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What is STM?

The strut-and-tie model (STM) is the design method for concrete structural members where ordinary beam theory no longer applies. Near concentrated loads, supports, openings, and abrupt geometry changes — the D-regions (Discontinuity or Disturbed Regions) — plane sections do not remain plane and the familiar flexure formulas are simply not valid.

STM idealizes the real, continuous stress field in a D-region as a truss that carries the same loads: compression flows through concrete struts, tension is carried by steel ties, and the zones where struts and ties meet are nodal zones. Once the truss is drawn, design reduces to three checks: strut strength, tie strength, and nodal-zone strength.

  • Adopted in ACI 318 (Chapter 23), AASHTO LRFD, and Eurocode 2
  • The standard tool for deep beams, corbels, pile caps, pier caps (copings), frame corners, anchorage zones, and other D-regions
  • Lower-bound (safe-side) plasticity method: a consistent truss in equilibrium that respects strength limits is a valid design
Concept sketch — dashed = strut (compression), solid = tie (tension), dots = nodes. Not to scale.

B-regions and D-regions

Every reinforced-concrete member splits into two kinds of zone. In B-regions (Bernoulli or Beam Regions) strain varies linearly over the depth — plane sections stay plane — so the familiar bending, shear, and axial formulas hold. In D-regions (Discontinuity or Disturbed Regions) that assumption fails, and those formulas quietly stop being valid.

Saint-Venant's principle locates the boundary: a statical disturbance — a concentrated load, a support reaction, a re-entrant corner, an opening — dies out roughly one member depth away. So a D-region reaches about one full depth on each side of every load, support, and geometric break. Deep beams, corbels, pile caps, and pier-cap copings are often D-region through and through, and STM is the method, adopted across design codes, for designing exactly these zones.

  • B-region — linear strain, beam theory valid (flexure, shear, torsion formulas)
  • D-region — nonlinear strain near loads, supports, openings, corners; the region reaching about one member depth from each stress disturbance
  • STM is the design method for D-regions; a member can be all D-region (a deep beam) or a mix of both