thirteen lecture - Texas A&M...
Transcript of thirteen lecture - Texas A&M...
S2015abnWood Beams 1Lecture 13
Elements of Architectural StructuresARCH 614
ELEMENTS OF ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 614
DR. ANNE NICHOLS
SPRING 2015
lecture
thirteen
wood construction:materials & beams
S2015abnWood Beams 2Lecture 13
Elements of Architectural StructuresARCH 614
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 ×=′≤
S2015abnWood Beams 3Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 4Lecture 13
Elements of Architectural StructuresARCH 614
Wood Properties
• cell structure and density
softwood
hardwoodhttp://www.swst.org/teach/set2/struct1.html
S2015abnWood Beams 5Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 6Lecture 13
Elements of Architectural StructuresARCH 614
Wood Properties
• load duration
– short duration
• higher loads
– normal duration
• > 10 years
• creep
– additional deformation with no additional load
S2015abnWood Beams 7Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 8Lecture 13
Elements of Architectural StructuresARCH 614
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 5.2 (pg 177)
S2015abnWood Beams 9Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 10Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 11Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 12Lecture 13
Elements of Architectural StructuresARCH 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)
S2015abnWood Beams 13Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 14Lecture 13
Elements of Architectural StructuresARCH 614
Beam Design Criteria • superpositioning
– use of beam charts
– elastic range only!
– “add” moment diagrams
– “add” deflection CURVES (not maximums)
+
=
+
=
S2015abnWood Beams 15Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood 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
S2015abnWood Beams 17Lecture 13
Elements of Architectural StructuresARCH 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 ≤
S2015abnWood Beams 18Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 19Lecture 13
Elements of Architectural StructuresARCH 614
Beam Design
6. Evaluate shear stresses - horizontal
•
• W and rectangles
• general
web
vA
V
A
Vf ≈=−
2
3max
Ib
VQfv =−max
( )vv Ff ≤
S2015abnWood Beams 20Lecture 13
Elements of Architectural StructuresARCH 614
pp FA
Pf ≤=
Beam Design
7. Provide adequate bearing
area at supports
S2015abnWood Beams 21Lecture 13
Elements of Architectural StructuresARCH 614
Beam Design
8. Evaluate torsion
• circular cross section
• rectangular
J
Tfv
ρ=
2
1abc
Tfv =
( )vv Ff ≤
S2015abnWood Beams 22Lecture 13
Elements of Architectural StructuresARCH 614
Beam Design
9. Evaluate deflections
allowableactualxy ∆≤∆=)(max
S2015abnWood Beams 23Lecture 13
Elements of Architectural StructuresARCH 614
Joists & Rafters
• allowable load tables
• allowable length tables for common
live & dead loads
• lateral bracing needed
• common spacings
S2015abnWood Beams 24Lecture 13
Elements of Architectural StructuresARCH 614
LRFD
• factored loads & reduced nominal capacity
φ - Resistance factor
γ - Load factor for (D)ead & (L)ive load
• nominal adjusted – no CD
u D D L L b nM M M Mγ γ φ= + ≤
n bnM F S′= ×
( )( )( )bn bn bF F product of adjustment factorsφ λ′ =
( )bn b FF F K conversion factor= ×
S2015abnWood Beams 25Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 26Lecture 13
Elements of Architectural StructuresARCH 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
S2015abnWood Beams 27Lecture 13
Elements of Architectural StructuresARCH 614
Engineered Wood
• I sections
– beams
• other products
– pressed veneer strip panels (Parallam)
• wood fibers
– Hardieboard: cement & wood
S2015abnWood Beams 28Lecture 13
Elements of Architectural StructuresARCH 614
Timber Elements
• stressed-skin elements
– modular built-up “plates”
– typically used for floors or roofs
S2015abnWood Beams 29Lecture 13
Elements of Architectural StructuresARCH 614
Timber Elements
• built-up box sections
– built-up beams
– usually site-fabricated
– bigger spans
S2015abnWood Beams 30Lecture 13
Elements of Architectural StructuresARCH 614
Timber Elements
• trusses
– long spans
– versatile
– common in roofs
S2015abnWood Beams 31Lecture 13
Elements of Architectural StructuresARCH 614
Timber Elements
• folded plates and arch panels
– usually of plywood
S2015abnWood Beams 32Lecture 13
Elements of Architectural StructuresARCH 614
Timber Elements
• arches and lamellas
– arches commonly laminated timber
– long spans
– usually only for roofs
S2015abnWood Beams 33Lecture 13
Elements of Architectural StructuresARCH 614
Approximate Depths