A concrete rectangular beam, $300$ mm wide x $600$ mm deep, is prestressed by post tensioned wires $($Ap$-640$ mm$^2),$ located at the constant eccentricity of $100$ mm and carrying an initial stress of $1000$ N$/$mm$^2.$ The span of the beam is $15$ m$.$ Calculate the percentage loss of stress in wires at the age of $3$ months, if the beam is loaded at $28$ days, M $40,$ Es $=210$ kN$/$mm$^2,$ Ec $=35$ kN$/$mm$^2,$ Initial prestress $=0\mathrm{.}76$ fp$,$ Slip at anchorage $=1\mathrm{.}5$ mm$,$ Coefficient

for wave effect $=0\mathrm{.}0018$ per m$,$ Coefficient of friction $=0\mathrm{.}55,$ RH $=50\%.$