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thirteen

wood construction:

column design

ARCHITECTURAL STRUCTURES:

FORM, BEHAVIOR, AND DESIGN

ARCH 331

DR. ANNE NICHOLS

SUMMER 2014

Wood Columns 1

Lecture 13

Architectural Structures

ARCH 331

lecture

Wood Columns 2

Lecture 16

Foundations Structures

ARCH 331

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Compression Members (revisited)

• designed for strength & stresses

• designed for serviceability & deflection

• need to design for stability

– ability to support a

specified load

without sudden or

unacceptable

deformations

Wood Columns 3

Lecture 16

Foundations Structures

ARCH 331

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Effect of Length (revisited)

• long & slender • short & stubby

Wood Columns 4

Lecture 16

Elements of Architectural Structures

ARCH 614

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Critical Stresses (revisited)

• when a column gets stubby, crushing will limit

the load

• real world has loads

with eccentricity

2

Wood Columns 5

Lecture 16

Foundations Structures

ARCH 331

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Bracing (revisited)

• bracing affects shape of buckle

in one direction

• both should be checked!

Wood Columns 6

Lecture 16

Elements of Architectural Structures

ARCH 614

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Wood Columns

• slenderness ratio = L/dmin

– d1 = smallest dimension

– le/d 50 (max)

– where is the allowable

compressive strength

parallel to the grain

– bracing common

– posts, round, built-up

cc FA

Pf

cF

Wood Columns 7

Lecture 16

Foundations Structures

ARCH 331

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Allowable Wood Stress

• where:

Fc = compressive strength parallel to grain

CD = load duration factor

CM = wet service factor (1.0 dry)

Ct = temperature factor

CF = size factor

Cp = column stability factor

pFtMDcc CCCCCFF

(Table 10.3)

F2008abnWood Columns 8

Lecture 16

Foundations Structures

ARCH 331

Strength Factors

• wood properties and load duration, CD

– short duration

• higher loads

– normal duration

• > 10 years

• stability, Cp

– combination curve - tables

pDcpcc CCFCFF *

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

3

Wood Columns 9

Lecture 16

Foundations Structures

ARCH 331

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Cp Charts – Appendix A

Wood Columns 10

Lecture 16

Foundations Structures

ARCH 331

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Column Charts – Appendix A, 12 & 13

Wood Columns 11

Lecture 16

Foundations Structures

ARCH 331

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Procedure for Analysis

1. calculate Le/dmin

– KL/d each axis, choose largest

2. obtain F´c

– compute

• KcE =0.3 sawn

• KcE = 0.418 glu-lam

3. compute Fc* FcCD

4. calculate FcE/Fc* and get Cp (Table 14)

5. calculate

2dL

cEcE

e

EKF

pcc CFF *

Wood Columns 12

Lecture 16

Foundations Structures

ARCH 331

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Procedure for Analysis (cont’d)

6. compute Pallowable = FcA• or find factual = P/A

7. is P Pallowable? (or factual Fc?)• yes: OK

• no: overstressed & no good

4

Wood Columns 13

Lecture 16

Foundations Structures

ARCH 331

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Procedure for Design

1. guess a size (pick a section)

2. calculate Le/dmin

– KL/d each axis, choose largest

3. obtain F´c

– compute

• KcE =0.3 sawn

• KcE = 0.418 glu-lam

4. compute Fc* FcCD

5. calculate FcE/Fc* and get Cp (Table 14)

2dL

cEcE

e

EKF

Wood Columns 14

Lecture 16

Foundations Structures

ARCH 331

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Procedure for Design (cont’d)

6. compute

7. compute Pallowable = FcA• or find factual = P/A

8. is P Pallowable? (or factual Fc?)• yes: OK

• no: pick a bigger section and go back to step 2.

pcc CFF *

Wood Columns 15

Lecture 16

Foundations Structures

ARCH 331

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Timber Construction by Code

• light-frame

– light loads

– 2x’s

– floor joists – 2x6, 2x8, 2x10, 2x12 typical at spacings of 12”, 16”, 24”

– normal spans of 20-25 ft or 6-7.5 m

– plywood spans between joists

– stud or load-bearing masonry walls

– limited to around 3 stories – fire safety

Wood Columns 16

Lecture 16

Foundations Structures

ARCH 331

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Design of Columns with Bending

• satisfy

– strength

– stability

• pick

– section

5

Wood Columns 17

Lecture 16

Foundations Structures

ARCH 331

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Design

• Wood

[] term – magnification factor for P-

F’bx – allowable bending strength

0.1

1

2

cEx

cbx

bx

c

c

Ff

F

f

F

f

Wood Columns 18

Lecture 16

Foundations Structures

ARCH 331

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Design Steps Knowing Loads

1. assume limiting stress

• buckling, axial stress,

combined stress

2. solve for r, A or S

3. pick trial section

4. analyze stresses

5. section ok?

6. stop when section is ok

Wood Columns 19

Lecture 16

Foundations Structures

ARCH 331

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Laminated Timber Arches

• two & three hinged

arches

• bent to wide range of

curves

• bending and

compression

• residual stress from

laminating, Cc

Wood Columns 20

Lecture 16

Foundations Structures

ARCH 331

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Laminated Arch Design

• radius of curvature, R, limited by lam

thickness, t

– R = 100t – southern pine & hardwoods

– R = 125t – softwood

• r = radius to inside face

of laminations

•

• Fb’ = Fb(CFCc)

2

Cr

t20001C