THE FIBERS OF THE STEEL REINFORCED CONCRETE SLAB STRAIN CHANGE FOR SHELTER OF THE CIVIL PROTECTION DURING THE TWO-STAGE TECHNOLOGY OF SLAB MANUFACTURING

There are often no technical specifications and technological maps for the installation of built-in shelters in existing structural and planning schemes of buildings. Atypical architectural and structural solutions for protective structures can be implemented using monolithic or precast concrete and steel-reinforced concrete structures. Due to the two-stage technology for the manufacture of precast steel reinforced concrete slabs, the geometric characteristics of their composite cross-sections change during the manufacturing process. Changes in cross-sections of building structure elements during the manufacturing process are commonly referred to as genetic nonlinearity. At the same time, in most cases, the change (increase due to the monolithic upper concrete shelf) of the cross-section of a steel-reinforced concrete slab element occurs under different deformations in its components: the existing deformations in the steel part from the dead weight of the monolithic concrete slab and the absence of deformations in the monolithic concrete slab. This fact is the reason for the bilinear appearance (discontinuity of the linear appearance at the border of the upper shelf of the steel I-beam and the bottom of the concrete slab) of the diagram of strains in the cross section of steel reinforced concrete slabs. It is possible to avoid the genetic nonlinearity of steel reinforced concrete slabs by installing temporary supports for steel beams during the concreting of the monolithic slab or by constructing the monolithic slab on inventory temporary formwork. By installing the above-mentioned temporary posts, it is possible, on the contrary, to achieve favorable pre-stresses in the components, which can be called self-stressing of such slabs. During the experimental loading of a fragment of a pre-stressing steel-reinforced concrete slab, the increase in relative deformations along the height of the cross-section, taking into account the pre-bending of steel beams, is similar to those predicted during the theoretical analysis of the stress-strain state of such beams. Taking into account the pre-stressing of the steel beams allowed us to reduce their cross-section from a rolled I-beam № 45 to an I-beam № 36 with a strip along the bottom girdle, which reduces steel consumption by 11.1 %.