Is it a water tank or an antenna?
Meeting the challenges of telecom antenna installations on municipal reservoirs
Daniel J. Zienty, Associate/Project Design Leader, Short Elliott Hendrickson Inc., St. Paul, Minnesota
Tom Struve, Manager of Street Maintenance, Equipment Maintenance and Central Operations, City of Eagan, Minnesota
Presenters, 2005 APWA Congress
With nearly 160 million cellular subscribers nationwide, public water storage facilities have become a popular location for installing antennas and associated ground equipment. This option has helped limit the construction of stand-alone towers, and the revenues that the telecommunications systems generate are a plus for local communities.
|Eagan's Yankee Doodle Reservoir|
The City of Eagan, Minnesota, is a second-ring suburb of Minneapolis/St. Paul that is successfully overcoming many of the challenges facing municipalities today as partners with public and private telecommunications carriers. Strict zoning provisions for communications sites in Eagan has limited the erection of monopoles, resulting in 21 current site leases on Eagan's six reservoirs and elevated water storage tanks.
This approach by the City posed numerous challenges since most of these facilities were not originally designed for routing cables and attaching antennas. Operator maintenance and safety issues were a main consideration in our assessment of current and future antenna sites. On the bright side, as new carriers entered the market or existing tenants performed equipment upgrades, new design opportunities presented themselves to lessen conflicts and accommodate the needs of both the City of Eagan and our new tenants.
This article details some of the challenges, issues and lessons learned as Eagan's water tanks evolved to accommodate telecommunication installations.
The City of Eagan signed its first tenant lease in 1990 for equipment placement on its 2 MG Safari Reservoir. This tank is one of two ideally located adjacent to a major freeway (Interstate 35). In the 15 years since, standardization has become a key element in reducing staff and engineering time allocated to taking this type of project from agreement to construction.
To address issues specific to construction, the City effectively outsources services related to plan review and inspection to two local engineering firms with whom they have established excellent working relationships. Initial radio frequency (RF) reviews (intermodulation studies) to identify potential interferences between existing and future tenants, as well as making sure that City emergency and public works communications is not compromised, are provided by Jeff Nelson of PSC Alliance. Review of the physical concept and final construction plans and installation inspection are performed by Dan Zienty, Associate/Project Design Leader for Short Elliott Hendrickson Inc. (SEH) in St. Paul, Minnesota.
Each project in Eagan begins with the completion of an Antenna Site Application Form (available electronically). Potential tenants are provided with an established fee structure based on the number of antennas and a standard ground footprint size. The next step typically includes a site walkthrough with City representatives and engineering personnel representing the City to handle construction questions. Agreement at that time should be reached on:
In the case of elevated storage, if the cabinets are to be located in the tank's interior, space must exist for both equipment and future operational maintenance activities. Considerations are made to accommodate future antennas and ground space needs.
Eagan's Deerwood Reservoir
If equipment is located outside the tank, consideration must be given to how placement will affect the mobilization of maintenance equipment. For example, tank repainting may necessitate the erection of a full containment (enclosure) system to limit dust emissions and paint drift. Enclosures may require the temporary installation of special outriggers that extend over the tank bowl or drip-line. Antenna-related equipment located inside this area could be damaged by exposure to excessive dust, or overheating from protective covers. Further, ground equipment that straddles this may interfere with anchor placement and daily operation of the containment system.
In Eagan, once the footprint of the lease space is established, a series of lease negotiations with the potential tenant's lease representative begins. During this design stage, RF information is requested and reviewed. City officials working together with City attorneys and the applicant then come to terms with the City's standard leasing arrangements and, following agreement, the lease, with exhibits detailing the leased space description and equipment location on the tank, is approved by the City Council. The next step is to agree on and approve construction plans with assistance from the City's construction consultant, followed by a Permit to Proceed issued by the City. Through careful preplanning, the City has effectively established a mechanism which permits both commercial collocation and shared use of the City's reservoirs for essential government wireless communications.
The City's engineering consultant, Dan Zienty with SEH, provides oversight of the construction plan approval and provides inspection or construction observation. Observation is important in protecting the City's interests throughout the process, with the pre-agreed-upon, not-to-exceed dollar amount based on the planned level of construction, quality of subcontractors and type of installation (new or upgrade).
Consideration of underground utility routing should include both electrical and telephone service lines and coaxial cables. The plan should indicate whether proposed lines cross existing municipal utilities. Further, based on tank design or location, the plan should indicate which method works best in routing antenna coax lines:
Finally, will the location of the utility trench allow for future use by other tenants?
The importance in routing coaxial cables cannot be stressed enough. Poor planning can produce major problems with respect to aesthetics, future facility maintenance, access and safety.
With standpipes and ground storage reservoirs, cables can be grouped at a single location on the shell wall, closest to the providers' ground equipment. Once the cables reach the center of the tank roof, cables can then be routed to the required antenna sectors. Clearance (approximately four inches or more) from the wall and roof of the tank is an important maintenance consideration. This can be accomplished with predesigned, off-the-shelf, and prepainted mounting brackets. These brackets can be used with additional components to provide for suitable alignment of multiple cables.
Eagan's Sperry Reservoir
In applications for legged tanks where antennas are mounted to a catwalk or horizontal struts, associated cables can be attached directly underneath the catwalk or the backside of struts, and along the backside of the support column to reduce visibility. Where exterior coax cable placement is used, multiple cables from numerous providers can make maintenance painting difficult as cables need to be wrapped to prevent possible damage from the coating removal operation. To combat this problem, Eagan now requires the installation of cables with manufactured white vinyl jackets (matching their tanks) on new installs and site upgrades.
Pedestal and fluted-column style tanks provide another challenge, as well as perhaps the easiest solution. The greatest difficulty in this situation arises when the antenna is placed on the roof for maximum elevation, which requires cables to be routed through the dry portion of the access tube. Safe access during cleaning and normal maintenance is a main consideration. Space limitations extend to the tank roof, where clearance must be maintained around the roof vent and other openings. If cables are not routed uniformly, tripping hazards can result.
Eagan's Southern Lakes and Sperry tanks illustrate that cable routing is easier, safer and cleaner when antennas can be mounted on a tank's exterior riser or column. Here, cables can be grouped using standard support components to create a cable tray, which can be attached using stud welds or clamped to the lip of the interior riser or flute stiffener ring.
Location of penetrations
In the case of elevated storage tanks, penetration placement on the main tank structure may require special design considerations to maintain structural integrity. Placement of penetrations that affect the tank's base, condensate ceiling or access tube should also be evaluated for conflicts that can prevent proper seal or bend radius.
Location of antennas
The antenna location (elevation and azimuth) is generally set by the leasing tenant to service a given area. Proper spacing is essential as new equipment must not interfere with existing or emergency equipment. As previously stated, Eagan's RF engineer also researches current licensing and band locations, and independently conducts intermodulation studies to obtain interference information in collocation situation. Eagan's approach has proven effective to preserve the appropriate vertical reservoir "real estate" for both government communications needs while also serving the commercial carriers. This strategy has maximized use of existing tall structures and limited the zoning and regulatory impact of new towers.
Antenna attachment is always a challenge. In Eagan, welding is used to install service line and coax penetrations, coax or antenna mounting brackets, and structural supports in certain situations. Whether shielded metal arc welding or stud welding, the process can have a negative impact on the tank's existing paint system. Where possible, stud welding is less detrimental. Bolting is an even better alternative where applicable. For smaller penetrations for single or smaller diameter cables, holes can be drilled and a nonferrous threaded coupling or firewall grommet can be installed.
Planning should include the substituting of non-corrosive materials to reduce the painting of exterior components. Eagan, in its planning, follows this practice as well as design applications that maximize environmentally controlled shop painting.
Even with the addition of telecommunications equipment, these are still water storage tanks, and the City of Eagan is responsible for the protection of their contents and for the protection of City staff in maintaining them. Safe access for all necessary personnel must be maintained in compliance with applicable OSHA guidelines for all employees. The City of Eagan contracted with its RF engineering consultant, Jeff Nelson at PSC Alliance, to provide required RF exposure training for employees including use of portable RF exposure meters so that exposure during site visits could be monitored and documented.
Tenant access is another issue, and with increased awareness in site vulnerability nationwide, control of site access has taken on greater significance. All of Eagan's tanks have been retrofitted with new security measures/locks, and more rigorous control (sign-in/out of access keys) is in place for contractor visits.
We are a society constantly in touch, in need of up-to-the-moment information. Wireless technology provides the City of Eagan, its citizens, businesses and public agencies with essential communications services. As with all forms of technology, advances will come, providing better service with less visual and physical impact to public structures. Understanding the importance in upfront planning will help ensure that Eagan's water tanks continue to pull double duty as telecommunications towers, while serving their original purpose of providing quality water storage.
The authors will give a presentation on this topic at the 2005 APWA Congress in Minneapolis. Their session is entitled "Are They Water Storage Tanks or Antennas?" and takes place on Monday, September 12, at 10:00 a.m. Dan Zienty can be reached at (800) 325-2055 or firstname.lastname@example.org; Tom Struve can be reached at (651) 675-5300 or email@example.com.