Design Contents |

Preliminary Matters

Design Methodology

..brain storming

..evaluation matrix

..QFD

Statistical Considerations

..variability in materials

..variability in dimensions

..variability in loading

..preferred sizes

Tolerances

Design Factor

Introduction to Failure

Failure Theories

Application of von Mises

..criterion in 2 D

Stress Concentration

..and notch sensitivity

Failure Under Combined Loading

..combined bending and torsion

Failure Under Cyclic Loading

..fatigue

..fracture mechanics

Instability - Buckling

Concentrically Loaded Strut

..slender columns

..Euler formula

..effective length

..short and intermediate columns

Eccentrically Loaded Strut

.. theory

Shock Loading

..deflection

..stress

BUCKLING OF STRUTS SUBJECT TO A CONCENTRIC LOAD |

**1. Introduction**

A type of failure that is sometimes overlooked for a body subject to compressive loading, is that due to instability,
called buckling.
The longer and more slender the column is, the lower the safe compressive stress that it can stand.
The slenderness of a column is measured by the slenderness ratio, L/k, where L
is the length of the column and (lower case) k = (I/A)^{0.5} the radius of
gyration of the cross sectional area about the centroidal axis. The minimum radius
of gyration is the one to be considered. This corresponds to the minimum value of I,
the second moment of area of the section. A is the cross section area.

A link to a 'buckling load calculator' based on the thoeory below, is given at the bottom of this page.

**2. Eulers Formula**

Euler analysis applies to slender columns, the formula, for the critical axial
concentric load that causes the column to be on the point of collapse for frictionless
pinned ends (no bending moment at the ends) is given below.

** **

Click here for a derivation of the above equation.

For a particular column cross section and length, the load capacity
F_{c} depends only upon the modulus of elasticity E. Since there is
little variation in E among diferent grades of steel, there is no advantage in using an
expensive, high strength alloy steel instead of structural steel for columns with L/k
greatcr than about l20.

**3. Effective Length**

Eulers equation as written can be applied to a column with ends fixed in any
manner if the length is taken as that between sections of zero bending moment.
This length is called the effective length, L_{e} and is equal to KL
(upper case) K, where L is the actual length and K is a constant
dependent upon the end fixings. Theoretical values for different types of column ends are
shown below. It should be noted that design codes issued by some organisations often
recommend values that are somewhat different to these theoretical values, see right hand
column.

End Fixings | Theoretical K value | Practical K value |

pinned frictionless ends: | K=1 | K=1 |

fixed ends: | K=0.5 | K=0.65 |

fixed - pinned and guided: | K=0.7 | K=0.8 |

fixed - free: | K=2 | K=2.1 |

**4. A typical factor of safety, or design factor, for Euler structural columns** is
between 2 and 3.5, **but** this is based on the **critical load**, not on the yield
or ultimate strength of the material.

If the long column remains straight and the load concentric, the average stress in
the column at the point of collapse is s_{c} = F_{c}/A and it is local
buckling at some point where the stress is below the yield stress of the material that
leads to failure.

**5. Short and Intermediate Columns**

If L_{e}/k is below a certain value for a particular material, the Euler
formula gives a critical load which causes a stress greater than the yield stress of the
material. Collapse in these cases is probably due to a combination of buckling and plastic
action. For very short columns the yield stress (with appropriate design factor) can be used. For columns that are not short, but where the Euler formula gives stress above the yield stress, empirical methods of design are used.
One popular equation in use since the early 1900s is the Johnson formula which can be used
for columns with slenderness ratios below a transition slenderness ratio or column
constant, C_{c}. Click here for
further explanation.

The value of *L _{e}/k* that indicates the transition slenderness ratio is
given by: and when L

As a very rough guide, for steel, the Euler buckling formula is only applicable for
columns with L_{e}/k exceeding about 100, depending upon the yield stress.

Link to Buckling Load Calculator and Interactive Graph

Link to Buckling Load Calculator

It should be noted that sheets and plates may suffer buckling. Where a fabricated 'I' section beam has an insufficiently thick web, this can also suffer from buckling. Standard sections are sized so this is unlikely to be a problem.

David J Grieve Revised: 2nd October 2013, 21st February 2010, 1st March 2004. Original: 27th August 1999.

**Contact the Author:**

Please contact me for comments and / or corrections or to purchase the book, at: davejgrieve@aol.com