A simply supported timber beam of rectangular cross$-$ section $(200$mm $*400$mm$)$ of $8$ m span is subjected to a concentrated load P kN at the mid$-$span. Determine the safe load$-$carrying capacity of the beam if the allowable stress in bending is $8$ N $/$ m $*$ m $^2$ What is the benefit of an I$-$ section made up of $20$ mm thickness Flanges and Web keeping the width and depth of the beam the same?