Determine the size mоdificаtiоn fаctоr, ψs, for а rectangular beam with b = 16 in. and d = 21 in., three galvanized No. 8 Grade 60 tension-reinforcement bars placed in the bottom of the beam, and No. 4 Grade 60 stirrups located every 6 in. along the span. Assume 8,000-psi normal-weight concrete and a clear cover of 1.75 in.