A Beginner's Guide to the Steel Construction Manual, 14th ed. Chapter 7 - Concentrically Loaded Compression Members © 2006, 2008, 2011 T. Bartlett Quimby
 Introduction Slenderness Limit State Limit State of Flexural Buckling for Compact and Non-compact Sections Limit State of Flexural Buckling for Slender Sections Limit State of Bolt Bearing on Holes Selecting Sections Chapter Summary Example Problems Homework Problems References Report Errors or Make Suggestions Purchase Hard Copy

Section 7.7

Compression Member Summary

Last Revised: 11/04/2014

Compressive strength is limited by:

• Material Strength
• General Buckling
• Local Buckling

Strength Limit States:

All strength limit states take the form:

 LRFD ASD Pu < fcPn Pa < Pn/Wc Req'd Pn = Pu/fc < Pn Req'd Pn = Pa Wc < Pn Pu / (fcPn)  < 1.00 Pa / (Pn/Wc) < 1.00 fc = 0.90 Wc = 1.67

Which is:  FORCE on a column < STRENGTH of a column

The STRENGTH of a Column is computed by:

The nominal capacity of the column, Pn = Fcr Ag

 Limit State Specification Critical Stress, Fcr Typical Design Variables Flexural Buckling E3 (Q=1), E7 If (KL/r) < 4.71 sqrt(E/QFy) or QFy/Fe < 2.25 Then: Fcr = [.658(QFy/Fe)] QFy Else: Fcr = .877 Fe Member selection (Ag and slenderness parameters), KL, support conditions
• Fe = p2 E / (KL/r)2 = Euler Critical Buckling Stress

• Q =1 for compact and non-compact sections

• Q = Qs Qa for slender sections

• Qs:  Slenderness factor for unstiffened elements (SCM E7.1).  A function of the slenderness (b/t) of the unstiffened element.
• Qa:  Slenderness factor for stiffened elements (SCM E7.2).  Different equations for flanges of square and rectangular sections, circular sections, and other uniformly compressed elements.
• KL = Effective Length.  The choice of K will depend on how the demand side of the inequality was computed.

Slenderness Limit State (SCM E2)

This serviceability limit state is not binding, but a good suggestion:   (KL/r) < 200

Computing Effective Length (SCM Commentary 7.2)

Principles of Mechanics are used to determine the effective length of columns.  The effective length is a function of the overall member length, length between lateral braces, end support condition, and the arrangement of these factors relative to each principle axis.