PSI - Issue 78

Paolo Morandi et al. / Procedia Structural Integrity 78 (2026) 1293–1301

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3. Design and Verification Approach and Modelling Criteria The approach proposed in the Guidelines for the design and verification of reinforced concrete (RC) structures with masonry infill walls is briefly illustrated in Fig. 3. In the design of new buildings, the first step consists of evaluating the seismic actions at the Ultimate Limit State (Severe Damage/Life Safety Limit State – SDLS or simply SD) through a linear elastic analysis (either equivalent static or multimodal response spectrum analysis) performed on a “bare” structural model, that is, a model including only the RC structural elements (beams, columns, joints, walls), while neglecting the stiffness and strength contributions of infills and partitions whose contribution is considered only in terms of mass. This practice aligns with current design standards (NTC18, EC8 2004, prEC8-1-2 2025). The resulting internal forces are used to design the structural elements, which need to be verified for strength at the SD. In-plane damage control of the infill walls is usually based on drift checks, by comparing the imposed inter-storey drift with the drift capacity of the infill panels. While current codes (NTC and EC8) require these checks only at Serviceability Limit States (Operational-OLS and Damage Limitation-DLS), the latest draft of prEC8-1-2 (2025) suggests extending these verifications also to the Ultimate Limit State (ULS or SD). The inter-storey drift demand at DLS/OLS and ULS is estimated through three alternative approaches: • using the bare structural model, neglecting infill stiffness; • using the bare model with a simplified approach to account for infill stiffness “a-posteriori”; • using a full "infilled model", with explicitly modelled infills. The document also presents criteria for linear and non-linear in-plane modelling of masonry infills, primarily with the approach of single concentric diagonal struts. In the case of non-linear models, the method proposed in the prEC8 1-2 (2025) is reported, along with other models (e.g., De Risi et al. 2018, Liberatore et al. 2018, Di Trapani et al. 2018). In addition to strength verifications of RC members at SD, local effects caused by infill thrust on adjacent columns must be evaluated, as also addressed in EC8. Simplified procedures have been developed to estimate the maximum shear force on columns caused by infill interaction. Lastly, out-of-plane resistance of infill walls is verified at SD by comparing the applied out-of-plane force per unit area with the panel’s out-of-plane strength. While NTC2018/Commentary and current EC8 provide guidance on evaluating out-of-plane actions, they do not specify how to calculate the panel resistance. In the Guidelines, methods for assessing the out-of-plane resistance of infill walls, also considering their interaction with in-plane seismic actions, have been introduced in line with the latest draft of EC8-1-2 (2025).

Fig. 3: Current seismic design approach of infilled RC buildings in Europe.

4. In-Plane Displacement Verifications The displacement verification simply consists in checking that the drift demands at DLS (and at OLS, where required) and at ULS for each storey j of the structure ( δ w,j,SLD and δ w,j,SLV ) are lower or equal than the corresponding drift limits ( δ DLS e δ ULS ). In addition to conventional drift assessments performed on a bare frame model, the Guidelines introduce a simplified method for incorporating the stiffening effect of infill walls “a-posteriori”. In this approach, developed by