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1、Assessment and Repair of Steel Tower & Steel Pole FoundationsNelson G. Bingel III and Kevin D. NilesOsmose Utilities Services, Inc., 215 Greencastle Rd., Tyrone, GA 30290-2944; PH(770) 632-6703; email: nbingelOsmose Utilities Services Inc., 980 Ellicott St., Buffalo, NY 14094-2398; PH (716)319- 3404
2、; email: knilesAbstractDuring the course of their service-life steel pole and lattice tower foundations aresubjected to significant environmental forces that can have a detrimental impact ontheir service life. Foundations are critical components of all transmission systems and therefore are crucial
3、to the national power grid.Deterioration identified in these foundations during regular inspections and assessment should be categorized into specific levels of structural severity. Each should be addressed accordingly by a varied regimen of mitigation and repair options.Coatings and anode installat
4、ion are most commonly used to help prevent deterioration. Repair options include but are not limited to: bolted steel repairs, welded steel repairs, reinforced concrete encasement of concrete foundations anddirect member replacement.This paper will address assessment and repairs specific to steel to
5、wer and steel pole foundations inclusive of structural durability, corrosion issues and standard repairmethods.Steel Transmission StructuresRegardless of their construction, steel transmission structures are composed of two basic components: the foundation which provides basic structural support for
6、 the entire structure and the structure itself, which creates clearance and provides support to the overhead electric conductors, ground wires and their associated hardware. Most steel transmission structures can be separated in to five distinct groups:Self Supporting Steel Lattice Towers which are
7、typically constructed bymechanically joining multiple steel members together to form a supporting lattice space truss.Guyed Steel Lattice Towers similar to the self supporting steel lattice structures described above, guyed towers are distinctly different because of their reliance onmultiple guys to
8、 provide structural stability. Self-Supporting Steel Poles which are usually constructed of only a few large components welded together to form a tapered hollow “pole”. Guyed Steel Poles similar to self-supporting steel poles, guyed steel poles rely on multiple guy wires to provide stability. Tubula
9、r Steel H-Frames consisting of two steel poles placed adjacent to ach other with a steel cross arm spanning the distance between the two.Foundation TypesAll steel structure types require an adequate foundation to support the loads imposed on the structure. Because of the varying types of environment
10、s and soil conditions where these structures may be located, it is necessary to utilize different types of foundations in order to provide a stable platform for the structure. These include:Direct-embedment, which can be utilized for both steel poles and steel grillage, involves the excavation of a
11、foundation hole in the earth by means of a drill rig or similar equipment. Excavations are made to accommodate the tower leg and grillage, which are set into the excavation and backfilled with either native soil, compacted gravel, crushed stone or concrete.Direct embedment of steel poles and steel g
12、rillages may sometimes be preferred over other foundation options due to their typically lower cost and ease of installation. However, they are usually not used where shallow rock and ledge are present or in areas where soil stability is a concern due to sandy soil conditions or in soil weakened by
13、the influence of water. Concrete foundations can encompass a wide range of variations depending on the application they are being utilized for. Two of the most common types are: Drilled shafts also called caissons, which are probably the most widely used concrete foundation type. These foundations a
14、re installed by drilling holes with a large auger and then filling them with concrete and reinforcing steel. In some cases a “belled” drill shaft is used instead of a straight shaft to help provide additional capacity. A belled hole is created in the same way as a regular drilled shaft but with a di
15、stinct modification. The bottom portion of the hole is hollowed out creating a type of anchor or “bell” at its base. Pier slabs (pads) encompass a wide range of applications and can either be pre-cast or formed and poured in place. Typically, these foundations are relatively shallow in comparison to
16、 drilled foundations, with most set at an average depth of approximately 10 or less. This usually entails the excavation of large foundation holes and a significant amount of non-native backfill which is compacted in place over the foundations. These foundations rely on their large surface area and
17、weight of backfill to provide stable support for the structure and resistance to uplift. One of the most significant advantages of reinforced concrete foundations is that thestructural steel members are protected from the environment by the concrete, whichlimits their exposure to corrosion and signi
18、ficantly increases their service life.Base Plates, Anchor Bolts and Stub AnglesWhen utilizing drilled shaft or pier slab foundations it is necessary to have a reliableinterface connection between the steel and concrete.Three of the most common connections are Base Plate and Anchor Bolts Lattice towe
19、rs and steel poles have used both base plates and anchors bolts of various designs to anchor the upper support structure to the foundation. Stub Angles usually a short piece of angle steel with shear connectors,which is set directly into the concrete. Pin Bases several designs have been engineered t
20、o create a pin connection between a tower and its foundations. These are normally used with guyed structures such as guyed vees.Reasons to InspectIt is important to understand that with all structures there is a need for periodicinspection to ensure structural integrity and public safety. Steel stru
21、ctures and theirfoundations are no exception, as these structures are sometimes placed inenvironments that are less than ideal relative to the longevity of their structuralcomponents.Environmental conditions can have a dramatic impact on the structural integrity of steel and concrete over relatively
22、 short periods of time. Detrimental influences suchas agricultural activity, changes in soil elevation from construction and emissionsfrom manufacturing facilities can all contribute to the degradation of structural components. These effects can be compounded by unseen defects in structuralmembers,
23、coatings and concrete at the time of construction.Often these environmental influences combine to create active corrosion cells which are detrimental to steel structural components.Definition of a Corrosion CellThe basic electrochemical corrosion cell consists of four components; a cathode, ananode,
24、 an electrolyte and a metallic path. Corrosion within the cell exists because ofthe flow of electrons and ions between these components. Anode the point where corrosion takes place within the cell due to the flow of positively charged ions away from it. Cathode negatively charged ions migrate from t
25、he cathode, which creates a polarization helping to protect the cathode from corrosion.Electrolyte is a substance or solution capable of conducting electricity.Metallic path conducts negatively charged electrons from the anode to thecathode.With a corrosion cell on steel towers and steel poles, the
26、electrolyte is represented by the soil the structure is buried in. The metallic path is the metal of the structure andthe cathode and anode can either be between the different legs of a steel tower oralong the surface of a steel pole where there is a difference of corrosion potential between two poi
27、nts.Development of Corrosion InspectionSome of the earliest studies into corrosion took place in England during the mid tolate 1800s, with additional detailed testing to come in the early 1900s. This early research into the chemistry of corrosion developed the fundamentals of research and inspection
28、 that are still in use today.Mainly used in gas, petroleum and other similar industries, corrosion inspection is a proven science with a long history of success. Inspection procedures and equipment have been developed and refined over many years, even though some of their basic principles have not c
29、hanged.In recent years, inspection methods and mitigation technology have been adopted bymany other industries, both public and private, which have an economic interest in protecting their infrastructure. Electric utilities are no exception and have begun to recognize the necessity for integrating c
30、orrosion inspection and mitigation efforts intotheir regular maintenance programs.Direct Inspection ProceduresA large majority of corrosion inspection is conducted through visual and physical assessment. These inspection methods include: Visual assessment by utilizing various grading criteria from r
31、ecognized experts in the industry, corrosion on metal can be quantified by various grades. A standard method for this purpose is the “SSPC VIS2 Standard Method for Evaluating Degree of Rusting on Painted Steel Surfaces”,developed by the Society for Protective coatings. This allows the inspector to c
32、ompare existing surface corrosion to samples in the standard so the corrosion can be classified by both type and severity. Physical measurements allow the inspector to measure the different components of a structure to help determine the amount of section loss cause by corrosion. The most common han
33、d devices for these measurements are calipers, micrometers and pit gauges. Photos are sometimes also used to support the physical measurements recorded in the field. Excavation because steel towers and steel poles may be buried in soil it is often necessary to make shallow inspection holes. The exca
34、vation extends the visual assessment and physical measurements through the initial transition area of the steel to soil interface. Usually inspection excavations are limited to a depth of 18” 24” but can occasionally be deeper if necessary to determine the extent of visible corrosion. Ultrasound mea
35、surements when visual inspection and physical measurements are limited due to a structures design or location, ultrasoundcan be successfully utilized in certain applications to help further assess a structures condition. Electromagnetic Acoustic Transducer (EMAT) technology has expanded the use of u
36、ltrasonic testing from rudimentary lateral scans reading through the structure to longitudinal scans along the length of its surface. This process can detect below ground corrosion on steel structures.Indirect Inspection MethodsEnvironmental factors have a significant affect on the corrosion process
37、. Indirect inspection methods have been developed to quantify their influence at specific locations. Several of these methods have become standard procedures in the pipeline industry and have been adapted for use in evaluating the environmental conditions surrounding steel structures.pH probably one
38、 of the most easily understood environmental influences, the pH measurement of soil surrounding a tower determines how acidic the soil is and helps to define its potential influence in the corrosion process. Measurements of a pH around 7 show a relatively neutral environment while measurements of 5.
39、5 or less are considered to be acidic. A pH reading of 8.0 or more usually indicates an alkaline environment and is typically less of a concern as it usually has little corrosive effect on steel. Redox otherwise known as the measurement of oxygen reduction, redox measures the dissolved oxygen conten
40、t in the soil. Distinct differences in oxygen content in the soil surrounding a buried steel structure can indicate the potential influence of microbial activity on corrosion.Although bacteria itself does not typically attack metals directly, variobacteria develop in certain Redox environments which
41、 can influence the corrosion process by creating corrosive byproducts. Environments free of oxygen support the presence of anaerobic bacteria, while oxygen rich environments tend to support aerobic bacteria. Environments that exist somewhere in the middle of the scale are relatively neutral and tend
42、 not to support microbial activity detrimental to steel. Redox measurements collectedin close proximity to the structure can help to determine whether a structure is at risk from microbial influence. Redox measurements of less than 100 mV can indicate the presence of aerobicbacteria and an increased
43、 potential for corrosion activity. While redox measurements between 100 and 350 mV which tend to support moreanaerobic activity tend to be more neutral.Soil Resistivity - is a significant indicator in determining how corrosive a soilenvironment is. Soils with low resistivity allow for the easy flow
44、of currentbetween an anode and cathode creating a higher level of corrosion activity. Soils with a resistivity of 1000 ohm-cm or less tend to be very corrosive, while soils with a resistivity of 10,000 ohm-cm are considered much less of aninfluence to corrosion.Half Cell Measurements measure the str
45、ucture to soil potential within theenvironment. Typically, this is done utilizing a digital or analog potentialmeter. The more negative the measurement is ( -.400 mV), the more likely it is to corrode.Corrosion RatingThe factors previously discussed are only indicative when considered alone. They mu
46、st be evaluated as to how they relate to each other in a specific environment. To dothis an algorithm was developed to measure these factors together in order todetermine a structures potential for corrosion or its “corrosion rating”. These ratings are divided into specific categories quantifying ea
47、ch structurespotential for future corrosion, as listed below: Low the potential for corrosion is low and no further action is necessary at this time. Typically the recommendation for these structures is to re-inspect in another ten years. Mild the potential for corrosion is slightly elevated and, wh
48、ile it may not be visually evident, it is possible that minor corrosion activity is present. Similar to the structures rated as low, it is recommended to re-inspect these structures in another ten yearsModerate structures rated as moderate usually show positive indicating factors for corrosion even though only minor corrosion may currently be visible. This corrosion may appear as minor surface corrosion or pitting. Structures in this condition should have mitigation efforts put in place soon to help avoid further c