In T-Beam construction, effective flange width is determined an applied automatically.
International Design Codes
ACI 318-19 : Beam Stability
TSC 2018 :
bf = effective flange width, in.
bw = web width or diameter of circular section, in.
h = overall thickness, height, or depth of member, in.
ln = length of clear span measured face-to-face of supports, in.
sw = clear distance between adjacent webs, in.
In nonprestressed T-Beam monolithic concrete construction, effective flange width, equlas to the beam width plus an effective overhanging flange width in accordance with Codes.