Assalam o alaikum,
I was reading ETABS concrete frame design manual ACI 318-08. In beam design section, it gives two formulae which I am unable to find in books. One formula for stress block depth and one formula for negative steel calculation. I have highlighted the formulae in below images. The manual design using these formulae and conventional formulae gives the same result. But kindly guide me from where I can get derivation of these formulae or theory behind this.