Structural Resistance and Stability of Masonry Walls with Thermal Break Elements

The implementation of highly demanding energy standards in Europe, that aim at houses with low energy consumption, has impacted the structural building systems and led to the introduction of composite masonry walls, e.g. a bearing masonry wall including thermal break elements at the first layer to eliminate thermal bridges. On the one hand, the presence of these thermal elements induces potential weaknesses in the walls, modifying their mechanical resistance at the local level. On the other hand, these elements also modify the boundary conditions of the walls and are thus likely to have an influence on the buckling behavior. Accordingly, it is necessary to adjust the design procedures, for instance, by EN 1996 to check the wall stability against vertical loading. However, experimental and numerical studies concerning the structural stability of composite masonry walls remain limited. In this work, extensive experimental investigations and numerical studies on the resistance and stability of composite masonry walls have been conducted. The research started with experimental tests on small to medium scaled specimens with the aim of studying the influence of the presence of thermal elements on the local resistance. The experimental campaign includes tests of different scales of homogeneous and composite masonry specimens (single units, stacked blocks, wallets) under uniaxial compression with and without eccentricity. The composite specimens considered herein are load-bearing masonry walls comprising two layers: aerated autoclaved concrete (AAC) as the first layer, and perforated clay or hollow concrete blocks for the other layers. The experimental results show that composite specimens generally fail due to the crushing of the AAC layer. Yet, the interaction between masonry and thermal elements proved to have a positive influence on the local bearing resistance due to the stiffening brought by the main masonry. In addition, the results indicate that the strength reduction due to eccentricity is slightly more severe for composite walls in comparison with the homogeneous specimens. In order to study the global stability and resistance of the composite specimens, numerical simulations were performed in two stages. Firstly, a FEM model using a mesoscale modeling approach was developed to reproduce the experimental tests of the current research project by means of calibrating proper material properties of the specimens and validate the numerical model against the experimental tests. Then, the modeling techniques and material inputs of the latter models are used to extend the observations on masonry walls made of composite masonry specimens in combination with other parameters such as slenderness, eccentricity, and boundary conditions. The numerical results indicate that composite masonry walls fail due to the weakest masonry layer, whether it is a thermal AAC or masonry layer. Composite walls with a low slenderness ratio (hef/t = 5 and 15) or which are imposed to a low amplitude of eccentricity (i.e. e < t/6) are likely to fail due to the crushing of the AAC. The walls with large slenderness ratio and/or eccentricity experienced stability failure, particularly at the upper layers. These results indicate that the AAC layer has a negligible influence on the stability of the main masonry as this layer does not influence significantly the boundary conditions at the bottom of the main masonry. Finally, as a first step towards the development of design procedures, available methods concerning the design of composite specimens under vertical compression are compared with the experimental and numerical observations. Based on the current experimental and numerical investigations, practical recommendations regarding the design of composite walls are considered including two approaches. First conservative approach consists of checking the resistance of each layer by considering them as separate walls (i.e. the AAC layer as equivalent short homogeneous AAC wall, the main masonry layers as a homogeneous masonry wall with slenderness and boundary conditions identical to one of the composite walls). The wall with lower strength governs the resistance of the wall. The latter approach does not take into account the favorable influence of the upper clay units on the local resistance of the AAC layer, proved experimentally on composite wallets, rendering this approach as a conservative one. A second realistic and favorable approach consists of considering the positive influence of the interaction brought by the upper layer on the AAC layer. This factor can be determined by applying an amplification factor u, derived from the characteristic values of the compressive strength obtained from tests on composite duplets referred to specimens made of two stacked AAC units with the same mortar. In such a case, the resistance of the AAC layer in a composite wall equals the strength of an equivalent homogeneous AAC wall multiplied by factor u. The resistance of the main masonry is equal to the resistance of a homogeneous wall with the same slenderness as the composite wall. The wall with the lower resistance (AAC wall or main masonry) should govern the resistance of the wall.

Publication year:2021

Accessibility:Embargoed