A Beginner's Guide to the Steel Construction Manual, 13th ed. (old) Chapter 9 - Combined Bending & Axial Forces © 2006, 2007, 2008 T. Bartlett Quimby
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Section 9.1

Introduction to Combined Effects

Last Revised: 11/04/2014

From statics and structural analysis engineers learn how to find forces on structural elements that can, at any given section, result in three translational forces and three rotational internal forces.  These internal forces cause stress at points on a given section.  As elastic behavior is assumed, the total normal stress at a point is the sum of the normal stresses at that point caused by the applicable internal forces. To be safe, the maximum total normal stress cannot exceed the capacity of the material to handle the stress.

Multiplying the material stress capacity by the appropriate section properties yields the strengths associated with each stress.  The strengths must exceed the internal forces, as has been seen in the various strength based limit states considered to date.  When combining forces, the ability to compute strengths associated with each internal forces is required.  Consequently, considering combined effects will necessitate a good ability to compute axial (SCM Chapter E) and bending strengths (SCM Chapter F) in members.

The simultaneous combination of axial compression with bending creates what are known as "second order" effects.  The bending forces, alone, create deflections in the member.  Adding axial force to the deflected member creates additional bending moment which, in turn, creates more moment, which creates more deflection, which creates more moment, which creates more deflection, etc until either the member either becomes unstable or the deflection/moment cycle converges.

The result of second order effects is that the internal moments, in the presence of axial force, are actually larger that those predicted from structural analysis on the undeflected shape.

A technique for accounting for second order effects is provided in the SCM and is covered in this chapter.

Finally, the SCM provisions for dealing with the total effect of combined axial and bending loads is covered.