Numerical evaluation of natural vibration frequencies of thermo-modernized apartment buildings subjected to mining tremors

Prefabricated, with load-bearing walls, apartment buildings represent a significant group of objects, especially occurring in the countries of Central and Eastern Europe. They were built in 50's, 60's and 70's of 20th century. Nowadays, the new European Union environmental regulations forced such buildings' modernization, mainly regarding energy efficiency. Usually, in order to improve their energy properties and, as the result, to reduce the energy losses, the external walls thermal insulation is performed. This is one of the fastest and cheapest method increasing the thermal resistance of walls of buildings constructed using relatively old technology. Extra thermo-modernization layers of the prefabricated walls (consisting of materials such as styrofoam, reinforced mortar, glass fibre textile mesh) bring additional mass to the building and could change its stiffness. The dynamic properties of the structure could be changed as the result. Changes in the natural frequencies of vibrations are the most important among them-calculations of these values are necessary especially in the case of buildings located in paraseismic or seismic areas. In the paper the influence of the additional multilayer thermal insulation of the external walls on the natural vibration frequencies of high (12 storey) actual apartment buildings subjected to mining tremors is investigated. Numerical calculations were carried out using finite element method software. Various approaches to the modelling of prefabricated load bearing walls before and after thermo-modernization were analyzed and discussed. A simple one layer model of walls with equivalent stiffness as well as multi-layer one were taken into consideration. Finally, the simple model reducing multi-layer panel to one layer panel with equivalent (substitute) Young modulus and equivalent (substitute) Poisson's ratio was recommended for the practical calculations of the dynamic properties of buildings. Numerically evaluated natural vibration frequencies of the buildings before and after thermo-modernization were verified by the experimental data obtained for actual structures.