For a multi$-$storey building shown in the following figure, structural analysis of a variety of specified loads results in axial force effects on column $\backslash ($ A B $\backslash )$ as follows:

$\backslash$begin$\{$tabular$\}\{$lcl$\}$

$\backslash$hline Load Effect & Intensity $\backslash ((\backslash$mathrm$\{$kN$\})\backslash )$ & Action $\backslash \backslash$

$\backslash$hline Dead Load & $150$ & Compression $\backslash \backslash$

Live Load & $85$ & Compression $\backslash \backslash$

Snow Load & $60$ & Compression $\backslash \backslash$

Wind Load & $120$ & Compression or Tension $\backslash \backslash$

Earthquake Load & $140$ & Compression or Tension $\backslash \backslash$

Differential Settlement & $20$ & Tension $\backslash \backslash$

$\backslash$hline

$\backslash$end$\{$tabular$\}$

a$)$ For the ultimate limit state $($ULS$),$ write all the possible load combinations for column $\backslash ($ A B $\backslash ),$ and report the critical case.

b$)$ For simplicity, assume that the resistance of the column is $\backslash (\backslash$mathrm$\{$AF$\}\_\{\backslash$mathrm$\{$y$\}\}\backslash ),$ in tension and $\backslash (0\mathrm{.}75\backslash$mathrm$\{$AF$\}\_\{\backslash$mathrm$\{$y$\}\}\backslash )$ in compression, where A is the cross$-$sectional area and $\backslash (\backslash$mathrm$\{$F$\}\_\{\backslash$mathrm$\{$y$\}\}\backslash )$ is the yield strength of the steel. Assuming further that the member is made of steel with $\backslash (\backslash$mathrm$\{$F$\}\_\{\backslash$mathrm$\{$y$\}\}=350\backslash$mathrm$\{$MPa$\}\backslash ),$ use the limit states approach to determine the cross$-$sectional area required.