PUNCHING OF REINFORCED CONCRETE FOUNDATION SLABS

The article provides an explanation of the secondary nature of the formation of a crushing pyramid as a consequence of the true brittle failure of slab reinforced concrete structures – crushing, shearing, or crushing of concrete, taking into account local stresses and the concentration of bending moments, and for foundations, the additional redistribution of contact stresses in the soil foundation and lateral forces, which allows us to reasonably reduce the variety of types of brittle failure with the formation of various forms of crushing pyramids to three: crushing, shearing, and crushing of concrete. Experimental studies were conducted with large-scale reinforced concrete foundation models on a sandy base, which allowed for the study of the parameters of the foundation and soil interaction and the determination of geometric and strength factors (console span, foundation slab height, reinforcement, and concrete strength) that play an important role in the formation of the foundation's brittle failure process, such as punching. Subject of research: stress-strain state of foundations during brittle failure in the form of crushing. Materials and methods: Experimental studies were conducted to obtain the penetration of foundation slabs, and 49 large-scale samples of foundation slabs measuring 1x1x0.1 m and 29 slabs measuring 2x2 m with a cross-section height ranging from 0.08 m to 0.4 m were tested. The tests were conducted in a 4x4x3(h) m reinforced concrete tray, with a sand layer of 2.2-2.4 m. The power equipment allowed for loading the foundation slabs with a force of up to 4000 kN, and the recording equipment, including specialized equipment for foundation research, provided high-precision measurements of the parameters of the soil-foundation system. Results: The methods of calculating the plates for the crushing and directions of improving the calculations of brittle strength are analyzed. The influence of the basic parameters of the foundations structures on the sandy soil base on the brittle strength during crushing is experimentally studied. The formation of different forms of the crushing pyramid as a secondary process of destruction from crushing, shearing or crushing of concrete is substantiated, constructive solutions are proposed to reduce the influence of the concentration of stresses and forces in the area of loading near the support. Conclusions: The calculation of foundations for according to formula 8.87 of SP 63.13330.2018 "Concrete and Reinforced Concrete Structures" often does not allow for sufficient accuracy in obtaining values of brittle strength, and the experimental values may significantly exceed the calculated values, with the excess being inconsistent. For foundation slabs, the minimum observed coefficient before the right-hand side of the formula is equal to 1.3. The actual mechanism of brittle failure of slabs can be considered to be crushing, shear of the compressed zone and crushing of concrete under the support, with subsequent (secondary) formations of the pyramid of the push-through. In the case of crushing of the compressed zone, the best results are given by the calculation of the critical combination of the principal stresses in the near-support part of the slab.