Abstract
The thickness of reinforced-concrete floor slabs is usually governed by deflection, and thickness is the largest cost item of a floor. This paper asks whether a triaxial 0°/60°/120° reinforcement net, whose cracked twisting stiffness has been derived analytically to exceed that of an orthogonal net, can reduce the thickness of deflection-governed two-way slabs, and under which conditions. Simply supported square slabs of 3–9 m span and 2–7.5 kN/m² imposed load are designed for strength and minimum reinforcement to ACI 318 and for long-term deflection to the Eurocode 2 interpolation method, using a plate model calibrated against pure-torsion tests. Where the full tensile strength is available, the governing thickness is that at which the slab just remains uncracked, and the triaxial layout gives no reduction. Where shrinkage restraint or construction loading lowers the effective cracking moment, the governing state is cracked and the triaxial slab is thinner by up to about 9% (effective tensile strength 0.5–0.67 f_ctm, mainly in long spans) and by 8.5–12.6% if fully cracked. Minimum reinforcement usually governs the steel, so the thinner triaxial slab generally uses less steel (k = 0.88–1.00 at q_k = 3 kN/m²), provided the bars are small enough for the spacing limit: at the same thickness the triaxial net needs bars about 18% smaller than the orthogonal net (Section 11.4). Closed-form thickness relations and first-order estimates for fixed edges and triangular panels are given; the benefit is roughly halved with fixed edges and is to be verified numerically and experimentally.