Buckling of Columns

By Vishwajeet Sinha|Updated : December 30th, 2016

Columns and Struts:

  • A structural member subjected to an axial compressive fore is called strut. As per definition strut may be horizontal, inclined or even vertical.
  • Vertical strut is called a column.

Euler’s Column Theory: This theory has the following assumptions.

  • Perfectly straight column and axial load apply.
  • Uniform cross-section of the column throughout its length.
  • Perfectly elastic, homogeneous and isotropic material.
  • Length of column is large as compared to its cross-sectional dimensions.
  • The shortening of column due to direct compression is neglected.
  • The failure of column occurs due to buckling alone.

Limitation of Euler’s Formula

  • There is always crookedness in the column and the load may not be exactly axial.
  • This formula does not take into account the axial stress and the buckling load given by this formula may be much more than the actual buckling load.

Euler’s Buckling (or crippling load): The maximum load at which the column tends to have lateral displacement or tends to buckle is known as buckling or crippling load. Load columns can be analysed with the Euler’s column formulas can be given as  

image001

image002

where, E = Modulus of elasticity, l = Effective Length of column, and I = Moment of inertia of column section.

  1. For both end hinged:

 n=1

image003

image004

  1. For one end fixed and other free:

image005 image006

  1. For both end fixed:

n=2,

image007image008

  1. For one end fixed and other hinged:

image009 image010  

 

Effective Length for different End conditions:

s1

Modes of failure of Columns

s2

 

Slenderness Ratio ( λ) Slenderness ratio of a compression member is defined as the ratio of its effective length to least radius of gyration.

s3

s5

Buckling Stress:

s6

 

  • Rankine’s Formula for Columns: It is an empirical formula, takes into both crushing PCS and Euler critical load (PR).image011

PR = Crippling load by Rankine’s formula

Pcs = σcs A = Ultimate crushing load for column

image012 Crippling load obtained by Euler’s formula

image013

Where, A = Cross-section are of column, K = Least radius of gyration, and A = Rankine’s constant.

Shape of kern in eccentric loading:

  • To prevent any kid of stress reversal, force applied should be within an area near the cross section called as CORE or KERN.
  • Shape of kern for rectangular and l-section is Rhombus and for square section shape is square for circular section shape is circular.

s7

 

s8

 

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Vishwajeet SinhaVishwajeet SinhaMember since Jun 2016
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Anand Sharma

Anand SharmaDec 31, 2016

Jeevith stress reversal means stresses of opposite nature that may reduce the stability,like the uplift forces in gravity dams
Anand Sharma

Anand SharmaDec 31, 2016

Crushing means to fail in compression,associated with short column.
Mohit Mathur
both ends fixed 4π^2EI/L^2
Manisha Ghosh
Very effective...👌
Vi Sh Nu

Vi Sh NuJan 29, 2017

Nice app

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