In which form is the permissible stress in columns, f by Merchant - Rankine's formula?
1
\(f\propto \frac{{{\pi ^2}E}}{{{\lambda ^2}}}\)
2
\(f\propto \frac{{fy}}{{1 + 0.2\sec (c.\lambda .\sqrt {f)} }}\)
3
\(\frac{1}{{{{(f)}^n}}}\propto \frac{1}{{{{({f_y})}^n}}} + \frac{1}{{{{({f_{cr}})}^n}}}\)
4
\(f\propto \frac{{{f_y}}}{{\phi + \sqrt {{\phi ^2} - {\lambda ^2}} }}\)