Prob. $2\mathrm{.}4-2.$ A "pencil" laser extensometer measures elongation $($from which extensional strain can be computed$)$ by multiplying the elongation. In Fig. P$2\mathrm{.}4-2$ the laser extensometer is being used to measure strain in a reinforced concrete column. The target is set up across the room from the test specimen so that the distance from the fulcrum, $C,$ of the laser to the reference point $O$ on the target is $d_{oc}=5m.$ Also, the target is set so that the laser beam points directly at point $O$ on the target when the extensometer points are exactly $L_0=150mm$ apart on the specimen, and the cross section at $B$ does not move vertically. At a particular value of $($compressive$)$ load $P,$ the laser points upward by an angle that is indicated on the target to be $=0\mathrm{.}0030$rad. Determine the extensional strain in the concrete column at this load value.