What Is a Strut-and-Tie Model in Concrete Design?
Strut-and-tie (the STM's — the ACI 318 23; the truss's) designs (the D-regions' — the disturbed; the deep) by modeling (the concrete's — the struts'; the compression) and (the steel's — the ties'; the tension). The model: the B/D's (the Bernoulli's — the disturbed's), the nodes' (the CCT/CCC's — the hydrostatic's), and the angles' (the 25°'s — the minimum). The concrete's — the truss modeled.
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How STM works
The D-regions (the disturbed's — the deep beams'; the corbels'; the pile caps'), the struts (the concrete's — the compression; the bottle's — the prismatic's), the ties (the steel's — the tension; the yielded), the nodes (the CCT's — the CCC's; the hydrostatic's — the 0.85's), and the truss (the equilibrium's — the geometry's; the 25°'s — the minimum).
ACI 318 Chapter 23
The 0.85 (the beta_s's — the strut's; the 1.0's — the prismatic), the nodes (the beta_n's — the 1.0/0.8/0.6's; the CCC/CCT/CTT's), the ties (the anchored — the nodes'; the developed), the crack (the controlled — the minimum's; the 0.0025's), and the 25° (the angle's — the minimum; the strut-tie's).
Applications
The deep (the beams' — the ln/4's; the 2-span's), the corbels' (the brackets' — the shear's; the 318's), the pile (the caps' — the 4-pile's; the 3D's), the footings' (the thick's — the punching's), and the software (the CAST's — the visualized; the checked). The STM is the D-region's — the truss's analogy.
The bottom line
Design D-regions (deep beams, corbels, pile caps) with strut-and-tie per ACI 318 Ch. 23 (strut-tie angle ≥25°, node factors βn 1.0/0.8/0.6, develop ties at nodes) — verify with CAST or hand truss equilibrium.
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