These provisions are for members in which the resultant of all loads is an axial load passing through the effective section calculated at the stress Fn as subsequently defined. Concentrically loaded angle sections should be designed for an additional moment according to the AISI Specification. The slenderness ratio, KL/ r, of all compression members should preferably be limited to 200 (300 during construction).
The nominal axial strength, Pn, is
Fe is the least of the elastic flexural, torsional and torsional-flexural buckling stresses (Arts. 10.14.110.14.3). Equation 10.59a is based on elastic buckling while Eq. 10.59b represents inelastic buckling, providing a transition to the yield point stress as the column length decreases.
Elastic Flexural Buckling
For doubly-symmetric, closed, or any other sections that are not subject to torsional or torsional-flexural buckling, the elastic buckling stress is
where E = modulus of elasticity (29,500 ksi or 203 000 MPa)
K = effective length factor (see Fig. 6.3)
L = unbraced length of member
r = radius of gyration of full, unreduced cross section
Symmetric Sections Subject to Torsional or Torsional-Flexural Buckling
Singly-Symmetric Sections. For singly-symmetric sections, such as C-sections, subject to torsional-flexural buckling, Fe is the smaller of Eq. 10.61 and that given by Eq. 10.62 or 10.63.
Doubly-Symmetric Sections. For doubly-symmetric sections, such as back-to-back Csections, subject to torsional buckling, Fe is taken as the smaller of Eq. 10.61 and the torsional buckling stress, t, given by Eq. 10.33.
For shapes with cross sections that do not have any symmetry, either about an axis or about a point, Fe should be determined by rational analysis or by tests.
Beams (C- or Z-Section) Having One Flange Through-Fastened to Deck or Sheathing
For such sections axially loaded in compression, refer to the AISI Specifications which give a special set of provisions. Based on the results of structural tests, they account for the partial restraint to weak axis buckling provided by the deck or sheathing. Strong axis buckling should be considered using the same equations as for members not attached to sheathing.