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Tag Archives: guyed towers

Hope for Overstressed Guyed Towers and Their Owners

The load path transfer method is a new way of reinforcing overstressed towers, enabling carriers to upgrade antennas and electronics, where it formerly seemed impossible.

By Tom Swan

With today’s seemingly endless proliferation of new wireless spectrum, the prospects for wireless tower owners seem glowing. Local communities, on the other hand, are adopting more codes and regulations that oppose new tower site development. Although citizens crave the expansive use of wireless technology, they would prefer it if wireless didn’t require towers. For these reasons, the collocation of antennas on existing structures is strongly promoted.

A one-third scale model with orange members that represent load path transfer method (LPTM) augmentation.

A one-third scale model with orange members that represent load path transfer method augmentation.

Tower owners are happy to welcome new tenants and added rental revenue. But prior to approval of a tower collocation, best practices dictate that towers be meticulously mapped and a structural analysis be performed to assess the current loading of antennas, mounts and cables.

Available tower space can also be deceptively burdened by a carrier trend of adding many hundreds of pounds of tower-mounted equipment adjacent to antennas. This trend has significantly increased tower loading. More and more, the structural evaluation process concludes that a tower is already overstressed. That means the collocation of new carrier facilities cannot proceed.

Several factors have contributed to this unfortunate condition. On older towers, the process of reinforcement by attaching additional structural members has often done more harm than good. Holes were drilled and members were welded, weakening the structures and promoting corrosion. EIA/TIA safety standards for ice and wind loading have also been modified, making them more stringent. Prior to the introduction of PCS cellular technology, and more recently LTE cellular technology, towers were built to a minimal standard without forethought to advancing industry requirements. Towers needed to be taller and stronger, but budgets prevailed.

An LPTM internally inverted truncated cone base attachment.

An LPTM internally inverted truncated cone base attachment.

Moreover, tower properties have been repeatedly sold to a succession of buyers, and records haven’t always made the transition to new owners. Structural analysis requirements have sometimes been circumvented while defunct antennas, supports and cables have not been removed.

Drop and Swap

Tower suppliers have a simple solution to this problem: drop and swap. Tower steel suppliers would never hold it against the tower owner for undersizing its requirement for a previous tower. They will simply deliver new steel. It’s not a problem at all.

But, wait. What if the community prefers the view with the old tower missing? Will jurisdictions approve the erection of a new, larger tower? How long will tenants be off the air? Will state historic preservation officers want to talk about “invisibility?” How about National Environmental Policy Act considerations? What will the Federal Aviation Administration want to know? Will birdwatchers protest?

Structural Reinforcement

Through the years, various methods of structural reinforcement have been tried with mixed success. They include removing and replacing overstressed or corroded members, or building up existing members. Adding strength by welding split sleeves to legs is costly. Many contractors, citing safety concerns, have ceased offering that option.

One solution encloses the existing tower in another tower or a portion of a tower. This exterior tower wrap is expensive and usually requires the relocation of mounts and antennas, and the rerouting of cables and ladders. Often, expensive foundation modifications are also required.

A Tower Within a Tower

The load path transfer method (LPTM) for strengthening a tower is nearly self-defining. LPTM relieves the load from each tower section’s legs by combining and delivering those loads to the tower’s foundation using a pre-engineered internal tower structure that’s assembled on-site. Once installed, a patented turnbuckle function tensions the combined structures from top to bottom. Each LPTM augmentation is designed for a specific tower. There is no welding. There is no drilling.

In most cases, antennas, mounts, cables and ladders need not be moved. Equipment remains on the air during the entire process. Also, depending on the jurisdiction, most LPTM projects require little or no additional zoning or permitting.

It is unfortunate that many professionally designed tower structures are being unnecessarily replaced. Tower professionals who followed all the right steps find that with the changing of rules or the evolution of technology, the value of their good work is depreciated. Now it is possible to add significant life to older towers.

First, let’s consider whether a tower is a good candidate for LPTM.

Foundation, Anchors and Guys

Given a few criteria, a well-designed and maintained tower and anchor foundation system can typically be used in an altered tower application, possibly with new guy wires at existing or different positions.

The original design and the construction should be inspected and documented. It is important to thoroughly inspect the anchor foundations and verify that installation was completed in accordance with the design. Extremely important: What is the condition of the anchors? Guy wire and anchor hardware corrosion that has occurred because of a lack of cathodic protection can be a problem.

A turnkey LPTM installation.

A turnkey LPTM installation.

Installing LPTM Components

The tower must be sound and climbable for an LPTM augmentation to proceed. Ideally, the tower face width should be at least 36 inches, providing ample room for workers to install LPTM components. LPTM essentially becomes a tower within a tower.

Beginning the Process

The first step in determining the feasibility for an LPTM project is to map the tower and its foundation. The mapping process is a tower owner’s necessary tool and is the basis for the LPTM design and quotation process. The resulting information is entered into a proprietary database written specifically for the LPTM. Database reports are used to develop a scope of work, a list of materials and the individual design of necessary components. The output will also generate the estimated end result of the process. At this point, the tower owner will know how its tower will benefit from LPTM and will receive a close estimate of the project’s cost.

LPTM Augmentation Examples

Oriental, North Carolina: The project involved a 480-foot guyed tower. A structural evaluation determined the feasibility of Verizon and AT&T each adding LTE overlays. Considering the proposed additions, the structural analysis indicated a 222.8 percent structural loading. An LPTM retrofit was shown to provide a low-cost, time-saving solution.

Along with the custom-designed LPTM augmentation, the project included removing the top 120-foot secondary tower addition, replacing existing guy wires, and adding a torque arm and an additional guy level.

After adding LTE overlays, the result was a 360-foot tower with a load rating of 78 percent.

Hawthorne, Florida: Near Gainesville, Florida, the project in Hawthorne was a 40-year-old, 480-foot guyed tower on which AT&T wanted to add LTE overlay equipment. Mapping and structural analysis indicated that the result would be a 228 percent structural overload. LPTM augmentation was recommended.

The tower’s top 180-foot portion was removed. Workers performed rust abatement, applied cold galvanizing and removed two tower leg structural modifications that were previously installed.

After the AT&T overlay was installed, an updated structural analysis showed a load rating of 63 percent.

Conclusion

The load path transfer method is a proven solution. Its results have been well documented.  Tower owners now have a reasonable alternative to high-cost tower replacement.


Tom Swan is national sales manager at Hemphill in Tulsa, Oklahoma. Hemphill manufactures LPTM components. His email address is Send Mail

 tswan@hemphill.com. Keypoint Construction of Mandeville, Louisiana, is the sole licensee of LPTM technology. At Keypoint, information is available from Dick Huddleston, managing partner. His email address is Send Mail 

For more information, go to www.hemphillbts.com/Towers/index.html