Use of analytical method for calculating stiffnesses of straight waffle slabs. Part 2. Calculation using relative stiffness of beams

Introduction. The analytical calculation of reinforced concrete waffle slabs calls for the determination of the components of total load on joists, depending on the dimensions of spans and the rigidity of central joists. In the case of slabs having rectangular coffers of various orthogonal rigidity, this theory leads to results differing significantly from those obtained by the finite element method. The waffle slab comprises a ribbed slab, characterized by cylindrical or relative beam stiffness.

Aim. This article discusses the accuracy of the calculated span bending moments in the joists of straight reinforced concrete waffle slabs using the span dimensions and relative stiffness of joists in analytical formulas.

Materials and methods. The work was carried out by comparing the bending moments for joists in the central zones of straight waffle slabs obtained analytically and using the SCAD software by the finite-element method. Square or rectangular in plan, 13 slabs having different aspect ratios of waffle were considered. A T-beam and girder construction were used in the computer model.

Results. The maximum deviations of the bending moments obtained by the analytical calculation from the computer-aided method amount to: -0.6% for a square slab having square coffers, from -2.8 to +2.5% for a square slab having rectangular coffers, from -6.2 to +2.0% at a span ratio of L_max/L_min ≤ 1.5 for a rectangular slab having rectangular coffers, and from -7.3 to +4.8% for those having square coffers. For rectangular slabs having any coffers at a span ratio of L_max/L_min ≥ 1.75, a computer-aided calculation revealed that the plot of bending moments in the long-side direction deviates from parabola.

Conclusions. The use of the span dimensions and relative stiffness of joists in the analytical formulas for calculating straight hinge-supported reinforced concrete waffle slabs allows the values of bending moments to be obtained that are in agreement with those obtained by the finite element method.

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