Page 226 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.6

CRITERIA FOR BUILDING DESIGN

5.6 CHAPTER FIVE

TABLE 5.1 Limiting Width–Thickness Ratios for Compression Elements (Continued)
Limiting width–thickness ratios
Width–
thickness λ ρ λ r
Case Description of element ratio (compact) (noncompact) Example
Stiffened elements

.
/
.
/
13 Flexure in webs of h/t 242 EF y 570 EF y
rectangular HSS

/
.
14 Uniform compression in all b/t NA 149 EF y
other stiffened elements

15 Circular hollow sections:
In uniform compression D/t NA 1.11E/F y
In flexure D/t 0.07E/F y 0.31E/F y

[a] K c = 4/ h t w and 0.35 ≤ K c ≤ 0.76.
/
[b] F L = 0.7F y for minor-axis bending, major-axis bending of slender-web built-up I-shaped members, and major-axis bending of compact- and
noncompact-web built-up I-shaped members with S xt /S xc ≥ 0.7; F L = F y S xt /S xc ≥ 0.5F y for major-axis bending of compact- and noncompact-web built-
up I-shaped members with S xt /S xc < 0.7.
Source: “Specification for Structural Steel Buildings,” American Institute of Steel Construction, Chicago, Ill., 2005, with permission.

5.2 DESIGN FOR STABILITY

Stability must be provided, both for individual compression elements and for the structure as a
whole. Lateral stability of a structure is typically provided by moment frames, braced frames, shear
walls, or combined systems.
The AISC Specification requires that the influence of the following be considered: second-order
effects (such as P–∆ and P–δ effects); flexural, shear, and axial deformations; geometric imperfections;
and member stiffness reduction due to residual stresses. P–∆ indicates the effect of loads acting on the
displaced joints, and P–δ indicates the effect of loads acting on the deflected shape of a member
between joints. Many structural analysis computer programs can provide second-order elastic analyses.

5.2.1 Determination of Second-Order Effects
In structures designed on the basis of an inelastic analysis, special provisions set forth in App. 7 to
the AISC Specification must be met. In structures designed on the basis of elastic analysis, individ-
ual member stability and structure stability can be provided by the following.

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