DES431 - Demystifying Diaphragm Design · Education Systems (AIA/CES), Provider # 50111237. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members.
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Participants may download the presentation here: http://www.awc.org/education/resources
The American Wood Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider # 50111237.
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.
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COURSE DESCRIPTIONThe 2018 International Building Code (IBC) specifies that structures using wood-framed shear walls and diaphragms to resist wind, seismic and other lateral loads shall be designed and constructed in accordance with AWC’s 2015 Special Design Provisions for Wind and Seismic (SDPWS) or 2018 Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings. Both code-referenced standards provide procedures for designing diaphragms for wood construction. This presentation will demystify diaphragm design by providing wind and seismic design examples for in-plane lateral design of wood- and gypsum-sheathed diaphragms including a brief overview of high-load diaphragms.
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GENERAL LATERAL LOAD PATH
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Load Path
Ties
ChordT T
ChordC C
w
Diaphragm reaction goes to shear walls
Diaphragms supporting Masonry or Concrete Structural Walls -Seismic Design Category C, D, E, & FDiaphragms shall be provided with continuous ties or struts between diaphragm chords to distribute these anchorage forces into the diaphragms (ASCE 7 - 16 sec. 12.11.2.2.1)
WFCMWOOD FRAME CONSTRUCTION MANUALfor One- and Two-Family Dwellings
AMERICANWOODCOUNCIL
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WOOD FRAME CONSTRUCTION MANUALfor One- and Two-Family Dwellings
WFCM2 0 1 8E D I T I O N
W O R K B O O KDESIGN OF WOOD FRAME BUILDINGS FOR HIGH WIND, SNOW, AND SEISMIC LOADS
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BUILDING DESCRIPTION
Figure 1: Isometric view (roof overhangs not shown).
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BUILDING DESCRIPTION
North
Wall Heights = 9' Windows Finished Grade to Foundation Top = 1' Typical 3'x4'-6" Floor Assembly Height = 1' Foyer 6'x4'-6" Roof Pitch = 7:12 Kitchen 4'x4'-6" House Mean Roof Height = 24.7' Bath 4'x6' Roof Overhangs = 2' Doors Building Length (L) = 40' Typical 3'x7'-6" Building Width (W) = 32' Foyer 6'x7'-6" Top plate to ridge height = 9.3' Kitchen 9'x7'-6"
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LOADS ON THE BUILDING
Structural systems in the WFCM 2018 Edition have been sized using dead, live, snow, seismic and wind loads in accordance with ASCE/SEI 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Provisions of the 2018 International Residential Code (IRC) are referenced as needed.
Lateral Loads:
Wind:
3-second gust wind speed in Exposure Category B (700 yr. return) = 160 mph
Vertical force distribution factor (F) – (ASCE 7-16 Section 12.14.8.1) = 1.2
Gravity Loads*:
Roof: Roof Dead Load = 10 psf Ground Snow Load, Pg = 30 psf Roof Live Load = 20 psf Ceiling: Roof Ceiling Load = 10 psf *Assumptions vary for wind and seismic dead loads Deflection limits per 2018 IRC Roof Rafters with flexible Ceiling Attached L/Δ = 240 Roof Rafters with no Ceiling Attached L/Δ = 180 Raised Ceiling Joists with flexible finish L/Δ = 240 Floor Joists L/Δ = 360 Exterior Studs (gypsum interior) H/Δ = 180
Note: See comparable deflection limits in 2018 IBC section 2308 for joists and rafters.
Floors: First Floor Live Load = 40 psf Second Floor Live Load = 30 psf Attic Floor Live Load = 30 psf Floor Dead Load = 10 psf
Walls: Wall Dead Load = 11 psf
d. Deflection for exterior walls with interior gypsum board finish shall be limited to an allowable deflection of H/180.
2018 International Residential Code for One- and Two-Family
Dwellings, International Code Council, Inc., Washington, DC. Reproduced with permission. All rights reserved. www.iccsafe.org
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WFCM APPLICABILITY LIMITATIONS
The following table is used to determine whether the building geometry is within the applicability limitations of the WFCM. Conditions not complying with the limitations shall be designed in accordance with accepted engineer practice (see WFCM 1.1.3).
Table W1.1 Applicability Limitations
Attribute Limitation Design Case
BUILDING DIMENSIONS
Mean Roof Height (MRH) maximum 33' 24.7'
Number of Stories maximum 3 3
Building Dimension (L or W) maximum 80' 40'
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PRESCRIPTIVE DESIGN LIMITATIONS
The following table is used to determine whether the building geometry is within the applicability limitations of the WFCM Chapter 3 prescriptive provisions. Conditions not complying with the limitations shall be designed in accordance with WFCM Chapter 2 (see WFCM 3.1.3).
Table W1.2 Prescriptive Design Limitations
Element Attribute Limitation Design
Case
FLOOR SYSTEMS (3.1.3.2)
Lumber
Joists
Joist Span 26' 16'
Joist Spacing 24" 16"
Cantilevers/Setback - Supporting loadbearing walls d N/A
Table 3.16A1 Roof Diaphragm Limits for Wind (Applicable to All Roof Slopes with and without Roof Irregularities)
Description: Minimum and maximum roof diaphragm lengths for given roof diaphragm widths and wind speeds based on roof diaphragm limitations.
Procedure: Using the procedures from Chapter 2 for calculating wind loads, determine the minimum and maximum roof diaphragm lengths permitted based on roof dia-phragm capacity. To account for possible irregularities in roof shape caused by roof features such as dormers or complex roof framing layouts, wind loads are cal-culated assuming conservative pressure coefficients regardless of the actual roof pitch associated with the top plate-to-ridge height and roof diaphragm width dimen-sions.
Background: Capacities for horizontal diaphragm as-semblies (roof and floor) are based on sheathing thickness, nail size, panel edge nailing, and supported panel edges.
Example: Given – 150 mph, Exposure B, 1 story slab-on-grade, 10' maximum top plate to ridge height, 24' roof diaphragm width.
RoofDiaphragmLength
RoofDiaphragm
Direc�on ofRoof Framing
Roof Sheathing Applied withLong DimensionPerpendicular to Roof Framing10 � Top Plate
to Ridge Height
24 �Roof Diaphragm
Width
The minimum diaphragm aspect ratio limit is equal to 0.33 (1:3) and the maximum diaphragm aspect ratio limit is equal to 3.0 (3:1).
Calculation of Minimum Roof Diaphragm Length
Calculate minimum roof diaphragm length, LMin, based on the maximum value obtained from the following two calculations: a) Calculate the roof diaphragm length required based on wind load acting parallel to the ridge (Note: pressure coefficients for wind acting parallel to ridge are constant and do not vary with roof pitch):
Roof diaphragm load for a top plate to ridge height of 10 feet (i.e. 10:12 roof pitch for a roof diaphragm width of 24 feet): WRD = 155 plf (Table 2.5B)
Calculate load into shear wall:
From roof diaphragm: V = [155 plf (24 ft)] / 2
= 1,860 lbs
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3:
vRD = 450 plf / (2.0) (3/8 nominal panel thickness, 2" nominal width framing, and Case 3)
b) Calculate the minimum roof diaphragm length needed based on aspect ratio limits:
LMin = 24 ft / 3
= 8 ft
The minimum roof diaphragm length is:
LMin = 9 ft (WFCM Table 3.16A1)
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Calculation of Maximum Roof Diaphragm Length
Calculate the maximum roof diaphragm length , LMax, based on the minimum value obtained from the following three calculations: a) Calculate roof diaphragm length needed based on the wind load acting perpendicular to the ridge:
Roof diaphragm load for a top plate to ridge height of 10 feet utilizing conservative pressure coefficients associated with a 12:12 roof pitch are calculated as follows:
The lateral load on the roof diaphragm will take load from half the wall below and load directly applied to the diaphragm.
Calculate wind forces in the roof diaphragm:
p = q(GCpf - GCpi)where:
p = pressure on the roof/walls
q = 21.15 psf (See Table C1.1)
Calculated the average pressure on the wall for a 12:12 roof pitch:
Interior Zone End ZoneWindward Leeward Windward Leeward
The roof diaphragm will take load from half the wall below and load directly applied to the diaphragm.
WRD = 20.9(5 ft) + 14.4(10 ft)
= 249 plf
Calculate load into the shear wall:
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3:
vRD = 600 plf / 2.0 (3/8 nominal panel thickness, 2" nominal width framing, and Case 1)
vRD = 300 plf
Maximum sidewall length:
LMax = [300 (24 ft)] / [249 plf / 2]
= 57 ft (rounded down from 57.8 ft)
b) Maximum roof diaphragm length shall not exceed 80' (WFCM limitation):
LMax = 80 ft
c) Calculate maximum roof diaphragm length based on aspect ratio limits:
LMax = 24(3)
= 72 ft
The maximum roof diaphragm length is:
LMax = 57 ft (WFCM Table 3.16A1)
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PRESCRIPTIVE DESIGN
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Table 3.16B Floor Diaphragm Limits for Wind
Description: Minimum and maximum floor diaphragm lengths for given floor diaphragm widths and wind speeds based on floor diaphragm limitations.
Procedure: Using the procedures from Chapter 2 for calculating wind loads, determine the minimum and maximum floor dia-phragm lengths permitted based on floor diaphragm capacity.
Background: Capacities for horizontal unblocked diaphragm assemblies are tabulated based on a nominal panel thickness of 15/32", 8d common nails, 6" nail spacing at dia-phragm boundaries, and supported panel edges.
b) Calculate the minimum floor diaphragm length needed based on aspect ratio limits: LMin = 36 ft / 3
= 12 ft
The minimum floor diaphragm length is: LMin = 14 ft (WFCM Table 3.16B)
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Calculation of Maximum Floor Diaphragm Length
Calculate the maximum floor diaphragm length, LMax, based on the minimum value obtained from the following three calculations:
a) Calculate floor diaphragm length based on the wind load acting perpendicular to the ridge:
Floor diaphragm load: WFD = 251 plf (Table 2.5A)
Calculate load into shear wall:
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms (unblocked) and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3 and specific gravity adjustment factor [1 – (0.5 – G)] for G = 0.42 (Footnote 2 Table 4.2C): vFD = 670 plf [1 - (0.5 - 0.42)] / 2.0 (Case 1)
vFD = 308 plf
Maximum sidewall length:
LMax = [308 (36 ft)] / [251 plf / 2]
= 88.4 ft
b) Maximum floor diaphragm length shall not exceed 80 ft (WFCM limitation): LMax = 80 ft
c) Calculate maximum floor diaphragm length based on aspect ratio limits: LMax = 36 (3)
= 108 ft
The maximum roof diaphragm length is: LMax = 80 ft (WFCM Table 3.16B)
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PRESCRIPTIVE DESIGN
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Tables 3.16C1-C3 Diaphragm Limits for Seismic
Description: Maximum diaphragm dimension, L, for seismic.
Procedure: Using shear loads at each level, calcu-lated in accordance with Table 2.6 and Commentary to Table 2.6, determine the maximum diaphragm dimension, L, for seismic such that the allowable unit shear capacity of the reference diaphragm con-struction is not exceeded.
Background: Tabulated requirements for maximum diaphragm dimension, L, are provided for the reference unblocked diaphragm construction in the WFCM. The aspect ratio of the diaphragm is limited to 3:1 and a maximum dimension, L, not to exceed 80 ft.
Example: Given – Two-story building above grade plane with rectangular dimensions of 36' x 80' and Seismic Design Category C.
Roof/Ceiling
For roof diaphragm construction of 3/8" wood structural panel sheathing (unblocked), 8d common nails ‐ 6" edge spacing:
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms (unblocked) and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3:
vRD = 430 plf / 2.0 (Case 1)
vRD = 215 plf
The effective seismic weight resisted by the roof/ceil-ing diaphragm assuming loading perpendicular to the L dimension:
WRD = Wroof + W(wall L)/2 + Wpartition/2 + Wgable
= 75,120 lbs
where:
Wroof = weight of the roof which includes consideration of 2′ overhangs
= 15 psf [(36 ft + 4 ft) x (80 ft + 4 ft)]
= 50,400 lbs
W(wall L) = weight of exterior walls in the L dimension
110 plf (80 ft + 80 ft) = 17,600 lbs
Wpartition = weight of partition walls
= 8 psf (36 ft) (80 ft) = 23,040 lbs
Wgable = weight of the gable end wall
= 110 plf (2) (1/2) (80 ft) / 2 = 4,400 lbs
V = (75,120 lbs) (1.1) (SDS/R) (0.7) = 4,450 lbs
v = 4,450lbs / (2W) = 62 plf
Factor for 2 story: 62/0.92 = 67 plf
67 plf < 215 plf OK
Maximum diaphragm dimension, L, tabulated = 80 ft (WFCM Table 3.16C1)
Diaphragm load ratio = 67/215 = 0.31 (WFCM Table 3.16C1)
Floor
For floor diaphragm construction of 15/32" wood structural panel sheathing (unblocked), G = 0.42 framing, 8d com-mon nails ‐ 6" edge spacing:
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms (unblocked) and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3:
Wfloor = weight of the roof which includes consideration of 2′ overhangs
= 12 psf (36 ft) (80 ft) = 34,560 lbs
W(wall W) = weight of exterior walls in W dimension
= 110 plf (36 ft + 36 ft) = 7,920 lbs
W(wall L) = weight of exterior walls in L dimension
110 plf (80 ft + 80 ft) = 17,600 lbs
Wpartition = weight of partition walls
= 8 psf (36 ft) (80 ft) = 23,040 lbs
V = (79,160 lbs ) (1.1) (SDS/R) (0.7) = 4,689 lbs
v = 4,689 lbs / (2W) = 65 plf
Factor for 2-story: 65/0.92 = 71 plf
71 plf < 221 plf OK
Maximum diaphragm dimension, L, tabulated = 80 ft
(WFCM Table 3.16C1)
Diaphragm load ratio = 71/221 = 0.32
(WFCM Table 3.16C1)
Footnote 1The tabulated requirements are based on reference con-struction of roof and floor diaphragms with long dimension of panels perpendicular to framing members and joints staggered (i.e. Case 1, unblocked diaphragm per SDPWS).
Footnote 2Tabulated maximum length requirements can be adjusted to determine the maximum length for other cases. The ref-erence case is for 3-story construction, floor weights = 12 psf, roof/ceiling = 15 psf, partition = 8 psf, and wall=110 plf. The maximum aspect ratio of unblocked diaphragms is limited to 3:1 and L is limited to 80 ft.
Footnote 3The diaphragm load ratio is used to account for other than reference conditions associated with the tabulated load ratio.
The number of stories factor accounts for vertical distribu-tion factors associated with 1, 2, and 3 story buildings in accordance with ASCE 7-16. The reference case for Table 3.16C is a three-story building, hence, the adjustment for three-story is 1.0. Reduced shears are associated with two-story and one-story buildings. The two-story factor of 0.92 results from the ratio of 1.1/1.2 and the one-story factor of 0.83 results from the ratio of 1.0/1.2 (see WFCM Commentary to Table 2.6).
Tabulated requirements are based on use of the reference vertical system which is wood frame walls sheathed with wood structural panels. Where other sheathing materials are used, increased seismic loads are applicable in propor-tion to the ratio of the applicable seismic R values (See Commentary to Table 3.3).
The diaphragm shear adjustment factor allows for adjust-ment of unit shear loads for other common load cases that may involve larger roof weight, floor weight, or wall weight than used for the reference conditions used in the tabulated requirements. Adjustment of tabulated values for other than the reference condition weights (denoted by the column of factors equal to 1.0) is by use of factors that account for the increase in forces for different weight materials. The largest applicable increase factor for a given building dimension W is tabulated rather than providing adjustment factors that vary by building aspect ratio.
Assuming all weights remained unchanged in the prior example except that roof weight is increased from 15 psf to 25 psf, an increase in shear load would result and could be calculated directly as follows:
WRD = Wroof + W(wall L)/2 + Wpartition/2 + Wgable
= 108,720 lbswhere:
Wroof = weight of the roof which includes consideration of 2′ overhangs
= 25 psf [(36 ft + 4 ft) (80 ft + 4 ft)]
= 84,000 lbs
V = (108,720) (1.1) (SDS/R) (0.7) = 6,440 lbs
v = 6,440 lbs / (2W) = 90 plf
Factor for 2-story: 90/0.92 = 98 plf
98 plf < 215 plf OK
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Ratio: V(25 psf roof)/V(15 psf roof
= 98 plf / 67 plf
= 1.46 (WFCM Table 3.3 Footnote 3)
Footnote 4Requirements are tabulated for ground snow load con-ditions of 30 psf, 50 psf, and 70 psf. Effective seismic weight includes 20% of ground snow load where ground snow load exceeds 30 psf. For 50 psf and 70 psf tables, effective seismic weight at the roof is increased by 10 psf and 14 psf to account for snow.
Values of SDS are associated with the upper boundaries of SDC A, B, C, D0, D1, and D2 used to tabulate requirements.
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Figure W4.1 Roof and Ceiling Framing - Finished Attic Details
Cross Section (East Section)
Roof Framing Plan (East Section)
Ceiling Framing Plan (East Section)
FINISHED ATTIC
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Figure W4.2 Roof and Ceiling Framing – Raised Ceiling Details
Cross Section (West Section)
Roof Framing Plan (West Section)
Ceiling Framing Plan (West Section)
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Ceiling Framing – Finished Attic
Floor Joists (WFCM 3.3.1.1) For habitable attics, use residential sleeping area with 30psf live load, choose floor joists from Table
3.18A: Live Load: ................................................................................................... 30 psf Dead Load: .................................................................................................. 10 psf Joist Vertical Displacement L/Δ: ............................................................... 360 Required Span: ............................................................................................ 16 ft Table W4.3 Selection of Specie, Grade, Size, and Spacing for Floor Joists: (Table 3.18A)
Specie Douglas Fir-Larch Hem-Fir Southern Pine Spruce-Pine-Fir
Spacing 16" 16" 16" 16"
Grade #2 #2 #2 #2
Size 2x10 2x10 2x12 2x10
Maximum Span 17'-5" Ok 16'-10" Ok 18'-6" Ok 17'-2" Ok Floor Sheathing (WFCM 3.3.4.1) Choose floor sheathing from Table 3.14: Floor Joist Spacing:..................................................................................... 16 in. Sheathing Type (Wood Structural Panel or Board Sheathing): .................. WSP Single Floor Span Rating or Grade: ................................................................................ 16 o.c. Tabulated Minimum Panel Thickness: ....................................................... 19/32 in. Ok Floor Diaphragm Bracing (WFCM 3.1.3.3g, 3.3.5, and Figure 3.7b) WFCM 3.3.5 prescribes floor diaphragm bracing in the first two bays at four feet on center for
wind speeds greater than 130 mph. Nailing requirements are per Table 3.1.
Blocking to Joist (toe-nailed): 2-8d Common
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Roof and Ceiling Sheathing
Sheathing (WFCM 3.5.4.1) Choose Roof Sheathing from Tables 3.12A and 3.12B: Ground Snow Load .............................................................................. 30 psf Live Load .......................................................................................... 20 psf Dead Load .......................................................................................... 10 psf Three second gust wind speed (700 yr) and exposure category: .......... 160 mph Exp. B Rafter/Truss Spacing: ........................................................................... 16 in. Sheathing Type: .................................................................................... WSP Sheathing Grade/OSB Tabulated Minimum Panel Thickness: From Table 3.12A (wind): .......................................................... 24/0 (3/8) in. Ok From Table 3.12B (live and snow): ........................................... 24/0 (3/8) in. Ok
Roof Diaphragm Bracing – Finished Attic (WFCM 3.1.3.3g and 3.5.5) WFCM 3.5.5 prescribes roof diaphragm bracing in the first two bays at four feet on center. However, the Exception in WFCM 3.5.5 permits roof diaphragm blocking to be omitted if the attic floor is used to brace the gable end wall which is what was prescribed in WFCM 3.3.5 (see p. 18).
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3.3.4.2 Sheathing Edge Support Edges of floor sheathing shall have approved
tongue-and-groove joints or shall be supported with block-ing, unless ¼ inch minimum thickness underlayment or 1½ inches of approved cellular or lightweight concrete is installed, or unless the finish floor is of ¾ inch wood strip.
3.3.5 Floor Diaphragm Bracing
For 3-second gust wind speeds greater than 130 mph (See Figure 1.1), blocking and connections shall be provid-ed at panel edges perpendicular to floor framing members in the first two bays of framing and shall be spaced at a maximum of 4 feet on center. Nailing requirements are given in Table 3.1 (see Figure 3.7b).
3.4 Wall Systems
3.4.1 Exterior Walls
3.4.1.1 Wood Studs Wall studs shall be in accordance with the maximum
spans for common species and grades of wall studs speci-fied in Tables 3.20A-B and spaced in accordance with Table 3.20C. Exterior loadbearing studs shall be limited to a height of 10 feet or less between horizontal supports as specified in Table 3.20C. Exterior non-loadbearing studs shall be limited to a height of 14 feet or less for 2x4 studs and 20 feet or less for 2x6 and 2x8 studs in accordance with Table 3.20C.
3.4.1.1.1 Notching and Boring Notches in either edge of studs shall not be located in the middle one-third of the stud length. Notches in the outer thirds of the stud length shall not exceed 25% of the actual stud depth. Bored holes shall not exceed 40% of the actual stud depth and the edge of the hole shall not be closer than 5/8 inch to the edge of the stud (see Figure 3.3b). Notches and holes shall not occur in the same cross-section.
EXCEPTION: Bored holes shall not exceed 60% of the actual stud depth when studs are doubled.
3.4.1.1.2 Stud Continuity Studs shall be continuous between horizontal supports, including but not limited to: girders, floor diaphragm assemblies, ceiling diaphragm assemblies, and roof diaphragm assemblies. When attic floor diaphragm or ceiling diaphragm assemblies are used to brace gable endwalls, the sheathing and fasteners shall be as specified in Table 3.15. The framing and connections shall be capable of transferring the loads into the ceiling or attic floor diaphragm (see Figures 3.7a-b).
3.4.1.1.3 Corners A minimum of three studs shall be provided at each corner of an exterior wall (see Figures 3.8a-b).
EXCEPTION: Reduced stud requirements shall be permitted provided shear walls are not con-tinuous to corners. Framing must be capable of
transferring axial tension and compression loads from above and providing adequate backing for the attachment of sheathing and cladding materi-als.
3.4.1.2 Top Plates Double top plates shall be provided at the top of all
exterior stud walls. The double plates shall overlap at corners and at intersections with other exterior or interior loadbearing walls (see Figure 3.8d). Double top plates shall be lap spliced with end joints offset in accordance with the minimum requirements given in Table 3.21.
3.4.1.3 Bottom Plates Bottom plates shall not be less than 2 inch nominal
thickness and not less than the width of the wall studs. Studs shall have full bearing on the bottom plate.
3.4.1.4 Wall Openings Headers shall be provided over all exterior wall open-
ings. Headers shall be supported by wall studs, jack studs, hangers, or framing anchors (see Figures 3.9a-b).
3.4.1.4.1 Headers Maximum spans for common species of lumber headers and structural glued laminated timber beams used in exterior loadbearing walls shall not exceed the lesser of the applicable spans given in Tables 3.22A-E and Table 3.23A. Maximum spans for common species of lumber headers used in exterior non-loadbearing walls shall not exceed spans given in Table 3.23B.
3.4.1.4.2 Full Height Studs Full height studs shall meet the same requirements as exterior wall studs selected in 3.4.1.1 (see Figures 3.9a-b). The minimum number of full height studs at each end of the header shall not be less than half the number of studs replaced by the opening, in accordance with Table 3.23C.
EXCEPTION: The minimum number of full height studs at each end of the header shall be permitted to be reduced in accordance with Table 3.23D. The capacity of the connection of the top
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EXCEPTION: Where ceiling joists are attached directly to rafters, the combined bearing thick-ness of the ceiling joist and rafter shall be used to determine the depth to thickness ratio.
3.5.1.4 Ridge Beams Ridge beams shall be installed at roof peaks. Ridge
beams shall be in accordance with the maximum spans for common species of lumber beams and structural glued laminated timber beams specified in Table 3.29. Rafters shall bear directly on the ridge beam or be supported by hangers or framing anchors (see Figure 3.10a). Ceiling joists or rafter ties shall not be required where a ridge beam is provided.
EXCEPTION: A ridge board shall be permit-ted to be substituted for a ridge beam when roof slopes equal or exceed 3 in 12. The ridge board shall be at least 1 inch nominal in thickness and not less than the depth of the cut end of the rafter. The rafters shall be placed directly opposite each other. Ceiling joists or rafter ties shall be used to provide a continuous tie between exterior walls. Ceiling joist/rafter tie to rafter connections shall be in accordance with Tables 3.9. Prescriptive solu-tions for ceiling joist/rafter tie to rafter connections are provided in Table 3.9A (see Figures 3.10b-c).
3.5.1.5 Hip and Valley Beams Hip and valley beams shall be in accordance with
the maximum spans (horizontal projection) for common species of lumber hip and valley beams specified in Table 3.28, respectively (see Figures 3.12a-c).
3.5.1.6 Ceiling Joists Ceiling joists shall be in accordance with the maxi-
mum spans for common species of solid sawn ceiling joists specified in Tables 3.25A-B, and shall be braced in accordance with 3.3.1.4.
3.5.1.7 Open Ceilings When ceiling joists and roof ties are omitted and the
rafters are used to create an open (cathedral) ceiling, rafter ends shall be supported on bearing walls, headers, or ridge beams. Rafters shall be attached to the support at each end in accordance with 3.2.
3.5.1.8 Roof Openings Trimmers and headers shall be doubled when the
header span exceeds 4 feet. Headers more than 6 feet in length shall be supported at the ends by rafter hangers or framing anchors unless they bear on a partition, beam, or
wall. Tail rafters which exceed 12 feet in length shall be supported on framing anchors (see Figures 3.11a-c). Nail-ing requirements are given in Table 3.1.
3.5.2 Wood I‑Joist Roof Systems
Wood I-joist rafter systems shall meet the requirements of 2.5.2.
3.5.3 Wood Roof Truss Systems
Wood roof truss systems shall meet the requirements of 2.5.3. See Table 3.27 for representative metal plate connected wood roof truss span tables. Actual design spans will vary by truss manufacturer as a result of specific design conditions.
3.5.4 Roof Sheathing
3.5.4.1 Sheathing Roof sheathing shall be in accordance with the
minimum requirements of Tables 3.12A and 3.12B.
3.5.4.2 Sheathing Edge SupportEdges of all 3/8-inch wood structural panel roof
sheathing, or 7/16-inch wood structural panel roof sheath-ing supported at 24 inches on center, shall be supported with blocking or edge clips.
3.5.5 Roof Diaphragm Bracing
For 3-second gust wind speeds greater than 130 mph (See Figure 1.1) blocking and connections shall be provid-ed, at panel edges perpendicular to roof framing members in the first two bays of framing, and shall be spaced at a maximum of 4 feet on center. Nailing requirements are given in Table 3.1 (see Figure 3.7b).
EXCEPTION: When an attic floor or ceiling diaphragm is used to brace the gable endwall or when a hip roof system is used, additional roof diaphragm blocking is not required.
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Roof Diaphragm Bracing – Raised Ceiling (WFCM 3.5.5 and 3.4.1.1.2) Blocking at 4 ft o.c. in first two rafter bays with full height studs on second floor end wall
framing is possible with balloon framing. The stud length of 12.1 ft to the raised ceiling plus
maximum gable height of 6.2 ft at the ridge gives 18.3 ft which is less than the 20 ft
maximum non-loadbearing stud height (3.1.3.3a). Balloon framed studs would have to be
designed for wind loads.
OR Bracing Gable Endwall with Attic Floor/Ceiling Sheathing Length from Table 3.15 with Gable
Brace Figure 3.7a. Three second gust wind speed (700 yr) and exposure category: ................. 160 mph Exp. B Roof Pitch: ................................................................................................... 7:12 Roof (diaphragm) Span (see raised ceiling calculations): ........................... 21.3 ft Diaphragm Length Available: ..................................................................... 14.5 ft Sheathing Type (wood structural panels or gypsum): ................................. WSP GYP Tabulated Min. Length of Attic Floor/Ceiling Diaphragm (interpolated): .. 7.1 ft 18.5 ft Bracing One Gable End Adjustment (Footnote 1): ..................................... 0.84 0.84 Wall Height Adjustment (Footnote 2): (13.1'/10') .................................... 1.31 1.31 Ceiling Framing Spacing Adjustment (Footnote 4): ................................... 1.0 0.78 Required Minimum Length of Attic Floor/Ceiling Diaphragm: Tabulated Minimum Length x Applicable Adjustment Factors: .......... 7.8 ft Ok 15.9 ft NG
WSP sheathing is required for the ceiling diaphragm since 15.9' required length of gypsum diaphragm is greater than the 14.5' length of the raised ceiling.
RAFTERSS
Endwall Blocking Detail (Floor or Roof) – floor blocking shown
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Exposure B
PRESCRIPTIVE DESIGN
Table 3.15 Minimum Attic Floor/Ceiling Length When Bracing Gable Endwall for Wind Loads
Attic Floor or Ceiling Diaphragm Bracing Gable Endwall(Wind Parallel to Ridge)
12:12
GypsumCeiling Diaphragm
Bracing Gable Endwall(Wind Parallel to Ridge)4
7:12
8:12
9:12
10:12
6:12
11:12
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AMERICAN WOOD COUNCIL
PRESCRIPTIVE DESIGN
1
2
3
4 Tabulated gypsum ceiling requirements shall be permitted to be multiplied by 0.78 when ceiling framing is spaced at 16 inches o.c. or less.
Attic floor or ceiling diaphragms are not required for hip roof systems.
Tabulated attic floor or ceiling length requirements assume sheathing continuous from gable‐end to gable‐end. For an attic floor or ceiling bracing only one gable‐end, the tabulated length requirement shall be permitted to be multiplied by 0.84. In no case shall the length requirement be less than one‐third of the distance between bracing walls for gypsum ceilings or one‐third of the distance for attic floors or ceilings sheathed with structural sheathing.
Tabulated length requirements are based on 10 foot wall heights. For other wall heights, H, the tabulated length requirements shall be multiplied by (H+1)/10.
Footnotes to Table 3.15
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COMMENTARY TO THE WOOD FRAME CONSTRUCTION MANUAL
AMERICAN WOOD COUNCIL
Table 3.15 Minimum Attic Floor/Ceiling Length when Bracing Gable Endwall for Wind Loads
Description: Minimum length of sheathing required from gable end on attic floor or ceiling.
Procedure: Using loads calculated from Chapter 2, determine the minimum sheathing length required to brace the gable end wall against those loads.
Background: Capacities for horizontal unblocked dia-phragm assemblies are tabulated based on sheathing thickness, nail size, and panel edge nailing.
Determine the length of sheathing required using wood structural panels:
Lateral diaphragm load from wind parallel to ridge:
From WFCM Table 2.5C: w = 101 plf
Calculate load into the floor / ceiling diaphragm: V = 101 plf (24 ft)
= 2,424 lbs
Using 2015 SDPWS Table 4.2C Nominal Unit Shear Ca-pacities for Wood-Frame Diaphragms and applying the (1/2.0) ASD reduction factor per SDPWS Section 4.2.3:
Capacity of structural sheathing: v = 505/(2.0) (15/32 nominal panel thickness,
Adjusting for specific gravity of framing per 2015 SDPWS Table 4.2C Footnote 2: v = 252.5 plf [1 - (0.5 - G)]
= 252.5 [0.92] plf
= 232 plf
Required structural sheathing attic floor or ceiling dia-phragm length: L = [2,424 lbs / 2] / 232 plf
= 5.2 ft
Given a maximum diaphragm aspect ratio of 3:1 for un-blocked diaphragms per SDPWS Table 4.2.4, for a building end wall width (roof span) equal to 24', the minimum length of attic floor/ceiling diaphragm is 8'.
(WFCM Table 3.15)
Determine the required attic floor/ceiling diaphragm length using Gypsum Wallboard:
From WFCM Supplement Table S-3:
ASD Unit Shear Capacity of gypsum wallboard: v = 70 plf
Required attic floor/ceiling diaphragm length using gyp-sum wallboard: L = [2,424 lbs / 2] / 70 plf
= 18 ft (rounded up from 17.3 ft) (WFCM Table 3.15)
Footnote 1:See Commentary for Table 2.5C Footnote 3 for calcula-tions.
Footnote 4:From Supplement Table S-3 Footnote 1:
For ceiling framing at 16" o.c. or less, the tabulated capac-ity for gypsum wallboard is 90 plf.
Dividing the tabulated capacity by the increased capacity: = 70 plf / 90 plf
= 0.78 (WFCM Table 3.15)
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ENG
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Table 2.5C Lateral Diaphragm Loads from Wind - Parallel to Ridge (For Attic Floor or Ceiling Diaphragm When Bracing Gable Endwall)
1 The total shear load equals the tabulated unit lateral load multiplied by the endwall length. 2 Tabulated unit lateral loads are based on 10 foot wall heights. 3 Tabulated unit lateral loads assume the attic floor/ceiling diaphragm is continuous between endwalls. When
the diaphragm only resists loads from one endwall, the tabulated unit lateral load shall be multiplied by 0.84. 4 Tabulated unit lateral loads are based on MWFRS wind loads and assume a building located in Exposure B
with a mean roof height of 33 feet. For buildings located in Exposures B with mean roof heights less than 33 feet, or in Exposures C and D, tabulated values shall be adjusted in accordance with Section 2.1.3.1.
5 Attic floor or ceiling diaphragms shall not be used to brace gable endwalls used with cathedral ceilings. 6 Attic floor or ceiling diaphragms are not required for hip roof systems. 7 When a ceiling diaphragm is used to brace the gable endwalls, the unit lateral loads on the roof diaphragm,
when the wind is parallel to the ridge, need not exceed the tabulated roof lateral load (from Table 2.5B) minus the ceiling lateral load (from Table 2.5C).
8 Shear capacity requirements for attic floor or ceiling diaphragms shall be calculated as follows:
1 Tabulated values are based on deflection criteria of L/240 under live loads and L/180 under total loads (assumed 10 psf for dead loads). 2 Edge supports (tongue‐and groove edges, panel edge clips at midway between supports, lumber blocking, or other) are required. Otherwise,
the maximum spacing is limited to 19.2 inches.
Table S-3 ASD Unit Shear Capacity for Horizontal Diaphragm Assemblies Sheathed with Gypsum Wallboard
Nail Spacing (in.) Sheathing Material
MaterialThickness
(in.) Nail Size
Diaphragm Construction Panel Edges
Intermediate Supports
Recommended ASD Unit Shear Capacity
(plf)
Gypsum Wallboard
1/2 5d Cooler Nails or 1‐1/4 Drywall
Screws Unblocked 7 10 70 1
1 Tabulated shear capacity can be increased to 90 plf when ceiling framing members are spaced not
more than 16 inches on center.
Table S-3 ASD Unit Shear Capacity for Horizontal Diaphragm Assemblies Sheathed with Gypsum Wallboard
Table S-2B Maximum Spans and Allowable Total Uniform Loads (Deflection) for Roof Sheathing for Normal Duration Loads
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AM
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LATERAL FORCE-RESISTING SYSTEMS 4
SPECIAL DESIG
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WIN
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Table 4.2C Nominal Unit Shear Capacities for Wood-Frame Diaphragms
1. Nominal unit shear capacities shall be adjusted in accordance with 4.2.3 to de-termine ASD allowable unit shear capacity and LRFD factored unit resistance. For general construction requirements see 4.2.6. For specific requirements, see 4.2.7.1 for wood structural panel diaphragms. See Appendix A for common nail dimensions.
2. For species and grades of framing other than Douglas-Fir-Larch or Southern Pine, reduced nominal unit shear capacities shall be determined by multiplying the tabulated nominal unit shear capacity by the Specific Gravity Adjustment Factor = [1-(0.5-G)], where G = Specific Gravity of the framing lumber from the NDS (Table 12.3.3A). The Specific Gravity Adjustment Factor shall not be greater than 1.
3. Apparent shear stiffness values, Ga, are based on nail slip in framing with moisture content less than or equal to 19% at time of fabrication and panel stiffness values for diaphragms constructed with either OSB or 3-ply plywood panels. When 4-ply or 5-ply plywood panels or composite panels are used, Ga values shall be permitted to be multiplied by 1.2.
4. Where moisture content of the framing is greater than 19% at time of fabrica-tion, Ga values shall be multiplied by 0.5.
5. Diaphragm resistance depends on the direction of continuous panel joints with respect to the loading direction and direction of framing members, and is independent of the panel orientation.
A
B
Sheathing Grade Common Nail Size
Minimum Fastener
Penetration in Framing
(in.)
Minimum Nominal Panel
Thickness (in.)
Minimum Nominal Width
of Nailed Face at Supported Edges and Boundaries
(in.)
SEISMIC WIND
6 in. Nail Spacing at diaphragm boundaries
and supported panel edges
6 in. Nail Spacing at diaphragm boundaries and
supported panel edges Case 1 Cases 2,3,4,5,6 Case 1 Cases
2,3,4,5,6 vs Ga vs Ga vw vw (plf) (kips/in.) (plf) (kips/in.) (plf) (plf) OSB PLY OSB PLY
Cases 1&3:Continuous Panel Joints Perpendicular to Framing
Cases 2&4: Continuous Panel Joints Parallel to Framing
Cases 5&6: Continuous Panel Joints Perpen-dicular and Parallel to Framing
Long Panel Direction Perpendicular to Supports
Long Panel Direction Parallel to Supportsa
(a) Panel span rating for out-of-plane loads may be lower than the span rating with the long panel direction perpendicular to supports (See Section 3.2.2 and Section 3.2.3)
3. Maximum roof & floor diaphragm lengths in WFCM Tables 3.16A & B are based on?a) Diaphragm capacityb) Maximum prescriptive building dimensionc) Aspect ratio limitsd) All of the abovee) Answers b) and c)
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POLLING QUESTION
4. SDPWS has tabulated shear stiffness for which of the following products?a) WSP with screwsb) WSP with nailsc) Lumberd) Gypsum boarde) All of the above except a)
D e m y s t i f y i n g D i a p h r a g m D e s i g n6 0
OUTLINE
Photo courtesy: KC Kim GB Construction
• Basic Lateral Load Overview and Code acceptance of 2015 SDPWS
Case 1 blocked diaphragm, 7/16” OSB sheathing, 8d common nails at 6”o.c. at all panel edges
A B
Allowable stress design load, =255 plf
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WOOD DIAPHRAGM DEFLECTION EXAMPLE2015 SDPWS CommentaryCalculate mid-span deflection for the blocked wood structural panel diaphragm shown in Figure C4.2.2-2. The diaphragm chord splice is sized using allowable stress design loads from seismic while deflection due to seismic is based on strength design loads in accordance with ASCE 7.
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FASTENER PATTERN – HIGH-LOAD DIAPHRAGMS
Note: Space panel end and edge joint 1/8”. Reduce spacing between lines as necessary to maintain minimum 3/8” fastener edge margin. 3/8” is minimum distance between rows.
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ASCE 7-10 Sec. 12.12 DRIFT AND DEFORMATION
FREQUENTLY ASKED QUESTIONS
Etc…..
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FREQUENTLY ASKED QUESTIONS
Q: Is diaphragm deflection cumulative with shear wall deflection?
A: Yes. Shear walls supporting a horizontal diaphragm would also be evaluated for deflection. Cumulative deflection would then be calculated to determine the maximum anticipated movement to compare with allowables.
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FREQUENTLY ASKED QUESTIONS
Q: Are there provisions to calculate the deflection for a diaphragm that is only partially blocked (i.e. at ends only) or is it proper to base the deflection on the entire diaphragm being unblocked?
A: There are no provisions for a partially blocked diaphragm. Suggest calculating for both a blocked and unblocked diaphragm to determine magnitude of difference and use engineering judgment.
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FREQUENTLY ASKED QUESTIONS
Q: When the nailing pattern in a horizontal diaphragm varies instead of being uniform, how is deflection calculated?A: One approach is to modify the nail-slip constant in the 4-term equation in proportion to the average load on each nail with non-uniform nailing compared to the average load with uniform nailing. APA’s Diaphragm and Shear Wall Design and Construction Guide (L350) provides an example: www.apawood.org.
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FREQUENTLY ASKED QUESTIONS
Q: Is there any benefit if wood structural panels are glued to the assembly?
A: We are not aware of any added benefit with respect to diaphragm deflection.
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FREQUENTLY ASKED QUESTIONS
Q: Is there a multiplier to use for a long-term consideration due to possible enlargement of nail holes or is that all included in the nail slip deflection calculations?
A: Testing done to verify diaphragm deflection calculations is based on full reverse cyclic loads, so the effects of nail hole enlargement has been addressed.
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FREQUENTLY ASKED QUESTIONS
Q: For large diaphragm, what adjustments to deflection equations need to be made to obtain inelastic diaphragm deflection?
A: ASCE 7 Minimum Design Loads for Buildings and Other Structures, Section 12.8.6 includes an amplification factor Cd which is used for story drift and seismic gaps.
D e m y s t i f y i n g D i a p h r a g m D e s i g n1 1 2
FREQUENTLY ASKED QUESTIONS
Q: What resources are available for designing NLT Diaphragms?
T h i s p r e s e n t a t i o n i s p r o t e c t e d b y U S a n d I n t e r n a t i o n a l C o p y r i g h t l a w s . R e p r o d u c t i o n , d i s t r i b u t i o n , d i s p l a y a n d u s e o f t h e p r e s e n t a t i o n w i t h o u t w r i t t e n p e r m i s s i o n o f A m e r i c a n W o o d C o u n c i l ( A W C ) i s