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Su2018abnWood Columns 1

Lecture 13

Architectural Structures

ARCH 331

ARCHITECTURAL STRUCTURES:

FORM, BEHAVIOR, AND DESIGN

ARCH 331

DR. ANNE NICHOLS

SUMMER 2018

lecture

thirteen

wood construction:

column design


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

Effect of Length (revisited)

• long & slender • short & stubby


Lecture 13


ARCH 331

Critical Stresses (revisited)

• when a column gets stubby, crushing will limit

the load

• real world has loads

with eccentricity


Lecture 13


ARCH 331

Bracing (revisited)

• bracing affects shape of buckle

in one direction

• both should be checked!


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

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

(Fig. 9.23)


Lecture 13


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


Lecture 13


ARCH 331

Cp Charts – Chapter 9, pg 478

min

2

0.822 cE

ld

e

EF

Table 9.3(c = 0.8 sawn, c = 0.9 glulam)

min

2

0.822 cE

ld

e

EF


Lecture 13


ARCH 331

Column Charts – Not in Appendix

Su2018abn

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 9.3)

5. calculate Fc= Fc

* CpWood Columns 11

Lecture 13


ARCH 331

Procedure for Analysis

min cE

cE 2 2l le e

d d

0.822E K EF

min min ( )( )( )( )M t T iE E C C C C


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

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 9.3)

min cE

cE 2 2l le e

d d

0.822E K EF

min min ( )( )( )( )M t T iE E C C C C


Lecture 13


ARCH 331

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 *


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

Design of Columns with Bending

• satisfy

– strength

– stability

• pick

– section


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

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


Lecture 13


ARCH 331

Laminated Timber Arches

• two & three hinged

arches

• bent to wide range of

curves

• bending and

compression

• residual stress from

laminating, Cc


Lecture 13


ARCH 331

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

•

• F'b = Fb(CFCc)

2

Cr

t20001C

wood construction: column designfaculty.arch.tamu.edu/anichols/courses/arch 331/summer...wood...

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