fifteen allowable stress adjustment factors terms, C with subscript - - PowerPoint PPT Presentation

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F2009abn

ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN

ARCH 331

• DR. ANNE NICHOLS

SPRING 2018

fifteen

wood construction:

materials & beams

Wood Beams 1 Lecture 15 Architectural Structures ARCH 331

lecture

F2008abn

• National Design Specification

– American Wood Council – ASD & LRFD (combined in 2005) – adjustment factors x tabulated stress = allowable stress – adjustment factors terms, C with subscript – i.e, bending:

Wood Beams 2 Lecture 15 Foundations Structures ARCH 331

Wood Beam Design  

factors adjustment

• f

product F F f

b b b

   

Wood Beams 3 Lecture 15 Foundations Structures ARCH 331 F2008abn

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 – adhesives

F2008abn Wood Beams 4 Lecture 15 Foundations Structures ARCH 331

Wood Properties

• cell structure and density

softwood hardwood

http://www.swst.org/teach/set2/struct1.html

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F2008abn Wood Beams 5 Lecture 15 Foundations Structures ARCH 331

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 Beams 6 Lecture 15 Foundations Structures ARCH 331 F2008abn

Wood Properties

• load duration

– short duration

• higher loads

– normal duration

• > 10 years
• creep

– additional deformation with no additional load

Wood Beams 7 Lecture 15 Foundations Structures ARCH 331 F2008abn

Structural Lumber

• dimension – 2 x’s (nominal)
• beams, posts, timber, planks
• grading

– select structural – no. 1, 2, & 3

• tabular values

by species

• glu-lam
• plywood

F2008abn Wood Beams 8 Lecture 15 Foundations Structures ARCH 331

Adjustment Factors

• terms

– CD = load duration factor – CM = wet service factor

• 1.0 dry  19% MC sawn
• 1.0 dry  16% MC glu-lam

– CF = size factor

• visually graded sawn

lumber and round timber

• > 12” depth

1 12

9 1

. ) d / ( CF  

• Fig. 9.23 (pg 477)

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F2008abn Wood Beams 9 Lecture 15 Foundations Structures ARCH 331

• terms

– Cfu = flat use factor

• not decking

– Ci = incising factor

• increase depth for pressure treatment

– Ct = temperature factor

• lose strength at high temperatures

Adjustment Factors

F2008abn Wood Beams 10 Lecture 15 Foundations Structures ARCH 331

Adjustment Factors

• terms

– Cr = repetitive member factor – CH = shear stress factor

• splitting

– CV = volume factor

• same as CF for glue laminated timber

– CL = beam stability factor

• beams without full lateral support

– Cc = curvature factor for laminated arches

Wood Beams 11 Lecture 15 Foundations Structures ARCH 331 F2008abn

Allowable Stresses

• design values

– Fb: bending stress – Ft: tensile stress – Fv: horizontal shear stress – Fc: compression stress (perpendicular to grain) – Fc: compression stress (parallel to grain) – E: modulus of elasticity – Fp: bearing stress (parallel to grain)

weak strong strong

Wood Beams 12 Lecture 15 Foundations Structures ARCH 331 F2008abn

Load Combinations

• design loads, take the bigger of

– (dead loads)/0.9 – (dead loads + any possible combination of live loads)/CD

• deflection limits

– no load factors – for stiffer members:

• T max from LL + 0.5(DL)

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Wood Beams 13 Lecture 15 Foundations Structures ARCH 331 F2008abn

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 – ponding

Wood Beams 14 Lecture 15 Foundations Structures ARCH 331 F2008abn

Beam Design Criteria

• superpositioning

– use of beam charts – elastic range only! – “add” moment diagrams – “add” deflection CURVES (not maximums)

+ = + =

Wood Beams 15 Lecture 15 Foundations Structures ARCH 331 F2008abn

Beam Deformations

• curvature relates to

– bending moment – modulus of elasticity – moment of inertia

EI M R  1



  dx EI x M slope ) (  EI x M curvature ) ( 

R  L y c ½ ½



   dx EI x M deflection ) (

Wood Beams 16 Lecture 15 Foundations Structures ARCH 331 F2008abn

Deflection Limits

• based on service condition, severity

L/480 L/240 L/240 L/180 L/120 DL+LL Roof or floor (damageable elements) L/360 Ordinary Usage Floor beams: L/360 no plaster L/240 plaster ceiling Commercial L/180 Industrial Roof beams: LL only Use

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Wood Beams 17 Lecture 15 Foundations Structures ARCH 331 F2008abn

• lateral buckling caused by

compressive forces at top coupled with insufficient rigidity

• can occur at low stress levels
• stiffen, brace or bigger Iy

Lateral Buckling

Wood Beams 18 Lecture 15 Foundations Structures ARCH 331 F2008abn

Timber Beam

Bracing

Wood Beams 18 Lecture 15 Foundations Structures ARCH 331 F2008abn

Design Procedure

• 1. Know Fall for the material or

FU for LRFD

• 2. Draw V & M, finding Mmax
• 3. Calculate Sreq’d
• 4. Determine section size

6

2

bh S 

h b

 

b b

F f 

Wood Beams 19 Lecture 15 Foundations Structures ARCH 331 F2008abn

Beam Design

4*. Include self weight for Mmax

– and repeat 3 & 4 if necessary

• 5. Consider lateral stability

Unbraced roof trusses were blown down in 1999 at this project in Moscow, Idaho.

Photo: Ken Carper

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Wood Beams 20 Lecture 15 Foundations Structures ARCH 331 F2008abn

Beam Design

• 6. Evaluate shear stresses - horizontal
• rectangles and W’s
• general

web v

A V A V f  



2 3

max

Ib VQ fv 

max

 

v v

F f 

F2008abn Wood Beams 21 Lecture 15 Foundations Structures ARCH 331

p p

F A P f   Beam Design

• 7. Provide adequate bearing

area at supports

F2008abn Wood Beams 22 Lecture 15 Foundations Structures ARCH 331

Beam Design

• 8. Evaluate torsion
• circular cross section
• rectangular

J T fv  

2 1ab

c T fv 

 

v v

F f 

Wood Beams 23 Lecture 15 Foundations Structures ARCH 331 F2008abn

Beam Design

• 9. Evaluate deflections

allowable actual

x y     ) (

max

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Wood Beams 24 Lecture 15 Foundations Structures ARCH 331 F2008abn

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

Wood Beams 25 Lecture 15 Foundations Structures ARCH 331 F2008abn

Joists & Rafters

• allowable load tables (w)
• allowable length tables for common

live & dead loads

• lateral bracing needed
• common spacings

Wood Beams 26 Lecture 15 Foundations Structures ARCH 331 F2008abn

Engineered Wood

• plywood

– veneers at different

• rientations

– glued together – split resistant – higher and uniform strength – limited shrinkage and swelling – used for sheathing, decking, shear walls, diaphragms

Wood Beams 27 Lecture 15 Foundations Structures ARCH 331 F2008abn

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

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F2008abn Wood Beams 28 Lecture 15 Architectural Structures ARCH 331

Engineered Wood

• I sections

– beams

• other products

– pressed veneer strip panels (Parallam) – laminated veneer lumber (LVL)

• wood fibers

– Hardieboard: cement & wood

Wood Beams 29 Lecture 15 Foundations Structures ARCH 331 F2008abn

Timber Elements

• stressed-skin elements

– modular built-up “plates” – typically used for floors or roofs

Wood Beams 30 Lecture 15 Foundations Structures ARCH 331 F2008abn

Timber Elements

• built-up box sections

– built-up beams – usually site-fabricated – bigger spans

Wood Beams 31 Lecture 15 Foundations Structures ARCH 331 F2008abn

Timber Elements

• trusses

– long spans – versatile – common in roofs

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Wood Beams 32 Lecture 15 Structural Systems I COSC 321 Su2008abn

Timber Elements

• folded plates and arch panels

– usually of plywood

Wood Beams 33 Lecture 15 Structural Systems I COSC 321 Su2008abn

Timber Elements

• arches and lamellas

– arches commonly laminated timber – long spans – usually only for roofs

Wood Beams 34 Lecture 15 Structural Systems I COSC 321 Su2008abn

Approximate Depths