping is the collapse of the Tacoma Narrows suspension bridge in 1940.

For tower structures, galloping is usually associated with the guy cables on tall towers, but in at least one instance it was related to a large rectangular wave-guide. There have been several different methods involving detuning and energy dissipation used for preventing galloping in guy cables that appear to be successful. In the case of the rectangular waveguide, galloping was controlled by moving the waveguide inside the tower along the centroidal axis from its original position on the outside of one face. This re-duced the torsional rotation of the structure, which was the source of the perpetuating force. Based on this experience, it would appear prudent to always install this type of waveguide inside the tower unless adequate torsional rigidity is provided throughout the height of the tower.

Allowable Stresses & Safety Factors Towers, like all other structures, are designed so that the maximum anticipated stresses are less than those which would cause failure. This ratio of failure stress to maximum allowable stress is known as the safety factor. It is intended to provide for several varia-tions from the ideal conditions assumed for design including loads greater than anticipated, imperfections in materials and tolerances in fabrication and con-struction.

The ANSI/TIA/EIA-222-F-1996 Standard refers to the American Institute of Steel Construction (AISC) Specification for the Design, Fabrication and Erection of Structural Steel for Buildingsfor the design of the structure’s members and to the American Concrete Institute (ACI) Building Code Requirement for Rein-forced Concrete Structuresfor the design of the rein-forced concrete foundations and guy anchors.

National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH2x2 Page 211 function of several variables, including the geometric configuration, the mechanical properties and the sizes of the individual members.

Each guy cable also has a stiffness with respect to movement of its attachment point to the tower that is a function of the amount of initial tension, the magni-tude of ice load, and the magnitude and direction of wind load on the cables. By evaluating all of these, it is possible to simulate all the guys at a given level as a spring having a specific stiffness. Because of the nonlinearity of some of the relationships involved, the spring constant derived is only valid for a specific set of conditions and for a finite range of translation.

Similarly, a torsional spring constant can be derived.

It is interdependent with the translation stiffness and is also valid for only a finite range of translation.

Another difference between a guyed and self-sup-porting tower is the magnitude and significance of the axial load. For a self-supporting tower this is composed only of the gravity loads from the tower, its appurte-nances and any ice load. It is independent of wind load, and its effects on individual member loads are relatively small. The axial load for a guyed tower includes in addition to the gravity loads, the vertical components of the tensions in the various guys. Since these tensions are directly affected by the wind load, the axial load is now dependent upon wind load and its effects on the individual leg members are relatively large. Tension in the guy wires also produces an additional bending moment on the tower equal to the product of the axial load and the deflection of the tower.

Despite the complexity of the relationships in-volved, the availability and widespread use of com-puter systems permits accurate structural analysis of guyed towers. There are several different structural models that may be used.

One of the most commonly used idealizes the tower shaft as a continuous beam-column on nonlinear elastic supports (the guys) subjected to simultaneous trans-verse (wind and or seismic) and axial (dead, ice and vertical components of guy tensions) loads.

The modes of failure are buckling of individual leg or bracing members under compressive loads; rupture of bracing members, guys, or guy anchor arms under tensile loading; and shear or tension failures of the con-nections.

Dynamic Considerations As previously mentioned, even though wind and earthquakes involve kinetic energy, their effects are simulated by equivalent static loads determined in ac-cordance with the design standards. In recent years there have been more sophisticated efforts to investi-gate the actual response of tower structures to the dynamic aspects of wind gusts. A conclusion drawn from these studies is that the bending moments in the upper portions of tall, guyed towers are considerably higher than those determined by the usual static analy-sis. Consequently, the loads imposed on the vertical legs and their splice connections would be amplified

For towers under 700 ft in height the allowable stresses given in the AISC specification may be in-creased by one third. For towers 1,200 ft or taller, no increase is permitted. For towers between 700 and 1,200 ft, the amount of increase permitted is deter-mined by linear interpolation.

In a similar manner, the required reinforced concrete strength for towers under 700 ft in height must equal

1.3 times the calculated reactions and for towers 1,200 ft and taller, 1.7 times the calculated reactions. Linear interpolation between these two values is used for towers between 700 and 1,200 ft to determine the required strength.

The minimum safety factor for guy cables is 2 for towers under 700 ft in height and 2.5 for towers 1,200 ft and taller with linear interpolation applied between these two heights.

EFFECTS OF ANTENNAS AND

TRANSMISSION LINES

Except for AM radiators, the tower is the necessary evil to support the broadcast antennas and transmission lines at a suitable height above ground. Thus the effects of this equipment are of paramount importance.

Loads Every antenna imposes a wind load and a dead load on the tower. If the antenna is mounted atop the tower, it also imposes an overturning moment. If it is mounted on a side of the tower, the antenna imposes a torsional moment. For TV and FM broadcast and microwave antennas, these loads are relatively large, and their location has a significant effect on the placement of guy cables.

Transmission lines feeding the various antennas also impose wind and dead loads on the tower. These loads are distributed uniformly between the antenna and their entry point near the base of the tower. The total pro-duced by a coaxial line or waveguide is frequently greater than that produced by the antenna itself. The shape of the transmission line influences the magnitude of the wind load, with circular or elliptical lines having loads that are 60% of those for rectangular lines with the same projected area.

It is important not to overlook the support system required for transmission lines. Some large waveguides have support systems that require nearly continuous vertical structural members that add substantial wind and dead loads.