ACI 318-19 Section 23 | Strut-and-Tie Method for Deep Beams
Deep beams have clear span-to-overall-depth ratios (ln/h) <= 4.0. In these deep members, Bernoulli's hypothesis (plane sections remain plane) fails completely. Internal stresses must be modeled as an idealized pin-jointed truss comprising concrete compression struts and rebar tension ties.
Governing Formula
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Notation & Variables
Worked Example
Classify D-region geometry
A transfer girder spanning 16 ft with clear depth h = 6 ft has clear span-to-depth ratio ln/h = 16 / 6 = 2.67 <= 4.0, classifying it as a deep beam.
Establish truss geometry
Formulate a simple triangular STM with two inclined compression struts and a bottom tension tie.
Verify bottle-shaped strut capacity
Provide minimum orthogonal skin reinforcement Av/s >= 0.0025 and Avh/s >= 0.0025 to enable beta_s = 0.75.
When does this govern?
Governs transfer girders carrying interrupted multi-story columns, pile caps, and corbel beam supports.
Common Mistake
Using standard flexural beam formulas (Mu / phi*fy*d) to size deep beams, which neglects massive diagonal tension forces across the shear span.