Explain the strut-and-tie model approach for designing discontinuity regions.

Strut-and-tie models (STM) are used for D-regions (disturbed regions like deep beams, pile caps, corbels) where beam theory doesn't apply. The model represents load transfer through compression struts (concrete), tension ties (reinforcement), and nodes (where they meet). Design steps: (1) Define geometry and loads, (2) Develop truss model following load path, (3) Calculate forces, (4) Design ties with adequate reinforcement, (5) Check strut and node stresses against allowable (reduced for cracking, anchorage). ACI 318 and Eurocode provide detailed provisions.