Shear Stress on Plate - Mystery Equation

I am reviewing an already approved (not by me) hand calculated stress analysis of applied loads on an aluminum plate which has structural ribs on its underside. The plate was cut in half, exposing the structural ribs and the Max Moment, Max Torque and Max bending stress was found. The engineer then "calculates" the Shear Stress using the Max Torque found however the equation used looks familiar, but I am unable to find its origin anywhere in Roark's Stresses and Strains book.

It's the equation for shear stress they used: Fs= (3*T*t)/(summation(b+t^3) <--This looks so familiar, but I cannot find any info about this equation? Thank you