1、Section 13 WOOD STRUCTURES 13.1 GENERAL AND NOTATIONS 13.1.1 General The following information on wood design is generally based on the National Design Specification for Wood Construction (NDP), 1991 Edition. See the 1991 Edition of the NDP for additional information. 13.1.2 Net Section In determini
2、ng the capacity of wood members, the net section of the member shall be used. Unless otherwise noted, the net section shall be determined by deducting from the gross section, the projected area of all material removed by boring, grooving, dapping, notching or other means. 13.1.3 Impact In calculatin
3、g live load stresses in wood, impact shall be neglected unless otherwise noted. See Article 3.8.1. 13.1.4 Notations a b c = coefficient based on support conditions for ta- = width of bending member (Article 13.6.4.3) = coefficient based on sawn lumber, round timber piles, glued laminated timber or s
4、tructural com- posite lumber (Article 13.7.3.3.5) pered columns (Article 13.7.3.4.2) CD C, = load duration factor (Article 13.5.5.2) = bending size factor for sawn lumber, struc- tural composite lumber, and for glued lami- nated timber with loads applied parallel to the wide face of the laminations
5、(Article 13.6.4.2) = compression size factor for sawn lumber (foot- notes to Table 13.5.1A) = tension size factor for sawn lumber (footnotes to Table 13.5.1A) and structural composite lumber (footnotes to Tables 13.5.4A and 13.5.4B) C, C, C, = sheer stress factor (footnotes to Table 13.5.1A) = beam
6、stability factor (Article 13.6.4.4) = wet service factor (Article 13.5.5.1) = column stability factor (Article 13.7.3.3) = volume factor for glued laminated timber with loads applied perpendicular to the wide face of the laminations (Article 13.6.4.3) = bearing area factor (Article 13.6.6.3) = form
7、factor (Article 13.6.4.5) = flat use factor for sawn lumber (footnotes to = repetitive member factor for sawn lumber (foot- = depth of member (Article 13.6.4.2.2) = maximum column face dimension (Article = minimum column face dimension (Article = representative dimension for a tapered column = tabul
8、ated modulus of elasticity (Article 13.6.3) = allowable modulus of elasticity (Article = tabulated unit stress in bending (Article 13.6.4.1) = allowable unit stress in bending (Article 13.6.4.1) = adjusted tabulated bending stress for beam sta- bility (Article 13.6.4.4.5) = tabulated unit stress in
9、compression parallel to grain (Article 13.7.3.2) = allowable unit stress in compression parallel to grain (Article 13.7.3.2) = adjusted tabulated stress in compression par- allel to grain for column stability (Article 1 3.7.3.3.5) = actual unit stress in compression parallel to grain (Article 13.7.3
10、.1) = tabulated unit stress in compression perpendicu- lar to grain (Article 13.6.6.2) = allowable unit stress in compression perpendic- ular to grain (Article 13.6.6.2) = tabulated unit stress in bearing parallel to grain (Article 13.7.4.1) = allowable unit stress in bearing parallel to grain (Arti
11、cle 13.7.4.1) Table 13.5.1A) notes to Table 13.5.1A) 13.7.3.4.2) 13.7.3.4.2) face (Article 13.7.3.4.2) 13.6.3) 357 358 HIGHWAY BRIDGES 13.1.4 F, = F: = F, = F: = f“ = Fe, = K= KbE = U i 13.5.5.1.1 DIVISION I-DESIGN 365 366 HIGHWAY BRIDGES 13.5.5.1.1 o ti d 8 3 E Y g o c, .e 2 3 Q a 8 .CI Q 13.5.5.1.
12、1 DIVISION I-DESIGN 367 3%8%8 .i+ 14.3 where: 1, = effective length in inches; 1, = unsupported length in inches; d = depth of bending member in inches. If lateral support is provided to prevent rotation at the points of bearing, but no other lateral support is provided throughout the bending member
13、 length, the unsupported length, lu, is the distance between points of bearing, or the length of a cantilever. If lateral support is provided to prevent rotation and lateral displacement at intermediate points as well as at the bearings, the unsupported length, lu, is the distance be- tween such poi
14、nts of intermediate lateral support. 13.6.4.4.4 The slenderness ratio for bending mem- bers, RE, is determined from the following equation: R, = d = depth of bending member in inches; b = width of bending member in inches. 13.6.4.4.5 The beam stability factor, CL, shall be computed as follows: 13.6.
15、4.4.5 DIVISION I-DESIGN - 379 (13-7) KbEE FbE = - where: (13 - 8) Ft = tabulated bending stress adjusted by all ap- plicable adjustment factors given in Equation (13-2) except the volume factor, C, the beam stability factor, CL, and the flat-use factor, C,; = 0.438 for visually graded sawn lumber 0.
16、609 for glued laminated timber, structural com- posite lumber, and machine stress rated lum- ber; = allowable modulus of elasticity in psi as de- I(bE E termined by Article 13.6.3. 13.6.4.5 Form Factor, Cf For bending members with circular cross sections the tabulated bending stress shall be adjuste
17、d by the form factor, Cf = 1.18. A tapered circular section shall be considered as a bending member of variable cross section. 13.6.5 Shear Parallel to Grain 13.6.5.1 General 13.6.5.1.1 The provisions of this article apply to shear parallel to grain (horizontal shear) at or near the points of vertic
18、al support of solid bending members. Refer to the 1991 edition of the NDS for additional design re- quirements for other member types. 13.6.5.1.2 The critical shear in wood bending mem- bers is shear parallel to grain. It is unnecessary to verify the strength of bending members in shear perpendicula
19、r to grain. 13.6.5.2 Actual Stress The actual unit stress in shear parallel to grain due to applied loading on rectangular members shall be deter- mined by the following equation: 2x7 JV f =- ” 2bd (13 - 9) where: f, = actual unit stress in shear parallel to grain in psi; b = width of bending member
20、 in inches; d = depth of bending member in inches; V = vertical shear in pounds, as determined in accor- dance with the following provisions. For uniformly distributed loads, such as dead load, the magnitude of vertical shear used in Equation (1 3-9) shall be the maximum shear occurring at a distanc
21、e from the support equal to the bending member depth, d. When members are supported by full bearing on one surface, with loads applied to the opposite surface, all loads within a distance from the supports equal to the bending member depth shall be neglected. For vehicle live loads, the loads shall
22、be placed to pro- duce the maximum vertical shear at a distance from the support equal to three times the bending member depth, 3d, or at the span quarter point, L/4, whichever is the lesser distance from the support. The distributed live load shear used in Equation (13-9) shall be determined by the
23、 following expression: where: VLL = distributed live load vertical shear in pounds; VLu = maximum vertical shear, in pounds, at 3d or L/4 due to undistributed wheel loads; VLD = maximum vertical shear, in pounds, at 3d or W4 due to wheel loads distributed laterally as specified for moment in Article
24、 3.23. For undistributed wheel loads, one line of wheels is as- sumed to be carried by one bending member. 13.6.5.3 Allowable Stress The allowable unit stress in shear parallel to grain shall be the tabulated stress adjusted by the applicable adjust- ment factors given in the following equation: whe
25、re: F: = allowable unit stress in shear parallel to grain in F, = tabulated unit stress in shear parallel to grain in CM = wet service factor from Article 13.5.5.1; CD = load duration factor from Article 13.5.5.2. psi; psi; 380 HIGHWAY BRIDGES 13.6.5.3 For sawn lumber beams, further adjustment by th
26、e shear stress factor may be applicable as described in the footnotes to Table 13.5.1A. For structural composite lumber, more restrictive ad- justments to the tabulated shear stress parallel to grain shall be as recommended by the material manufacturer. 13.6.6 Compression Perpendicular to Grain 13.6
27、.6.1 General When calculating the bearing stress in compression perpendicular to grain at beam ends, a uniform stress dis- tribution shall be assumed. 13.6.6.2 Allowable Stress The allowable unit stress in compression perpen- dicular to grain shall be the tabulated stress adjusted by the applicable
28、adjustment factors given in the following equation: Fc; = FciCvlCb (13-12) where: Fc; = allowable unit stress in compression perpendic- Fc, = tabulated unit stress in compression perpendic- C, = wet service factor from Article 13.5.5.1; Cb = bearing area factor from Article 13.6.6.3. ular to grain,
29、in psi; ular to grain, in psi; 13.6.6.3 Bearing Area Factor, Cb Tabulated values in compression perpendicular to grain apply to bearings of any length at beam ends, and to all bearings 6 inches or more in length at any other loca- tion. For bearings less than 6 inches in length and not nearer than 3
30、 inches to the end of a member, the tabulated value shall be adjusted by the bearing area factor, cb, given by the following equation: (13 - 13) where Fc, F; sin2 e + F; cos2 e FA = allowable unit stress for bearing on an inclined surface, in psi; allowable unit stress in bearing parallel to grain f
31、rom Article 13.7.4; allowable unit stress in compression perpendic- ular to the grain from Article 13.6.6; angle in degrees between the direction of load and the direction of grain. 13.7 COMPRESSION MEMBERS 13.7.1 General 13.7.1.1 The provisions of this article apply to simple solid columns consisti
32、ng of a single piece of sawn lumber, piling, structural composite lumber, or glued laminated timber. Refer to the 1991 Edition of the NDS for design requirements for built-up columns, consisting of a number of solid members joined to- gether with mechanical fasteners, and for spaced columns consisti
33、ng of two or more individual members with their longitudinal axes parallel, separated and fastened at the ends and at one or more interior points by blocking. 13.7.1.2 The term “column” refers to all types of compression members, including members forming part of a truss or other structural componen
34、ts. 13.7.1.3 Column bracing shall be provided where necessary to provide lateral stability and resist wind or other lateral forces. 13.7.2 DIVISION I-DESIGN 381 13.7.2 Eccentric Loading or Combined Stresses Members with eccentric loading or combined stresses shall be designed in accordance with the
35、provisions of the NDP, 1991 Edition. 13.7.3 Compression 13.7.3.1 Net Section The actual unit stress in compression parallel to grain, fc, shall be based on the net section as described in Article 13.1, except that it may be based on the gross section when the reduced section does not occur in the cr
36、itical part of the column length that is most subject to potential buckling. 13.7.3.2 Allowable Stress The allowable unit stress in compression parallel to grain shall not exceed the tabulated stress adjusted by the applicable adjustment factors given in the following equa- tion: Fi = FcCMC C, = loa
37、d duration factor from Article 13.5.5.2; CF = compression size factor for sawn lumber from C, = column stability factor from Article 13.7.3.3. grain in psi; grain in psi; footnotes to Table 13.5.1A; 13.7.3.3 Column Stability Factor, Cp 13.7.3.3.1 Tabulated values in compression parallel to grain are
38、 applicable to members which are adequately braced. When members are not adequately braced, the tabulated stress shall be modified by the column stability factor, C,. 13.7.3.3.2 When a compression member is supported throughout its length to prevent lateral displacement in all directions, Cp = 1.0.
39、For other conditions, the column sta- bility factor shall be determined in accordance with the . following provisions. 13.7.3.3.3 The effective column length, I, shall be determined in accordance with good engineering practice. Actual column length, I, may be multiplied by an effective length factor
40、 to determine the effective column length: 1, = KI (13-16) where: I, = effective column length in inches K = effective length factor from Table C-1 of Appen- I = actual column length between points of lateral dix C support in inches. 13.7.3.3.4 For columns of rectangular cross section, the column sl
41、enderness ratio, led, shall be taken as the larger of the ratios, Ze,dl or Ze2/d2. (See Figure 13.7.1A.) The slenderness ratio shall not exceed 50. I, Ud f disunm kiwecn poinu of hicd suppon in plmn 1 and 2. inck d, and d: mr.urrionrl dimensions of rrcirn;uirr compression mem- ber in plmm of Irtcnl
42、suppon. inches. FIGURE 13.7.1A 13.7.3.3.5 The column stability factor, Cp, shall be as given by the following expressions: (13 - 17) (13- 18) 13.7.3.4.2 382 HIGHWAY BRIDGES TABLE 13.7.1A Support Condition Coefficients for Tapered Columns support Condition SUUDO Condition Coefficient. a Large end fix
43、ed, small end unsupported Small end fixed, large end unsupported Both ends simply supported 0.70 0.30 Tapered toward one end 0.50 Tapered towards both ends 0.70 where: FC = tabulated stress in compression parallel to grain adjusted by all applicable modification factors given in Equation (13-14) exc
44、ept Cp; I(cE = 0.300 for visually graded sawn lumber; 0.418 for glued laminated timber, structural compos- ite lumber, and machine stress-rated lumber; = 0.80 for sawn lumber; 0.85 for round piles; 0.90 for glued laminated timber and structural composite lumber. c For especially severe service condi
45、tions or extraordi- nary hazardous conditions, the use of lower design values than those obtained above may be necessary. Refer to the 1991 Edition of the NDSB. 13.7.3.4 Tapered Columns 13.7.3.4. 1 For rectangular columns tapered at one or both ends, the cross-sectional area shall be based on the re
46、presentative dimension of each tapered face. The repre- sentative dimension, drep, of each tapered face shall be based on the support condition coefficient given in Table 13.7.1A. 13.7.3.4.2 For support conditions given in Table 13.7.1A, the representative dimension, drep, of each ta- pered face sha
47、ll be as given by the following equation: d, = d, +(dmax -dmi,) a - 0.15 1 - - ( :)I (13 - 19) where: d, = representative dimension for a tapered column d, = minimum column face dimension, in inches; d, = maximum column face dimension, in inches; a = coefficient based on support conditions. face, in
48、 inches; 13.7.3.4.3 For support conditions other than those in Table 13.7.1A, the representative dimension of each ta- pered face shall be as given by the following equation: d, = d, + 0.33(dm, - d“) (1 3-20) 13.7.3.4.4 For any tapered column, the actual stress in compression parallel to grain, f, s
49、hall not exceed the al- lowable stress determined by Equation (13-14), assuming the column stability factor Cp = 1.0. 13.7.3.5 Round Columns The design of a round column shall be based on the de- sign of a square column of the same cross-sectional area with the same degree of taper. 13.7.4 Bearing Parallel to Grain 13.7.4.1 The actual stress in bearing parallel to grain shall be based on the net area and shall not exceed the tab- ulated stress for bearing parallel to grain adjusted by the applicable adjustment factor given in the following equation: Fi = F,CD (1
