fourteen wood construction: column design Wood Columns 1 Elements - - PowerPoint PPT Presentation

S2019abn Wood Columns 1 Lecture 14 Elements of Architectural Structures ARCH 614

ELEMENTS OF ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN

ARCH 614

• DR. ANNE NICHOLS

SPRING 2019 lecture

fourteen

wood construction: column design

S2019abn Wood Columns 2 Lecture 14 Elements of Architectural Structures ARCH 614

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

S2019abn Wood Columns 3 Lecture 14 Elements of Architectural Structures ARCH 614

Effect of Length (revisited)

• long & slender
• short & stubby

S2019abn Wood Columns 4 Lecture 14 Elements of Architectural Structures ARCH 614

Critical Stresses (revisited)

• when a column gets stubby, crushing will limit

the load

• real world has loads with eccentricity

S2019abn Wood Columns 5 Lecture 14 Elements of Architectural Structures ARCH 614

Bracing (revisited)

• bracing affects shape of buckle

in one direction

• both should be checked!

S2019abn Wood Columns 6 Lecture 14 Elements of Architectural Structures ARCH 614

Wood Columns

• slenderness ratio = L/dmin = L/d1

– d1 = smaller dimension – le/d  50 (max) – where is the allowable compressive strength parallel to the grain – bracing common

c c

F A P f   

c

F

S2019abn Wood Columns 7 Lecture 14 Elements of Architectural Structures ARCH 614

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

    



p F t M D c c

C C C C C F F  

(Table 5.2)

 

* c cE

F F

f 

S2019abn Wood Columns 8 Lecture 14 Elements of Architectural Structures ARCH 614

Strength Factors

• wood properties and load duration, CD

– short duration

• higher loads

– normal duration

• > 10 years
• stability, Cp

– combination curve - tables

 

p D c p c c

C C F C F F   

*

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

S2019abn Wood Columns 9 Lecture 14 Elements of Architectural Structures ARCH 614

Cp Charts

(c = 0.8 sawn, c = 0.9 glulam)

 

min 2

0.822  

cE l d

e

E F

S2019abn Wood Columns 10 Lecture 14 Elements of Architectural Structures ARCH 614

Procedure for Analysis

• 1. calculate Le/dmin

– KL/d each axis, choose largest

• 2. obtain F´c

– compute

• where
• 3. compute Fc

*  FcCD

• 4. calculate FcE/Fc

* and get Cp (chart)

• 5. calculate

 

min 2

0.822

e

cE L d

E F  

p c c

C F F

*

 

min min (

)( )( )( )

M t T i

E E C C C C  

S2019abn Wood Columns 11 Lecture 14 Elements of Architectural Structures ARCH 614

Procedure for Analysis (cont’d)

• 6. compute Pallowable = FcA
• r find factual = P/A
• 7. is P  Pallowable? (or factual  Fc?)
• yes: OK
• no: overstressed & no good

S2019abn Wood Columns 12 Lecture 14 Elements of Architectural Structures ARCH 614

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

• where
• 4. compute Fc

*  FcCD

• 5. calculate FcE/Fc

* and get Cp (chart)

 

min 2

0.822

e

cE L d

E F  

min min(

)( )( )( )

M t T i

E E C C C C  

S2019abn Wood Columns 13 Lecture 14 Elements of Architectural Structures ARCH 614

Procedure for Design (cont’d)

• 6. calculate
• 7. compute Pallowable = FcA
• r find factual = P/A
• 8. is P  Pallowable? (or factual  Fc?)
• yes: OK
• no: pick a bigger section and go back to step 2.

p c c

C F F

*

 

S2019abn Wood Columns 14 Lecture 14 Elements of Architectural Structures ARCH 614

Specific Column Charts

S2019abn Wood Columns 15 Lecture 14 Elements of Architectural Structures ARCH 614

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

S2019abn Wood Columns 16 Lecture 14 Elements of Architectural Structures ARCH 614

Design of Columns with Bending

• satisfy

– strength – stability

• pick

– section

S2019abn Wood Columns 17 Lecture 14 Elements of Architectural Structures ARCH 614

Design

• Wood

[] term – magnification factor for P- F’bx – allowable bending strength

. 1 1

2

                

cEx c bx bx c c

F f F f F f

S2019abn Wood Columns 18 Lecture 14 Elements of Architectural Structures ARCH 614

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