$2$ rectangular$-$section flat wooden planks are nailed together to form a $T-$section bar as shown in the figure. If the nail spacing is $"$b$"$ and shear force that a nail can carry is $"V",$ calculate the shear force that the section can safely carry.

nail spacing $b=(40+a)mm$

shear force, $V=(10+a)kN$

$a=(250+a)mm$

$d=(300+)mm$

$t_w=(10+)mm$

$t_f=1\mathrm{.}5t_w$