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The problem involves an aluminum column with a rectangular cross-section subjected to a centric load \( P \), fixed at one end (B) and supported at the other end (A) by smooth, rounded plates. The plates allow movement in one plane while restraining movement in the other. The goal is to determine the optimal ratio \( \frac{a}{b} \), where \( a \) and \( b \) are the dimensions of the rectangular cross-section, to minimize buckling under the applied load.

### Key Concepts:

- **Buckling of columns:** Buckling refers to the sudden lateral deflection of a structural member subjected to a compressive force. For long, slender columns, buckling is a critical mode of failure.
  
- **Euler’s buckling formula:** The critical buckling load for a column with a rectangular cross-section can be analyzed using Euler's formula:
  \[
  P_{cr} = \frac{\pi^2 E I}{L^2}
  \]
  where:
  - \( P_{cr} \) = critical load for buckling,
  - \( E \) = modulus of elasticity,
  - \( I \) = moment of inertia about the buckling axis,
  - \( L \) = length of the column.

  In this problem, we are given that the column is restrained in one plane and free to move in the other. Therefore, buckling can occur in the direction where movement is permitted.

### Moment of inertia:
The moment of inertia for a rectangular cross-section is:
\[
I = \frac{1}{12} \times b \times a^3 \quad (\text{for buckling in the direction of } a)
\]
\[
I = \frac{1}{12} \times a \times b^3 \quad (\text{for buckling in the direction of } b)
\]

The column will buckle around the axis that has the lower moment of inertia, as this will result in a lower critical load.

### Determining the optimal ratio \( \frac{a}{b} \):
For the most efficient design, the critical buckling loads about both axes (in both planes) should be equal, meaning that the moments of inertia in both directions should be equal. Setting the two moments of inertia equal to each other:
\[
\frac{1}{12} b a^3 = \frac{1}{12} a b^3
\]
Simplifying:
\[
b a^3 = a b^3
\]
Divide both sides by \( ab \) (assuming \( a \neq 0 \) and \( b \neq 0 \)):
\[
a^2 = b^2
\]
Thus:
\[
a = b
\]
Therefore, the optimal ratio \( \frac{a}{b} \) is \( 1 \), meaning the cross-section should be square to provide the most efficient design against buckling.

### Final Answer:
The ratio of the sides \( \frac{a}{b} \) of the rectangular cross-section corresponding to the most efficient design against buckling is \( 1 \).

---

Do you need further clarification or details about this calculation? Here are five related questions for deeper exploration:

1. How would the analysis change if the column were not aluminum but another material?
2. What are the assumptions in Euler's buckling theory, and when do they not hold?
3. How would the optimal ratio \( \frac{a}{b} \) change if the column had different boundary conditions?
4. What effect would changing the length \( L \) of the column have on the buckling behavior?
5. Can you explain how to derive Euler’s buckling formula for different types of column constraints?

**Tip:** To avoid buckling in columns, it is essential to balance both the material properties and the geometry. Matching moments of inertia for the two planes of symmetry ensures more uniform buckling resistance.

An aluminium column with a length of L and a rectangular cross section has a fixed end B and supports a centric load at A. Two smooth and rounded fixed plates restrain end A from moving in one of the vertical planes of symmetry of the column but allow it to move in the other plane. Determine the ratio a/b of the two sides of the cross section corresponding to the most efficient design against buckling.

Analyze how to determine the optimal ratio a/b of a rectangular cross section in a column subjected to buckling. This engineering problem involves Euler’s buckling theory, moment of inertia calculations, and structural optimization techniques. Perfect for students in undergraduate structural engineering courses.

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