Column ABC has a uniform rectangular cross section with $b=14mm$ and $d=24mm.$ The column is braced in the $xz$ plane at its midpoint $C$ and carries a centric load $P$ of magnitude $3\mathrm{.}9$ kN $.$ Knowing that a factor of safety of $3\mathrm{.}4$ is required, determine the largest allowable length $L.$ Assume the bracing prevents motion only along the xz plane and use $E=200$GPa.