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A rigid square frame ABCD has four edge bars (AB, BC, CD, DA) and two diagonal bars (AC, BD) that merely touch freely at the centre. All joints are hinged. By an external arrangement, the bar AB is held in tension T at its ends, and the configuration is symmetric. Which statement is correct?
- AD and BC are also under tension.
- AD and BC are under compression.
- AC and BD are under tension.
- CD is under compression.
Correct answer: CD is under compression.
Solution
With AB pulled in tension, equilibrium at the joints requires the diagonals to push outward and the far edge CD to be compressed, so CD is under compression.
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