Chapter 3 Wood Wood Products Fundamentals of Building Construction Materials Methods 5th Edition Copyright 2009 J Iano All rights reserved TREES Tree Growth Bark Outermost protective layer A dead B living Cambium layer C Source of new wood cells Sapwood D Living cells that store and transport nutrients Heartwood E Dead cells that contribute to structural strength Pith F Innermost first year s growth Annual growth rings Result from differences in rate of tree growth and density of cells from spring to summer Softwoods From cone bearing coniferous trees Relatively simple cell structure Generally plain figure pattern of grain and surface features top pine bottom fir Softwoods Mostly originating from North American forests Fast growing plentiful relatively inexpensive Generally soft easily worked Uses structural wood products finish trim shingles and siding flooring But not all softwoods are soft For example Douglas Fir is harder than some hardwoods Hardwoods From broadleafed deciduous trees More complex cell structure Often more interesting figure top walnut with readily apparent pores bottom red oak with pores and rays Hardwoods Harvested from around the world Slower growing generally more expensive than softwoods Denser with greater variety of colors and figure fine trim paneling flooring fine cabinet work furniture Certified Wood Sustainable forestry management Protect forest ecosystem Maintain long term forest economic viability Some programs also address social responsibilities for example the land rights of indigenous peoples Certified Wood Forest Stewardship Council FSC Only certifying organization currently recognized for LEED certification LUMBER Sawing Plainsawn growth rings roughly parallel to wider face of board right Quartersawn growth rings close to perpendicular to wider face of board left Plainsawn lumber Broader grain pattern on wide face Greater distortion during drying More uneven surface erosion or wear More efficiently sawn from log less costly Also called flatsawn flat grain left red oak right white oak Quartersawn lumber More narrowly spaced grain pattern on wide face Less distortion during drying More even surface erosion or wear More costly to saw from log Riftsawn left angle of grain falls between perfectly quarter sawn and plainsawn Also called edge sawn edge grain vertical grain red oak Seasoning After lumber is sawn it is seasoned dried either in air or in kilns Seasoned lumber is lighter stronger and stiffer than green unseasoned lumber Decay causing fungi cannot survive in wood with a moisture content MC below 20 Seasoning As wood dries below approximately 30 percent MC it shrinks mostly in cross section and only slightly in length Difference between rates of tangential and radial shrinkage cause distortions in shape especially in flatsawn lumber Seasoning Wood eventually dries to equilibrium with the surrounding air reaching its equilibrium moisture content EMC EMC for exterior uses 15 19 EMC for interior uses 5 11 right Surfacing Lumber is surfaced to make it smooth and more dimensionally precise Framing lumber usually surfaced four sides S4S Finish lumber may be S4S or surfaced two sides S2S the other sides to be sawn and surfaced by the woodworker Surfacing after seasoning S DRY removes some drying distortions Surfacing before seasoning S GRN sometimes more economical best for wood species that don t distort excessively as they dry Construction planking unsurfaced resulting in a plank that is stronger no material has been removed and more slip resistant Structural Grading Framing lumber pieces are graded and stamped for structural strength and stiffness Higher structural grades have fewer defects and when left exposed are generally also more attractive Grading is most commonly performed visually by trained inspectors or it may be done by machine Example Appearance Grading Finish lumber is graded for the extent of defects such as knots and other appearance characteristics Appearance grading is always performed visually Example Structural Properties of Wood Wood has both useful tensile and compressive strength Strength varies significantly with direction of grain species and presence of knots or other defects Defect free wood is close to the strength of steel on a per weight basis but typical grades are weaker Strength also varies with duration of load moisture content chemical treatments temperature size and shape of piece Lumber Dimensions Actual sizes are less than nominal size E g 1x4 actual size is approximately x 3 2x4 is 1 x 3 2x10 is 1 x 9 In U S lumber is priced by the board foot based on nominal not actual dimensions 12 sq in nominal cross section 1 foot long 1 board foot E g an 8 foot 2x12 2x12 12 x 8 16 board feet WOOD PRODUCTS Manufactured Wood Products Solid wood and wood fiber materials can be used in the manufacture of various wood products More sizes and shapes Fewer defects More consistent quality More efficient use of wood materials In some cases stronger and stiffer than solid wood members Glue Laminated Wood Beams made from glued solid lumber pieces Readily available in larger sizes stronger and stiffer than solid wood Can be curved Uses high grade pieces efficiently only in the parts of the beam with high stress levels while using lower grade materials in other areas Structural Composite Lumber In order of increasing strength Laminated strand lumber LSL and oriented strand lumber OSL glued shredded wood strips top right Laminated veneer lumber LVL glued full depth veneers bottom shown resting on its side Parallel strand lumber PSL glued veneer strips top left Used as beams headers short beams over openings studs nonstructural rim boards etc Wood Plastic Composites WPCs Made from mixtures of wood fibers and plastics May be prefinished Used for exterior decking wood trim etc Less stiff and greater thermal expansion and contraction than solid lumber Also plastic lumber greater than 50 plastic has both finish and structural applications Structural Wood Panels Plywood laminated veneers Composite panels face veneers bonded to solid core Oriented strand board OSB shown at right glued shredded strands like LSL Particleboard glued particles smaller than OSB strands Fiberboard glued very finegrained wood particles Structural Wood Panels Standard panel sizes 4 x 8 5 16 to 1 1 8 thick Used as sheathing on framed walls and roofs and subflooring over floor framing Span Rating Example 32 16 may be used as roof sheathing over rafters spaced