Page 237 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.17

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.17

distance. It is considered good practice to keep the inclination of lacing bars to the axis of the mem-
ber to not less than 60° for single lacing or 45° for double lacing. Also, use double lacing or lacing
made up of angles where distance between the lines of bolts or welds exceeds 15 in (380 mm).
For limitations on the longitudinal spacing of connectors between elements in continuous contact
consisting of a plate and a shape or two plates, and other limitations, see Art. 5.9.7.

5.5 DESIGN OF FLEXURAL MEMBERS

The design flexural strength φM n and the allowable flexural strength M n /Ω are determined using
φ b = 0.90 (LRFD) and Ω b = 1.67 (ASD) for all cases. The nominal flexural strength, M n , is calcu-
lated from the applicable equations, which depend on the member cross section and the axis of bend-
ing. For cross sections and bending cases not discussed in this article, refer to the provisions given
by the AISC Specification.
These design criteria apply to members subject to simple bending, loaded in a plane parallel to a
principal axis that passes through the shear center, or restrained against twisting at load points. Also, at
all points of support, members must be restrained against twisting (rotation about the longitudinal axis).
A common term used in the strength provisions is C b , the lateral-torsional buckling modification
factor for nonuniform moment diagrams, applicable when both ends of the unsupported segment are
braced:
C = 12 5 . M max R ≤ 30 . (5.39)
b
25 . M max + 3 M + 4 M + 3 M C m
A
B
where M max = absolute value of maximum moment in the unbraced segment, kip⋅in (N⋅mm)
M A = absolute value of moment at quarter point of the unbraced segment, kip⋅in (N⋅mm)
M B = absolute value of moment at centerline of the unbraced segment, kip⋅in (N⋅mm)
M C = absolute value of moment at three-quarter point of the unbraced segment, kip⋅in (N⋅mm)
R m = cross-section parameter
= 1.0, doubly symmetric members
= 1.0, singly symmetric members subjected to single-curvature bending
2
= 0.5 + 2(I yc /I y ) , singly symmetric members in reverse-curvature bending
4
4
I y = moment of inertia of section about y axis, in (mm )
I yc = moment of inertia about y axis referred to compression flange (use smaller flange if
4
4
reverse curvature bending), in (mm )
In singly symmetric members subjected to reverse-curvature bending, the lateral-torsional buckling
strength must be checked for both flanges. The available flexural strength must be greater than or equal
to the maximum moment causing compression within the flange under consideration.
Note that important simplifications apply. C b can conservatively be taken as 1.0 for all cases.
Also, for all cantilevers or overhangs where the free end is unbraced, C b = 1.0. For doubly symmetric
members with no transverse loading between brace points, C b reduces to 2.27 for the case of equal
end moments of opposite sign, and to 1.67 when one end moment equals zero.
5.5.1 Doubly Symmetric Compact I-Shaped Members and Channels—
Major Axis Bending
This article applies to doubly symmetric I-shaped members and channels, subjected to bending
about their major axis, and having compact webs and compact flanges (see Art. 5.1.5). All current
W, S, M, C, and MC shapes listed in ASTM A6, except W21 × 48, W14 × 99, W14 × 90, W12 × 65,
W10 × 12, W8 × 31, W8 × 10, W6 × 15, W6 × 9, W6 × 8.5, and M4 × 6 have compact flanges for
steels with F y ≤ 50 ksi (345 MPa); all current W, S, M, HP, C, and MC shapes listed in ASTM A6
have compact webs for F y ≥ 65 ksi (450 MPa).

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