Despite being one of the oldest but still most used building materials on the planet, (micro-)structure-(macro-)property relations of fired clay bricks and masonry structures are still mostly limited to experience or empirical laws formulated based on experimental campaigns. To meet the demands of modern bricks with a low CO₂ footprint but improved performance, brick compositions, block geometries or mortar joints have been altered. Compositional changes include the mix of raw clays with pore-forming additives (e.g., sawdust or paper sludge), tempers (e.g., quartz sand or fly ash) before the mixture is extruded, dried, and fired at different temperatures. Regarding block design, vertically perforated bricks with thin webs are produced to minimize the thermal conductivity. Thick mortar beds have been reinforced or entirely replaced by thin joints or adhesives. This complexity, however, requires predictive tools to develop and optimize the material performance of masonry structures, a challenge that is tackled in this paper. To predict the mechanical behavior of fired clay, a micromechanics multiscale material model was developed [1,2,3,4] which resolves the material heterogeneities of fired clay bricks across three scales of observation. Mineral grains (such as quartz and feldspar) and pores are considered, at the three observation scales, with a matrix phase hosting them. Morphometry data are obtained from extensive experimental characterization attempts including mercury intrusion porosimetry, micro-computed tomography, nanoindentation, SEM-EDX [5,6,7]. Based on analytical micromechanics homogenization, stiffness, elastic limits stresses, and thermal conductivity are upscaled to the level of fired clay and corroborated by experimentally measured stiffness, conductivity, and strength data. To predict the failure of vertically perforated fired clay blocks, three-dimensional finite element models based on unit cells with periodic boundary conditions were developed [8,9,10]. Discrete cracks were modeled using extended finite element modeling for bricks. Mortar joints were modeled using the concrete damage plasticity framework. The model is extended to consider shear loading and even to predict spalling of webs in fire situations. This way, a better understanding of the relationship between brick geometry and strength properties was gained as a result. Additionally, the potential benefit of mortar joint reinforcement was identified. [1] Buchner, T., Kiefer, T., Königsberger, M., Jäger, A., & Füssl, J. (2021). Materials and Design, 212, 110212. [2] Buchner, T., Königsberger, M., Jäger, A., & Füssl, J. (2022). Mechanics of Materials, 170. [3] Kiefer, T., Füssl, J., Kariem, H., Konnerth, J., Gaggl, W., & Hellmich, C. (2020). Journal of the European Ceramic Society, 40(15), 6200–6217. [4] Buchner, T., Königsberger, M., Gaggl, W., Früh, G., and Füssl, F. (2023) Construction and Building Materials, 132601. [5] Buchner, T., Kiefer, T., Zelaya-Lainez, L., Gaggl, W., Konegger, T., & Füssl, J. (2021). Applied Clay Science, 200 [6] Kariem, H., Hellmich, C., Kiefer, T., Jäger, A., & Füssl, J. (2018). Journal of Materials Science, 53(13), 9411–9428. [7] Kariem, H., Füssl, J., Kiefer, T., Jäger, A., & Hellmich, C. (2020). Construction and Building Materials, 231, 117066. [8] Kiefer, T., Kariem, H., Lukacevic, L., Füssl, J. (2017). Construction and Building Materials,150:24-34. [9] Suda, R., Kiefer, T., Schranz, C., Füssl F. (2021), Engineering Structures, 239:112277. [10] Reismüller, R., Königsberger, M., Jäger, A., Füssl, J. (2023). Fire Safety Journal, 103729.