Look to the E.2.2 NOTATIONS
wt =Tank and roof weight acting at base of shell, N/m (lbf/ft)
Ws =Total weight of tank shell and appurtenances, N (lbf)
wrs= Roof load acting on the shell, including 10% of the specified snow load N/m (lbf/ft)
Wrs =Roof load acting on the tank shell including 10% of the roof design snow load, N (lbf)
In your case Wrs = 11444 lbf , and wrs = Wrs /πD apparently, the roof is self-supporting roof. In case of column supported roof , Roof load acting on the tank shell will be calculated with tributary area.
For example if center supported roof, the load acting on the shell will be, Wrs =11444 *(31.16**2 -15.58**2)/(31.16**2)= 0.75* 11444
and wrs =(0.75* 11444)/(pi*31.16)