I imagine that they developed an equation for strength of flare bevel groove weld based on the effective area(effective throat times length...figure 2.1) and allowable stress(see Table 2.1)....then equated this to allowable stength of rebar in force units....divided to get length of weld, and after reviewing experimental data, followed by discussions with a lot of give and take, they came up with an equation that was dimensionally correct.
Just winging it here. Wish someone who knew how they arrived at the equation will share some thoughts on this matter.