Tower augmentation
A radical technology to meet to meet radical changes within the wireless industry involves in-place tower enhancement.
The wireless industry is undergoing the most radical change that I have witnessed since becoming involved with towers 40 years ago. This change is forcing tower designers, fabricators and erectors to find innovative ways of meeting the demands that are now presenting themselves to the industry as the new wave of “opportunities.”
As if the challenges for the wireless industry were not enough, broadcasters are now busily engaged in finding new locations for their new digital television antennas. My company has received many requests for engineering analysis of existing structures as well as requests for quotations on new broadcast structures.
Although the engineering and fabrication requirements will represent an awesome expenditure of labor to meet the expected demands, these requirements can be accommodated. How to find enough workers to build these structures has also been a source of concern within the industry.
A radical way has been found to relieve the demand on labor to a considerable extent. Relief is achieved by way of a more efficient and less labor- and time-consuming process of in-place tower enhancement.
Having built tall towers, both self-supporting and guyed, for many years, I agree with others who have concluded that a shortage of experienced manpower may also exacerbate safety problems. Several years ago, we were called upon to construct in Cincinnati a self-supporting tower that was 954 feet in height. Because of a short project time line, at one point 85 tower climbers were working on the project. We quickly learned that a large number of men working together on a project can be a potential hazard because they lack experience working as a team.Despite the many manpower problems, solutions must be found that will help the operators meet their schedules for implementing their systems.
A patent-pending solution will greatly relieve many tower construction problems. Although this solution was developed to solve the needs of the wireless industry, it is also being used to relieve the tower problems of the broadcasters.
The problems associated with the broadcast industry are similar to those beginning to manifest themselves to the wireless industry. The growth of cellular and personal communications services (PCS) industries has nearly brought about a revolution within some cities, causing many local governments to declare a moratorium on new towers.
The demand for tower space has never been greater. However, analysis of existing towers reveals that most already are overloaded. The rush has begun to determine the structural status of numerous towers and, as a result, many owners are now looking for space on other new towers as well as existing towers.
An axiom in the tower business is “Towers attract antennas” and, owning several towers myself, I can attest to the truth of that statement. It also has been said, “Necessity is the mother of invention.” I know this to be true also. One tower in our inventory was located in a popular location and reached the point of overload quickly. In fact, the potential for continued loading was still growing, and we determined that something innovative needed to be done to bring the towerinto compliance with the Electronics Industries Association code requirements.
For many years, we have analyzed towers of all shapes, sizes and descriptions and have designed and implemented “fixes” to bring structures into code. However, we had never had a requirement where we needed to bring a structure to a capacity of four to five times its original loading.
At first, we could see no way in which this could be accomplished. The task seemed impossible, and we sought to build a new tower. We quickly learned that the land was not available, and even if it were to become available, the chance of obtaining a building permit was at best, “slim to none.”
Our next thought was to construct a new tower beside the existing tower. Our idea was again frustrated by the fact that the space was quite simply too small. The original tower was less than 50% guyed and it stood immediately adjacent to an interstate highway. The tower topped out at 500 feet, and as such, should it ever collapse, it could conceivably fall across the highway. The fact that guy lines would have to be interlaced only added to the complexity and increased the possibility of collapsing an already overloaded structure. Additionally_the tower being greatly loaded_the prospect of moving existing transmission lines and antennas onto an adjacent structure posed a staggering cost. This process was not only expensive, but it held a high potential for damage to antennas and transmission lines, especially those that had become weathered.
While all of the above options were considered and rejected for various reasons, the pressure was intensifying daily. The facts that the structure was already heavily loaded, and that new customers were clamoring daily to be allowed on the structure, only intensified the complexity of the problems.
For many years, my company has constructed its guyed and self-supporting towers with a “U-shaped” member. Because many towers are quite large, the practice of “nesting” several of these shapes into ever-increasing thicknesses of composite members solves any loading problem. For example, one proposed design of a 1,149-foot, self-supporting tower for the Far East resulted in leg sections 24″ 3 24″ 3 24″ by 3 inches thick. This required four layers of U-shapes nested and stitch-bolted together.
Suddenly, the solution became obvious. We would simply design the required “U” shape and place the formed leg member around the existing leg member in such a way as to construct “a tower around a tower,” as shown in Figure 1 on page 12.Once on the trail of this new solution, other factors began to fall rapidly into place. The new leg and lacing members could be placed in a manner that would shield the existing members from wind load. In so doing, the tower wind loading to be considered would be greatly reduced. The best part of the solution resulted from a structural benefit that I will refer to as the “Ad2” effect. All structural engineers will quickly recognize what I am referring to here; but, for those who practice in other disciplines I will describe the augmented benefits by saying that “The total of the structural benefits of the composite shape is much greater than the sum of the individual parts.” When one considers the composite load-carrying capacity of the “built-up” cross section (consisting of the old leg working in conjunction with the new leg), the resulting cross-section structural capacities increase significantly.
The actual benefit of this procedure is enhanced because the contribution to the composite cross section may be factored to allow for deficiencies in the existing leg member, due to deterioration. Many of our tower inspectors have found towers with pipe legs that have rusted from the inside, thus rendering them dangerously close to collapse. In cases like these, the existing leg is determined to contribute nothing to the strength of the cross section, and the new U-shaped member is increased in size or thickness to carry the extra load.
The old and new leg members are clamped together as frequently as needed to effect the transfer of forces to ensure that the legs will work together in the appropriate sharing of forces. However, the clamps are not brought to a final tightened condition until the tower is completely stacked. The new tower compresses, due to added weight, as the new tower is stacked. To avoid placing additional loading on the old tower, because of this shortening effect, the clamps are finally torqued only after the new steel is completely in place.
Because the objective is to completely shield all existing members with new members to minimize wind loading, the new steel is detailed with splice plates of differing gap lengths that vary the gap between successive vertical sections. The compressive effect mentioned above will always allow the proper alignment of the shear bracing of the new tower lacing steel to coincide with the existing tower lacing members, thus always maintaining proper shielding.
With respect to the clamping mechanism, although the lightweight clamp shown on Figure 1 is usually sufficient for small towers, the clamp that interconnects large members would require fabrication from heavy structural plate to apply sufficient clamping strength to effect the structural transfer.
In Figure 2, below left, the original tower is represented by the dotted lines. The broken circles forming the triangular cross section represent the existing tower. The smaller circles represent existing transmission lines. Because the new U-shaped legs must be open to fit around the flanged connection of the existing legs, the coaxial transmission lines will be moved out onto the face at these flange intersections. In most cases, the transmission lines are already on the tower faces, and the new tower steel is simply configured to pass over the transmission lines, which remain undisturbed. In some cases, it is desirable to configure the new augmentation steel much closer to the existing steel. This is accomplished by loosening the coaxial transmission lines from the existing tower (over a distance of approximately 40 feet above and below the panel where the new steel is being worked) and sliding the new lacing member in behind the coax. The new lacing steel is thus connected to the tower legs, and the coax is fastened to the new steel as work progresses upward.
Figure 2 also shows the means by which a large quantity of new coax is mounted to the tower. Because many of the towers to be augmented have narrow face widths, it has been necessary to use small bars that extend perpendicular to the tower face. This gains sufficient space to allow for the large quantity of coaxial lines to be deployed. The bars can be configured to use the popular snap-in coax mounting devices or any other mounting device requested by the customer. Because most waveguide is mounted inside broadcasting structures, the problem is eliminated in these cases.
On towers with small face-widths, a new climbing device is configured on the heel of the outside corner surface of the new U-shaped leg because, in most cases, the coax will be configured so closely and densely that climbing the tower by any other means will be impossible. (See Photo 2 on page 14.)
The “antenna stand off” mounting devices are configured to facilitate extending the existing antenna mounting structures at a sufficient distance from the tower face to clear the stack of new coaxial lines running up the faces of the tower. The new stand-off brackets are configured to allow the existing antenna mounting arm simply to be unbolted from the existing tower leg and swung outward from the tower face while suspended by a rope or a cable sling from above. The new tower steel is passed behind the mounting arm, and it is attached to its new stand-off bracket with the same hardware originally used. The new antennas to be mounted to the tower also use the antenna stand-off brackets to allow clear passage of the coaxial lines up the face of the tower.
The augmentation tower steel is stacked to an elevation about 2 to 5 feet below existing guy line elevation. At this point, plates A and B are installed along with the associated hardware and steel as shown in Figure 3 on page 16. The new augmenting guy lines are attached to the plates A and B. Once the guy lines A and B are attached to their new augmentation footings, the lines are tensioned to ensure the vertical alignment of the new construction to this elevation. Once this is done, the original existing guy line is removed from the original mounting bracket and placed on the new hardware provided below, as shown in Figure 3. This existing guy line is then retensioned on the existing footing, and the stacking process is continued.
Because poor soils usually require extreme measures to sufficiently anchor the guy lines, we have been conducting research with Foresight Products, Commerce City, CO, regarding its “Manta Ray” anchor system. We have developed a system of “ganging” several of these anchors together to develop fairly high load capacities. It is expected that for smaller towers, this system will function well in cohesive soils.
Summary There are many advantages to be realized by the augmentation system: * Towers that are badly deteriorated may be kept in service. * Payload capacities of existing towers can be increased by many times the original capacity. * In most cases, the customer does not have to be taken off the air, even for a very short duration. * In many cases, a building permit for augmentation is not required. * There is no requirement for additional land. * The method is simple and safe because no gin pole is required. The lift line is rigged from an appropriate point on the existing tower. * At no time is the original or new structure vulnerable. * Should an increase in height be required, a gin pole is installed as the process reaches the height of the original structure, and the process simply continues, with the new steel, to the desired height. * A computer model established the condition of the augmented tower. This model can be used to monitor and maintain future management of the structure. * The overall cost of augmentation is usually considerably less than going through the process of land acquisition, permitting, construction and landscaping for a new structure. * The augmentation is done in a manner to allow for future “augmentation of the augmentation.” * Only a portion of an affected structure needs be augmented to achieve a desired elevation. * Dual guying provides a system of redundancy that allows safe simple future maintenance of both tower and footings. * A simple system of obtaining field data allows an accurate computer model to be configured. * Erectors may work directly with their customers in selling and installing the tower augmentation kits. * Should erectors desire, training can be provided for their crews, including time-saving techniques for kit installations.
With continual auctioning of spectrum by the FCC, mobile telephony companies and investors have new opportunities to provide wireless services to the general public.
To facilitate the installation of wireless systems, carriers must find suitable structures to support the antennas and equipment necessary to operate a wireless system. As a result, new communications towers are being constructed across the country at a tremendous rate. Tower proliferation is becoming an increasing concern to municipalities and to the general public.
Municipalities can benefit from proper handling of this situation. The use of consultants to the wireless industry for radio frequency (RF) engineering design and program management (PM) of systems implementation can be a proactive step in assisting municipalities in facilitating the planning and construction of new towers within their jurisdiction. Assistance can be provided through education, feasibility studies, tower development, tower management and financing. Several steps can be used in implementing a program for municipal jurisdictions.
The planning and feasibility process begins with the municipality identifying available parcels of land suitable for new tower construction. For example, an RF engineering team can use propagation tools and measurement test equipment to validate the site locations and to analyze existing carriers’ signal strengths. Site locations meeting RF parameters can be reviewed for capital requirements. This cost assessment would be conducted through a detailed analysis of soils conditions, tower capacity requirements, structure type, and wind loading requirements. If the feasibility proves positive, there are several options for the municipality, ranging from consulting services for municipal tower ownership, working with regional carriers to build on selected location, or building and managing each tower location. At this point consultants typically engage the municipality contractually to lease municipal lands for tower ownership.
There are numerous benefits to the municipality for strategic tower deployment planning. Proactive movement by the municipality will facilitate the implementation of wireless service and satisfy the general public as a whole with advanced wireless services and specific tower placement. This process will reduce or completely eliminate tower proliferation. Although not any one portfolio of towers will satisfy all wireless designs, this process will cover more than 50%. Strategic tower placement will also generate revenue for municipalities. As consideration for land usage, tenants will pay rent on each parcel of land leased. In addition, arrangements can be made for local government installations for fire and police department wireless dispatch. In some cases, the consultants can be retained to manage the towers. Tower management includes facilitating co-location rights, interference protection, monitoring and controlling site access, maintenance, and negotiating with future tenants.
This type of arrangement benefits not only the municipality but the wireless service providers as well. With competition increasing for wireless services, speed to market is critical. One common delay in system implementation is zoning resolution. Jurisdictional approval for new towers can delay a site for more than six months_sometimes indefinitely. Typically, in most jurisdictions, co-location on existing towers does not require jurisdictional approval and reduces overall timelines. Also, if the carrier is aware of the existing tower, it will incorporate the site in its initial design as an anchor tenant. This process provides carriers speed to market and favorable reception by the local government and the public. Otherwise, disputes can arise from the public that put both the municipality and the carrier in an awkward position.
A well-managed municipal wireless deployment strategy helps governments develop new services within the community, and generate revenue for the jurisdiction.
Nichols and Schneider are vice president general manager, respectively, for program development with LCC International, McLean, VA, which provides RF engineering and consulting services.
