The seismic response of low-energy buildings founded on a thermal insulation layer – A parametric study

Abstract

Modern passive and low-energy houses, which strictly follow the requirements of passive house standards for the prevention of thermal bridges, are often founded on a thermal insulation (TI) layer, which is placed under the building’s RC foundation slab. Such buildings, which have already been developed for non earthquake-prone areas, might not perform well in earthquake-prone areas, so their suitability in such areas needs to be verified, and appropriate solutions found. The insertion of a flexible layer under the foundations can affect the seismic response in several different ways, as follows: (a) by increasing the compressive and shear stresses in the TI layer, (b) by increasing the negative effects on the superstructure (increased rocking phenomena), and (c) by prolonging the fundamental period of the superstructure (changing the dynamic characteristics of the building). The latter is particularly important in the case of stiffer, low-rise buildings with short fundamental periods, for which the movement to the right hand side into the resonance part of the response spectrum might increase the maximum top accelerations. The main goal of the paper was to quantify the differences in the earthquake response of buildings with/without the inserted TI layer under the RC foundation slab, and to identify the possible negative influences of already developed technical solutions, as well as to propose structural measures and limitations for their application in earthquake-prone regions. In the paper an extensive parametric study of the seismic behaviour of buildings founded on thermal insulation made of extruded polystyrene (XPS) is presented. Selected results are shown in the form of nonlinear response spectra, and IDA curves. The results show that amplifications of selected observed parameters can be expected in the case of stiffer buildings with fundamental period of the fixed based superstructure shorter than approximately 0.3–0.5 s. The amplifications, as expected, in general increase with the number of storeys, the slenderness of the building (i.e. the H:B ratio), and the seismic weight of the building. It was also shown that the inelastic behaviour of the superstructure appears to be favourable for the TI layer since it reduces the forces and transfers smaller moments onto the foundations. In general, no critical negative effects were observed in the case of regular buildings having up to three storeys, providing that sliding on a waterproofing layer is prevented. Some suggestions for the reduction of the seismic risk of such buildings are also presented.