Given:

• $A = 6,000 \, \text{mm}^2$
• $r_x = 150 \, \text{mm}$
• $r_y = 100 \, \text{mm}$
• $E = 200,000 \, \text{MPa}$
• $L = 4 \, \text{m} = 4000 \, \text{mm}$

Step-by-step solution:

1. Calculate Effective Length (hinged at both ends):
   $L_{\text{eff}} = L = 4000 \, \text{mm}$

2. Calculate Slenderness Ratio (we use the smaller $r_y$ for conservative calculation):
   $\lambda = \frac{L_{\text{eff}}}{r_y} = \frac{4000}{100} = 40$

3. Euler’s Critical Stress Formula: $\sigma_{\text{cr}} = \frac{\pi^2 E}{\lambda^2}$

4. Substitute values: $\sigma_{\text{cr}} = \frac{\pi^2 \times 200,000}{40^2}$

5. Calculate $\sigma_{\text{cr}}$: $\sigma_{\text{cr}} = \frac{9.8696 \times 200,000}{1600}$ $\sigma_{\text{cr}} = \frac{1,973,920}{1600}$ $\sigma_{\text{cr}} = 1233.7 \, \text{MPa}$

Answer: $\sigma_{\text{cr}} \approx 1233.7 \, \text{MPa}$