A Beginner's Guide to the Steel Construction Manual, 15^th ed.

Chapter 7 - Concentrically Loaded Compression Members

© 2006, 2008, 2011, 2017 T. Bartlett Quimby

Section 7.1

Introduction

Last Revised: 04/19/2021

Compression members are common in engineered structures. They are used as columns to support loads from beams, floors, roofs, and other areas. They are also found as chord and web members in trusses. As introduced in the prior chapter, compression members are subject to buckling instability as well as material failure.

Compression member failures due to buckling are generally sudden and dramatic. The lack of warning of impending failure is a safety concern. Consequently, extensive research has been conducted to determine safe limits on column strength where buckling is a possibility.

Chapter 6 introduced the basics of buckling behavior and then focused on both general member buckling and buckling of sub-elements of a cross-section. This chapter looks at the overall strength of a compression member as governed by the limit states of material failure, overall member stability, and local buckling.

A quick review of Euler's equation,

s_cr= p²E/(L_e/r)²

shows that member strength is heavily influenced by the slenderness (L_e/r) of the member.

Test results show that Euler's predicted failure load is rarely attained in real members. The reasons for this include:

1. compression members are often not perfectly straight,
2. residual stresses in the member cause uneven stress distribution,
3. end restraints are rarely perfectly pinned or perfectly fixed, and
4. the load is not perfectly concentric.

To account for this behavior, compressive strength curves have been developed that skirt the lower bound of compression member strength test data.

The remainder of this chapter looks at the limit states that are related to concentrically loaded compressive members.

Choosing the Appropriate Specification

Member compressive strength is highly dependent on cross-sectional geometry so it is appropriate that there be separate sections of the specification used to determine the compressive strength of members. SCM Chapter E has quite a few different parts to it to address the various sectional needs. SCM Table User Note E1.1 (SCM page 16.1-34) is a helpful index to the sections. You will want to refer to it often as you learn how to determine the capacity of members in compression.

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