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

E LEMENTS OF A RCHITECTURAL S TRUCTURES : F ORM, B EHAVIOR, AND D ESIGN ARCH 614 D R. A NNE N ICHOLS S PRING 2019 lecture fourteen wood construction: column design Wood Columns 1 Elements of Architectural Structures S2019abn Lecture 14 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 Wood Columns 2 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Effect of Length (revisited) • short & stubby • long & slender Wood Columns 3 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Critical Stresses (revisited) • when a column gets stubby, crushing will limit the load • real world has loads with eccentricity Wood Columns 4 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Bracing (revisited) • bracing affects shape of buckle in one direction • both should be checked! Wood Columns 5 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Wood Columns • slenderness ratio = L/d min = L/d 1 – d 1 = smaller dimension – l e /d  50 (max) P    f F c c A F  – where is the allowable compressive c strength parallel to the grain – bracing common Wood Columns 6 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Allowable Wood Stress         F  F C C C C C c c D M t F p • where: F c = compressive strength parallel to grain C D = load duration factor C M = wet service factor (1.0 dry) C t = temperature factor (Table 5.2)   C F = size factor  F C p = column stability factor f cE * F c Wood Columns 7 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Strength Factors • wood properties and load duration, C D – short duration • higher loads – normal duration • > 10 years • stability, C p http://www.swst.org/teach/set2/struct1.html – combination curve - tables      * F F C F C C c c p c D p Wood Columns 8 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

C p Charts 0.822  E  min F   ( c = 0.8 sawn, c = 0.9 glulam) cE 2 l e d Wood Columns 9 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Procedure for Analysis 1. calculate L e /d min – KL/d each axis, choose largest 2. obtain F ´ c  0.822 E  min – F compute   cE 2 L e d   • E E min ( C )( C )( C )( C ) where min M t T i *  F c C D 3. compute F c * and get C p (chart) 4. calculate F cE /F c   * F F C 5. calculate c c p Wood Columns 10 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Procedure for Analysis (cont’d) 6. compute P allowable = F  c  A • or find f actual = P/A 7. is P  P allowable ? (or f actual  F  c ?) • yes: OK • no: overstressed & no good Wood Columns 11 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Procedure for Design 1. guess a size (pick a section) 2. calculate L e /d min – KL/d each axis, choose largest 3. obtain F ´ c  0.822 E  – min F compute   cE 2 L e d   • where E E min ( C )( C )( C )( C ) min M t T i *  F c C D 4. compute F c * and get C p (chart) 5. calculate F cE /F c Wood Columns 12 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Procedure for Design (cont’d)   * 6. calculate F F C c c p 7. compute P allowable = F  c  A • or find f actual = P/A 8. is P  P allowable ? (or f actual  F  c ?) • yes: OK • no: pick a bigger section and go back to step 2 . Wood Columns 13 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

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

Design of Columns with Bending • satisfy – strength – stability • pick – section Wood Columns 16 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614

Design • Wood 2   f f   c bx   1 . 0       f F  F 1 c c     F bx cEx [] term – magnification factor for P-  F’ bx – allowable bending strength Wood Columns 17 Elements of Architectural Structures S2019abn Lecture 14 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 Wood Columns 18 Elements of Architectural Structures S2019abn Lecture 14 ARCH 614