thirteen wood construction: column design ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN DR. ANNE NICHOLS SUMMER 2017 lecture

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1 ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN DR. ANNE NICHOLS SUMMER 2017 leture thirteen wood onstrution: olumn design Wood Columns 1

2 Compression Members (revisited) designed for strength & stresses designed for servieability & defletion need to design for stability ability to support a speified load without sudden or unaeptable deformations Wood Columns 2

3 Effet of Length (revisited) long & slender short & stubby Wood Columns 3

4 Critial Stresses (revisited) when a olumn gets stubby, rushing will limit the load real world has loads with eentriity Wood Columns 4

5 Braing (revisited) braing affets shape of bukle in one diretion both should be heked! Wood Columns 5

6 Wood Columns slenderness ratio = L/d min d 1 = smallest dimension l e /d 50 (max) where is the allowable ompressive strength parallel to the grain braing ommon posts, round, built-up Wood Columns 6 F f P F A

7 Allowable Wood Stress F F C C C C C where: F = ompressive strength parallel to grain C D = load duration fator C M = wet servie fator (1.0 dry) C t = temperature fator C F = size fator C p = olumn stability fator D M t F p (Fig. 9.23) Wood Columns 7

8 Strength Fators wood properties and load duration, C D short duration higher loads normal duration > 10 years stability, C p ombination urve - tables F F * C p F C C D p Wood Columns 8

9 F E 0.822E le d min 2 C p Charts Chapter 9, pg 478 Table 9.3 F E 0.822E le d min 2 ( = 0.8 sawn, = 0.9 glulam) Wood Columns 9

10 Column Charts Not in Appendix Wood Columns 10

11 Proedure for Analysis 1. alulate L e /d min KL/d eah axis, hoose largest 2. obtain F ompute F min E E (K E =0.3 sawn) l 2 2 e le d d (K E = glu-lam) E min Emin ( CM )( Ct )( CT )( Ci ) 0.822E K E 3. ompute F * F C D 4. alulate F E /F * and get C p (Table 9.3) 5. alulate F = F * C p Wood Columns 11

12 Proedure for Analysis (ont d) 6. ompute P allowable = F A or find f atual = P/A 7. is P P allowable? (or f atual F?) yes: OK no: overstressed & no good Wood Columns 12

13 Proedure for Design 1. guess a size (pik a setion) 2. alulate L e /d min KL/d eah axis, hoose largest 3. obtain F ompute Wood Columns 13 F min E E K E =0.3 sawn l 2 2 e le K E = glu-lam d d E min Emin ( CM )( Ct )( CT )( Ci ) 4. ompute F * F C D 5. alulate F E /F * and get C p (Table 9.3) 0.822E K E

14 Proedure for Design (ont d) 6. ompute F 7. ompute P allowable = F A or find f atual = P/A 8. is P P allowable? (or f atual F?) F * C yes: OK no: pik a bigger setion and go bak to step 2. p Wood Columns 14

15 Timber Constrution by Code light-frame light loads 2x s floor joists 2x6, 2x8, 2x10, 2x12 typial at spaings of 12, 16, 24 normal spans of ft or m plywood spans between joists stud or load-bearing masonry walls limited to around 3 stories fire safety Wood Columns 15

16 Design of Columns with Bending satisfy strength stability pik setion Wood Columns 16

17 Design Wood f F 2 F bx f 1 bx f F Ex 1.0 [ ] term magnifiation fator for P- F bx allowable bending strength Wood Columns 17

18 Design Steps Knowing Loads 1. assume limiting stress bukling, axial stress, ombined stress 2. solve for r, A or S 3. pik trial setion 4. analyze stresses 5. setion ok? 6. stop when setion is ok Wood Columns 18

19 Laminated Timber Arhes two & three hinged arhes bent to wide range of urves bending and ompression residual stress from laminating, C Wood Columns 19

20 Laminated Arh Design radius of urvature, R, limited by lam thikness, t R = 100t southern pine & hardwoods R = 125t softwood r = radius to inside fae of laminations C C F' b = F b (C F C ) t r 2 Wood Columns 20