2005 NDS- LRFD


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2005 NDS- LRFD

  1. 1. 2005 NDS: LRFD Provisions Paul W. McMullin, P.E., Ph.D. Sr. Project Engineer Dunn Associates, Inc.
  2. 2. Philosophy “decisions not to utilize specific practices require the same background knowledge as those which lead to the utilization of fracture-safe design practices” (Pellini 1969, p. 1)
  3. 3. Outline Development of Beam and Column Wood Design Codes Provisions Why LRFD Lateral Supplement What is LRFD Wood Mechanical Design Connectors General Terminology Proprietary and Requirements Connectors Resources
  4. 4. Last Decade of Wood Design 1991 NDS 1996 LRFD- New state of the art 1997 NDS- Based on 1991 2001 NDS (ASD)- Updated to technology of 1996 LRFD 2005 NDS (ASD/LRFD)
  5. 5. 2005 NDS
  6. 6. 2005 NDS Combined ASD/ LRFD provisions LRFD in same format as ASD, except: Behavioral equations same 3 LRFD based adjustment factors KF—Format Conversion Factor φ—Strength Reduction Factor λ—Time Effect Factor No CD—Load Duration Factor Consistent with ASCE 7-02 and 7-05
  7. 7. NDS 2005: Chapter Layout 1) General Requirements for Structural Design 2) Design Values for Structural Members 3) Design Provisions and Equations 4) Sawn Lumber 5) Structural Glue Laminated Timber 6) Round Timber Poles and Piles 7) Prefabricated Wood I- Joists 8) Structural Composite Lumber
  8. 8. NDS 2005: Chapter Layout (cont.) 9) Wood Structural Panels 10)Mechanical Connections 11)Dowel-Type Fasteners 12)Split Ring and Shear Plate Connectors 13)Timber Rivets 14)Shear Walls and Diaphragms 15)Special Loading Conditions 16)Fire Design of Wood Members
  9. 9. NDS 2005: Appendix Layout a) Construction and Design Practices b) Load Duration (ASD) c) Temperature Effects d) Lateral Stability of Beams e) Local Stresses in Fastener Groups (New 2001 NDS) f) Design for Creep and Critical Deflection Applications g) Effective Column Length h) Lateral Stability of Columns
  10. 10. NDS 2005: Appendix Layout i) Yield Limit Equations for Connections j) Solution of Hankinson Formula k) Typical Dimensions for Split Ring and Shear Plate Connectors l) Typical Dimensions for Dowel-Type Fasteners m) MFG Tolerances for Rivets and Steel Side Plates for Timber Rivet Conn. n) Load and Resistance Factor Design (LRFD)
  11. 11. Design Value Supplement: Chapter Layout 1) Sawn Lumber Grading Agencies 2) Species Combinations 3) Section Properties 4) Reference Design Values
  12. 12. Why Use LRFD? State-of-the-art More rational treatment of loads ASD—Deterministic (inconsistent safety factors) LRFD—Reliability based ASD load combinations not maintained Ease of designing with multiple materials that use LRFD Size benefits with multiple transient loads Consistent with FEMA 356 (performance based design codes)
  13. 13. LRFD Design Concepts Safety Limit State Serviceability Limit Factored Loads State (Demand)—Larger Unfactored Reduced Nominal Loads—same Strength– φRn Mean material (Capacity)—Larger strength and stiffness—same
  14. 14. LRFD Statistical Methodology AF & PA, 2005 NDS e-course
  15. 15. LRFD Nuts and Bolts Basic Relationship αQ ≤ φλRn Specific f ≤ F 'n f—demand stress (e.g. Pu/A) F’n—LRFD level adjusted stress (capacity) Example (Sawn Lumber in Bending) F’b=FbCMCtCLCFCfuCiCrKFφbλ
  16. 16. LRFD Adjustment Factors 2005 NDS
  17. 17. LRFD Nuts and Bolts (Cont.) “Adjusted” strength, rather than “allowable” Three new adjustment factors KF—Format Conversion Factor (ASTM D 5457) Generally 2.16/φ Rearranged φKF=2.16 φ—Strength Reduction Factor λ—Time Effect Factor Essentially only one additional adjustment factor
  18. 18. LRFD Specific Adjustment Factors 2005 NDS 2005 NDS
  19. 19. LRFD Specific Adjustment Factors (Cont.) 2005 NDS
  20. 20. Beams and Columns Behavioral equations typically same Very minor adjustments to FbE and FcE Shear increase from 2001 NDS maintained ' FbE = 1.20 Emin 2 RB ' FcE = 0.822 Emin (le d )2 E’min—for stability, 5th percentile, App D Use E’ for deflection
  21. 21. Wind and Seismic Supplement Design guidance for lateral loads Chapters 1) Designer Flowchart 2) General Design Requirements 3) Members and Connections 4) Lateral Force-Resisting Systems General Wood Diaphragms Wood Shear Walls Appendix A (Plywood tables)
  22. 22. Diaphragms Rigid/Flexible defined Based on relative deflection Deflection equations Limits on diaphragms resisting load from concrete and masonry walls 3-sided diaphragms discussed Cantilevered diaphragms permitted
  23. 23. Diaphragms (Cont.) Capacities for structural sheathing Wind and Seismic values different Values for lumber diaphragms Tables contain Ga (shear stiffness) Ga includes nail slip term Multiply table values by φD=0.8
  24. 24. Diaphragm Table 2005 NDS
  25. 25. Diaphragm Table 2005 NDS
  26. 26. Shear Walls Deflection equation Guidelines for double sheathed walls Aspect ratio criteria remains same as IBC 2003 Perforated shear wall values Tables Structural Panels [Tab 4.3A] Gypsum and Cement Plaster [Tab 4.3B] Lumber [Tab 4.3C] Plywood [Tab A.4.3A]
  27. 27. Shear Walls (Cont.) Shear Wall Framing 2” Nominal—Typical 3” When Nails 2” O.C. or less 10d @ 3” O.C. Nominal Capacity (vs)>700 lb/ft • For seismic design category D, E, F Requirements irrespective of ASD or LRFD
  28. 28. Shear Wall Table 2005 NDS
  29. 29. Shear Wall Table 2005 NDS
  30. 30. Mechanical Connectors Same behavioral equations Check Local Wood Stresses Multiple Fasteners Based on Mechanics OR Appendix E For both ASD & 2005 NDS LRFD
  31. 31. Mechanical Fastener: Local Wood Failure Modes Local Wood Stress Modes Net Section Tension Row Tear-out Group Tear-out 2005 NDS
  32. 32. Mechanical Fastener Adjustment Factors 2005 NDS
  33. 33. Pre-Engineered Connectors Convert using ASTM D5457-93 (1998) Multiply ASD value by φ and λ (CM, Ct) KF 1996 LRFD Manual- Pre-Engineered Metal Connectors, Supplement
  34. 34. 2005 NDS Learning AF&PA e-course http://www.awc.org/Hel pOutreach/eCourses/S TD104/NDS2005.pdf AWC and AF&PA web sites—misc. papers Structural Wood Design Using ASD and LRFD (Workbook) ASD/LRFD Manual for Engineered Wood Construction 2005 Edition
  35. 35. 2005 NDS Design Aids 2005 NDS 2005 Design Value Supplement 2005 Wind & Seismic Supplement ASD/LRFD Manual for Engineered Wood Construction 2005 Edition—Available December 2005?
  36. 36. Summary 2005 NDS—dual format LRFD adds essentially 1 LRFD advantageous adjustment factor because: Behavioral equations State-of-the-art same as ASD Reliability based Design guide not Consistent with other available material codes Code currently available Size benefits Adopted in 2006 IBC Consistent with FEMA 356