Both methods design the same thing: a D-region, using the same lower-bound plasticity idea that a stress field in equilibrium and within strength limits is safe. The difference is how the stress field is obtained. In a strut-and-tie model the engineer chooses a discrete truss — a finite set of struts, ties, and nodes — and the design is then a set of explicit strength checks on those members. In a compatible stress field method the field is computed continuously by a non-linear finite-element analysis with assumed material laws, and the answer comes back as a field rather than as members.
That difference has practical consequences. A truss is auditable: every strut, tie, and node maps to a code clause, a checker can follow it by hand, and the reinforcement layout is a direct output. A computed field captures compatibility and gives deformation and crack-width information a truss cannot, but its result depends on the constitutive assumptions and the mesh, and reviewing it means reviewing an analysis rather than a set of checks.
Neither replaces the other. Design codes — ACI 318 Chapter 23, AASHTO LRFD 5.8.2, Eurocode 2 §6.5 — are written around the strut-and-tie form, which is why STM remains the method that produces a directly checkable submission. Continuous stress-field analysis is a strong complement when serviceability, crack width, or an unusually complex geometry is the question.
- Same basis: lower-bound plasticity — an equilibrium field within strength limits is safe
- STM: engineer-chosen truss · explicit strut/tie/node checks against clauses · auditable by hand
- CSFM: computed continuous field · adds compatibility, deformation, crack width · depends on material law and mesh
- Codes are written in strut-and-tie form (ACI 318 Ch. 23 · AASHTO LRFD 5.8.2 · Eurocode 2 §6.5)
- Practical split: STM for the design submission, stress-field analysis when serviceability or geometry demands it