CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 J. LIU. Wood Structural Panels

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1 CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 J. LIU Wood Structural Panels

2 Wood Structural Panels Plywood Oriented Strand Board (OSB) Composite Panels Generally, wood veneer and reconstituted wood-based material bonded Particleboard Medium density fiberboard

3 Wood Structural Panels Composite Panels Particleboard made from small wood particles pressed together with glue under extreme heat and pressure to make a solid panel. Medium density fiberboard made the same way except the wood particles are further refined into even smaller particles to provide a smooth edge to the panel.

4 Uses Roof, floor and wall sheathing Horizontal and vertical (shearwall) diaphragms Structural components Prefab. I-joists Sandwich panels Gusset plates Concrete formwork

5 Sheathing

6 Sheathing Thickness requirements Often determined by sheathing-type loads (normal to surface of panel) Nailing requirements Determined by unit shears in horizontal or vertical diaphragm More later on both topics

7 Panel Dimensions and Installation Standard size 4 x 8 ft Can be available in 9, 10, 12 ft lengths Dimensionally stable But some changes can be expected under varying moisture conditions Installation recommend clearance between panel edges and ends Typically 1/8 in to permit panel movement Other spacing provisions may apply, depending on type of panel, moisture conditions

8 Installation Typical tolerances +0, -1/16 in., and +0, -1/8 in. Some panel grade stamps: sized for spacing Allow for panel spacing while maintaining basic 4 ft. module Nail popping If installed on green beams, nails should be set below surface of sheathing (will appear to pop upwards as supports dry)

9 Plywood Plywood panel made up of a number of veneers (thin sheets of wood) Veneer obtained by rotating peeler logs in a lathe

10 Plywood Veneer then clipped to proper size Dried to low moisture content (2 5 %) Graded according to quality Veneer spread with glue and cross-laminated Adjacent layers have wood grain at right angles Odd number of layers Glue bond obtained is stronger than wood in plies Panel is grade-stamped

11 3 ply, 3 layer panel

12 4 ply, 3 layer panel Extension of simple 3-ply construction Two center plies have grain running in same direction

13 5 ply, 5 layer panel

14 Plywood Layers Face Outside ply (if outside plies are different veneer quality, face is the better veneer grade) Grain oriented typically parallel to 8 ft dim. Back The other outside ply (grain also parallel to 8 ft. dim.) Crossband Inner layer(s) placed at right angles to face and back Center Inner layer(s) parallel with outside plies

15 Plywood orientation Strong orientation Grain of outside layers parallel to span Weak orientation Grain of outside layers perpendicular to span Thus, two sets of cross-sectional properties for use with stresses, depending on orientation (stress and properties tables) Also, load tables for sizing

16 Design Values / Load Tables 2005 ASD/LRFD Manual For Engineered Wood Construction We ll use this: Load-Span Tables for APA Structural-Use Panels

17 Transformed sections (used to develop load tables)

18 Strong Orientation Weak Orientation

19 Species Groups for Plywood

20 Species Groups for Plywood Group1 highest stiffness and strength; Group 5 lowest Groups 1 to 4 reference design values Mixing of species? Reference design values based on species group of face and back plies Species group of outer plies included in grade stamp

21 Veneer Grades N special order natural finish veneer A smooth, paintable surface, without knots B solid-surface veneer, may contain knots up to 1 in. in width if sound and tight-fitting C-plugged improved C-grade with knotholes less than ¼ x ½ in C may contain open knotholes up to 1 in. in width and some up to 1-1/2 in. D open knotholes up to 2-1/2 in., occasional up to 3 in. across grain (not allowed in exterior applications)

22 Exposure Durability Exterior Insoluble waterproof glue Minimum of C-grade veneers Required when permanently exposed to weather or when MC will exceed 18% (continuously or in repeated cycles) Interior Not exposed to weather or exceed 18% MC Typically available with exterior glue, but can be with exterior, interior, or intermediate Exposure 1 if exterior glue; might be used if long construction delays (exposure) expected

23 Marine grade plywood Gap and void-free in all layers No voids even on cut edges Water-proof glue Water has no effect on glue when immersed Typically constructed of harder woods such as Douglas Fir or Western Larch

24 Plywood Grade Stamps Veneer grades of face and back Minimum species group of the outer plies Highest species group number Exposure durability classification Mill number Inspection agency PS-1 (U.S. Product Standard PS-1 Construction and Industrial Plywood)

25 Plywood Grade Stamp APA American Plywood Association GRADE OF VENEER ON PANEL FACE GRADE OF VENEER ON PANEL BACK SPECIES GROUP NUMBER DESIGNATES TYPE ON EXTERIOR OR INTERIOR PLYWOOD STANDARD GOVERNING MANUFACTURE MILL NUMBER

26 Plywood Grades Sheathing Grades Normally used for roof, floor, wall sheathing C-C (exterior type) C-D (typically Exposure 1) When added strength required: STRUCTURAL 1 can be added to designation CC-STR 1 or C-D STR 1

27 Plywood Grades Span Rating (e.g. 24/16) Number on left is maximum recommended span in inches when used as roof sheathing Second number is maximum recommended span in inches when used a subflooring Meant for use of panels without any structural calculations Panel must be oriented in the strong direction Certain edge support requirements must be satisfied May use panels for longer spans if justified by structural calculations

28 Plywood May be sanded Improves surface condition However, reduces thickness of outer veneers Different cross-sectional properties are used in strength calculations for sanded, touch-sanded, and unsanded panels Or, different tables, such as Table 3 for Group 1 Sanded Plywood panels (APA, 2011)

29 Oriented Strand Board Non-veneer panel manufactured from reconstituted wood strands or wafers, bonded with resin Strands directionally oriented, typically in 3 to 5 perpendicular layers, cross laminated in similar manner to plywood

30 Nonveneer (OSB) Grade Stamp

31 Adjustment Factors NDS Table Applicability of Adjustment Factors for Wood Structural Panels C M and C t obtain from approved source (such as APA, NDS commentary) New for panels C S Panel Size Factor If panel width < 24 NDS Table 9.3.4

32 Adjustments ASD only Span Adjustments: 2-span to 1-Span 3-span to 1-Span 3-span to 2-Span Wet Locations, C M MC 16% or more

33 Roof Sheathing Example Consider roof system with sheathing span of 24 inches, dead load of 10 psf, snow load of 75 psf, total load deflection limit of L/240. Determine the minimum span-rated sheathing using LRFD. Use strength axis across supports. Assume sheathing grade, 3-span, 4-ply, normal temperatures, dry service conditions. 1.4 D = 1.4 (10 psf) = 14 psf 1.2 D S = 1.2 (10 psf) (75 psf) = 132 psf D + S = 10 psf + 75 psf = 85 psf

34 Roof Sheathing Example, cont d. Note: Tables are ASD! Assumes 3 spans (No span adjustment needed) Assumes 2 spans

35 Roof Sheathing Example, cont d. Note: Tables are ASD!

36 Roof Sheathing Example, cont d. Note: no adjustment for, or mention of, 3-ply, 4-ply, etc. in Load Tables May not know # of plies (i.e., not on grade stamp), therefore tabulated values based on most conservative construction (Table 6)

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38 Roof Sheathing Example, cont d. For Deflection, no modification; same for ASD and LRFD 32/16 does not satisfy deflection limit, but 40/20 does 163 > 85 psf (D+S)

39 Roof Sheathing Example, cont d. Bending: K F (130psf) = 2.54 (130 psf) = 330 psf Shear: K F (218 psf) = 2.88 (218 psf) = 628 psf

40 Roof Sheathing Example, cont d. = 0.8 for 1.2 D S b = 0.85 and v = 0.75 Bending: 330 psf (0.8)(0.85) = 224 psf Shear: 628 psf (0.8)(0.75)= 377 psf Both > 132 psf OK! Use 40/20 sheathing

41 Roof Sheathing Span ratings assume panel edge support Support intended to limit differential movement between adjacent panels Unsupported edge may require thicker panel or closer joist spacing Lumber blocking cut and fitted between roof joists Tongue and groove edges Panel clips metal H-shaped clips placed between plywood edges

42 Floor Sheathing Floor systems may use two layers of panels or a single layer Subfloor bottom layer in a two-layer system (the basic structural sheathing material) Underlayment the top layer in a two-layer system Combined subfloor-underlayment a single layer system (usually with nontextile flooring) A finish floor such as tile, hardwood, carpeting usually provided

43 Underlayment Purpose is to provide solid surface for application of floor finish Grade: Underlayment Typical thickness ¼ in. for remodeling and use over panel subfloor and 3/8 to ½ in. for use over lumber subfloor or new construction When finish floor has some structural capacity (e.g. wood strip flooring, lightweight concrete) underlayment is not required

44 Subfloor Panels must be used in strong direction and continuous over two or more spans Differential movement at panel edges must be limited by: T&G edges Blocking ¼ in. underlayment with panel edges offset over subfloor Finish floor of ¾ in. wood strips 1-1/2 in. of lightweight concrete

45 Single-Layer Floor Systems Performance rated panels known as Sturd-I-Floor Include plywood, composite, and non-veneer panels

46 Floor Sheathing Example APA-rated Plywood Sturd-I-Floor is considered for an office building subfloor. The floor utilizes a twolayer floor system with a separate subfloor and underlayment. A ¼ in. plywood UNDERLAYMENTgrade panel is used over the subfloor; its joints are staggered with respect to the joints in the subfloor (no special edge support is required). Assume strength axis perpendicular to supports, 3-span, 4- ply, normal temperature, but MC at 16% or more.

47 Floor Sheathing Example, cont d. Dead load 12 psf, distributed (moveable)partition load of 15 psf, floor live load of 50 psf. (L from occupancy) Floor framing members spaced at 16 in. o.c. Floor system must satisfy live load deflection limit of L/360 and total deflection limit of L/ D L = 1.2 (12 psf) (65 psf) = 118 psf D + L = 12 psf + 65 psf = 77 psf

48 Floor Sheathing Example, cont d.

49 Floor Sheathing Example, cont d.

50 Floor Sheathing Example, cont d. For Deflection, modify by C M only 270(0.85)=230 > 65 LL And 405(0.85)=344 > 77 TL OK! Bending: K F (213 psf) = 2.54 (213 psf) = 541 psf Shear: K F (338 psf) = 2.88 (338 psf) = 973 psf

51 Floor Sheathing Example, cont d. = 0.8 for 1.2 D L (occupancy) b = 0.85, v = 0.75, C M = 0.75 Bending: 541 psf (0.8)(0.85)(0.75) = 276 psf Shear: 973 psf (0.8)(0.75)(0.75) = 438 psf Both > 118 psf OK! USE 16 oc APA-rated Sturd-I-Floor

52 Floor Example What if we have a 1-span condition? For Deflection, 270(0.85)(0.53) =122 > 65 LL 405(0.85)(0.53)=182 > 77 TL OK! Bending: 541 psf (0.8)(0.85)(0.75)(0.8) = 221 psf Shear: 973 psf (0.8)(0.75)(0.75)(1.2) = 527 psf > 118 psf OK! USE 16 oc APA-rated Sturd-I-Floor

53 Wall Sheathing Two basic uses: Structural only: as sheathing to distribute normal wind forces to the studs or function as basic shearresisting elements (shearwalls) Combined sheathing-siding

54 Wall Sheathing Sizing for sheathing-type loads (wind loads normal to panels) same procedure as for floor and roof sheathing Design as shear walls later topic!