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Guide for Heat-Straightening of Damaged Steel Bridge Members

Table of Contents

• 1. Introduction
  • 1.1 History of Heat Straightening
  • 1.2 Typical Types of Damage
    • 1.2.1 Category S
    • 1.2.2 Category W
    • 1.2.3 Category T
    • 1.2.4 Category L
  • 1.3 Classification Use
  • 1.4 Objectives of This Guide
• 2. Heat Straightening Basics
  • 2.1 What is Heat Strightening?
  • 2.2 Why Heat Straightening Works
  • 2.3 Fundamental Heating Patterns
    • 2.3.1 Vee Heat
    • 2.3.2 Edge Heats
    • 2.3.3 Line Heats
    • 2.3.4 Spot Heats
    • 2.3.5 Strip Heats
  • 2.4 Defining Basic Damage Patterns and Yield Zones
  • 2.5 Basic Heating Patterns
    • 2.5.1 Flat Plate Bent About the Major Axis (Category S)
    • 2.5.2 Structural Members Bent About Their Strong (Major) Axis (Category S)
    • 2.5.3 Structural Members Bent About Their Weak (Minor) Axes (Category W)
    • 2.5.4 Structural Members Subject to Twisting Damage (Category T)
    • 2.5.5 Flanges and Webs with Local Buckles (Category L)
    • 2.5.6 Angles
  • 2.6 Complex Damage
  • 2.7 Equipment and Its Use
  • 2.8 Safety Considerations
  • 2.9 Temperature Control
  • 2.10 Restraining Forces
  • 2.11 Practical Considerations
    • 2.11.1 Torch Tip Size and Intensity
    • 2.11.2 Material configuration
    • 2.11.3 Judging the Temperature
    • 2.11.4 Jacking Forces
    • 2.11.5 Heating Patterns
    • 2.11.6 Sequencing of Heats
    • 2.11.7 Lack of Movement
    • 2.11.8 Cooling the Steel
• 3. Assessing, Planning and Conducting Successful Repairs
  • 3.1 Role of Engineer, Inspector and Contractor
  • 3.2 Keys to a Successful Repair
  • 3.3 Steps in the Assessment Process
    • 3.3.1 Initial Inspection and Evaluation for Safety and Stability
    • 3.3.2 Detailed Inspection for Specific Defects
      • 3.3.2.1 Signs of Fracture
      • 3.3.2.2 Degree of Damage
      • 3.3.2.3 Material Degradation
      • 3.3.2.4 Geometry of the Structure
  • 3.4 Steps in the Planning and Design Process
    • 3.4.1 Analysis of Degree of Damage and Determination of the Maximum Strain due to Damage
    • 3.4.2. Conduct a Structural Analysis of the System
    • 3.4.3. Select Regions Where Heat Straightening is Applicable
    • 3.4.4. Select Heating Patterns and Parameters
      • 3.4.4.1 Vee Depth
      • 3.4.4.2 Vee Angle
      • 3.4.4.3 Number of Simultaneous Vee Heats
    • 3.4.5. Develop a Constraint Plan
    • 3.4.6 Estimate the Heats Required to Straighten the Members
    • 3.4.7 Repair Plans and Specifications
  • 3.5 Supervision of Repairs
    • 3.5.1 Monitoring the temperature
    • 3.5.2 Controlling restraining forces
    • 3.5.3 Review of Proposed Heating Patterns
    • 3.5.4 Checking Tolerances
    • 3.5.5 Safety
    • 3.5.6 Checklist of Procedures for Supervisors and Inspectors
• 4. Effects of Heat Straightening on the Material Properties of Steel
  • 4.1 Introduction
  • 4.2 Residual Stresses in Heat-straightened Steel
  • 4.3 Effect of Heat Straightening on Material Properties of Steel
  • 4.4 Limits on Jacking Force to Minimize Risk of Fracture
  • 4.5 Limits on Maximum Damage Strains
  • 4.6 Fatigue and Fracture Performance
• 5. Heat Straightening of Flat Plates
  • 5.1 Introduction
  • 5.2 Variables Affecting the Movement of Heat–straightened Plates
    • 5.2.1 Temperature
    • 5.2.2 Effect of Vee Angle
    • 5.2.3 Restraining Forces
  • 5.3 Analytical Development
• 6. Heat Straightening Rolled Shapes
  • 6.1 Fundamental Damage Patterns
  • 6.2 Composite Deck–Girder Bridges
    • 6.2.1 Factors Affecting Heat–Straightening Behavior of Composite Girders
      • 6.2.1.1 Heat Patterns
      • 6.2.1.2 Residual Moments
      • 6.2.1.3 Restraining Forces
      • 6.2.1.4 Stiffening Effect of Web
    • 6.2.2 Model for Heat–Straightening Response
    • 6.2.3 Modeling Statically Indeterminate Spans with Intermediate Diaphragms
  • 6.3 Trusses and Axially Loaded Members
    • 6.3.1 Introduction
    • 6.3.2 Response of Columns to Heat Straightening
• 7. Heat Straightening Repair of Localized Damage
  • 7.1 Damage Classification
  • 7.2 Heat Straightening Procedures for Unstiffened Local Damage
    • 7.2.1 Phase I. Initial Heating Patterns and Jacking Locations
      • 7.2.1.1 Restraining forces
      • 7.2.1.2 Vee heats
      • 7.2.1.3 Line heats
      • 7.2.1.4 Web line heat
    • 7.2.2 Phase II. Heating/Jacking Pattern if θ_n = 0 or θ_f = 0
    • 7.2.3 Phase III. Heating Pattern if θ_f = θ_w
    • 7.2.4 Flange Damage in Opposite Direction
  • 7.3. Heat straightening Procedures for Stiffened Elements
    • 7.3.1 Initial Heating Pattern
    • 7.3.2 Final Heating Pattern
  • 7.4 Determination of Jacking Forces
  • 7.5 Conclusions
• Appendix I: Specifications for the Selection of Contractors and the Conduct of Heat-Straightening Repairs
  • A1 Selection of Contractor (or the Contractor's field supervisor)
  • A2 Technical Specifications for the Conduct of Heat-Straightening Repairs
• Appendix II: Nomenclature
• Appendix III: References and Other Sources of Information

• Figures

  • Figure 1: Graphic illustration of Category S damage.
  • Figure 2: Examples of Category W damage.
  • Figure 3: Examples of Category T damage.
  • Figure 4: Category L damage showing flange buckles on wind bracing on Mississippi River Bridge in Greenville, MS.
  • Figure 5: Conceptual example of shortening a steel bar.
  • Figure 6: Stages of movement during vee heat.
  • Figure 7: Schematic diagram of edge heats used to heat–curve a beam.
  • Figure 8: Line heat in progress on the web of a wide flange beam.
  • Figure 9: Schematic of line heat mechanism.
  • Figure 10: Strip heat in progress with a completed strip heat in the foreground.
  • Figure 11: Schematic of strip heat on the flange of a rolled beam.
  • Figure 12: Yield zones for basic damage patterns.
  • Figure 13: Yield zone and vee/strip heat layout for a category S damage to a rolled beam.
  • Figure 14: Plate vee heat pattern over yield zone.
  • Figure 15: Heating patterns for wide flange beams and channels bent about their major axes (Category S).
  • Figure 16: Heating patterns for wide flanges and channels bent about their minor axes (Category W).
  • Figure 17: Wide flanges and channels with twisting damage (Category T).
  • Figure 18: Typical heating patterns for local damage.
  • Figure 19: Heating patterns for angles.
  • Figure 20: Characteristics of plastic flow and restraint during heat straightening.
  • Figure 21: Brittle fracture during heat straightening.
  • Figure 22: Offset measurements to calculate degree of damage and radius of curvature.
  • Figure 23: Radius of curvature for a damaged beam of curvature and cord length.
  • Figure 24: Diaphragm damage due to vehicle impact on girder.
  • Figure 25: Jacking arrangements for global and local damage on a composite girder bridge.
  • Figure 26: Temperature sensing crayons.
  • Figure 27: Jacks in place on a Wisconsin bridge.
  • Figure 28: Iron–carbon equilibrium diagram.
  • Figure 29: Residual stress distribution for plates damaged and then vee heated
  • Figure 30: Typical residual stress distribution for a heat straightened angle
  • Figure 31: Typical residual stress distribution for a heat straightened angle
  • Figure 32: Typical residual stress distribution for a heat straightened channel
  • Figure 33: Typical residual stress distribution for a Category S wide flange beam
  • Figure 34: Typical residual stress distribution for a Category W heat straightened wide flange beam
  • Figure 35: Yield stress versus number of damage/repair cycles for heat straightened beam.
  • Figure 36: Tensile stress versus number of damage/repair cycles for heat straightened beam.
  • Figure 37: Percent elongation versus number of damage/repair cycles for heat straightened beam.
  • Figure 38: Influence of heating temperature on plastic rotation for 3/4 depth vee heats and a jacking ratio of 0.16.
  • Figure 39: Influence of jacking ratio on average plastic rotation for 650°C (1200°F) heating temperatures
  • Figure 40: Primary and stiffening plate elements for a channel bent about its major axis (Category S damage).
  • Figure 41: Weak axis bending resulting in a yield line in the plate element.
  • Figure 42: Typical deformed shape and yield zones in damaged composite girders.
  • Figure 43: Heating patterns for composite girder.
  • Figure 44: Diaphram stiffened composite girder.
  • Figure 45: Dead load conditions on a simply supported beam.
  • Figure 46: PΔ effect on an axially loaded column.
  • Figure 47: Plastic rotation versus jacking ratio for axially loaded Category W column.
  • Figure 48: Typical localized damage classified as Category L.
  • Figure 49: Typical Category L/U damage.
  • Figure 50: Heat straightening local flange damage (Category L/U).
  • Figure 51: Arrangement of restraining forces during various stages of repair.
  • Figure 52: Arrangement of vee and line heats.
  • Figure 53: Curvature and line heating patterns for category L/S damage.

• Tables

  • Table 1: Recommended torch tips for various material thicknesses.
  • Table A1: Recommended Tolerances for Heat Straightening Repair.
  • Table A2: Recommended torch tips for various material thicknesses.