Tower Selection

bullet Selecting a Tower Supplier
bulletSite Selection
bulletDesign Options
bullet Environmental Factors
bulletTwist and Sway
bullet Loading
bulletAccessories
bulletErection

Foreword

Tower specification is created to help buyers evaluate and select all types of guyed and self-supporting towers and the options required to meet their long-term tower needs. Developed for all types of self-supporting and guyed towers, the information contained in Tower Specification can help in making valid tower value comparisons. It also provides important tower specification criteria and the environmental considerations critical to optimizing tower value and return on tower investment.

Note: Material contained in this book is general in nature. Be sure to check all local codes and regulations before you invest money or otherwise begin tower planning or construction.

Selecting a Tower Supplier

Most reputable tower manufacturers are able to provide towers which meet the performance requirements typically specified. In addition, most types of towers can deliver adequate performance if properly designed and erected. What the buyer must consider is the engineering behind the tower design, the long-term maintenance, the future use of the tower and the ease of working through the design-to-construction process. Therefore, it may be as important for the buyer to select the best supplier as it is to select the best tower type. The best tower manufacturers will be those who share the following attributes:

1. In-House Engineering Support. Look for a supplier who has the in-house capability to design a tower to meet the particular criteria of your location and usage. Beware of suppliers who sell ready-made catalog solutions to your tower needs. Tall towers, like buildings and other structures, must be individually designed to meet the unique loading requirements of each location and application. If there is a question whether a particular manufacturer is capable of providing you with the custom design you require, request a tower analysis with your quotation.

2. Professional, Registered Engineers. Be sure that professional, registered engineering in your state is available through the tower manufacturer. This certification assures you that the manufacturer has the credentials and capability to design a tower which will provide the required performance, and that the documentation will be timely and certified.

3. Tower Experience. As with nearly any industry, the best, most qualified suppliers are those who have been in the business the longest. Look for a supplier with at least 20 years' experience. Be sure to ask for a list of references, and call them for an unbiased evaluation of the supplier's ability to design, manufacture and service your new tower.

4. Responsiveness. Finally, look for a tower supplier who has the ability to quickly give you the right answers to your tower design questions, During the design construction process, prompt response to design changes and modifications of documentation can be the key to maintaining critical path scheduling. In the future, you may also require fast service or an opinion about the viability of a new application. A timely response to your tower questions can be a valuable benefit of selecting the right manufacturer.

Identifying Performance Criteria It is extremely important that the performance criteria be clearly defined and that future tower usage be anticipated in order to meet the long-term needs of all potential users. No matter how flexible the design, no tower manufacturer, consultant or installer can ensure our satisfaction unless the tower is properly specified. The following tips are presented as a guide to good tower specification practices.

Site Selection

It should be considered that site selection may require zoning board, planning commission, FAA and FCC approvals. The ideal site from the standpoint of the tower will be flat, level ground with good soil compaction qualities and a low water table. If a guyed tower is under consideration, the site should be of sufficient size to accommodate a guy radius, from the tower base to the outer anchor heads, of 70-80 percent of the overall tower height, plus 15-20 feet to accommodate the foundations.

Actual conditions, in fact, rarely meet all the above criteria. However, a reputable tower manufacturer will be able to help you work through the problems with a particular site by custom designing the tower, foundations and guy system. This special engineering should be accomplished swiftly and at no additional cost.

If the tower required is to be self-supporting and the land area is very small, the tower manufacturer should be able to custom engineer the structure to accommodate the antenna tower height, antenna loading and wind load requirements while maintaining a desired base size dimension.

Guyed or Self-Supporting
In most cases, the decision on whether a tower will be guyed or self-supporting will be dictated by the site selected. As a general rule, above 100 feet high, guyed towers are less expensive. As the height is increased, the differential in price will increase greatly. However, if the desired location is in a downtown or metropolitan area where land is very expensive, it may still make economical sense to consider a self-supporting tower.

 

Guyed Tower Land Requirements Guyed tower land requirement guidelines are based on tower height:
Guy radius = 70%
Side A = 125%
Side B = 110%
 


 

Guyed Towers: Most manufacturers suggest a standard outer guy radius of 70 percent (80 percent for microwave) of the overall tower height. At this radius, downloads on the tower structure from the guys are minimized resulting in the least-cost tower to meet a particular antenna and wind load requirement. As the guy radius is reduced below 70 percent, the downward pulling force of the guy system is increased as a result of the sharper angle of the guy relative to the structure. This increased downward force will result in the need for a stronger, more expensive tower and a heavier, more expensive guy system. If properly designed, shortening the guy system to 40-50 percent of tower height does not compromise the tower's ability to support the load intended or to meet the requirements of the EIA RS-222 code. However, tower and foundation costs will definitely be greater.

Self-Supporting Towers: Land areas required for self-supporting towers can vary by the tower design used by the various manufacturers. Generally, for the same application, pipe and angle-constructed towers are wider at the base than solid rod-type towers. This difference relates to the wind loading imparted on the structure itself. Solid rod towers have lower overall wind-catching surface area, thus reducing the loading of the structure and permitting a smaller base dimension. Before selecting a site for a self-supporting tower, it would be advisable to contact several tower manufacturers in order to establish the base size of the tower, considering the height and antenna loading requirements.

Caution: A common error in tower site selection is inadequate land availability. Soil type, land contour and even the effects of nearby structures or mountains may alter the true land requirements. The best approach is to consult an engineer with tower design expertise prior to land acquisition.

Design Options

Design options and their corresponding costs should be readily available to you from the tower manufacturer. A tower quotation package should contain:

bulletA list of all materials included in the quoted price
bulletLand area requirements
bulletTower drawings which summarize the tower components
bulletFoundation specification

The tower vendor should have the ability to supply a set of sealed analysis and prints for the attainment of zoning permits. A good tower manufacturer should be able to provide all this in two or three days.

Types of Construction
Solid steel rod, hollow round tube and angular steel are the most popular types of steel used to construct towers. Each material has advantages and disadvantages. These include variations in strength, durability, maintenance requirements and reactions to the environment. The following comparison of advantages and tradeoffs reveals that welded solid steel rod construction offers numerous advantages when compared to alternative types of construction.

 


Solid Steel Rod
 
 
Advantages: Disadvantages:
bulletLower drag coefficients than angular steel construction to minimize wind and ice loading
bulletHighest corrosion resistance for a longer predictable life
bulletSolid construction eliminates potential for corrosion from the inside out as may occur in hollow round tube construction
bulletIncreased material thickness compared to angular members
bulletIn-factory welded construction is superior from a maintenance and performance perspective and yields exceptionally easy in-field installation
bulletFactory-painted, easy-to-maintain appearance
bulletPre-fitted design allows faster, lower-cost erection
bulletMay be heavier than angular steel or hollow tube towers
bulletMay have slightly higher cost

Hollow Round Tube
 
 
Advantages: Disadvantages:
bulletMost efficient engineering use of steel
bulletEasy to paint, like solid steel rods
bulletDrain holes in tower legs tend to plug with debris, hold moisture and begin to rust from the inside out
bulletUsually field assembled (rather than factory assembled), increasing likelihood of discovering a poor fit in the field

Angular Steel
 
 
Advantages: Disadvantages:
bulletMost economical to manufacture
bulletMost economical to ship to tower site
bulletTowers are essentially manufactured in the field, so the quality of the tower is in the hands of the field installation crew
bulletDifficult to paint due to angular design
bulletRequires additional maintenance, such as retightening of the mechanical fasteners which hold the tower together to torque specifications
bulletMost difficult and costly erection
bulletAerodynamically inferior to hollow tube and solid steel rod on a component-by-component basis

 

Environmental Factors

Perhaps the most challenging aspect of tower specification is understanding the impact of variables in the environment which affect tower life and performance. The key factors are the wind and ice loading conditions which the tower must endure and the structure of the soil on which the tower stands.

A. Wind Loading and Tower Codes

It is extremely important that the tower consumers  understand and clearly state there requirements (to the tower vendor) relative to wind load and ice load. Ideally, from the user's and the tower manufacturer's standpoint, the referenced specification would be the latest revision of the Electronics Industry Association Structural Standards for Steel Antenna Tower and Antenna Supporting Structures - EIA/TIA-222-E.

The EIA/TIA code is uniformly accepted by the tower industry. Other codes may be referenced including the Uniform Building Code, the Standard Building Code, the Southern Building Code and the National Building Code, or the wind load requirements as defined by ANSI (American National Standards Institute).

Each of the above codes approaches the effect of wind and ice load on the structures and the guy system somewhat differently. Wind load may be stated in wind pressure (psf) or velocity (mph). Safety factors of tower members and guys are expressed differently, all of which result in confusion to the user. The current EIA/TA-222-E attempts to consolidate the codes and define the appropriate code requirements specifically for tower structures.

A former version of the tower code (EIA RS-222-"C"), which was in effect for over 10 years, expressed wind load in pounds per square foot of pressure as applied against the full height of the tower structure and guys. A U.S. map was supplied dividing the country using three wind zone categories (A, B and C). Up to 300 feet, a Zone A tower was to be rated with specified safety factors given at 30 psf, Zone B at 40 psf, and Zone C at 50 psf. Above 300 feet, the pressure was increased; and above 650 feet, the pressure was increased again. 

Most people think in terms of miles per hour (not psf) and a new code, EIA RS-222-D, was developed which had U.S. maps divided into  wind load areas defined in basic wind speed (mph) at a reference height of 33 feet above ground. The tower calculations for this code differed considerably from Version "C" and took into account the gradual increase in basic wind speed as a function of height above ground, closely approximating the actual conditions of the tower. Thus, direct correlations between converted wind speed in Version "C" and Version "D" cannot be made. The EIA/TIA-222-E code further defines tower design criteria and has been accepted by the American National Standards Institute (ANSI) and is now jointly controlled by the Electronics Industry Association (EIA) under the auspices of the Telecommunications Industry Association (TIA).

Using the EIA/TIA-222-E map or the county wind speed provided in the code will determine the minimum wind speed requirement for the tower site. If local conditions are known to exceed the specified wind speed, an appropriate increase should be discussed with the tower manufacturer at the time of quotation.

B. Ice Loading

All towers can withstand some ice accumulation. For years, the majority of towers have been specified and designed without formal consideration for ice loading. Ice can endanger tower survival if it is simultaneous with extremely high wind loads and/or if the ice accumulation on tower membranes and guy wires is excessive. Unusual, isolated ice storms have increased the diameter of a one-inch guy wire to five to six inches.

To factor in the proper ice loading consideration, the tower specifier should refer to the TIA-222-F Code appendix. The code requires ice load consideration but does not stipulate ice load amounts or wind velocity.

If the tower location is known to have a history for high wind and ice conditions, or is in an area where ice storms are likely, the tower specifier may wish to add an ice criteria to the design condition. Even if the area is not at high ice risk, the tower specifier may elect to include an ice consideration, thereby resulting in a more conservative design.

The TIA-222-F code provides for two options when ice is included in the design loading. One option takes into account that ice normally does not occur at full wind speeds and permits analyzing the tower with radial ice loading at 75 percent of the required wind load (translating to 87 percent of the wind velocity). The other option is to analyze the tower with radial ice simultaneously with the full wind velocity.

Depending on the tower height, antenna and line loading and wind speed specified, the addition of ice in the tower analysis can substantially change the tower design and may greatly increase the cost of the overall structure. The need to include ice in the tower analysis should be discussed with your tower vendor at the time of quoting to get a better definition of the effects on your particular tower requirements.

C. Soil Type and Structure

Soil type and structure of the subsoil can be a big factor affecting erection costs and possibly long-term suitability of a location. For taller towers (200-300 feet and up) it is essential to provide a geological analysis (soil report) for your tower manufacturer prior to tower design and quotation.

Some typical foundation types are illustrated on the following page with the tower type generally associated with them.

 

Soil reports typically include the following data:

bulletAllowable bearing capacity of the soil at the tower base
bulletThe allowable passive pressure at all boring locations
bulletThe depth that water was encountered
bulletThe blows/feet if a penetrometer is used
bulletThe visual classification of the soil and the associated depths
bulletThe dry unit weight and the buoyant unit weight
bulletThe foundation system which is best suited to the soil conditions encountered
bulletAny construction problems anticipated
bulletAny other variables which will affect the installation and design of the foundation system
bulletIf the soil is expected to be saturated at any time, it should be noted and the expected
bulletdepth of saturation given

To ensure an adequate soil report, be certain the number of borings and the depth of borings are sufficient for the type tower being considered.

 

Guyed Tower Foundations Typical "Dead man" Guy-Anchor and Base Foundation for Guyed Towers

Guyed Towers: Ideally a boring should be taken at each guy location as well as at the base of the tower. At a minimum one boring should be placed at the base location and at the outer guy anchor positions. If a large disparity in the soil conditions is encountered from one boring to another or if there is a large elevation difference from one position to the next, additional borings should be taken at all anchor locations.

At the guy anchor points, a boring depth of 10 to 20 feet is typically recommended. If the reactions are relatively small (20 kips or less), 10 feet is acceptable. But 15 feet borings are required at the base of the tower, unless unusual conditions exist. For example, a high water table, high organic soil content and recent disturbance of the top layers of soil due to excavation and fill may cause soil strength to diminish and mandate deeper borings.

 

Self-Supporting Tower Foundation Typical Self-Supporting Tower Foundations

Self-Supporting Towers: For self-supporting towers, the number of borings depends on the width of the tower face and the type foundation used. The chart below summarizes the general recommendations.

 

Foundation Face Width Borings Depth (see note)
Drilled pier * 6'-25' 1 25'+
Drilled pier *25+ 1 at each leg 25'+
Pier & pad *6'-25' 1 25'+
Pier & pad *25'+ 1 at each leg 25'+
Mat All 1 at each leg 25'+
* Not recommended for a width less than 6'-0"


 

Note: The type of foundation the geotechnical engineer recommends will influence the depth of the borings. A minimum boring depth of 25 is required. The depth should be varied if the recommended depth of 25 feet will not provide the required resistance. It is common to have a boring extend to a depth of 30 to 50 feet so that the drilled pier can be founded on a solid soil or rock layer.

A drilled pier foundation is usually best for towers with faces of 6 feet wide or larger providing the site may be economically reached by drilling equipment.

Twist and Sway

When towers are designed in full or in part for microwave loading, a second design criteria must be considered. In addition to the consideration of tower structural integrity, the twist and sway of the structure may affect the performance of the microwave transmission.

A microwave antenna transmits its signal within a limited beam width. This width is a characteristic of the antenna size and the signal frequency. Especially in the case of higher frequencies, excessive movement in the tower may result in signal loss. The EIA/TIA design code includes graphs which permit the determination of allowable tower twist and sway (in degrees) when the antenna size and frequency are known. The tower specfier may stipulate the required twist and sway either by stating the maximum allowable movement in degrees, or by stating the antenna size and frequency.

The wind load criteria for allowable twist and sway must also be stated. This performance wind loading is usually less than the wind and ice loading stipulated for the structural design. The "C" version of the EIA code recommended a minimum 20 psf wind load for twist and sway. The "F" version recommends a minimum 50 mph basic wind speed. As in the case of design loading, the tower specifier may wish to stipulate a higher criteria than these recommended. (top)

 

Loading

Transmission Lines The transmission line which runs up the tower to the antenna often represents a greater wind loading to the tower design than the antenna itself. When there are numerous transmission lines, their aggregate loading may be the most serious design consideration.

The loading of transmission lines may be theoretically reduced by assumed "bundling" of the lines. When transmission lines are clustered into larger bundles, their accumulated wind load is significantly reduced. If this practice is acceptable, the specifications should specify that "bundling" is permitted. However, bundling makes it more difficult to service lines or to replace individual lines at a future date; therefore, a cable support systems is often desired so that each individual line is installed separately on the tower. If this method is desired, the specification should state that bundling is not permitted, ensuring that the loads are not rationalized and the tower strength not compromised.

If the latter alternative is chosen, the specifier may wish to request a cable support system (cable ladder) so that the transmission line hardware (either bolt-on, snap-in or both) may be installed conveniently to the tower.

Future Loading

Effort should be made to define future loading at the time the tower is initially specified. While some post-construction modification is possible, the ultimate tower capacity is limited by consideration in the initial design. It is also more costly to modify a tower at a future date than to build the capacity into the tower at the time of original construction.

The tower specifier should consider all possible future owner uses of the tower. Future applications, microwave additions, auxiliary antennae and possible height extensions should be evaluated. In addition, consideration should be made to possible lease of tower space to other users.

It is sometimes assumed that specifying a heavier than necessary wind and/or ice design loading will provide sufficient tower strength to allow for future unknown antenna additions. While this is somewhat true, it is preferable to specify future antenna loading by number, height and type of the antennas. If an increase in tower height may be necessary, the ultimate tower height and ultimate antenna loading should be incorporated in the initial design.

 

Accessories

A. Lighting and Painting

The FAA determines the required aviation warning marking for each tower on a site-by-site basis. General guidelines exist as to when a tower will require marking (usually height 200 feet and over) and as to what type of marking will be required. FAA circulars are available describing the guideline marking requirements for each tower height. However, a specific FAA permit is required for each site and will stipulate the marking required.

Towers under 500 feet in height are most commonly marked with orange and white paint bands for day marking and red flashing beacons for night. Alternatively, towers in these heights are marked with medium-intensity strobe lighting, which utilizes flashing, omni-directional beacons that vary in their intensity during day and night conditions. The FAA may stipulate which type of lighting is required or may allow the owner his choice.

Taller towers may require either of the above lighting systems, or may require directional, high-intensity strobe lighting. When this lighting is specified, painting is usually not required.

In extreme cases, a dual lighting system may be required, incorporating strobe lighting for day marking and red lighting for night marking.

The tower specifier should not rely on the manufacturer for lighting recommendations; rather, lighting requirements should be defined through application to the FAA and should then be made part of the tower specification.

B. Lighting Alarms

When tower lighting is part of the specification, a lighting controller with remote sensing alarms may be requested. Such a controller offers dry contacts which can be incorporated with the site alarm system to indicate various modes of lighting failure.

C. Climbing

The tower needs to be easily climbed so that access to all antennas, lighting components, and structural members is practical for maintenance and inspection. Climbing may involve use of horizontal tower members, step bolts or climbing ladders. A separate ladder adds to the design loading on the tower and the initial cost and future maintenance. However, a separate ladder may be necessary if it is not practical to climb the structure using the horizontal members.

In the case of larger self-supporting towers, it is advisable to allow for a climbing provision on each of the tower legs. Since lighting fixtures and antennas may be distributed on any of the three tower legs, it must be practical to access each leg individually.

D. Safety Climb Devices

When possible, maintenance of the tower and antennas should be performed by professional tower personnel. Professional tower climbers rarely use safety climb devices. However, if it is necessary for others to climb the tower, a safety climb device may be desired. The safety climb device incorporates a steel cable or rigid rail which is installed on the climbing ladder or on the climbing face of the tower. A special climbing belt with sliding sleeve allows the climber to ascend the tower while constantly attached to the cable or rail.

E. Antenna Mounting Brackets

The tower specifier should detail any special requirements involved with antenna mounting. If multiple antennae are to be mounted at the same level, horizontal separation requirements should be included. The tower manufacturer may then choose from existing brackets and/or platform arrangements available, or may custom design an appropriate mounting system. In the case of cellular installations, various antenna mounting systems will involve wide differences in cost, wind loading on the tower and maintenance convenience. Once the number of antennae and spacing requirements have been identified, dialogue with the tower manufacturer can ensure that an appropriate system is selected.

F. Transmission Line Support

Transmission lines may be installed on any tower using available adapting hardware supplied by the transmission line manufacturer. This hardware allows the line hangers to be adapted to round members, angle members, etc. More systematic installation of the transmission line can be accomplished with a support system.

Such a system eliminates the need for adapting hardware, allowing the waveguide hangers (whether bolt-on or snap-in) to be installed directly on the tower. The number of transmission lines to be accommodated and the type of line hanging hardware to be used should be specified.

G. Transmission Line Bridge

The transmission line bridge supports and protects the lines for the horizontal run from the tower to the building entry ports. The length of the bridge, height above ground, number of transmission lines, width of building port and feasibility of intermediate supports should all be specified.

 

Erection

Typically, you will have the option of purchasing a turnkey installation where the manufacturer provides the erection crew or supervises the erection using a private erection company. The lowest cost method is usually for you to subcontract the erection on the project. If you have selected a tower from a reputable manufacturer, they will have the capability to respond quickly to your installation questions, and the need for turnkey installation service is greatly diminished. Typically, installation is 40 to 100 percent of tower cost. The taller the tower, the lower the erection cost as a percentage of tower cost.

Note: Choose a reputable erection company. Your selected tower vendor can supply a list of erectors. Ask for their references and call them to assure that the company has experience with the type of tower you have selected. It is a good idea to make this selection early, and let the tower erector assist you with your tower decision. Often they have erected towers from several manufacturers and can offer suggestions which may prove invaluable to your final installation.


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