Overhead Line Support Column Condition Assessment

1. Background

During the modernization of the tram network on the Grand Boulevard by BKV, Siemens Combino type tram vehicles were put into operation, requiring a larger cross-sectional catenary for operation. Following the replacement of the catenary, it became apparent from the leaning of some columns that a portion of the old columns had suffered corrosion over the past years to the extent that they could no longer withstand the heavier catenary load and the dynamic forces resulting from its sway. However, since not all columns have corroded and it is not economically feasible to replace all columns simultaneously, it is important to assess the condition of the columns and replace them in order of necessity. For this purpose, a testing method is required that allows the condition of the columns to be assessed non-destructively and unambiguously. In the following, we present such a method.

2.1. Theoretical Background - Physics of Solids

Every solid body is capable of vibrating in different directions at various frequencies. The largest amplitudes are observed at the body-specific natural frequency, as the body "resonates" at that frequency in the given direction (hence the concept of resonance frequency). Naturally, no body starts vibrating on its own. Excitation - i.e., external forces - is required for this. The greater this force and - in the case of alternating forces - the more precisely it matches the body's natural frequency, the greater the vibrations the solid body performs (in the direction induced by the force). If it can move freely, it vibrates at its natural frequency, but if, for example, due to a constant force, it cannot move freely, it will vibrate at the frequency forced by the excitation. To understand this, let's compare this physical fact to the vibration of a bell. The bell has a "natural" sound that is heard when struck. The current frequency of the sound depends on the bell's rigidity (i.e., its material, shape, and wall thickness) and the mass set in motion (its size). This frequency is the bell's natural frequency or resonance frequency. It is easy to see that this is a direction-dependent property: if the bell is struck in a different direction, it will produce a weak and "false" sound or may not sound at all.

2.2. Resonance of Mechanical Structures

Mechanical structures exhibit different stiffness at different frequencies. At frequencies where the structure's stiffness is low (i.e., high vibration susceptibility), even very small forces can induce strong oscillations (vibrations) in the structure. These frequencies are called resonance frequencies, and the surrounding frequency range is referred to as the resonance range. The frequency range where the structure exhibits particularly high stiffness is called the anti-resonance range. Therefore, if the excitation frequency falls within the resonance range, significant vibrations occur. If the excitation frequency falls within the anti-resonance frequency range, it is possible that no vibrations occur in the excited material. Another important parameter is resonance damping. Damping indicates how quickly the resonance vibrations induced by a single excitation decay. Hard materials (such as glass, steel, brass) generally have low damping and continue to vibrate for a long time after excitation (an everyday example of this is a bell and a xylophone). The frequency range of such resonances is usually very narrow, characterized by high, distinct amplitude peaks. Soft and plastic (elastic, soft) materials (such as rubber, wood) have high damping. These materials typically have a very wide frequency range of resonance, with less distinct amplitude peaks.

2.3. Characteristics of Overhead Line Support Columns

Support columns naturally - as mechanical structures - have resonance frequencies. The frequency depends on the structural design of the support column, the load it bears (the tensile force exerted by the overhead line), and its condition (wall thickness). Assuming uniform column structures and constant - identical - loads, the resonance of the support columns depends only on the condition of the columns when compared to the resonance properties of appropriately conditioned reference columns, the resonances of corroded columns can be identified through resonance measurements.

3.1. Basis for Determining the Condition of Support Columns

Based on the above, we know that the frequency range of resonances depends on the materials and the structural design. Focusing on a single column type, assuming uniform manufacturing processes and low manufacturing tolerances, we can start from the premise that a new support column has high stiffness and therefore a high-frequency resonance. The corroded support column, however, has reduced wall thickness, resulting in decreased stiffness and consequently a lower natural frequency. The essence of the discussed testing method lies in this: the more advanced the corrosion of the support column, the lower the wall thickness and stiffness, which can be detected based on the lower frequency of the natural frequency. It only requires experiments to determine the frequency range in which the natural frequency of properly conditioned columns is located and at what value the resonance frequency of these columns becomes unacceptable in the case of severe corrosion.

3.2. Method for Determining the Condition of Support Columns

The simplest resonance test is the impact test, where the structure of the column is excited by a single impact, and the spectrum of vibrations generated by the impact is recorded. The resonance frequencies appear as peaks in the spectrum. The anti-resonance ranges, on the other hand, appear as valleys. The width of the peaks and valley-like ranges characterizes the damping. The frequency range that can be excited by impact depends on the duration of the impact impulse τ. In the 1/τ frequency range, 90% of the energy imparted to the structure by the impact acts as vibration excitation. Therefore, soft (blunt) impacts, such as those made with a rubber mallet, can only excite vibrations up to a few hundred Hz, while short (hard) impacts (e.g., with a metal hammer) can induce vibrations in a frequency range of several kHz. Since support columns are considered large structures and exhibit soft - low stiffness - behavior, the "rubber mallet" excitation seems applicable.

In order to excite the columns with equal force, we recommend the use of an arm-type auxiliary structure that should be temporarily attached to the column under investigation. It is also possible that the column load caused by passing electric trams can be utilized to perform the resonance test. (This needs to be determined by experiment.)

Figure: spectrum showing four resonance frequencies recorded with impact test

3.3. Expected results of the condition assessment of support columns

Depending on the type, column structures are expected to show significantly different resonance frequencies. Therefore, the test can only be applied based on the comparison of columns belonging to the same type. However, for each type, it is expected that significantly different resonance frequencies occur depending on the corrosion condition, making it possible to clearly distinguish between good and bad condition columns. The thresholds required for decision-making need to be determined experimentally separately for each type.

4. Summary

The non-destructive testing method presented above is fast, cost-effective, and non-invasive. The total cost of the necessary equipment for the test (two-channel vibration analyzer with vibration sensors, excitation hammer structure) does not exceed 4 million forints. The test itself is expected to take less than 10 minutes per column, allowing for the inspection of many columns in a short time. The results can be stored and used for later inspections - even to demonstrate the rate of column deterioration in so-called trends. Note: The described method is considered the intellectual property of PIM Professional Industrial Measurement Technology Ltd., therefore it is protected by copyright.

Rahne Eric (PIM Ltd.) pim-kft.hu, gepszakerto.hu

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