Code of Practice for Use of Structural Steel In Overhead Transmission Line Towers, Part 3 Testing
1978 Edition

Scope Overview of Testing Structural Steel Transmission Towers

• Aim: Defines standardized procedures for testing steel lattice towers for overhead transmission.

• Essentials:

  • Testing verifies structural behavior under predetermined load conditions (Clause 12.1).
  • Design loads, permissible stresses, and construction requirements referenced from Part I of IS 802 (Clause 1.1.1).
  • Test documentation must comprehensively include:
    • Tower classification, manufacturer, and client information.
    • Dates, test location, and involved personnel.
    • Assembly and shop drawings along with any modifications.
    • Load application diagrams and rigging configurations.
    • Details on the testing facility and equipment calibration.
    • Tabulated load and deflection data for each test stage.
    • Failure analysis data if applicable, including maximum load and failure mode.
    • Photographic records depicting the tower and any failures (Clause 12.1).

• Rounding Guidelines: Test outcomes should be rounded per IS 2-1960, preserving significant figures of specified values (Clause 0.4).

• References: Procedures align with ASCE guidelines and IEC draft standards on overhead line tower testing (Clause 0.3).

Test Report Content Summary (Clause 12.1)

This section ensures thorough and uniform testing and documentation procedures.

Overview of Testing Protocols for Steel Transmission Line Towers

• Reporting Essentials (Clause 12.1):

  • Include tower type, manufacturer, client details.
  • Record test dates, location, and personnel involved.
  • Provide assembly/shop drawings and dimensioned diagrams indicating load points and directions.
  • Present rigging arrangement diagrams.
  • Describe test facilities, including number, location, range, calibration, and accuracy of load transducers.
  • Tabulate applied loads and recorded deflections separately for each test.
  • Report failure details including maximum load before collapse, failure descriptions, and failed component specifications.
  • Include photographs covering the overall tower and any failure points.

• Rounding Off (Clause 0.4): Final results are to be rounded in accordance with IS 2-1960 maintaining the same number of significant figures as the specified values.

Example Format for Load Table

Example Format for Deflection Table

flowchart LR
    A[Setup Testing] --> B[Apply Loads]
    B --> C[Measure Loads and Deflections]
    C --> D[Record Data]
    D --> E[Analyze Failures if Any]
    E --> F[Compile Test Report]

Calibration Requirements for Testing Instruments

• Erection Tolerance: The tower should be erected within a vertical deviation of 1 in 360 (Clause 2.3).

• Calibration Process (Clause 3.1):

  • Employ standard calibrated weights to systematically calibrate all measuring devices.
  • Calibration must span the full range of expected test loads prior to testing each tower.
  • Develop calibration curves to adjust test load readings accordingly.

• Measurement Accuracy:

  • Graduated scales on the tower should be about 1 meter in length with marking accuracy within ±5 mm (Clause 8.2).
  • Final data must be rounded off following IS 2-1960 with consistent significant figures (Clause 0.4).

Calibration Curve Example

Plotting these corrections ensures accurate load measurements.

graph LR
  A[Standard Weights] --> B[Calibrate Instruments]
  B --> C[Create Calibration Curve]
  C --> D[Correct Load Readings]
  D --> E[Obtain Accurate Test Data]

Key Aspects of Load Application in Tower Testing

• Instrument Calibration (Clause 3.1):

  • Calibrate all devices with standard weights up to maximum expected load.
  • Establish calibration curves to adjust load readings.

• Loading Scenarios (Clause 4.3):

  • Load magnitudes, directions, and application points are defined by the client.

• Load Steps (Clause 6.2):

  • Apply loads progressively up to the ultimate design load (Design Load × Factor of Safety).
  • Recommended incremental steps: 25%, 50%, 75%, 90%, 95%, and 100%.

• Load Application Techniques (Clause 4.1):

  • Loads are applied using rigging diagrams via:
    • Standard wire attachments
    • Angled or bent plates
    • U-bolts, D shackles, or swinging brackets (with purchaser approval)
  • Rigging must be secure and meet safety standards.

Load Application Steps Summary

flowchart LR
    A[Begin Calibration] --> B[Calibrate Instruments Up to Max Load]
    B --> C[Generate Calibration Curves]
    C --> D[Apply Loads Gradually]
    D --> E[Follow Load Steps: 25%, 50%, 75%, 90%, 95%, 100%]
    E --> F[Use Rigging Diagram for Load Application]
    F --> G[Ensure Rigging Safety]
    G --> H[Complete Testing Procedure]

Procedures for Measuring Loads and Deflections

• Load Cases (Clause 4.3):

  • Clients provide details on load magnitudes, directions, and application points.

• Load Measurement (Clause 5.1):

  • Utilize strain gauges or calibrated weights.
  • Account for and eliminate friction in pulleys or measure and correct for it.

• Deflection Measurement (Clauses 5.2 & 8.1):

  • Measure deflections at the top cross arm on the front faces of transverse and longitudinal sides.
  • Record deflections at three stages: before loading, during load application, and after load removal.
  • Use theodolites and graduated scales to ensure precise readings.

Typical Deflection Measurement Process

Important Considerations

• Deflection readings are critical to assess structural response under loading.
• Accurate load measurement is essential; friction effects must be accounted for.
• Allowable deflection limits depend on design and client specifications (refer to IS 802 Part 3 annexes).

flowchart LR
    A[Load Application] --> B[Measure Load]
    B --> C{Is Pulley Friction Present?}
    C -- Yes --> D[Measure and Correct]
    C -- No --> E[Record Load]
    E --> F[Apply Load to Tower]
    F --> G[Measure Deflection]
    G --> H[Record Before, During, and After Load]
    H --> I[Analyze Deflection Data]

Overview of Normal and Broken Wire Load Tests

• Load Application (Clauses 6.2, 7.1, 7.2, 9.3):

  • Loads are increased gradually as percentages of ultimate design load (Design Load × F.O.S): 25%, 50%, 75%, 90%, 95%, 100%.

• Observation Times:

  • At load steps up to 95%, maintain load for 2 minutes to observe tower response.
  • At 100% load, hold for 5 minutes.

• Destruction Testing:

  • After reaching 100% ultimate load, apply increments of 5% load until failure occurs.
  • All normal and broken wire test requirements apply during this phase.

Load Application and Observation Summary

Ultimate Load Formula

[ Ultimate, Design, Load = Design, Load \times Factor, of, Safety ]

Additional Notes

• Loads must be applied and released gradually following the same sequence.
• Tower must be monitored for cracks or failures throughout.
• Mechanical strength must be verified as per Clause 10.

flowchart TD
    A[Begin Load Testing] --> B{Apply Load Steps}
    B -->|25-95%| C[Observe for 2 minutes]
    C --> D[Apply 100% Load]
    D --> E[Observe for 5 minutes]
    E --> F{Perform Destruction Test?}
    F -- Yes --> G[Increase Load in 5% Steps]
    G --> H[Check for Failure]
    F -- No --> I[End Test]

Observation Periods and Loading Steps in Testing

• Observation (Clause 7):

  • The code does not specify exact observation durations during load application.

• Load Application (Clause 6.2):

  • Loads are applied incrementally to the ultimate design load (Design Load × F.O.S) in stages: 25%, 50%, 75%, 90%, 95%, and 100%.
  • Loads should be applied and removed gradually in the same sequence.

• Test Report Requirements (Clause 12.1):

  • Include tower and manufacturer details.
  • Record test dates, location, and personnel.
  • Provide assembly and shop drawings with modifications.
  • Include dimensioned tower diagrams with load points and rigging arrangements.
  • Describe test facility and calibration details.
  • Present load and deflection tables per test.
  • Report failure details if any, including max load and element specifications.
  • Include photographs of the tower and any failures.

Load Step Progression Diagram

graph LR
    A[0% Load] --> B[25% Load]
    B --> C[50% Load]
    C --> D[75% Load]
    D --> E[90% Load]
    E --> F[95% Load]
    F --> G[100% Load]
    G --> F
    F --> E
    E --> D
    D --> C
    C --> B
    B --> A

Note: Observation durations at each step are typically defined by project-specific procedures, as the standard does not prescribe them explicitly.

Deflection Measurement and Recording Guidelines

• Measurement Locations (Clause 5.2):

  • Deflections are recorded at the top cross arm on the front faces of transverse and longitudinal sides or front and rear faces of transverse sides.

• Load Conditions (Clause 5.2):

  • Measurements are taken at three stages: before loading, during applied load, and after load removal.

• Instrumentation and Calibration (Clause 3.1):

  • Use calibrated theodolites and graduated scales.
  • Instruments must be calibrated with standard weights up to maximum test loads.
  • Calibration curves are used to correct test readings.

• Load Case Details (Clause 4.3):

  • Load values, directions, and application points are provided by the client.

Typical Deflection Recording Procedure

Important Notes

• Deflection change (\Delta) is computed as: [ \Delta = L_{loaded} - L_{unloaded} ] where (L) is displacement from a fixed reference.
• Theodolite readings of angular displacement are converted to linear deflection using geometry.

flowchart LR
    A[Setup Tower] --> B[Calibrate Instruments]
    B --> C[Apply Client-Defined Loads]
    C --> D[Measure Deflections at Top Cross Arm]
    D --> E[Record Deflections Before, During, and After Load]
    E --> F[Analyze and Adjust Data Using Calibration]
    F --> G[Complete Testing]

Key Points on Destruction Testing

• Optional Test (Clause 9.1): Performed only if requested by the purchaser.

• Test Conditions (Clause 9.2): Can be conducted under normal or broken wire conditions as agreed.

• Load Increment (Clause 9.3):

  • Follow all normal/broken wire load test requirements.
  • After reaching ultimate design load, increase load in 5% increments until structural failure.

Specifications

Load Increment Flow

graph LR
A[Start at Ultimate Design Load] --> B{Increase Load in 5% Steps?}
B -- Yes --> C[Apply Incremental Load]
C --> D{Has Failure Occurred?}
D -- No --> B
D -- Yes --> E[Terminate Test]

Mechanical Strength Validation of Transmission Towers

• Ultimate Load Testing (Clauses 9.3 & 2.1.1):

  • Apply ultimate design loads incrementally.
  • After reaching 100% ultimate load, increase loads in 5% increments until failure or test completion.
  • Tower must sustain 100% ultimate load without failing.

• Load Application (Clause 4.1):

  • Loads applied using rigging diagrams.
  • Employ normal wire attachments, angled or bent plates.
  • U-bolts, D shackles, or hangers permitted with purchaser approval and safe rigging.

• Failure Handling (Clause 11.1):

  • Replace any failed component with one of higher mechanical strength.
  • Retest structure at 100% ultimate load to verify integrity.

Load Increment Formula

[ P_n = P_u \times (1 + 0.05 \times n) ]

Where:

• (P_u) = Ultimate design load
• (n) = Number of 5% increments beyond ultimate load

Load Step Summary

flowchart TD
    A[Start Load Test] --> B[Apply Load up to 100% Ultimate Load]
    B --> C{Load Held Successfully?}
    C -- Yes --> D[Increase Load by 5%]
    D --> E{Exceeds Failure Load?}
    E -- No --> C
    E -- Yes --> F[End Test]
    C -- No --> G[Replace Failed Component]
    G --> H[Retest at 100% Ultimate Load]
    H --> F

Actions to Address Premature Tower Failures

• Replacement of Failed Parts (Clause 11.1):

  • Substitute failed members with components possessing higher mechanical strength.
  • Modified towers must successfully pass the ultimate load test at 100% specified load again.

• Material Testing (Clause 10.3):

  • Coupons may be extracted from test members for laboratory material analysis upon request.

• Clarifications (Clause 10.2):

  • Ovalization of bolt holes or permanent bolt deformation does not constitute failure.

• Test Reporting (Clause 12.1):

  • Comprehensive documentation including tower details, test conditions, drawings, load and deflection data, failure analysis, and photographs.

Remedial Process Summary

flowchart TD
    A[Detect Premature Failure] --> B[Identify Failed Member]
    B --> C[Replace with Stronger Member]
    C --> D[Retest at Ultimate Load]
    D -- Pass --> E[Approve Structure]
    D -- Fail --> B
    E --> F[Prepare Final Test Report]

Essential Elements of the Tower Test Report

• Tower Information: Type, manufacturer, client details, test dates, and site location.

• Personnel: Names of all individuals present during testing.

• Drawings: Complete list of assembly and shop drawings including any modifications.

• Diagrams: Dimensioned line schematic showing load points and directions, rigging arrangement details.

• Test Facility Description: Details of load transducers (quantity, locations, ranges, calibration, accuracy).

• Load Tables: Recorded loads at various points and increments for each test.

• Deflection Tables: Measured deflections corresponding to loading stages.

• Failure Data (if any): Maximum loads before failure, failure descriptions, and properties of failed elements.

• Photographic Documentation: Images of the entire structure and detailed failure areas.

• Additional Requirements: Replacement members post-failure must have superior mechanical strength and pass ultimate load tests (Clause 11.1).

• Certified steel producer and physical test reports for components must be included (Clause 12.2).

• All numerical data should be rounded as per IS 2-1960, keeping consistent significant figures.

Example Table Format for Loads and Deflections

flowchart TD
    A[Setup Tests] --> B[Apply Loads]
    B --> C[Measure Loads & Deflections]
    C --> D[Record Data]
    D --> E[Compile Test Report]
    E --> F[Review and Certification]