Effect of Torsional Rigidity of Concrete Elements with Normal Cracks onto Spacial Work of Bridges and Floorings

The article shows the effect of torsional rigidity of beam elements onto spatial work of bridges, floorings and other plate-ribbed systems It is shown that, in determining the torsional rigidity using existing methods it is necessary to have spatial spiral cracks, but torsional rigidity when normal cracks is not researched. A method is described for determining the torsional rigidity of reinforced concrete elements when normal cracks. Analysis of publications and task assignment It is known that the spatial work of plate-ribbed systems is significantly affected by the torsional rigidity of their elements. In the reinforced concrete plate-ribbed systems (bridges, ribbed monolithic and prefabricated floorings) the bending and torsional rigidity is affected by various cracks [1, 2, 5, 6, 8, 10, 11]. Under action of local loads (e.g. traffic load on one or more bridge beams) in some beams cracks can appear, while in others they may be absent. Thereby the torsional and bending rigidity in beams without cracks and with cracks will differ. In works [2, 10, 11] it is shown that the redistribution of local load depends substantially on the bending rigidity, as well as on torsional rigidity of individual beams. This dependence is essential. Consequently, the definition of bending and torsional rigidity is an important and relevant task. At the same time, nowadays there are a large number of studies on the bending rigidity of reinforced concrete elements with cracks, and a very small number of studies on torsional rigidity of such elements. Most studies on torsion in reinforced concrete are devoted to investigation of the strength of such elements. Existing methods for determining of torsional rigidity [9] refer only to concrete elements with spatial (spiral) cracks, although experimental studies established a significant effect of normal cracks on torsional rigidity of reinforced concrete elements. B.V. Karabanov [8] concluded that during formation of cracks the torsional rigidity decreases faster than bending rigidity, based on experiments of V.N. Gornov [5]. However, theoretically this fact is not justified. The article of author [1] shows methods of resolving the problem for determining the torsional rigidity of reinforced concrete elements with normal cracks. One of the most reliable and common way is to use software packages (such as, for example, "Lira-Windows"), which implements the finite elements method (FEM). However, it should be used only for part of the problem solution, because movement of reinforcing rod under effect of dowel force is an issue subject to experimental determination. Object of the present article is to develop methods for determining the torsional rigidities of reinforced concrete elements with normal cracks using experimental data, as well as data of numerical experiment with application of FEM, and to proof the influence of change in torsional rigidity of structural elements of bridges and floorings on redistribution of local loads acting on them.. Summary of main material As stated above, the rigidity of reinforced concrete elements under torsion was considered previously only for cases of spiral spatial cracks presence. Furthermore, in these works simple types of sections: a rectangle with symmetrical reinforcement, ring, and cylindrical elements are considered. In practice one can meet a variety of sections types: T-sections, I-sections, hollow triangles, boxshaped sections, etc. The first steps in this direction are made in the Odessa State Academy of Construction [3, 4]. However, these studies are only at the initial stage of research. That's why it is necessary to develop methods for determining the torsional rigidities, which would have common hypothesis for all types of sections, and include ways to calculate the elements with the presence of both spatial and inclined normal cracks.