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S2017abnWood Beams 1
Lecture 13
Elements of Architectural Structures
ARCH 614
ELEMENTS OF ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 614
DR. ANNE NICHOLS
SPRING 2017
lecture
thirteen
wood construction:materials & beams
S2017abnWood Beams 2
Lecture 13
Elements of Architectural Structures
ARCH 614
Wood Beam Design
• 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:
factorsadjustmentofproductFFf bbb
S2017abnWood Beams 3
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 4
Lecture 13
Elements of Architectural Structures
ARCH 614
Wood Properties
• cell structure and density
softwood
hardwoodhttp://www.swst.org/teach/set2/struct1.html
S2017abnWood Beams 5
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 6
Lecture 13
Elements of Architectural Structures
ARCH 614
Wood Properties
• load duration
– short duration
• higher loads
– normal duration
• > 10 years
• creep
– additional
deformation with no additional load
S2017abnWood Beams 7
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 8
Lecture 13
Elements of Architectural Structures
ARCH 614
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
0112 91
.)d/(CF Table 5.2 (pg 177)
S2017abnWood Beams 9
Lecture 13
Elements of Architectural Structures
ARCH 614
• 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
S2017abnWood Beams 10
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 11
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 12
Lecture 13
Elements of Architectural Structures
ARCH 614
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)
S2017abnWood Beams 13
Lecture 13
Elements of Architectural Structures
ARCH 614
Beam Design Criteria • strength design
– bending stresses predominate
– shear stresses occur
• serviceability– limit deflection and cracking
– control noise & vibration
– no excessive settlement offoundations
– durability
– appearance
– component damage
– ponding
S2017abnWood Beams 14
Lecture 13
Elements of Architectural Structures
ARCH 614
Beam Design Criteria • superpositioning
– use of beam charts
– elastic range only!
– “add” moment diagrams
– “add” deflection CURVES (not maximums)
+
=
+
=
S2017abnWood Beams 15
Lecture 13
Elements of Architectural Structures
ARCH 614
Deflection Limits
• based on service condition, severity
Use LL only DL+LL
Roof beams:
Industrial L/180 L/120
Commercial
plaster ceiling L/240 L/180
no plaster L/360 L/240
Floor beams:
Ordinary Usage L/360 L/240
Roof or floor (damageable elements) L/480
S2017abnWood Beams 16Lecture 13
Elements of Architectural StructuresARCH 614
• 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
S2017abnWood Beams 17
Lecture 13
Elements of Architectural Structures
ARCH 614
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 size6
2bhS
h
b bb Ff
S2017abnWood Beams 18
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 19
Lecture 13
Elements of Architectural Structures
ARCH 614
Beam Design
6. Evaluate shear stresses - horizontal
•
• W and rectangles
• general
web
vA
V
A
Vf
2
3max
Ib
VQfv max
vv Ff
S2017abnWood Beams 20
Lecture 13
Elements of Architectural Structures
ARCH 614
pp FA
Pf
Beam Design
7. Provide adequate bearing
area at supports
S2017abnWood Beams 21
Lecture 13
Elements of Architectural Structures
ARCH 614
Beam Design
8. Evaluate torsion
• circular cross section
• rectangular
J
Tfv
2
1abc
Tfv
vv Ff
S2017abnWood Beams 22
Lecture 13
Elements of Architectural Structures
ARCH 614
Beam Design
9. Evaluate deflections
allowableactualxy )(max
S2017abnWood Beams 23
Lecture 13
Elements of Architectural Structures
ARCH 614
Joists & Rafters
• allowable load tables
• allowable length tables for common
live & dead loads
• lateral bracing needed
• common spacings
S2017abnWood Beams 24
Lecture 13
Elements of Architectural Structures
ARCH 614
LRFD
• factored loads & reduced nominal capacity
- Resistance factor
- Load factor for (D)ead & (L)ive load
• nominal adjusted – no CD
u D D L L b nM M M M
n bnM F S
( )( )( )bn bn bF F product of adjustment factors
( )bn b FF F K conversion factor
S2017abnWood Beams 25
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 26
Lecture 13
Elements of Architectural Structures
ARCH 614
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
S2017abnWood Beams 27
Lecture 13
Elements of Architectural Structures
ARCH 614
Engineered Wood
• I sections
– beams
• other products
– pressed veneer strip
panels (Parallam)
• wood fibers
– Hardieboard: cement & wood
S2017abnWood Beams 28
Lecture 13
Elements of Architectural Structures
ARCH 614
Timber Elements
• stressed-skin elements
– modular built-up “plates”
– typically used for floors or roofs
S2017abnWood Beams 29
Lecture 13
Elements of Architectural Structures
ARCH 614
Timber Elements
• built-up box sections
– built-up beams
– usually site-fabricated
– bigger spans
S2017abnWood Beams 30
Lecture 13
Elements of Architectural Structures
ARCH 614
Timber Elements
• trusses
– long spans
– versatile
– common in roofs
S2017abnWood Beams 31
Lecture 13
Elements of Architectural Structures
ARCH 614
Timber Elements
• folded plates and arch panels
– usually of plywood
S2017abnWood Beams 32
Lecture 13
Elements of Architectural Structures
ARCH 614
Timber Elements
• arches and lamellas
– arches commonly
laminated timber
– long spans
– usually only for roofs
S2017abnWood Beams 33
Lecture 13
Elements of Architectural Structures
ARCH 614
Approximate Depths