ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN DR. ANNE NICHOLS SUMMER 2017 lecture twelve wood construction: materials & beams Wood Beams 1 Lecture 12 Architectural Structures F2009abn Wood Beam Design National Design Specification f Wood Beams 2 National Forest Products Association ASD & LRFD (combined in 2005) adjustment factors x tabulated stress = allowable stress adjustment factors terms, C with subscript i.e, bending: b F F b b product of adjustment factors Timber lightweight : strength ~ like steel strengths vary by wood type by direction by flaws size varies by tree growth renewable resource manufactured wood assembles pieces Wood Beams 3 adhesives Wood Properties cell structure and density Wood Beams 4 softwood http://www.swst.org/teach/set2/struct1.html hardwood 1
Wood Properties moisture exchanges with air easily excessive drying causes warping and shrinkage strength varies some temperature steam volatile products combustion http://www.swst.org/teach/set2/struct1.html Wood Properties load duration short duration higher loads normal duration creep > 10 years additional deformation with no additional load Wood Beams 5 Wood Beams 6 Structural Lumber dimension 2 x s (nominal) beams, posts, timber, planks grading Wood Beams 7 select structural no. 1, 2, & 3 tabular values by species glu-lam plywood Adjustment Factors terms Wood Beams 8 C D = load duration factor C M = wet service factor 1.0 dry 19% MC sawn 1.0 dry 16% MC glu-lam C F = size factor visually graded sawn lumber and round timber > 12 depth C F 9 ( 12 / d ) 1. 0 1 Fig. 9.23 (pg 477) 2
Adjustment Factors terms C fu = flat use factor not decking C i = incising factor increase depth for pressure treatment C t = temperature factor lose strength at high temperatures Adjustment Factors terms C r = repetitive member factor C H = shear stress factor splitting C V = volume factor same as C F for glue laminated timber C L = beam stability factor beams without full lateral support C c = curvature factor for laminated arches Wood Beams 9 Wood Beams 10 Allowable Stresses design values F b : bending stress F t : tensile stress strong F v : horizontal shear stress weak F c : compression stress (perpendicular to grain) F c : compression stress (parallel to grain) strong E: modulus of elasticity F p : bearing stress (parallel to grain) Load Combinations design loads, take the bigger of (dead loads)/0.9 (dead loads + any possible combination of live loads)/c D deflection limits no load factors for stiffer members: T max from LL + 0.5(DL) Wood Beams 11 Wood Beams 12 3
Beam Design Criteria strength design bending stresses predominate shear stresses occur serviceability limit deflection and cracking control noise & vibration no excessive settlement of foundations durability appearance component damage Beam Design Criteria superpositioning use of beam charts elastic range only! add moment diagrams add deflection CURVES (not maximums) = + Wood Beams 13 Wood Beams 14 ponding Beam Deformations curvature relates to bending moment modulus of elasticity moment of inertia Wood Beams 15 1 R M EI c ½ M ( x) curvature EI M ( x) slope dx EI M ( x) deflection dx EI R L ½ y Deflection Limits based on service condition, severity Wood Beams 16 Use Roof beams: Industrial Commercial plaster ceiling no plaster Floor beams: Ordinary Usage LL only L/180 L/240 L/360 L/360 Roof or floor (damageable elements) DL+LL L/120 L/180 L/240 L/240 L/480 4
Lateral Buckling lateral buckling caused by compressive forces at top coupled with insufficient rigidity can occur at low stress levels Timber Beam Bracing stiffen, brace or bigger I y Wood Beams 17 Wood Beams 18 Design Procedure 1. Know F all for the material or F U for LRFD Beam Design 4*. Include self weight for M max and repeat 3 & 4 if necessary 2. Draw V & M, finding M max 3. Calculate S req d 4. Determine section size fb F b b h 2 bh S 6 5. Consider lateral stability Unbraced roof trusses were blown down in 1999 at this project in Moscow, Idaho. Photo: Ken Carper Wood Beams 18 Wood Beams 19 5
Beam Design 6. Evaluate shear stresses - horizontal fv F v 3V rectangles and W s fvmax 2A V A web Beam Design 7. Provide adequate bearing area at supports f P A p F p general f v max VQ Ib Wood Beams 20 Wood Beams 21 Beam Design 8. Evaluate torsion fv F v Beam Design 9. Evaluate deflections circular cross section f v T J rectangular f v T 2 c 1 ab y max ( x) actual allowable Wood Beams 22 Wood Beams 23 6
Decking across beams or joists floors: 16 in. span common ¾ in. tongue-in-groove plywood 5/8 in. particle board over ½ in. plywood hardwood surfacing roofs: 24 in. span common ½ in. plywood Joists & Rafters allowable load tables (w) allowable length tables for common live & dead loads lateral bracing needed common spacings Wood Beams 24 Wood Beams 25 Engineered Wood plywood veneers at different orientations glued together split resistant higher and uniform strength limited shrinkage and swelling used for sheathing, decking, shear walls, diaphragms Engineered Wood glued-laminated timber glulam short pieces glued together straight or curved grain direction parallel higher strength more expensive than sawn timber large members (up to 100 feet!) flexible forms Wood Beams 26 Wood Beams 27 7
Engineered Wood I sections beams other products pressed veneer strip panels (Parallam) (LVL) wood fibers Hardieboard: cement & wood Timber Elements stressed-skin elements modular built-up plates typically used for floors or roofs Wood Beams 28 Architectural Structures Wood Beams 29 Timber Elements built-up box sections built-up beams usually site-fabricated bigger spans Timber Elements trusses long spans versatile common in roofs Wood Beams 30 Wood Beams 31 8
Timber Elements folded plates and arch panels usually of plywood Timber Elements arches and lamellas arches commonly laminated timber long spans usually only for roofs Wood Beams 32 Structural Systems I COSC 321 Su2008abn Wood Beams 33 Structural Systems I COSC 321 Su2008abn Approximate Depths Wood Beams 34 Structural Systems I COSC 321 Su2008abn 9