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Evaluating Damage & Repairing Wood Trusses PowerPoint Presentation
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Evaluating Damage & Repairing Wood Trusses

Evaluating Damage & Repairing Wood Trusses

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Evaluating Damage & Repairing Wood Trusses

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  1. Evaluating Damage & Repairing Wood Trusses By Jim Vogt, P.E.

  2. Evaluating Damage & Repairing Wood Trusses Outline • Introduction to Wood Trusses • Common Causes of Damage • Evaluating the Damage • Repair Materials • Repair Examples

  3. Introduction to Wood Trusses • Materials • Basic Design Concepts

  4. panel length peak 12 top chord slope continuous lateral brace truss plate overall height web wedge heel panel point bearing splice bottom chord span (out-to-out of bearings) overhang bottom chord length cantilever

  5. Wood Trusses • Materials • Log • Heavy Timber/Glued-Laminated Timber/Structural Composite Lumber • Sawn Lumber • Connectors • Split Rings/Shear Plates • Bolts • Timber Rivets • Wood Structural Panel Gussets • Metal Connector Plates

  6. Originally used in bridge design as early as mid 1500s

  7. Various Types of Wood Trusses

  8. Dimension Lumber &Plywood Gussets

  9. Metal Connector Plates

  10. Metal Connector Plates

  11. Metal Plate Connected Wood Trusses • Trusses are the major engineered wood component used today • $13 billion industry • 80% residential market share • Roof systems greater than 60% market share • Floor systems about 20% market share

  12. Metal Plate Connected Truss Associations

  13. Load Basic Design Concept • Load vs. Resistance • Externally applied loads become internal forces

  14. Typically proprietary software from the connector plate manufacturer Dedicated exclusively to truss design Contains lumber and connector plate properties to design trusses Design Software for MPCW Trusses

  15. Common Causes of Damage • Storage and handling problems • Installation problems • Field modifications • Loading/design problems • Adverse environments • Fire • Manufacturing mistakes

  16. Storage

  17. Handling

  18. Plates & Lumber Damage

  19. Installation

  20. Insufficient Bracing • Top chord buckling under its own weight • Insufficient Lateral Restraint. • No Diagonal Bracing

  21. Compression Buckling • Stabilize the column to increasing compression load capacity by 4x

  22. Check Truss Orientations • Check upside-down and left-to-right

  23. Check Bearing Locations • Are supports in the correct locations?

  24. Check Truss Orientations • Important for cantilevered trusses

  25. Check Truss Orientations • Important for unevenly loaded girders

  26. Field Modifications

  27. Trusses cut for pipes, stairs, chimneys?

  28. Loading & Design Problems

  29. Construction Loading

  30. Check for Overloading

  31. Adverse EnvironmentsFactors Affecting Wood Strength: • Moisture • Temperature • Decay Fungi • Insects • Load Duration • Chemicals • Fire

  32. Moisture Content

  33. Temperature Temperature Strength Strength Temperature • Inverse relationship between strength & temperature

  34. Decay Fungi • Exposure to chronic moisture • Attack beyond the wood surface

  35. Decay Hazard Courtesy USDA Wood Handbook

  36. Other Fungi Mold • Strength unaffected • Needs food, temperature, water, & air to survive

  37. SBCA Documents Regarding Mold on Wood Structural Building Components TTB

  38. Insects • Many different types of insects attack & destroy wood • Subterranean Termites • Carpenter Ants

  39. Ranges for Subterranean & Dry-wood Termites Courtesy USDA Wood Handbook