Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-01-11
2001-11-06
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06311565
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to techniques and equipment for assessing integrity of anchor rods such as those used to support transmission towers and more particularly to methods and associated apparatus for making these assessments without damaging the load-bearing capability of the rods or excavating the terrain in which they may be implanted.
BACKGROUND OF THE INVENTION
The boom in cellular telephone usage and other forms of wireless communication has dramatically increased the number of transmission towers employed today. These towers frequently are guyed, with multiple elongated wire braces spaced about the base of a tower supporting and assisting in counteracting the effects of upper-level winds on the position and integrity of the tower. Several wire braces may terminate in a device such as an equalizer plate, to which a steel rod, typically (although not necessarily) ten to fourteen feet long, is connected at one end. In such situations the other end of the rod may be embedded in concrete and implanted in the ground, thus serving to anchor its associated wire braces and thereby moor the tower.
These anchor rods may, of course, have flaws existing as a consequence of their formation. The acts of connecting an end to an equalizer plate, embedding the opposite end in concrete, and burying a rod partially underground may also create cracks or other flaws in the rod. Because subjected to concrete, soil, and groundwater in use, the anchor rods—even if initially without flaws—further may corrode over time. Wind-related vibrations of the rods additionally cause wallows, or depressions circumscribing their circumferences, in the ground beginning at their entry points therein, so that potentially-harmful surface water may traverse the entire subterranean lengths of the rods.
As the absolute number of guyed structures increases, so too does the number of failures of rods used to anchor the guy wires. According to the December 1995 issue of
Cellular Business
magazine, in the preceding five years approximately nine guyed towers failed because of anchor corrosion. See K. Boney, “Getting a Grip on Towers,”
Cellular Business,
December 1995, pp. 42, 46, 48, 52. A study commenced after an injury-causing tower collapse in 1990
found that 50% to 75% of (anchors) show some signs of corrosion . . . Of that 50% to 75%, maybe 5% to 10% have been corroded to the point that their structural integrity is in question.
Because anchors are built to support specific capacity load, any corrosion also leaves the anchor overstressed. If a 1-inch round steel anchor shaft is corroded to 0.9″ or 0.95″ instead of its full-inch design, the structural integrity of that anchor is compromised.
Id. at p. 42.
Historically, structural integrity of buried anchor rods was not routinely assessed. As described in Cellular Business, “buried anchors used to be ‘out of sight, out of mind.’ People didn't see them, so they didn't worry about them.” Id. When assessment was necessary, the conventional technique for doing so involved excavating the soil in which a rod was set in order to examine some or all of its length visually. of course, excavating the soil about a rod decreases its load-bearing capability, destabilizing the associated tower until the soil can be replaced. Contacting a galvanized rod with metallic digging equipment, for example, might also damage the rod, and disturbing carcinogenic tars coated on rods might preclude replacement of the soil under existing environmental rules and regulations. Further, of course, mere visual examination of a rod may not result in detection of myriad types of flaws possibly present in it.
SUMMARY OF THE INVENTION
There thus exists significant need for techniques and equipment for evaluating structural integrity of buried anchor rods in situ. The present invention fulfills this need, providing methods useful for in-field (or remote) analysis of rod integrity without excavation of soil. Practicing the methods of the invention additionally is unlikely to damage the rods or the concrete in which they may be embedded. In some circumstances a small amount of metal might need to be removed from the cosmetic weld joining the rod to an equalizer plate (or similar device); this act does not degrade performance of the rod or plate to any measurable extent, however.
Techniques encompassed by the present invention include transmitting ultrasonic (or other suitable) energy from the above-ground end of a rod to its buried end and receiving the energy returned therefrom. Such returned energy, if ultrasonic, manifests in both direct and mode-converted states, either or both may be analyzed to assist in detecting flaws, corrosion, wastage, or other degradation of the rod. Data from a field evaluation may be compared with baseline data maintained either for a specific rod or for rods of similar composition and length (or both), and periodic field evaluations of a rod may be used to analyze trends in its structure over time.
Many conventional metal utility poles or towers are guyed so that the above-ground (“head”) ends of any supporting rods are both visible and immediately accessible. Other towers, however, particularly those used in connection with the cellular telephone industry, utilize equalizer plates such as those discussed above. An example of such a plate
10
is shown in FIG.
1
. Contrary to the design of many other connectors, the equalizer plate
10
of
FIG. 1
contains a recess into which the head end of rod
14
is fitted. The head end thereafter is welded to plate
10
, rendering it inaccessible from the exterior of the plate
10
.
Because welds typically used for this purpose are not completely of the “full-penetration” type, the plates and head ends typically are not acoustically coupled well. As a consequence, ultrasonic signals transmitted via an equalizer plate to a rod are subject to distortion caused by the poor acoustical coupling. This distortion obviously can affect the results of any analysis performed of a rod, rendering the results ineffective (or at least other than optimal) for decisionmaking about the integrity of the rod.
The invention thus also contemplates preparing the surface of the equalizer plate and head end of a rod, if necessary, to improve the quality of the acoustic signal transmitted through the rod. Such preparation may include removing a portion of the weld or otherwise creating a small flat surface at or adjacent the head end of the rod and perpendicular (or substantially so) to the longitudinal axis of the rod. A portable transducer may then be acoustically coupled to the prepared surface and used to transmit ultrasonic energy through the length of the rod and receive any returned energy for analysis. After the transmissions are complete and the transducer removed, the prepared surface may in some cases be coated with a cold-galvanize compound (such as, but not necessarily, a sprayable material containing zinc) to protect the prepared surface against rust. Thereafter, the prepared surface need merely be dusted to remove dirt or debris thereon before conducting further tests.
Commercially-available anchor rods utilized with some towers are (nominally) either ten or fourteen feet long, similar to those described above. Ten-foot rods have nominal diameter of 1.25 inches, while rods whose nominal length is fourteen feet have nominal diameter of 1.45 inches. Preferred center transmission frequency for evaluation of the former rods is approximately 5 mHz, while that for the latter rods is approximately 2.25 mHz.
Additionally encompassed by the present invention are tools and equipment useful for practicing the innovative analytical methods. One such device, usable when necessary to prepare the surface of the equalizer plate and rod for ultrasonic transmissions, includes a cutting tool for removing a portion of the weld joining the head end of the rod to the plate. The cutting tool is adapted for movement both horizontally and vertically to create, as much as reasonably possible, a flat surface at or adjace
Hinz William R.
Parker Matthew J.
Davis Oetavia
Fuller Benjamin R.
Kilpatrick & Stockton LLP
Russell Dean W.
Westinghouse Savannah River Company
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