Evaluating Damage & Repairing Wood Trusses

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

9. Dimension Lumber &Plywood Gussets

10. Metal Connector Plates

11. Metal Connector Plates

12. 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

14. Metal Plate Connected Truss Associations

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

16. 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

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

18. Storage

19. Handling

20. Plates & Lumber Damage

21. Installation

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

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

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

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

31. Check Truss Orientations • Important for cantilevered trusses

32. Check Truss Orientations • Important for unevenly loaded girders

33. Field Modifications

34. Trusses cut for pipes, stairs, chimneys?

38. Loading & Design Problems

39. Construction Loading

41. Check for Overloading

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

43. Moisture Content

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

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

46. Decay Hazard Courtesy USDA Wood Handbook

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

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

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

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