Two blocks A and B are connected by two short $20mm$ diameter bars as shown, Fig. $18\mathrm{.}11.$ Calculate the maximum pull $F$ at a distance of $10mm$ from the axis of symmetry which may be exerted if the tensile stress at the section $x-x$ is limited to $150\frac{N}{m}{m}^2.$

$(65\mathrm{.}8kN)$

Fig. $18\mathrm{.}11$