Understanding Critical Load Calculations for Structural Analysis

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Homework Statement

in the 3rd photo , i m having problem finding the critical load

Homework Equations

The Attempt at a Solution

to find the critical load , we should use the maximum stress multiply by the area which is 5500(250MPa) , am i right ? why the author used 59.4MPa(5500) ? is it wrong ?

 

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anyone can help ?