Composite Beam Design Calculator

The calculator places the PNA in the slab whenever the steel section's tension force (area × yield strength) is less than or equal to the concrete's compression force (0.85 × f'c × effective width × slab thickness) — which is the typical case for a slab wide and thick enough to develop more compression capacity than the steel beam can pull. When steel force exceeds concrete force instead, the PNA gets pushed down to the top of the steel section.

The calculator takes the smaller of the two competing forces — concrete's compression capacity and steel's tension capacity — as the governing composite capacity, since the connection can only transfer as much force as the weaker side can develop. With a typical slab and beam combination, concrete usually has the larger capacity, so steel tends to control, but a thin or narrow slab can flip that.

Because it can't derive one from what you enter. The real stiffness improvement over a non-composite beam comes from building a transformed composite section, which needs the steel shape's moment of inertia (Ix) — and this calculator only asks for the steel area and depth. Composite action typically does reduce deflection substantially by increasing effective I, but the actual percentage for your specific beam and slab requires a genuine transformed-section calculation, so no figure is shown here rather than a constant that would look computed without being computed.

The calculator divides the total composite force (the smaller of the concrete and steel force capacities) by the capacity of a single stud, Qn = 0.5 × Asc × √(f'c × Ec), capped by the stud's shear-fracture limit of area × 450 MPa. The result is rounded up to a whole number of studs, with a minimum of two enforced regardless of how small the calculated demand is.

The overall method — the plastic-stress-block approach, effective slab width, and the shear-stud capacity equation Qn — comes from ANSI/AISC 360-22, Chapter I of the American Institute of Steel Construction's Specification for Structural Steel Buildings (specifically §I3.1a for effective width and §I8.2 for stud strength). The concrete modulus of elasticity, Ec = 4700√f'c, is a separate approximation from ACI 318-25 §19.2.2.1, published by the American Concrete Institute, which AISC's own composite provisions rely on for that one input.