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WATER STORAGE TANK ANTENNA INSTALLATION CONSIDERATIONS by: Ira M. Gabin, P.E., Vice President, Dixon Engineering |
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The rapid expansion of wireless communication services throughout the
United States has resulted in the construction of many cellular antenna
towers. To save on the cost of erecting these towers, communications
companies look for existing structures that are suitable for locating
their antennas. Also, in some areas zoning restrictions have severely
limited the ability of cellular companies to locate their towers. For
these reasons, water storage tanks are prime sites for antennas.
Existing tanks are often the highest structure in a community and
obviously do not need FAA and zoning approvals.
Leasing revenue from antenna installations has been a welcome development for hard pressed water utility budgets. Leasing rates range from a few hundred dollars to over 1000 dollars per month depending on the location and suitability of the storage tank. Since tanks usually have room for multiple antenna installations, leasing revenue can be doubled, tripled, or more by adding additional cellular carriers. In some cases, it is possible to pay for all future tank maintenance and painting with these revenues. While the income provided to water utilities from placement of antennas is certainly worth considering, care must be taken to avoid the adverse effects of these installations. Many problems have occurred from antenna installations that were improperly designed and constructed. Many installers have viewed the tank as simply a platform for their antennas, without understanding the important function the tank serves and the purpose of tank appurtenances. Problem areas include:
Structural damage can occur from improper reinforcement of basebell and column penetrations and roof attachments. Since the steel plate in most tank roofs is fairly thin, heavy antenna frames can buckle the plates if not properly attached and reinforced. In extreme cases, damage to the tank structure and foundations can occur from poor design. Coating damage occurs from welding operations. While exterior paint is usually repaired, the interior paint which has also been burned is usually ignored. Severe corrosion damage can occur on the wet interior of tanks if the affected areas are not power tool cleaned and recoated with a fast curing coating. Coatings are also subject to damage from cable bracket clamps and set screws that move and abrade the paint. The dissimilar metals between the clamp material and the tank steel can also establish an active corrosion cell at these locations. Cable routing is a major problem with many installations. Installers often route the cables in a manner that is the easiest for them to install. This frequently causes access restrictions to hatches, ladders, and manholes. OSHA violations for confined space access and ladder standards are common. The cables are also often installed so that they are difficult or impossible to paint around when exterior or dry interior repainting is required. This increases costs to the water utility as painting contractors must factor cable removal and reinstallation into their bids. Contamination of the water supply is a major concern. Cable roof penetrations are often improperly sealed or in some cases left completely open. Roof vents have also been used as penetrations with their screens removed and cables routed through them. It is also common for roof hatches to be left open after work is completed. Aesthetics are another important consideration. A wide range of mounting designs are available which should be reviewed for appearance by the utility. The routing of the cables on the exterior of legged tanks should be selected on a side of the tank that is least visible. Damaged exterior paint should be properly repaired and color matched. Horizontal cables laid flat on the roof should be avoided as they act as dams for bird droppings. When a heavy rainfall causes the accumulated material to flow over the cables, it can stain the exterior of the tank and can also run through a nearby roof opening such as a misaligned cathodic cap and cause contamination. Vandalism is another important consideration. Cables attached to legged tanks should have vandal guards to prevent climbers from using them to access the tank to express their deepest thoughts with spray paint. A few basic steps should be followed by storage tank owners to avoid these problems. First, the cellular company should be required to provide a drawing of its complete installation including site utilities, ground structures, equipment panels, cable routing, and antenna structures. Secondly, a qualified engineering firm experienced with both storage tanks and antenna installations should be retained to inspect the tank, and review the drawings, welding procedures and coating repair specifications. Structural analysis should be performed to ensure that the tank can safely support the antennas. Also, care must be taken that the new installation does not interfere with existing cellular or utility antennas. Only after all necessary drawing and specification revisions are completed should the installer be allowed to begin. Keys to the tank or tank site should be returned when the project is finished. It is not recommended that antenna companies be allowed to access the tank to service their equipment without utility personnel providing authorization and access. The final step is inspection of the completed installation. The same firm that reviewed the drawings should inspect the entire installation including the interior paint repairs. In some cases paint repairs can only be completed during low demand times of the year. In other cases the repairs must wait for warmer weather. This will require coordination with the cellular company to ensure that the interior paint is properly repaired. When negotiating a lease with the cellular company, the design review and inspection services should be included in the contract at the company’s expense. Most cellular companies are very cooperative in including these services in the lease. It is a minimal expense and also provides them with the assurance that their equipment will be properly installed. A number of other legal and financial considerations should be carefully evaluated including length of contract, cost of living adjustments, access, liability, exclusivity clauses, and future painting expenses. Since many of your neighboring utilities may already have antenna installations, it would be worthwhile to inquire about their leases and hopefully benefit from their experience. Water storage tank antennas are certainly worth investigating. By following these steps you should be able to obtain the benefit of this new source of revenue while avoiding the problems that can be caused by deficient antenna design and installation. |
