National Design Specification for Wood Construction. Copyright Materials. Learning Objectives

Transcription

1 National Design Specification for Wood Construction The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of Completion for both AIA members and non-aia members are available upon request. This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Presented by: Michelle Kam-Biron, PE, SE Director, Education Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. Copyright Materials This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written permission of the speaker is prohibited. American Wood Council 2012 Learning Objectives At the end of this program, participants will be knowledgeable of: 1. Load Resistance Factor Design (LRFD) and how it applies to wood structural design. 2. Similarities and differences with respect to ASD, design values, and behavioral equations. 3. Format and content within the 2005 NDS. 4. Changes in the 2012 NDS and Supplement relative to previous editions. 1

2 Outline NDS History 1991 Overview LRFD Primer Chapter-by-chapter discussion Changes from previous editions Summary More information NDS History IBC 2006 SECTION 2305 GENERAL DESIGN REQUIREMENTS FOR LATERAL-FORCE-RESISTING SYSTEMS General. Structures using wood shear walls and diaphragms to resist wind, seismic and other lateral loads shall be designed and constructed in accordance with the provisions of this section. Alternatively, compliance with the AF&PA SDPWS shall be permitted subject to the limitations therein and the limitations of this code. SECTION 2306 ALLOWABLE STRESS DESIGN Allowable stress design. The structural analysis and construction of wood elements in structures using allowable stress design shall be in accordance with the following applicable standards: American Forest & Paper Association. NDS National Design Specification for Wood Construction Load and resistance factor design. The structural analysis and construction of wood elements and structures using load and resistance factor design shall be in accordance with AF&PA NDS. 2

3 NDS and Supplement Governing Codes for Engineered Wood Design Chapters 14 Appendices 2005 SDPWS (Special Design Provisions for Wind and Seismic) html Free download Future for Engineered Wood Design 2009 IBC (International Building Code) IBC 2009 SECTION 2305 GENERAL DESIGN REQUIREMENTS FOR LATERAL- FORCE-RESISTING SYSTEMS General. Structures using wood shear walls and diaphragms to resist wind, seismic and other lateral loads shall be designed and constructed in accordance with the provisions of this section. Alternatively, compliance with the AF&PA SDPWS shall be permitted subject to the limitations therein and the limitations of this code. and the provisions of Sections 2305, 2306 and SECTION 2306 ALLOWABLE STRESS DESIGN Allowable stress design. The structural analysis and construction of wood elements in structures using allowable stress design shall be in accordance with the following applicable standards: American Forest & Paper Association. NDS National Design Specification for Wood Construction SDPWS Special Design Provisions for Wind and Seismic Load and resistance factor design. The structural analysis and construction of wood elements and structures using load and resistance factor design shall be in accordance with AF&PA NDS and AF&PA SDPWS. 3

4 NDS and Supplement Chapters 14 Appendices Engineered Wood Design 2008 SDPWS (Special Design Provisions for Wind and Seismic) mandatory mic.pdf Free download NDS 2012 Approval ANSI approval August 15, IBC Reference IBC 2009 SECTION 2305 GENERAL DESIGN REQUIREMENTS FOR LATERAL- FORCE-RESISTING SYSTEMS General. Structures using wood frame shear walls and or wood frame diaphragms to resist wind, seismic and or other lateral loads shall be designed and constructed in accordance with AF&PA SDPWS and the applicable provisions of Sections 2305, 2306 and SECTION 2306 ALLOWABLE STRESS DESIGN Allowable stress design. The design structural analysis and construction of wood elements in structures using allowable stress design shall be in accordance with the following applicable standards: American Forest & Paper Association. NDS National Design Specification for Wood Construction SDPWS Special Design Provisions for Wind and Seismic Load and resistance factor design. The structural analysis design and construction of wood elements and structures using load and resistance factor design shall be in accordance with AF&PA NDS and AF&PA SDPWS. 4

5 Engineered Wood Design 2008 SDPWS (Special Design Provisions for Wind and Seismic) mandatory mic.pdf Free download NDS and Supplement Chapters 14 Appendices Chapters 14 Appendices NDS & SDPWS Engineered Wood Design Publications Updates/Errata Comprehensive List Free download 2005 NDS & SDPWS 2012 NDS & 2008 SDPWS 5

6 Outline Design Process Overview LRFD Primer Chapter-by-chapter discussion Changes from previous editions Summary More information Demand Capacity Design Process Design Concepts Load Support Conditions Geometry Materials Performance Fire Economics Aesthetics. Demand Capacity Two Limit State concerns: safety against failure or collapse Serviceability (performance in service) 6

7 Serviceability LRFD - Safety Unfactored loads Mean (avg) material strength values Factored loads Material strength values - modified Property Variability Engineered Wood Design Normal Distribution Curves x = mean x x x x = standard deviation SCL Relative Frequency x COV x = x Coefficient of variation Load I-Joist Glulam MSR Lumber Visually Graded Lumber S > R Failure Material Property Values 7

8 Statistical Model Normal Distribution Curves for Safety Function, Z f Z = f R -f S m Z = m R -m S z 2 R 2 S Safety (or reliability ) Index Probability of Failure P f = one failure expected for x number of structures designed and built with a given Ex. = 2.7 represents 1 failure for every 100 structures or members designed. P f : 10,000, : 1,000, : 100, : 10, : 1, : : 10 m z z Probability of failure of structure Performance Distribution (Z) LRFD - Range on Structural Design Range for Strength Various Materials Low Typical High P f 1 : 25 1 : 63 1 : 251 LRFD Design Equation Demand Capacity n Q R n i=1 = Load Factor = Reliability Index = time effect factor (replaces LDF) 8

9 Allowable Stress Design What stays the same? Same basic equation format Same adjustment factors Same behavioral equations LRFD vs. ASD Three new notations -,, and K F Design loads (factored) for safety are bigger Design loads (unfactored) for serviceability are the same Material resistance values are bigger Load Duration Factor changes to Time Effect Factor LRFD vs. ASD LRFD vs. ASD ASD applied stress allowable stress Theoretical safety margin applied to material stresses LRFD Member performance factored load factored resistance factor Load factors to account for variations in loads Estimated loads Design Load Adjusted Resistance Tested material strength Estimated loads Factored Design Load Factored Design Resistance Tested member resistance Design values Design values 9

10 2012 NDS Factored Load Combinations ASCE NDS tied to ASCE 7 Factored Loads: Baseline 10 minutes (ASD uses 10 years) Permanent Long term Short term 2012 NDS NDS 2012 LRFD Specification Format Conversion Factor K F : ASD R N = R ASD Reliability indices or data confidence factors LRFD R N = K F R ASD R ASD reference strengths 10

11 2012 NDS R N = K F R ASD K F converts reference design values (ASD normal load duration) to LRFD reference resistance Why use LRFD? Ease of designing with multiple materials Does not penalize material strength for unknowns on loads Realize efficiencies with multiple transient live loads extreme event loads ASD load combinations have not been maintained in deference to LRFD load combinations Outline Overview LRFD Primer Chapter-by-chapter discussion Changes from previous editions Summary More information NDS 2012 Chapters 1. General Requirements for Building Design 2. Design Values for Structural Members 3. Design Provisions and Equations 4. Sawn Lumber 5. Structural Glued Laminated Timber 6. Round Timber Poles and Piles 7. Prefabricated Wood I-Joists 8. Structural Composite Lumber 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 Commentary!!! 11

12 NDS 2012 Supplement 1 Sawn Lumber Grading Agencies 2 Species Combinations 3 Section Properties 4 Reference Design Values - Lumber and Timber - Non-North American Sawn Lumber - Structural Glued Laminated Timber - MSR and MEL NDS 2012 Appendices A. Construction and Design Practices B. Load Duration (ASD Only) C. Temperature Effects D. Lateral Stability of Beams E. Local Stresses in Fastener Groups F. Design for Creep and Critical Deflection Applications G. Effective Column Length H. Lateral Stability of Columns I. Yield Limit Equations for Connections J. Solution of Hankinson Equation K. Typical Dimensions for Split Ring and Shear Plate Connectors L. Typical Dimensions for Standard Hex Bolts, Hex Lag Screws, Wood Screws, Common, Box, and Sinker Nails M. Manufacturing Tolerances for Rivets and Steel Side Plates for Timber Rivet Connections N. Appendix for Load and Resistance Factor Design (LRFD) Mandatory NDS Chapter 1 Chapter 1 - Terminology f b F b ' Reference design values (F b, F t, F v, F c, F c, E, E min ) Adjusted design values (F b ', F t ', F v ', F c ', F c ', E', E min ') Allowable (changed in the 2005) 12

13 Chapter 1 Design Loads NDS Chapter 2 Reference loads Minimum load standards ASCE 7 10 Chapter 2 Adjustment Factors Adjusts from reference to site conditions C D, time-dependent C M wet service C t temperature Chapter 2 Adjustment Factors Adjust from reference conditions K F LRFD format conversion factor LRFD resistance factor Revised 13

14 Chapter 2 Adjustment Factors Wet Service Factor, C M Wet Service Conditions 30 Wood EMC % Relative Humidity % Temp 30 deg F Temp 70 deg F Temp 130 deg F Wet Service Conditions Wet Service Factor, C M %Strength at 12% Moisture Content Impact Strength Modulus of Elasticity Modulus of Rupture Crushing Strength NDS Supplement for lumber Moisture Content of Wood (%) 14

15 NDS Chapter 3 Chapter 3 Behavioral Equations ASD vs LRFD adjusted stresses from reference ASD F n = F n C D x adjustment factors LRFD F n = F n K F n x adjustment factors Chapter 3 Behavioral Equations Beams C L beam stability Chapter 3 Behavioral Equations Beams F be Equivalence F be 1.20E R ' min 2 b K E ' be 2 Rb 2012/ NDS NDS - E min adjusted for safety for both ASD and LRFD processes - R B = Slenderness Ratio Critical Buckling Design Value for bending members 15

16 Chapter 3 Behavioral Equations Columns C P column stability Chapter 3 Behavioral Equations Columns F ce equivalence F ce 0.822E le d ' min /2005 NDS K E d ' ce 2 le 2001 NDS Critical Buckling Design Value for compression members Chapter 3 Behavioral Equations E min 1.03E( (COV ))/1.66 E = reference MOE 1.03 = adjustment factor to convert E to a pure bending basis (shear-free) (use 1.05 for glulam) 1.66 = factor of safety COV E = coefficient of variation in MOE (NDS Appendix F) OR E min values published in NDS Supplement E Chapter 3 Behavioral Equations Tension members (tension parallel to grain) ASD F t = F t C D x adjustment factors LRFD F t = F t K F t x adjustment factors 16

17 Chapter 3 Behavioral Equations Wood and tension perpendicular to grain Not recommended per NDS Chapter 3 Behavioral Equations Combined bi-axial bending and axial compression initiators: notches moment connections hanging loads Chapter 3 Behavioral Equations Combined bi-axial bending and axial compression Chapter 3 Behavioral Equations Bearing perpendicular to grain F c = F c C M C t C i C b (ASD) F c = F c C M C t C i C b K f c (LRFD) New 17

18 NDS Chapter 4 Chapter 4 Lumber Design values Visually graded lumber MSR / MEL Timber Decking Chapter 4 Lumber Lumber adjustment factors K F and Chapter 4 Lumber Lumber adjustment factors New K F and 18

19 Chapter 4 Lumber Lumber adjustment factors C F - size factor Chapter 4 Lumber Lumber adjustment factors Chapter 4 Lumber Lumber adjustment factors repetitive member C r = lumber < 24 o.c. 3 or more Load distributing element NDS Chapter 5 19

20 Chapter 5 Glued Laminated Timber Significant changes New adjustment factors Stress interaction Shear reduction Clarified or added Curved members Double-tapered Tapered straight Chapter 5 Glulam New adjustment factors Stress interaction Shear reduction Chapter 5 Glulam Clarified or added Curved members Double-tapered Tapered straight Chapter 5 Glulam Adjustment factors C V volume Not cumulative with C L Min (C v, C L ) 20

21 NDS Chapter 6 Chapter 6 Poles & Piles Poles - post-frame Piles - foundations Chapter 6 Timber Piles Design values Significant changes from 2005 NDS Design values moved to NDS Supplement Chapter 6 Timber Poles 2005 NDS 2005 NDS 2012 NDS 2012 NDS 21

22 Chapter 6 Poles & Piles Adjustment factors C ct condition treatment C ls load sharing C cs critical section NDS Chapter 7 Chapter 7 I-joists Design values M, V, EI, K no changes Evaluation Reports Contain proprietary design Chapter 7 I-Joists Beam stability factor Braced compression flange C L = 1.0 Unbraced compression flange Design as unbraced column 22

23 NDS Chapter 8 Chapter 8 Structural Composite Lumber No changes from 2005 NDS Evaluation Reports Contain proprietary design Relative Frequency x = mean COV x x = x Load x SCL I-Joist Glulam MSR Lumber Visually Graded Lumber Material Property Values Chapter 8 Structural Composite Lumber Adjustment factors C V volume C v < 1.0 Not cumulative with lateral stability factor, C L then min. (C v, C L ) C v > 1.0 Cumulative with lateral stability factor, C L Chapter 8 Structural Composite Lumber Adjustment factors C r Repetitive Member Factor= 1.04 C r is different than lumber (C r lumber = 1.15) Applies to F b only 23

24 NDS Chapter 9 Chapter 9 Wood Structural Panels Design values obtain from an approved source F b S F t A F v t v F s F c A EI EA G v t v F c Chapter 9 Wood Structural Panels Adjustment factors C G - grade & construction Removed C s - panel size Clarified Moved from commentary Chapter 9 Wood Structural Panels Adjustment factors C M - wet service C t - temperature 24

25 NDS Chapter 10 Chapter 10 Mechanical Connections Design issues Reference design values Chapter 11 dowel-type connectors (nails, bolts, lag/wood screws) Chapter 12 split rings and shear plates Chapter 13 timber rivets Adjustment factors No significant changes Connections session NDS Chapter 11 Chapter 11 - Tabulated Values Consistent titles and footnotes Penetration assumptions in titles 25

26 Chapter 11 - Tabulated Values New post frame ring shank tables Based on ASTM F1667 Chapter 11 - Dowel Bearing Length Fastener Type Tip Length, E Lag Screws Wood Screws Nails & Spikes Appendix L 2D 2D E, l s, l m < p E / 2 Chapter 11 - Dowel Bearing Strength Wood Structural Panels D <¼ Chapter 11 - Perp to Grain Distance Glulam only Moisture content 26

27 NDS Chapter 12 Chapter 12 Split Rings and Shear Plates Geometry factor, C Side Grain Chapter 12 Split Rings and Shear Plates NDS Chapter 13 Geometry factor, C End Grain 27

28 Chapter 13 Timber Rivets Many applications Chapter 13 Timber Rivets Chapter 13 Timber Rivets Parallel to grain Timber rivet capacity Proper application of C D Timber Rivets Design 2005 NDS 28

29 Timber Rivets Design 2012 NDS Chapter 13 Timber Rivets Maximum distance perpendicular to grain between outermost rows of rivets shall be 12 Consistent with glulam NDS Chapter 14 NDS Chapter 14 29

30 Chapter 14 Shear Walls and Diaphragms NDS Chapter 15 ANSI / AWC SDPWS 2008 standard Recorded Webinar 2005/2008 SDPWS Diaphragm Deflection Chapter 15 Special Loading NDS Chapter 16 Built-up columns Flatwise bending check Consistent with Chapter 3 30

31 Chapter 16 Fire Design No significant changes Chapter 16 Fire (ASD) Fire resistance up to two hours Columns Beams Tension Members ASD only Products Lumber Glulam SCL Decking Chapter 16 Fire (ASD) NDS Appendices DCA 2 Design of Fire-Resistive Exposed Wood Members Limited to 1-Hour IBC Simplified approach Beams/Columns 31

32 NDS 2012 Appendices 2012 A Construction and Design Practices B Load Duration C Temperature Effects D Lateral Stability of Beams E Local Stresses in Fastener Groups F Design for Creep and Critical Deflection Applications G Effective Column Length H Lateral Stability of Columns I Yield Limit Equations for Connections J Solution of Hankinson Equation K Typical Dimensions for Split Ring and Shear Plate Connectors L Typical Dimensions for Standard Hex Bolts, Hex Lag Screws, Wood Screws, Common, Box, and Sinker Nails M Manufacturing Tolerances for Rivets and Steel Side Plates for Timber Rivet Connections N Appendix for Load and Resistance Factor Design (LRFD) Appendix E Example E.8 A critical check Appendix L Appendix N 32

33 NDS Commentary Outline Overview LRFD Primer Chapter-by-chapter discussion Changes from previous editions Summary More information 2012 NDS Notable Changes Chapter 5 Glulam Chapter 6 Poles and Piles Chapter 12 Split Rings & Shear Plates NDS 2012 Supplement New nominal and minimum Timber sizes per PS Section properties distinguish lumber, P&T, B&S New Coast Sitka Spruce & Yellow Cedar values Revised Northern Species bending and tension values Clarify Timber size factor adjustments New and revised values for several foreign species Revised glulam values - primary changes to shear New Tables 6A & 6B for Timber Poles and Piles 33

34 Southern Pine Design Values Wood Design Package ALSC approves design values June 1, 2012 AWC compiles them NDS Supplement More information Ww.spib.org Support Documents 2 nd Quarter 2012 Wood Design Package NDS + Commentary NDS Supplement ASD/LRFD Manual MANUAL Print copies available then Supplement: Design Values for Wood Construction More Details White paper - Comprehensive table Section-by-section changes Structure Magazine January 2012 Engineered Wood Design NDS Changes Web.pdf Free download 34

35 More Details 2008 SDPWS (Special Design Provisions for Wind and Seismic) Changes 08.pdf Free download More Details RFDStrucEng.pdf White paper LRFD Questions? This concludes The American Institute of Architects Continuing Education Systems Course 35