The article is concerned with the experimental investigation considering the stressed state of essential constructions of hydropower buildings: the pressure pipeline “fork” of large hydropower unit and elastically restrained heavy arches weakened with round holes. The precise knowledge of the stress-strain state (SSS) of the structure during the design phase is essential to the construction’s durability. Analytical methods of calculations, including numerical ones (e.g, finite element method - FEM), are being developed and improved. Experimental methods for studying the stress-strain state of structures on models complement the analytical calculations and combine with them effectively. The research was carried out by the method of photoelasticity "freezing" of deformations using the ability of epoxide polymers to maintain the constant optical anisotropy, caused with loading of the model, after unloading. The results allowed to specify the stress distribution in significant critical structural sections, to evaluate the reliability of estimates of stress concentrations by the holes in arches, fulfilled by finite-element method.