Purpose of research. Experimental determination and analysis of the bearing capacity, deformability and crack resistance of a reinforced concrete floor slab supported on three sidesMethods. Testing a specimen of a reinforced concrete floor slab supported on three sides (loggia floor) by a controlled load simulating the operational one. Measurement of deflections. Analysis and justification of test resultsResults. The slab was loaded to a level of 325 kN or 20.1 kPa. This load exceeded the control load by a factor of 1.64 with a safety factor of 1.6, exceeded the design load by a factor of 2.64, and exceeded the standard load by a factor of 3.08. The maximum recorded deflection at maximum load was 3.01 mm in or in span fractions of 0.0009, which is 7.41 times less than the allowable deflection from the standard load - 0.00667 of the span length. The testing has been terminated for safety reasons.Conclusion. The points that could allow the developing of more economical building structures are: refinement of the design scheme for the operation of structures, the optimal choice of a finite element in the calculation, as well as the gradual inclusion in the sets of rules of methods for calculating building structures based on the theory of structural systems with distributed parameters at critical energy levels.