Influence of cracking on the forces in reinforced concrete slab elements.

The article presents a method for calculating reinforced concrete floors using an approximation in the form of a cross-rod system. It is shown that the appearance of cracks in the concrete changes the bending and torsional stiffness of the floor elements. However, almost all software solutions for structural analysis account only for the change in the bending stiffness of the rod elements, as a result of the formation of cracks. The torsional stiffness, on the other hand, remains the same and does not change during the iterative calculations. As a result of this lack of consideration, the forces in complex statically indeterminate systems are obtained with errors. To eliminate this drawback, it is proposed to account for the change in the torsional stiffness of the rods as a result of cracking. Formulas are provided to account for the change in bending and torsional stiffnesses, due to the formation of cracks, as part of the iterative calculations of floors and cross-rod systems. It is shown that after calculating the cross-rod system, it is easy to account for the forces in a solid slab. The formulas for the transition from the bending and torsion of the rod system, to the bending and torsion of a solid slab are provided. A method for determining the reactions in the supports of the elements that are used in the calculation of rod systems, by the displacements method is provided, taking into account the variable stiffness along the length of each element. This, as a result, increases the accuracy of the forces being determined. Using a reinforced concrete cross-rod system with plan dimensions of 6000x6000 mm, as an example, it is shown that when the change in the torsional stiffness of the rods, due to the formation of cracks is accounted for, the torsional moments in the rods can vary by more than 260%, compared to the calculation without taking into account the change in torsional stiffness. Bending moments differ up to 20%. These results show that when designing statically indeterminate reinforced concrete systems, it is important to account for the variation in the torsional stiffness of the elements, as a result of concrete cracking. [ABSTRACT FROM AUTHOR]