Effective performance evaluation of structural anchors, especially those in overhead catenary systems (OCS), is critical for ensuring operational reliability and safety. In this case, a comprehensive instrumentation program was implemented to monitor and evaluate the load-deformation response of two OCS anchors under controlled load conditions. The focus was on recording deformation, strain, and load responses during both loading and unloading phases. This review highlights the challenges encountered in the instrumentation for on-site OCS anchor pull test, the solutions devised, and the test setup employed for this ambitious field study. The insights gained from the instrumentation process underline the importance of advanced monitoring techniques for real-world structural assessments.

Instrumentation for On-Site OCS Anchor Pull Tests

1. Environmental and Site Constraints

The outdoor nature of the testing posed significant environmental challenges. The instrumentation had to endure variable weather conditions while maintaining precision.

2. Anchor-Specific Design Complexity

The unique half-circle bent design of the anchors, combined with varying material properties (e.g., high-strength concrete and steel), required customized sensor placement strategies to accurately capture strain and deformation. The anchors were to be tested in tension, and a special rigid steel frame was designed by the client to facilitate the test program in the field.

3. Large Deformations and Complex Load Paths

OCS anchors experience large deformations under high loads (300% of the nominal design load), with stress concentrations varying across their geometry. This required precise sensor placement and robust wiring to avoid sensor damage during testing.

4. Data Fidelity and Synchronization

With a large number of sensors capturing high-frequency data simultaneously, ensuring accurate synchronization between strain gauges, displacement transducers, and load cells was critical.

 

Instrumentation Solutions Implemented

 

1. Optimized Sensor Placement and Preparation

A total of seven strain gauges and five linear variable displacement transducers (LVDTs) were carefully positioned to monitor critical regions of the anchors. Prior to installation, the surface preparation process included:

  • Grinding: Ensured smooth and even surfaces for strain gauge adhesion.
  • Cleansing: Removed contaminants using market-standard cleaning agents.
  • Epoxy Adhesive: Secured strain gauges in place, preventing detachment during large deformations.

2. Robust Instrumentation Configuration

  • Strain Gauges: Positioned on the top and bottom faces of each anchor to capture tensile and compressive strains.
  • LVDTs: Mounted radially and tangentially to track displacements along critical load paths.
  • Load Cell: A high-precision, 200,000 lb capacity load cell recorded applied loads with 0.5% accuracy.

3. High-Frequency Data Acquisition System (DAS)

All sensors were connected to a DAS with a sampling rate of 100 Hz, ensuring high-resolution data capture during rapid load changes. For analysis, data was down-sampled to 1 Hz to balance fidelity and manageability.

4. Structured Loading and Unloading Protocol

The test procedure followed a multi-stage loading and unloading schedule, incrementally increasing loads while allowing controlled relaxation. This approach minimized risks to instrumentation and ensured comprehensive data capture over the full load spectrum.

Instrumentation Setup

Instrumentation for On-Site OCS Anchor Pull Tests

 

1. Test Frame and Loading System

The test frame, designed by the client, provided structural support and stability during testing. Anchors were subjected to tensile loads using a manual hydraulic jack. The frame’s connection to the foundation ensured that applied loads were efficiently transferred to the anchors.

2. Strain Measurement

  • Strain Gauge Type: Vishay Micro-Measurements and SR-4 gauges were selected for their reliability under high strains.
  • Wiring Management: U-shaped wiring loops and tie wraps minimized the risk of wire damage during deformation.
  • Placement: Gauges were distributed across areas of expected maximum strain, including the pin connection and extreme fibers of the anchor.

3. Deformation Monitoring

  • LVDTs: Five LVDTs were deployed for radial and tangential displacement measurements. A custom wooden frame securely held the sensors in position.
  • Configuration: Tangential displacement was recorded at the crown, while radial measurements were taken along the load direction.

4. Load Monitoring

  • Load Cell: Positioned between steel plates, the load cell monitored real-time loads applied by the hydraulic jack.
  • Calibration: Factory calibration ensured that load readings remained within the specified accuracy range.

5. Data Acquisition System (DAS)

The DAS centralized input from all sensors, synchronizing strain, displacement, and load data. This system was critical for identifying time-correlated responses across the instrumentation network.

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