Structural Design for Wind Loads: An Overview of Engineering Considerations for Wood

Transcription

1 Structural Design for Wind Loads: An Overview of Engineering Considerations for Wood Lori Koch, PE Manager, Educational Outreach American Wood Council Buildings

2 Copyright Materials This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written permission of AWC is prohibited. American Wood Council

3 The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course 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 course 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. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. 3

4 Course Description This course is intended for structural engineers and building designers seeking an overview of design steps, considerations and detailing best practices related to the wind-resistive design and analysis of non-residential and multi-family wood buildings. Developed in response to the fact that engineering curricula does not typically include a wood design course, it provides an overview of relevant 2012 International Building Code (IBC) provisions and American Wood Council (AWC)-referenced standards, a discussion of common design errors, and guidance related to load path continuity, shear walls, diaphragms, and uplift restraint. Topics covered in detail include: 2012 IBC and AWC-referenced standards applicable to wood-frame wind-resisting systems Uplift-resistant design solutions, including combined shear and uplift Out-of-plane wall design for wind loads, including tall walls, gable end walls, and tall walls with large openings, as well as deflection criteria for a variety of wall finishes Design and stiffness assumptions for rigid, semi-rigid and flexible diaphragms Diaphragm deflection and design of components such as chords and collectors Shear wall design methods, including segmented, perforated and force transfer around openings Design of shear wall components, including holdowns and drag struts Multi-story wood-frame design example for high winds, including accommodation of accumulated forces and deflections as well as design and detailing for wood s interface with concrete podiums and foundations 4

5 Objectives Upon completion, participants will understand: 1. Review applicable building codes and reference standards for the design of non-residential and multi-family wood-frame windresisting systems. 2. Discuss design and detailing of wood-frame diaphragms, including flexibility, deflection, boundary fasteners, and chords and collectors. 3. Explore design steps for wood-frame shear walls, including panel edge nailing, end posts and holdowns, drag struts, and transfer mechanisms to lower floors and foundations. 4. Examine other wind-related design considerations, such as uplift and out-of-plane wall forces, and discuss the design and detailing effects of stacking multiple stories of wood-frame lateral forceresisting systems. 5

6 Outline ANSI NDS SDPWS WFCM Summary More Info 6

7 ANSI Accreditation AWC ANSI-accredited standards developer Consensus Body Wood Design Standards Committee 7

8 Outline ANSI NDS SDPWS WFCM Summary More Info 8

9 NDS History

10 Governing Codes for Wood Design 2015 NDS referenced in 2015 IBC 10

11 2015 NDS Primary Change New Provisions to Address CLT Charging Language Design Values Design Equations Product Chapter Connection Design Fire Design 11

12 2015 NDS Chapter Reorganization 2012 NDS 1-3 General 4-9 Products Connections 14 Shear Walls & Diaphragms 15 Special Loading 16 Fire 2015 NDS 1-3 General 4-10 Products +CLT Connections Shear Walls & Diaphragms 15 Special Loading 16 Fire 12

13 NDS 2015 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. Cross-Laminated Timber 11. Mechanical Connections 12. Dowel-Type Fasteners 13. Split Ring and Shear Plate Connectors 14. Timber Rivets 15. Special Loading Conditions 16. Fire Design of Wood Members 13

14 NDS 2015 Supplement 1. Sawn Lumber Grading Agencies 2. Species Combinations 3. Section Properties 4. Reference Design Values Sawn Lumber and Timber MSR and MEL Decking Non-North American Sawn Lumber Structural Glued Laminated Timber Timber Poles and Piles 14

15 NDS 2015 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 15

16 NDS Chapter 1 16

17 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 ') 17

18 Chapter 1 Design Loads Reference loads Minimum load standards ASCE

19 NDS Chapter 2 19

20 2015 NDS Format Conversion Factor K F ASD R N = C D R ASD LRFD R N = φ λ K F R ASD R ASD reference strengths 20

21 NDS Chapter 3 21

22 Chapter 3 CLT Design Equations New 22

23 Chapter 3 CLT Design Equations New NDS Commentary guidance on C P 23

24 NDS Chapter 4 24

25 Chapter 4 Lumber Design values Visually graded lumber MSR / MEL Timber Decking 25

26 Chapter 4 Lumber Lumber adjustment factors 26

27 NDS Chapter 5, 6, and 7 27

28 NDS Chapter 8 28

29 Chapter 8 Structural Composite Lumber New products Laminated Strand Lumber (LSL) Oriented Strand Lumber (OSL) ASTM D

30 NDS Chapter 9 30

31 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^ 31

32 Chapter 9 Wood Structural Panels Adjustment factors 32

33 NDS Chapter 10 New 33

34 What is Cross Laminated Timber (CLT)? Photos provided by FPInnovations

35 Chapter 10 Cross-Laminated Timber New 35

36 Chapter 10 Cross-Laminated Timber New 36

37 Chapter 10 Cross-Laminated Timber New 37

38 NDS Chapter 11 Ch 10 Ch 11 38

39 NDS Chapter 12 Ch 11 Ch 12 39

40 Chapter 12 Dowel-type Fasteners New 40

41 NDS Chapter 13 Ch 12 Ch 13 41

42 NDS Chapter 14 Ch 13 Ch 14 42

43 NDS Chapter 15 43

44 Chapter 15 Special Loading 44

45 NDS Chapter 16 45

46 Chapter 16 Fire (ASD) Fire resistance up to two hours Columns Beams Tension Members ASD only Products Lumber Glulam SCL Decking CLT - NEW 46

47 2015 NDS Supplement New Southern Pine Design Values ALSC approves design values June 1, 2013 AWC compiles them NDS Supplement More information

48 Outline ANSI NDS SDPWS WFCM Summary More Info 48

49 Code Acceptance of Standard 2015 IBC References 2015 SDPWS in Section 2305 for lateral design and construction

50 General Overview Outline Chapter 1: Flowchart Chapter 2: General Design Requirements Chapter 3: Members and Connections Chapter 4: Lateral Force Resisting Systems

51 Chapter 2 General Requirements Terminology Definitions New Flexible and Rigid Diaphragm removed

52 Chapter 3 - Members and Connections Framing Sheathing Connections Covers out-of-plane wind load resistance of shear walls and diaphragms

53 Chapter 3 - Members and Connections Framing walls Accounts for composite action Strength and Stiffness Applies now to EI Up to 24 o.c. New Extension of 1.15 repetitive member factor, C r

54 Chapter 3 - Members and Connections Uplift Resisting Systems New

55 Chapter 3 - Members and Connections 3.4 Uplift Force Resisting Systems New

56 Chapter 4 Anchorage of Concrete or Masonry Walls Anchorage of Concrete or Masonry Walls to Diaphragm SDC C, D, E, or F New

57 Chapter 4 Anchorage of Concrete or Masonry Walls Pre-fabricated roof sections lifted into place Safety, cost, speed Two common types All wood Hybrid

58 Chapter 4 Nominal Design Value Revised

59 Chapter 4 Nominal Design Value Diaphragm Configuration Figures Direction with respect to load of Continuous panel joints Framing members Independent of panel orientation Revised

60 Chapter 4 Horizontal Distribution Revised Horizontal Distribution of Shear Idealized as Flexible ASCE , or D DIAPHRAGM > 2 x D SHEARWALLS tributary area Idealized as Rigid ASCE , or D DIAPHRAGM < 2 x D SHEARWALLS relative lateral stiffness of vertical LFRS Semi-rigid complex analysis or envelope (NEW)

61 Chapter 4 - Lateral Force-Resisting Systems Torsional Irregularity Rigid or Semi-rigid Torsionally Irregular Story Drift D A max > 1.2 D B & C Average

62 Chapter 4 - Lateral Force-Resisting Systems Revised Torsional Irregularity SDC A Exempt Rigid or Semi-rigid WSP diaphragms L/W < 1.5:1 Diagonal Lumber (single or double layer) L/W < 1:1 Δ A max < ASCE 7 allowable story drift

63 Chapter 4 Open Front Diaphragms Revised

64 Chapter 4 Open Front Diaphragms Revised Open Front Structures Not Torsionally Irregular WSP diaphragms L /W < 1.5:1 Diagonal Lumber (single or double layer) L /W < 1:1 Torsionally Irregular > 1-story L /W < 0.67:1 1-story L /W < 1:1

65 Chapter 4 Open Front Diaphragms Revised Open Front Structures Load parallel to opening - model as semi-rigid or rigid D A max < ASCE 7 allowable story drift L < 35 Exception: Cantilever < 6 beyond nearest vertical LFRS need not comply to

66 Chapter 4 Open Front Diaphragms Revised Open Front Structures 1 story L < 25 L /W < 1:1 Idealized as rigid - distribution of torsional shear 20 40

67 Chapter 4 High Load Diaphragms Blocked Diaphragm Configuration Figures Direction with respect to load of Continuous panel joints Framing members Independent of panel orientation Revised

68 Chapter 4 High Load Diaphragms High Load Blocked Diaphragms 4. The depth of framing members and blocking into which the nail penetrates shall be 3" nominal or greater The width of the nailed face of framing members and blocking at boundaries and adjoining panel edges shall be 3" nominal or greater. The width of the nailed face not located at boundaries or adjoining panel edges shall be 2" nominal or greater.

69 Chapter 4 - Lateral Force-Resisting Systems Wood Shear Walls

70 Chapter 4 - Lateral Force-Resisting Systems Structural Fiberboard Shear Walls NEW

71 Chapter 4 Design Value Format Nominal design values tabulated for shear walls ASD reduction factor (2.0) LRFD resistance factor f(0.80)

72 Chapter 4 - Lateral Force-Resisting Systems Shears Walls in a Line: same materials and construction Individual full height shear walls provide all same deflection, d sw Exception: WSP h/b s > 2:1 v s x 2b s /h Fiberboard h/b s > 1:1 v s x ( b s /h) Shear distribution proportional to capacities Shear capacity reduction not combined with aspect ratio adjustment ( ) REVISED 2:1 unless v s = 2(b s /h) 2:1 3:1 3½:

73 Chapter 4 - Unblocked Shear Walls Section Nails 6" panel edge spacing Up to 2:1 aspect ratio 16' height limit Based on cyclic testing Shear capacity reduction

74 Chapter 4 - Unblocked Shear Walls Deflection ( ) Less stiffness Deflection amplified by C ub v C ub

75 Chapter 4 - Lateral Force-Resisting Systems Wood Shear Walls Footnotes moved to text and revised to reflect recent testing

76 Chapter 4 Aspect Ratios & Capacity Adjustments Revised Shear Wall Aspect Ratio Factors h/b s > 2:1 WSP v s x ( h/b s ) h/b s > 1:1 Struct. Fiberboard v s x ( h/b s ) 2:1 unless v s = ( (h/b s )) 2:1 3:1 3½:

77 Chapter 4 Aspect Ratios & Capacity Adjustments Revised Perforated Shear Walls h/b s > 3.5:1 Not considered h/b s > 2:1 L i = L (2b s /h) Aspect Ratio Factors ( ) do not apply Shear distribution exceptions ( ) do not apply

78 What s Missing for CLT? Lateral Design! ASCE 7 Minimum Design Loads for Buildings and Other Structures Response Modification Coefficient, R CLT not recognized system in ASCE 7 Table Options Performance-based design Demonstrate equivalence to existing ASCE 7 system ASCE 7-10, FEMA P695, and FEMA P795 Quantification of Building Seismic Performance Factors; Component Equivalency Methodology CLT floors with traditional shear walls 78

79 Outline ANSI NDS SDPWS WFCM Summary More Info 79

80 WFCM and IRC/IBC 2001 WFCM 2003, 2006, 2009 IRC/IBC 2012 WFCM 2012 IRC/IBC 2015 WFCM 2015 IRC/IBC

81 WFCM and IRC R Alternative Provisions WFCM permitted R Wind Design For bldgs. where wind design is required or > 110 mph use one or more of the following: WFCM ICC 600 (newly referenced) ASCE 7 IBC

82 WFCM and IBC Chapter 16 Determination of Wind Loads Section of IBC ( ) Wind loads in accordance with ASCE 7 Exception: residential structures per WFCM Not used for structures on hills, ridges, or escarpments Chapter 23 Wood design & 2309 conventional construction

83 2015 IBC

84 2015 IBC Applications Single-story Slab-on-grade L and W < 80 Examples Restaurants Office Buildings Design Lateral Gravity

85 WFCM Chapter 1: General Chapter 2: Engineered Design Chapter 3: Prescriptive Design General outline Chapters 2-3 Connections Floor systems Wall systems Roof systems Supplement

86 Chapter 1: General Mean roof height < 33 ft < 3 stories Building length/width < 80 ft Loads ASCE psf ground snow load mph wind speed 700 yr. return, 3-sec gust, Exp. B, C, D Seismic Design Categories A, B, C, D 0, D 1, D 2

87 Chapter 2: Engineered Loads and load path Continuous Continuity created by connections Always ends in supporting soil Building has hundreds of load paths Source: FEMA

88 Chapter 2: Engineered Suction loads

89 Chapter 3: Prescriptive Segmented shear walls - wind

90 2015 WFCM Changes Rafters/Ceiling Joists Brittle Finishes New

91 2015 WFCM Changes Header Spans Supporting Roof and Ceiling New

92 Availability PDF versions Free view-only Buy a printable PDF Summer 2015 Commentary Printed version 92

93 Questions? This concludes The American Institute of Architects Continuing Education Systems Course 93