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ARCHITECTURAL STRUCTURES:

FORM, BEHAVIOR, AND DESIGN

ARCH 331

DR. ANNE NICHOLS

SUMMER 2014

Wood Beams 1Lecture 12

Architectural StructuresARCH 331

lecture

twelve

wood construction:

materials & beams

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Foundations Structures ARCH 331

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 ×=′≤

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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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Wood Properties

• cell structure and density

softwood

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

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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

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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

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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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• 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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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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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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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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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

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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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Beam Deformations

• curvature relates to

– bending moment

– modulus of elasticity

– moment of inertia

EI

M

R=

1

∫== dxEI

xMslope

)(θ

EI

xMcurvature

)(=

R θ

L y c

½δ ½δ

∫ ∫==∆ dxEI

xMdeflection

)(

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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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Timber Beam

Bracing

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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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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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pp FA

Pf ≤=

Beam Design

7. Provide adequate bearingarea at supports

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Beam Design

8. Evaluate torsion

• circular cross section

• rectangular

J

Tfv

ρ=

2

1abc

Tfv =

( )vv Ff ≤

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Beam Design

9. Evaluate deflections

allowableactualxy ∆≤∆=)(max

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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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Joists & Rafters

• allowable load tables (w)

• allowable length tables for common live & dead loads

• lateral bracing needed

• common spacings

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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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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 ARCH 331

Engineered Wood

• I sections

– beams

• other products

– pressed veneer strip panels (Parallam)

– (LVL)

• wood fibers

– Hardieboard: cement & wood

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Timber Elements

• stressed-skin elements

– modular built-up “plates”

– typically used for floors or roofs

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Timber Elements

• built-up box sections

– built-up beams

– usually site-fabricated

– bigger spans

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Timber Elements

• trusses

– long spans

– versatile

– common in roofs

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Structural Systems I

COSC 321

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Timber Elements

• folded plates and arch panels

– usually of plywood

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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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Structural Systems I

COSC 321

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Approximate Depths