A beam is deep when its span-to-depth ratio is small enough that shear deformation and the direct compression path dominate — in ACI 318 terms, a clear span not more than four times the member depth, or a load applied within twice the depth of a support. Below that limit the beam does not bend the way beam theory assumes: plane sections do not stay plane, the strain profile is non-linear, and sectional flexure-and-shear design has no theoretical basis. The whole member is a D-region.
The physical picture is simple. Load applied near the top travels to the supports along a direct inclined compression path — a strut — rather than through a chain of flexural sections. That strut pushes outward at the bottom, and something has to hold the two ends together: the bottom reinforcement, acting as a tie in nearly uniform tension over its whole length. That is why deep-beam bottom steel is not curtailed toward the supports the way ordinary beam steel is, and why its anchorage at the support is decisive rather than incidental.
Transfer girders, wall-like beams over openings, coupling beams, and pile caps are all deep members. So are ordinary beams close to a heavy concentrated load, even when the rest of the span behaves normally — the D-region reaches about one member depth on each side of the disturbance.
- When it applies: clear span ≤ 4 × depth, or a concentrated load within 2 × depth of the support (ACI 318 Chapter 9 / Chapter 23)
- The load path: an inclined strut from load to support, closed by a bottom tie in near-uniform tension
- Modelling: bearing plates set the node sizes; the tie sits at the centroid of the bottom bars; struts follow the compression flow
- What usually governs: the tie steel and its anchorage behind the support node, then the node under the bearing plate
- Detailing: distributed horizontal and vertical web reinforcement controls crack widths and gives the assumed ductility
- In AStrutTie: the Deep beam (simple) and Continuous deep beam templates build the truss automatically, then check strut, tie, and node against the chosen code