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The effective lengths of the columns with ideal boundary conditions shown in Case-I, Case-II, and Case-III are respectively

Three columns of length L under axial load P: Case-I hinged at both ends, Case-II fixed at the base and free at the top, Case-III fixed at both ends, each drawn in its buckled shape

  • A.\(L\), \(4L\), \(2L\)
  • B.\(L\), \(2L\), \(L/4\)
  • C.\(L\), \(L\), \(L\)
  • D.\(L\), \(2L\), \(L/2\)

Answer: D

The effective lengths are \(L\), \(2L\), \(L/2\).

The effective length is the length of a pin-ended column that buckles at the same load, measured between points of zero moment (inflection points) in the buckled shape:

  • Hinged-hinged: the whole column is one half sine wave, so \(L_{e} = L\).
  • Fixed-free: the column is half of a pin-ended half wave, so \(L_{e} = 2L\).
  • Fixed-fixed: the inflection points lie at \(L/4\) from each end, leaving \(L/2\) between them, so \(L_{e} = L/2\).

The options with \(4L\) or \(L/4\) confuse the effective length with the factor in the buckling load (\(P_{cr}\) changes by 1/4 and by 4 for these cases), and \(L, L, L\) ignores the end conditions.

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