In using this code, the designer can choose cotθ between 1.0 and 2.5, which corresponds to 21.8° ≤ θ ≤ 45°. More rigorously, the modified compression field theory (MCFT) developed by Vecchio and Collins (1986) solves the inclination of diagonal cracks using a strain compatibility procedure involving average stresses and average strains. Simplified shear design equations derived from the complete MCFT have been incorporated into the AASHTO load and resistance factor design (LRFD) (AASHTO 2014) bridge design specifications since the first edition.