D4.E.6.2Members Subject to Bending
The laterally unsupported length of the compression
flange for the purpose of computing the factored moment resistance is specified
in
STAAD.Pro through the
UNT
and
UNB
parameters (Refer to D4.E.7Design Parameters). The
LAT
parameter is used to specify if the member is
laterally supported against lateral torsional buckling.
- The factored moment
resistance, Mr, developed by a member subjected to uniaxial bending moments
about a principal axis where effectively continuous lateral support is provided
to the compression flange or where the member has no tendency to buckle
laterally, is calculated as:
-
For Class 1 and Class 2 sections (Cl. 13.5(a) ):
Mr = φ·Z· Fy = φ·Mp -
For Class 3 sections (Cl. 13.5(b) ):
Mr = φ·S· Fy = φ·MyNote: For S16-19, the exception for single angle profiles is included per Cl. 13.5 D) I. -
For Class 4 sections (Cl. 13.5(c) ):
Mr = φ·Se· FyNote: For S16-19, the exceptions for circular hollow profiles per Cl. 13.5 C) IV and for single angle profiles per Cl. 13.5 d) ii) is included.
-
- For laterally unsupported
members, flexural resistance is calculated as follows:
- For doubly symmetric Class 1 and Class
2 sections (Cl 13.6(a) ):
Note: The value for ω2 can be specified using the
CB
parameter. Otherwise, it is calculated as indicated here.Note: For S16-19, Cl. 13.6 H) I pertaining to cantilevers with different bracing conditions is included. - For doubly symmetric Class 3 and Class
4 sections –except closed square and circular sections– and for channels: but not greater than φMy for Class 3 sections and the value specified in Cl.13.5(c)(iii) for Class 4 sections.Note: For S16-19, Cl. 13.6 H) II pertaining to cantilevers with different bracing conditions is included in accordance with Cl. 13.6(b), except for Mu and ω2, which depend on Cl. 13.6 H) I.
- For singly symmetric (monosymmetric)
Class 1, Class 2, or Class 3 sections and T-shape sections, lateral torsional
buckling strength shall be checked separately for each flange under compression
under factored loads at any point along its unbraced length:
- when Mu > Myr :
-
when M ≤ Myr :
when singly symmetric beams are in single curvature,
- ω3 = ω2 for beams with two flanges, = 1.0 for T-sections
in all other cases,
- ω3 = ω2[0.5 + 2(Iyc/Iy)2] f, but ≤ 2.0 for T-Sections
Note: For single angles (asymmetric sections) designed per S16-19, Cl. 13.6 G (I) and (II) are used. - For doubly symmetric Class 1 and Class
2 sections (Cl 13.6(a) ):