Tower Solutions: No More Guessing

Resistance to new tower construction has increased as community groups and municipalities adopt the "not-in-my-neighborhood" philosophy. Because of this resistance and the rising costs of new towers, providers need to maximize the use of towers already in service.

The usefulness of a tower to a service provider often hinges on the ability of the structure to support additional equipment. If a tower is structurally adequate, you can add more equipment to increase the tower's capability to meet demand. However, if the structure is not adequate, you will have to search for an alternate site. Usually, consulting engineers will perform tower analysis to determine if it can withstand additional equipment. To do this, an engineer needs a substantial amount of structural information, including how far the foundation penetrates into the ground, i.e. its depth or length.

Unfortunately, construction drawings that provide this information aren't available for many of today's towers. These construction plans have been misplaced and lost over the years. Therefore, engineers don't have the necessary data to make an accurate assessment of a tower's ability to accommodate increased loads. But thanks to a non-destructive testing technology called dispersive wave propagation, you don't have to play guessing games. Dispersive wave propagation has been researched, tested and successfully applied to problems with unknown foundation depths. This technology can help your engineers determine the usefulness of existing sites.

CRUCIAL KNOWLEDGE Placing additional equipment on existing towers increases the total surface area, which can bring the structure close to, or beyond, its original design limit. This increased surface area generates larger wind loads that increase the stress on the tower. If the tower is in an environment where ice accumulation is likely, then the surface area increases even more, and the tower will experience larger loads. The foundation of a tower must provide adequate resistance.

The depth to which a foundation penetrates into the ground and its condition are integral components in a complete structural analysis. These two factors determine whether a tower can resist the overturning forces generated by the superstructure. If there is not adequate penetration, then the foundation may not generate adequate resistance to prevent overturning. If penetration is sufficient but major fractures or material deterioration exist (e.g. rotted, driven timber piles), then the effective depth of the foundation can be reduced. In either case, the risk of structural failure rises.

Without accurate foundation data, engineers will have access only to local foundation design practices, local soil conditions and construction drawings from the tower manufacturer, which may show "suggested" foundation designs. You should be cautious when using these methods for determining foundation depths. If depths are underestimated, you may find a tower unacceptable, when in fact it has adequate support. Conversely, if penetrating depths are overestimated, you may deem a tower adequate and increase the risk of structural failure unknowingly.

Unknown foundation depths have plagued civil engineers for years. The North Carolina Department of Transportation (NCDOT) faced this problem with its existing bridges. As in many states, bridges were built so long ago that installation records and penetrating lengths for the piles no longer exist. The absence of this information led to a need for a non-destructive test to determine how far bridge foundation piles penetrated into the ground so that engineers could make complete structural evaluations. This information is critical when bridge evaluations are performed for structural stability and erosion (scour) potential.

This need for information prompted the principals of FDH to develop a technology to determine the depths of foundations that support bridges and other structures. This technology, known as dispersive wave propagation testing, encompassed nearly 10 years of research. In 1994, the NCDOT adopted the technology for use with timber piles. The technology now can be applied to timber, steel and concrete foundations. With the increased demand for tower evaluations and retrofits, engineers have adapted dispersive wave testing for use in the tower industry. This non-destructive test can determine foundation lengths and detect major defects or material deterioration. With this information now available, engineers don't have to guess penetrating depths. They can perform a complete tower analysis.

Engineering companies have incorporated dispersive wave propagation testing into tower evaluations for several wireless providers, including Sprint PCS and 360 Degrees Communications as well as MCI, CBS, the Minnesota Department of Transportation and various private tower owners. In the upper Midwest and the Southeast, foundations of both self-supporting and guyed towers have been tested. The technology is applicable to a variety of conditions, including concrete mat foundations, caisson (drilled shaft) and guyed towers.

DISPERSIVE WAVE PROPAGATION Dispersive wave propagation considers how mechanical properties and geometry (i.e. boundaries, discontinuities) of the material affect wave motion. Any blow (strike) to the surface of a solid creates a disturbance that propagates throughout the solid as a wave. Ideally, if the speed (velocity) of the wave and the time required for it to travel up and down a solid can be measured, the physical length of the solid can be calculated as the product of velocity and time. However, a wave in a solid material with bounded surfaces continuously changes its shape and elongates as time passes and does not have a single velocity. This phenomenon is known as dispersion.

Dispersive waves can be represented mathematically as the algebraic sum of many separate frequencies, each traveling at its own velocity. These different frequencies traveling at various speeds cause the dispersion phenomenon. As they travel through a solid, the frequencies reflect and refract from internal boundaries until they dissipate and eventually die out.

The unique feature of dispersive wave propagation is the ability to analyze dispersive waves effectively. Individual frequencies comprising a wave must be isolated and analyzed by Fourier- and non-Fourier-based digital signal analysis methods to determine their velocities and travel times. This allows a computation of the solid's total length. For tower foundations, frequencies are analyzed to determine the distance they have traveled from the foundation's top to its bottom surface.

There are a variety of other features that make dispersive wave propagation testing a versatile field test:

*You will not have to disrupt operations by taking a tower out of service.

*All equipment is waterproof (except for the digital oscilloscope) so that divers can conduct testing in underwater environments.

*If a tower is supported by driven piles, you don't need access to a pile's top (as with traditional pile-testing methodologies). You need to access only a small portion of a pile's side.

*You can test foundations whether they were recently installed or have been in place for many years.

CONDUCTING DISPERSIVE WAVE TESTS Equipment needed for the data generation and acquisition in dispersive wave testing is transported to the field easily and assembled quickly. Equipment items include a digital storage oscilloscope, piezoelectric accelerometers (henceforth "gages"), signal conditioners, power supplies and hand-held hammers for creating the dispersive waves.

To conduct tests, technicians temporarily mount the gages directly onto a foundation. Then they strike the foundation either on the side, or from the top, with a hammer to create dispersive waves that travel throughout its length. Striking the foundation on its side, as with a driven pile, generates bending (flexural) waves. Striking the foundation from the top, as in caissons or mats, creates a high-frequency dilatational (longitudinal) wave.

After the wave is created, component frequencies reflect from the bottom surfaces of the foundation, or from major defects or areas of material deterioration, and return to the point of impact. These reflections are recorded by the gages, and the data is digitized and stored on the portable oscilloscope. A computer using specially designed software completes a dispersive wave analysis on the stored data. The analysis yields the velocities of a selected group of frequencies and the time needed for them to travel the foundation's length. Then, engineers compute the depth of penetration, or location of a major fracture or area of material deterioration, to determine whether the tower's foundation can adequately support additional loads.

Dispersive wave testing requires an accessible foundation. For driven piles, this means a small portion of the piles' sides must be exposed in order to mount the equipment and perform the tests. If a tower is supported by a spread footing and driven piles, for example, you will need to excavate and expose the piles' sides. Similarly, if a tower is supported by other foundation types (caissons, mats), you will need to expose a small portion of the foundation's tops in order to perform the tests.

FINAL ANALYSIS Knowledge of foundation lengths and conditions is a prerequisite for predicting load capacity of telecommunications towers. Engineers must have accurate information to perform a complete tower analysis. When foundation lengths are unknown, dispersive wave propagation testing is a versatile tool for obtaining this information.

Engineering companies have applied this technology to tower foundations successfully. It provides critical information necessary for a complete tower analysis and avoids the risks associated with making uneducated guesses.