A Beginner's Guide to the Steel Construction Manual, 13^th ed. (old)

Chapter 8 - Bending Members

© 2006, 2007, 2008 T. Bartlett Quimby

Section 8.2

Flexure

Last Revised: 06/16/2011

Bending members are subjected to internal moments which create both tensile and compression stress in the cross section.  The compression effects are of significant concern as buckling (both local and overall) can limit the strength of a beam section. The tension effects do not cause buckling, however, they may cause yielding in the beams.  Chapter F of the SCM specification covers the various limit states associated with flexural strength.

The SCM separately defines the different flexural limit states as:

• Y:  Yielding.  The yielding limit state is the ultimate limit state where the whole section is considered to be yielded.
• LTB: Lateral Torsional Buckling.  This is overall column buckling of the compression flange of the section.  This may occur when a compression flange of beam lacks lateral support.
• FLB:  Flange Local Buckling.  This is plate buckling of the compression flange (either stiffened or unstiffened).
• WLB:  Web Local Buckling.  This is plate buckling of the web.
• TFY:  Tension Flange Yielding.  This limit state is particularly applicable to sections where the centrodial axis is not at mid-height of the section.  This may occur with built-up sections where the tension flange is smaller than the compression flange.
• LLB:  Leg Local Buckling.  This limit state is specifically applicable to single angles in flexure.
• LB: Local Buckling.  This limit state applies only to round "pipe" sections.

SCM Table F1.1 (SCM pg 16.1-45) is useful in determining the limit state that applies to each section and situation.  The table also directs you to the appropriate part of SCM Chapter F for each case.  Take some time to review this table.  Note that the applicable limit states are dependent on the compact criteria for the flanges and webs.  This is because most of the limit states are concerned with buckling behavior of the compression elements in the cross section.

While Chapter F is very thorough in covering most possible conditions, the most common condition is covered by section F2:  Doubly symmetric compact I-shaped members and channels bent about their major axis.  The discussion in this text will focus primarily on this condition plus minor axis bending.  The limit states that are covered are Y, LTB, and FLB.

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