A simply supported reinforced-concrete beam carries a center load. In a pre-cracked elastic section, the upper concrete carries compression and the lower reinforcing bars carry tension. Change steel area and watch neutral-axis depth, stress, curvature and deflection change.
• A real-time 3D cutaway of a simply supported concrete beam (width 300 mm, total depth 500 mm, effective depth 440 mm) with longitudinal reinforcing bars, stirrups, an enlarged cracked section, compression and tension resultants, pin/roller supports and a load head, with home view, focus-selected-part, auto-rotate, expand, show/hide instrument covers and hide-labels controls. • Adjustable peak center load (10-200 kN), number of tension bars (2-6), bar diameter (12-28 mm) and beam span (3-6 m), with an option to cycle the applied load. • Live measurements of midspan bending moment, neutral-axis depth from the top, elastic steel tensile stress, top concrete compression, midspan deflection and tension steel area. • A Curves & measurements tab with a model response curve, parameter comparison, the model equations and snapshot measurements. • An Experiments tab with presets (increase steel area, longer span, beyond the teaching range), pause and time-step controls, a model-verification bench, a timestamped event log and a copyable trial report. • A Learn & assess tab with lessons, a knowledge-check quiz with reset, and a written model-scope statement with a technical reference link.
The model uses a modular ratio n = Es/Ec = 200/30 and a steel area As = Nπdb²/4. The cracked neutral axis satisfies b c²/2 = nAs(d − c), the cracked moment of inertia is Icr = b c³/3 + nAs(d − c)², and the maximum moment for a center point load is Mmax = PL/4 with support reactions P/2. Concrete compression is σc = Mc/Icr, steel stress is σs = nM(d − c)/Icr, and midspan deflection is δmid = PL³/(48Ec Icr) with Ec = 30 GPa. Tension concrete is neglected, so the lower bars carry all the tension.
This is a pre-cracked linear elastic transformed-section model throughout. It does not predict first cracking, plastic capacity, bond, shear, development length or collapse, and concrete tension is omitted even at small load. Stresses above 30 MPa concrete or 500 MPa steel are flagged as outside this illustrative elastic range; these are not code design resistances, and deformation is exaggerated.
No. Tension is assigned to the reinforcing steel while compression remains in the concrete above the neutral axis.
No. Ultimate capacity requires material nonlinearity and additional checks; the simulator flags stresses above 30 MPa concrete or 500 MPa steel as outside its illustrative elastic range and does not claim an ultimate capacity.
Both the bending moment and the deflection increase; at a fixed section and load, elastic midspan deflection scales with the span cubed (δmid = PL³/(48Ec Icr)).
The cracked section becomes stiffer and the steel tensile stress falls at the same applied moment.