twelve Wood Beams 1 Architectural Structures F2009abn ...
Transcript of twelve Wood Beams 1 Architectural Structures F2009abn ...
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F2009abn
ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 331
DR. ANNE NICHOLS
SUMMER 2013
Wood Beams 1
Lecture 12
Architectural Structures
ARCH 331
lecture
twelve
wood construction:
materials & beams
F2008abnWood Beams 2
Lecture 15
Foundations Structures
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Wood Beam Design
• National Design Specification
– 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:
factorsadjustmentofproductFFf bbb
Wood Beams 3
Lecture 15
Foundations Structures
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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
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Lecture 15
Foundations Structures
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Wood Properties
• cell structure and density
softwood
hardwoodhttp://www.swst.org/teach/set2/struct1.html
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Foundations Structures
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Wood Properties
• moisture
– exchanges with air easily
– excessive drying causes warping and
shrinkage
– strength varies some
• temperature
– steam
– volatile products
– combustionhttp://www.swst.org/teach/set2/struct1.html
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Wood Properties
• load duration
– short duration
• higher loads
– normal duration
• > 10 years
• creep
– additional
deformation with no additional load
Wood Beams 7
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Foundations Structures
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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
Wood Beams 8
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Foundations Structures
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Adjustment Factors
• terms
– CD = load duration factor
– CM = wet service factor
• 1.0 dry 16% MC
– CF = size factor
• visually graded sawn
lumber and round timber >
12” depth
0112 91
.)d/(CF Table 10.3 (pg 376)
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Foundations Structures
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• 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
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Foundations Structures
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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
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Foundations Structures
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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
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Foundations Structures
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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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Foundations Structures
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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
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Foundations Structures
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Beam Design Criteria • superpositioning
– use of beam charts
– elastic range only!
– “add” moment diagrams
– “add” deflection CURVES (not maximums)
+
=
+
=
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Foundations Structures
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Beam Deformations
• curvature relates to
– bending moment
– modulus of elasticity
– moment of inertia
EI
M
R
1
dxEI
xMslope
)(
EI
xMcurvature
)(
R
L y c
½ ½
dxEI
xMdeflection
)(
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Foundations Structures
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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/360Ordinary Usage
Floor beams:
L/360no plaster
L/240plaster ceiling
Commercial
L/180Industrial
Roof beams:
LL onlyUse
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• 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
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Foundations Structures
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Timber Beam
Bracing
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Foundations Structures
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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
2bhS
h
b bb Ff
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Foundations Structures
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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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Beam Design
6. Evaluate shear stresses - horizontal
•
• rectangles and W’s
• general
web
vA
V
A
Vf
2
3max
Ib
VQfv max
vv Ff
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Foundations Structures
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pp FA
Pf
Beam Design
7. Provide adequate bearing
area at supports
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Foundations Structures
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Beam Design
8. Evaluate torsion
• circular cross section
• rectangular
J
Tfv
2
1abc
Tfv
vv Ff
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Foundations Structures
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Beam Design
9. Evaluate deflections
allowableactualxy )(max
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Foundations Structures
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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
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Foundations Structures
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Joists & Rafters
• allowable load tables (w)
• allowable length tables for common
live & dead loads
• lateral bracing needed
• common spacings
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Foundations Structures
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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
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Foundations Structures
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Engineered Wood
• glued-laminated timber
– glulam
– short pieces gluedtogether
– 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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Architectural Structures
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Engineered Wood
• I sections
– beams
• other products
– pressed veneer strip
panels (Parallam)
– (LVL)
• wood fibers
– Hardieboard: cement & wood
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Foundations Structures
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Timber Elements
• stressed-skin elements
– modular built-up “plates”
– typically used for floors or roofs
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Foundations Structures
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Timber Elements
• built-up box sections
– built-up beams
– usually site-fabricated
– bigger spans
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Foundations Structures
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Timber Elements
• trusses
– long spans
– versatile
– common in roofs
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Lecture 15
Structural Systems I
COSC 321
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Timber Elements
• folded plates and arch panels
– usually of plywood
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Lecture 15
Structural Systems I
COSC 321
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Timber Elements
• arches and lamellas
– arches commonly
laminated timber
– long spans
– usually only for roofs
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Lecture 15
Structural Systems I
COSC 321
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Approximate Depths