Design Approach: 3) Full Sawn: In this case a rough surface is obtained with actual size equal to the nominal size.

Transcription

1 E 479 Introduction: Sizes of Structural Lumer and Use Text hapter 4 Design Approach: The design of structural wood is carried on the asis of allowale stresses at service load levels. Structural calculations are ased on the standard net size of a piece of lumer. ost structural lumer is dressed lumer. 1) Dressed Lumer: Lumer that has een surfaced to the standard net size, which is less than the nominal size (stated)(textook Section 4.11 and NDS 01 Supplement). i.e. 8 x12 memer (nominal size = 8 x 12 in.) actually is 7 ½ x 11 ½ in. (Standard net size) NDS Tale 1A Sec. 3 Supplement Lumer is dressed on a planning machine for the purpose of otaining smooth surfaces and uniform sizes. Typically lumer will e S4S (surfaced on 4 sides). 2) Rough Sawn: Large timers are usually rough sawn to dimensions that are close to standard net sizes, roughly 1/8 larger than the standard dressed size. Rough surface is usually ordered specially for architectural purposes in smaller sizes. 3) ull Sawn: In this case a rough surface is otained with actual size equal to the nominal size. 1

2 E 479 Wood Rating The majority of sawn lumer is graded y visual inspection, and material graded in this way (visually) is known as visually graded structural lumer. As the lumer comes out of the mill, a person familiar with lumer grading rules examines each piece and assigns and stamps a grade. There are two road size classifications of sawn lumer: Dimension Lumer: smaller (thinner) sizes of structural lumer. Dimension lumer usually ranges in the size from 2x2 through 4x16. In other words, dimension lumer is any material with a thickness (smaller dimension of a piece of wood, and width is the larger dimension) of 2 to 4 inches. Timers: are the larger pieces and have a minimum nominal dimension of 5 inches. Thus, the smallest practical size timer is a 6x6 inch. The design properties given in the NDS supplement are ased on two different sets of AST Standards (Textook Sections 4.3 and 4.4): In-grade procedures applied to Dimension lumer lear wood procedures applied to timers The lumer grading rules which estalish the allowale stresses for use in structural design have een developed over the years. The relative size of the wood was used as a guide in anticipating its use. Although most lumer is visually graded, a small % of lumer is AHINE STRESS RATED y sujecting each piece of wood to a nondistructive test. This process is highly automated. As lumer comes out of the mill, it passes through a series of rollers. In this process, a ending load is applied aout the minor axis of the cross section, and the modulus of elasticity of each piece measured. In addition the piece is visually inspected. The material graded using SR is limited to a thickness of 2 or less. SR has less variaility in mechanical properties than visually graded lumer. onsequently, is often used to faricate engineered wood products. Glulam eams Wood I joists and light frame However, stress rated oards are not commonly used for structural framing ecause they are very thin. So we will focus on dimension lumer. It must e remarked that the allowale stress depends on the species and on the size of the memer. Species (Sec. 4.5 Textook): A large numer of species can e used to produce structural lumer. The 2001 NDS supplement (Sec. 4, Page 29) contains allowale stresses for a large numer of species. The choice of species for use in design is a matter of economics typically. or a given 2

3 E 479 location only a few species groups may e availale and it is prudent to check with local distriutors as well as a wood products agency. The species of tress used for structural lumer are classified as hardwoods and softwoods owing not necessarily to a description of the wood properties. or example evergreens aka conifers are a large majority of the structural lumer. This will e either Douglas-ir or Southern Pine. Allowale Stresses/Design Values (NDS Taulated values in the NDS Supplement 01): Are determined y multiplying the taulated (stresses) y the appropriate adjustment factors (Textook, Sections , and design example in Section 4.23). Thus ecoming allowale design value ( ). or example for tension parallel to the grain: ( adjustment factors) t = t x = Design value 3

4 E 479 or an acceptale design, the axial tensile stress due to loads, f t, should not exceed the allowale (adjusted) stress: ft t Design Value (stress) Bending Tension parallel to grain Shear parallel to grain ompression perpendicular to grain ompression parallel to grain odulus of elasticity t P u c1 c E Taulated Stress t v ci c E Allowale (adjusted) Stress Adjustment actors: Some decrease other increase taulated value (Textook, Sections , and design example in Section 4.23, and NDS 2001 Sections 2 and 4) Examples: D fu f = load duration factor = we service factor (moisture = size factor = flat use factor = form factor content) Stresses and adjustment factors: Stresses due to known loads :(NDS 2001, Section 3)) P f =, f = t A S, etc. 4

5 E 479 Taulated Values (Stresses): Taulated design values listed in the NDS Supplement 2001 ED. These values include reduction for safety () and are for normal load duration under the specified moisture service condition. odulus of elasticity (E) does not include reduction for safety and represent average values. Dimension Lumer Page 29 NDS Supp Tale 4A, page 30, 31: Base design value for visually graded dimension lumer (except southern pine) Tale 4B, page 36,37: Base design value for visually graded southern pine Tale 4, page 39-42: Design values for mechanically graded dimension lumer (SR) Timers (5x5 and larger) Tale 4D, page 43-49: Design values for visually graded timers (all species) Adjustment actors (Sec 4.3 NDS 01 and Suplement to NDS Tales): A. Wet Serviced actor: E = Equilirium moisture content = the average moisture content that lumer assumes in service. oisture designation in grade stamp S-Grn (surface green) = 19% (in service) S-Dry (surfaced dry) = 15% (in service) These values can vary depending on environmental conditions (in most uildings ranges from 7-14% E). Special conditions must e analyzed individually. 5

6 E 479 Taulated values in NDS supplement apply to memers with E of 19% or less (regardless of S-GRN or S-Dry). If E exceeds 19%for an extended period of time, tale values should e multiplied y (see Page 30 and others for values in Tale 4 NDS-Supp 01) B. Load Duration actor: D Wood can handle higher stresses if loads are applied for a short period of time. All taulated values apply to normal duration loading (10 years) The term duration of load refers to the total accumulated length of time that a load is applied during the life of a structure. Tale in NDS 01 provides D to e used in the one associated with the shortestduration of time. Whichever comination of loads, together with the appropriate load duration factor produces the largest memer size is the one that must e used in design. 6

7 E 479. Size actor: The size of the memer has an effect on its unit stress. - See Supplement 01 Tales: 4A, 4B, 4, 4D and 4E D. Repetitive emer actor: r only!! The system performance of a series of small closely spaced wood memers, where failure of one memer is not fatal (see Supplement 01) 7

8 E 479 E. lat use actor: fu Except for decking, taulated stress for dimension lumer apply to wood memers that are stressed in flexure aout the strong axis edgewise or load applied to narrow face. If however load is a applied to the wide face the stresses may e increased y fu. Taulated ending stresses also for timer Beams & stringers apply for ending aout x- axis. NDS does not provide fu for these cases. 8

9 E 479. Temperature actor: t The strength of the wood in service is increased as the temperature cools elow the normal temp in most uildings. On the other hand, the strength decreases as temperatures are increased. The factor t is the multiplier that is used to reduce taulated stresses if higher than normal temperatures are encountered in a design situation. Values of t are given in NDS Sec for T > 100 o. Important to note that strength will e regained when temperature returns to normal values! Thus this factor applies for sustained conditions. G. orm actor: f The purpose of this factor is to adjust taulated ending stress for non-rectangular sections (see Section in NDS 01). 9

10 E 479 Example: Determine the taulated and allowale design values for the following memer and loading condition. No. 2 Hem-ir (ending aout strong axis) loor eams 4x6 4 on centers. Loads are (D+L). High-humidity conditions exist, and moisture content may exceed 19%. Stresses Bending (NDS Supp 01) Taulated value, = 850 psi (Ta. 4A Supp. NDS 01) 10

11 E 479 actors (NDS 01 Sec. 4.3) (Tale 4.3.1, NDS 01 Page 27) In many practical situations, a numer of adjustment factors may have a value of 1.0. A comprehensive summary of the modification factors for wood memers is given in NDS Tale D D t t = load duration factors (Sec NDS) (Tale controlled y live load) = 0.85 since = size factor (Sec. 4.3, NDS 01and Tale 4A) = 1.3 = = since x Temperature factor (Sec NDS) Wet service (Sec NDS 01, Supp h 4, Tale 4A) > 19% or 1.0 if = 850 x 1.3 = 1105 psi < 1150 psi 1150 psi T 100 o 11

12 E (Sec ) d / = = 1.5 < 2.0; no lateral support is required 4 (Sec ) (Also could e invoked if needed) L i i fu r r f = Beam Staility actor (Sec and NSD 01) (Element is not loaded on its flat side) = f orm L = Incising actor (Sec.4.3.8) done to increase treatment penetrations = Repetitive emer actor (Sec NDS) factor (Sec &3.3.4) inally calculate allowale stress for ending = 850 x 1 x... x 1.3 x... = 1105 psi Tension II to Grain T T = Tension parallel to grain (Tale 4A Supp) = 525 psi actors (NDS 01, Sec. 4.3 & Tale 4.3.1) D t i T (Tale 4A, Supp Adj. actors) = 1.3 = 525 x 1.3 = 683 psi Shear II to Grain, V V = 150 psi actors (NDS 4.3) 12

13 E 479 D = 0.97 Tale 4A Supp Adj.actor t i V = 150 psi x 0.97 = 146 psi ompression to grain = 405 psi actors (NDS 4.3) Tale (Sec ) and Tale 4A = 0.67 t i = Bearing area factor (Sec ) Assume l 6" = 405 x 0.67 = 271 psi ompression II to grain c = 1300 psi actors (NDS Tale 4.3.1) D (Tale 4A, Supp) = 0.8 or 1.0 when ( c )( ) 750 psi = 1.1 (Tale 4A, Supp) = 0.8 since1300 x 1.1> 750 t i p (This is a eam!) c = 1300 x 0.8 x 1.1 x K1.0 = 1144 psi 13

14 E 479 odulus of Elasticity, OE rom Tale 4A E = 1,300,000 t i T N.A. = 0.9 (actors in Tale 4A, Supp) psi E = 1,300,000 x 0.9 = 1,170,000 = temperature factor t = repetitive factor r = Buckling Stiffness factor for wood tresses (4.4.2) psi 14

15 E 479 Design Summary Beams (hapter 6 Text) 1. Determine trial eam size ased on ending stress considerations (long. Bending stress, II to grain see ig. 6.1a). or sawn lumer loaded-edgewise only are given taulated values. ( S ) = reqd Select trial memer with (use Tale for dressed S4S) ( S ) prov ( S ) reqd recheck for appropriate size factor,, since initially is unknown (eam size) so that f = S ( ) act (with actual ) 2. heck shear (Sec. 3.4 NDS) f f V = 1.5 A v v v = V t w d supp. with app. factors In this calculation a reduced shear (d- away from support face, d = overall depth) can e used V (Sec )(a) f v V = 1.5 A If this check shows the eam size selected to e inadequate, the size is revised to provide sufficient A. 15

16 E 479 Deflection riteria (IB 2003 Sec. 3.5 NDS 01) Limits are estalished for deflections for eams, trusses, and similar memers that are not to e exceeded under certain gravity loads. Tale in the IB 2003 gives the necessary limits and other information necessary to ensure user comfort and to prevent excessive cracking of plaster ceilings. or Green Lumer ( > 19%) TOTAL =.0 ( long term ) + short term Live < L/240 2 < L/180 or Seasoned Lumer ( < 19%) 16

17 E 479 < L/180 TOTAL = 1.5( Long Term ) + Short Term Live < L/240 where Long Term = immediate deflection due to the long term portion of the design load (usually dead load) Short Term = immediate deflection due to short term component of the design load (usually live load) Bearing - Sec NDS 01 17

18 E 479 Example: Sawn Beam Design (Dimension Lumer) Beams are spaced 16 inches on center (roof eam) Buckling of the compression zone is prevented y the plywood roof sheathing aterial is No. 1 Douglas ir larch Loads wd = 19 l / ft (Dead load per lineal ft.) wl = 27 l / ft (Live load per lineal ft.) TOTAL = 46 l / ft Required load comination (Sect NDS 01 and Tale 2.3.2) and Duration actors Dalone D = 0.9 D + L = = 1.15 ( snow load) D Determine trial size ased on ending and then check other criteria. (Sec , NDS 01 and Tale 4A of the supplement, Spacing < 24 ) = (Tale 4A Supp to NDS01) =1. 20 ( < 19%, normal temperature conditions, compression edge of ending memer supported throughout in accordance with and no incision),, and T L i = 1000 (1.15) (1.2) (1.15) = 1587 psi max 12,515 3 Reqd S = = = 7.9 in 1587 Try 2x6 S = 7.56 in 3 (rom Tale 1B Supp). rom NDS Supplement Tale 4A S 1.3 = 1.3 = 1587 x = 1719 psi = = 7.32 in < 7.56 o. k reqd r 18

19 E 479 heck for Shear (NDS Sec. 3.4) f v 3 = 2, t V d (Rect.) A = 8.25 in 2 (Tale 1B Supp) onservative to use V V = 46.0 (13.5) = 3110 = AX fv = 3 2 (311) (8.25) = 56.5 psi V = V ( D ) ( ) ( t ) ( i ) = 180 (1.15) = 207 psi > 56.5 psi ok heck Deflections E = E ( ) ( ) ( )( ) = E T i t = 1, 7000 Ksi (Tale 4A Supp NDS01) w L L 5( 27.0 )(13.5 ) (1728 ) = = = 0.57" L 384E I ( 384 ) (1, 700,000 )( 20.8 ) = L L x 12 = 240 = 0.67" > 0.57" ok Also check for long term effects y calculating dead plus live deflection does not exceed L/180 (Do in-class). Use 2x6 No. 1 D-L 19% Bearing Stress heck (Sec NDS01) = R Re qd. = Re qd. l ( A > = ) ( ) ( = t = 0.49 in 625 ) = 625 psi 2 = 0.33: < 6" ok 19