All Content © 2011
KM Consulting Engineers, Inc.



Site by: Jason Rodriguez
 

Communities are looking for ways to reduce the number of new tower structures in their jurisdictions. As a result, newly adapted ordinances and local codes call for maximum use on existing towers. Co-location has become a popular method of decreasing the influx of new towers. But before a wireless carrier can co-locate on an existing tower, the tower must be analyzed for new antennas, mounts and cable loading. Often, when tower structures are analyzed using the "Structural Standard for Steel Antenna Tower and Antenna Supporting Structures" (EIA/TIA~222F) carriers discover that the tower is overstressed and cannot handle the new loads. This overloading is blocking many co-location efforts.

 

Causing Stress

There are several factors that contribute to overstressed towers. First, the EIA standard has changed over the years and has added more stringent load and safety factors. Many older towers just do not meet today's standard requirements. Before the surge in paging, cellular and PCS site demand, tower owners did not have any reason to spend additional money on stronger, taller structures. Owners ordered towers for current requirements with some moderate growth potential. Few tower owners envisioned the need for towers that would hold multicarrier equipment.

But even as tower procurement began, owners have continued to hamper tower structures by not taking the necessary steps to ensure their structures' capabilities.

Owners have added antennas, mounts and coax cable to structures without re-analyzing these extra loads. Often owners do not remove old inactive antennas and associated mounts and cable. Another problem is that tower ownership is frequently transferred without updated inventories, structural drawings and current analysis.

Now the damage is done, and the industry is discovering that a high percentage of existing towers fail the structural analysis when they are evaluated for new antennas and associated equipment. In fact, even when the analysis does not consider the new antennas and equipment, many towers fail the existing load evaluation.

 

Fixing the Problem

Despite this problem, there are limited options to solve overloading. The easiest method is to remove some of the existing sail area (antennas, mounts and cable) such as obsolete equipment. Before the tower is analyzed, the owner should inform the structural engineer that certain antennas will be obsolete and, therefore, the calculations should not include the sail area and dead weight of those antennas and related tower-mounted equipment (mounts and cable). Removing dead systems is a key factor in keeping the sail area of a tower in check.

After inactive systems have been cleared from the tower, you may want to reinforce the tower so that it can handle additional loads. You can do this by changing out overstressed members and replacing them with larger capacity members or building up existing members to increase their capacity.

Changing out horizontals and diagonals is relatively easy. You brace the tower with temporary supports, remove the undersized member and insert the new member. Alternatively, you can build up diagonals and horizontals by either welding or bolting new members onto the existing member. Welding is more time-consuming and costly because the galvanized existing member must be removed to allow for welding. After welding, the member will require cold galvanizing or other appropriate rust protection.

Changing out tower legs is not a popular option because it is a difficult process. The conventional method to reinforce legs is to build up the leg with longitudinally split pipe. This method of increasing the cross-sectional area of steel is time consuming and expensive. At each climbing lug, horizontal gusset plate and other tower leg attachment, the split section of pipe must be fitted or notched around the leg item. After fitting, the split section and the existing tower leg must be prepared for welding and then protected against rust. As with the diagonal/horizontal welding a qualified inspector should inspect the tower legs. This method of leg build-up is detailed and labor intensive. The required reinforcement cost often will outweigh the economics of the project.

 

A New Option

In addition to these traditional methods, there is a new alternative of leg reinforcement that is much easier and more cost-effective on hollow leg towers, the composite column construction method. When a tower leg is filled with structural non-shrinking grout, the fill is completely confined, which will increase the strength capacity of the leg. The structural grout reinforces the tubular steel section, thereby providing better stability and protection from local buckling. Because the steel tube provides the framework, you will not need reinforcement in the fill.

Historically, concrete or structural grout filled tube sections were used in constructing columns and caissons. Composite columns have been used for many years in multistory buildings. Researchers have conducted extensive testing on composite (steel/concrete) members. They have completed some of this testing on circular tubes filled with concrete and with no reinforcement. The research took into account both axial and bending loading.

Some research also has analyzed the effects of cyclic loading. These tests show that cyclic loading does not deteriorate bending strength. Using developed equations, researchers have recorded excellent agreement between calculated loads and experimental data. The design of grout reinforcement is based on equations that research papers developed.

In addition to the research documents, AISC addresses composite column design in its Manual of Steel Construction, Load Resistance Factor Design. KM Consulting Engineers, Inc. design criteria for reinforcing tower legs is based on the review of research documents, which were more conservative than the AISC guidelines. Tower leg reinforcement is developed by injecting structural grout into the legs. The structural formulas and method of injection have been developed to allow for a conservative, safe capacity increase while ensuring that the leg has a homogenous filling of specified structural non-shrink grout.

If you decide to use this method of reinforcement, it is important that you use an experienced grout contractor who has knowledge of pressure injection grouting and the proper equipment. During a pressure-injection sequence, an interruption of the filling process could result in a partially filled leg. Pressure monitoring, correct capacity of hoses and fittings, and an equipment break-down back-up plan are critical to the success of the filling operation. The structural non-shrink grout is just as important as the injection process. The flow ability, design strength, working time and set time must be designed, specified and adhered to during the construction process. You must take into consideration the temperatures of materials, ambient and pipe legs.

Before you actually inject grout into the leg cavity, you must work out materials, equipment, water and power calculations to avoid grouting interruptions. The capacity of the grout pump must meet the desired lift for a given viscosity. The entire operation must be monitored and recorded.

When this reinforcement method is properly engineered and executed by a qualified contractor, the result is an expeditious and efficient reinforcement of a tower leg system. A typical 180-foot tower can be reinforced in 12 to 16 working hours. The average grout (7,000psi to 9000psi) has a working time of 30 minutes and a set time of three to four hours.

The 16-hour reinforcement process includes the preparation time for the tower legs and the staging of equipment and materials. The actual grouting runs between 15 minutes to 30 minutes per leg for a 180-foot tower.

The addition of compressive strength to a tower leg varies with the leg material specifications, the unbraced length of leg and the diameter of the leg. A typical 6-inch diameter leg can gain approximately 60% additional capacity. As the diameter decreases the additional capacity rapidly decreases. For example, a 3-inch diameter leg gains approximately 25% additional capacity.

 

Saving Tool

Reinforcing tower legs to composite section columns is an economical alternative reinforcement method. However, this method alone will not save extremely overstressed tower legs or moderately overstressed slender legs. It will not cure overstressing in diagonals or horizontals, and it is applicable only to hollow leg towers. Yet, if a tower is experiencing moderate overstressing in leg compression and is considered a candidate for composite leg reinforcement, it is a tool that can save towers.



 
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