Bracing for a Storm

Are new wind-loading standards in our future?

What good is your new technology and bundled plans if Mother Nature gives your tower a whack upside the head?

Problems faced by tower operators and carriers vary with the geography they intend to serve: Western fires, blizzards triggered by Nor'easters, Midwestern summer storms, East Coast hurricanes.

A storm's direct hit could topple anything in its path, no matter how secure the tower. But what can you do to minimize the damage?

Standards - primarily TIA/EIA-222 version F - require towers to withstand different wind speeds depending on their geographic locations, although the majority of inland U.S. towers only must sustain wind speeds of up to 80 mph. But when Hurricane Floyd buffeted the Carolina coast in 1999 with 100-mph winds, there was cause to reconsider the standards, industry experts say. Now some are thinking that version G - in the form of higher national wind-loading requirements - may be just down the road.

For towers that would not meet new standards for wind durability, additional support may have to be added. For carriers, the trend toward tower consolidation means overall construction will be the tower owner's responsibility. But beware: Any additional construction costs are likely to go back to carriers in the form of higher rents. So it pays to have an active hand in tower planning, whether or not your firm actually builds the structure.

Many believe a new version of 222 requirements would standardize assumptions on loading for wind, ice and other conditions, based on geography.

"What has to be understood is that all engineering guidelines leave room for engineering judgment," said Dave Snyder, Crown Castle CTO. "In the case of 222-F, there was ample room for an engineer to take either aggressive or conservative assumptions on things like maximum ice loading, wind conditions, and other parameters, which directly affect the allowable load on a structure."

If an engineering firm or tower manufacturer had been very aggressive in its assumptions when applying 222-F, the new version would tend to represent an increase relative to the previous design assumptions, Snyder said.

"If the firms were being conservative, the new version could represent a "decrease" relative to the previous assumptions," Snyder said. "Additionally, whether the new standard will increase or decrease the wind-loading requirements will be a very location-specific thing."

In many cases, local municipalities already require coastal towers to exceed TIA/EIA specs, due to higher incidents of hurricane-strength winds. For instance, Crown Castle has towers located in Puerto Rico, which have been subjected to hurricane-force winds. Towers in such areas must have larger and stouter foundation designs and larger cross members and other structural components not necessary in less vulnerable areas.

"The maximum specification for a tower is very location and design-load specific," Snyder said. "It also needs to be understood that the maximum wind speed a tower can withstand is a direct function of the load on a tower."

Hurricanes: Biggest Threat Hurricanes may represent the most direct large-scale threat to tower operators and carriers in population-heavy coastal regions. U.S. Cellular has towers in many such regions, including Florida, North Carolina and south Texas. Here, towers typically have more cross bracing, thicker steel, and thicker guy wires, said Ragon Linde, U.S. Cellular director of engineering.

"As a carrier, we buy what is required to not only meet but to try to exceed the required building codes," Linde said.

During hurricanes, experts say freestanding and support towers are better engineered to handle sustained winds. The lattice design of both types of towers allows a single-directional wind to pass through more easily. Because there usually are more carriers co-located on such towers, the steel must be thicker to support a higher loading requirement. This may help the tower maintain its foundation hold.

Aerodynamics also can play a role in whether a tower survives a storm. Shorter monopole design, with a tubular design and fewer connecting pieces, can make towers less vulnerable to high winds, such as those from tornadoes - though even the most heavily wind-girded tower likely won't survive a twister's worst.

"There isn't much that will withstand a direct hit from an F5 tornado," Linde said. "Many municipalities require a letter from the tower manufacturer stating that the tower is designed to collapse on itself."

Ice Is Not Nice Wind speed isn't the only factor needing consideration before disaster strikes. Towers in northern states must handle ice loading along with high winds.

Another factor: temporary loads, such as when tower riggers perform maintenance work. The assumption, and indeed the requirement for tower maintenance, is that the wind condition is either low or moderate, Snyder said.

"There was a tragic incident in Texas a few years back when a large broadcast tower collapsed with several tower riggers on the tower, which illustrates why they need to be adequately considered," he said.

A tower becomes overloaded when all the forces on the tower exceed the breaking point of the tower's weakest member. Twist and sway can cause paths to blow off alignment, leading to overstress on the cross members, legs and guy wires.

Tower operators also must make sure their sites aren't overloaded. All sites - not just those in disaster-prone areas - should get regular inspections to ensure proper loading. (One legal note: It's a good idea to get the tower's engineering drawings when investigating its loading capacity. Even for carriers that don't own their own towers, getting a copy of the tower's drawings is the only way to get insurance on your equipment, important in disaster-prone areas. Also, if the tower fails due to overloading - and you're the last carrier to co-locate on it - liability is possible.)

Some things that carriers should keep in mind about wind loading:

1 The ultimate wind speed that a tower can withstand is a function of its original design, current load, and quality of construction.

2 The allowable design load on a tower involves a good deal of engineering judgment and assumptions. One engineer may consider a tower overloaded, while another may not, depending on their assumptions and risk-management philosophy.

3 Monitor the effect of microwave dishes. Mounting them at different levels changes the loading/tower beef-up requirements.

4 The load rating of a tower is dependent on the structural characteristics of the tower and the foundation, not just the tower itself.

5 A tower that is rated to handle 10 antennas in one location may be rated to handle fewer in another spot due to the change in maximum design wind speed and other assumptions.

6 Soil conditions affect the design of the tower foundation and the tower's loading capabilities, both initially and for the future.

7 It's important to keep records of the foundation and tower design for future loading analyses of the tower.