PSI - Issue 78

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

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Hak et al. (2018), it is sufficient to calculate the displacement demand at the DLS and ULS, and the corresponding interstorey drifts, through a linear elastic analysis (either equivalent static or multimodal response spectrum analysis) performed on a “bare” structural model of the building. The stiffening effect of the infills is then considered a posteriori and in a simplified manner by introducing a coefficient ( C j ) that allows estimating the drift of the infilled frame at storey j ( δ w,j ) starting from the drift of the bare frame ( δ j ). The actual displacements of an infilled frame are smaller than those of the corresponding bare frame, and the difference increases with the relative stiffness of the infills compared to the bare RC structure. The inter-storey drift of the infilled frame at the j -th floor ( δ w,j ) can be derived from Equation (1) or graphically from Fig. 4, as a function of the bare frame drift at storey j ( δ j ) and of the relative stiffness-density parameter ( C j ) between the infills and the bare structure at that floor: , = � , ’ , ’+ , , ≤ , ’+ , − , , > , ’+ , (1) where δ m,j ’ represents the equivalent drift capacity, considering the different infill types present at storey j. For C j =1, the difference between δ w,j and δ j equals δ C,j , which can be taken as 2/5· δ m,j ’. To compute δ m,j ’ at floor j , it is necessary to know the deformation capacity of each infill type used in the building, such as those listed in Table 1. If only one infill type is used throughout the building, δ m,j ’ equals the corresponding δ m ’ . The values of δ m,j ’ correspond to the drift limits at DLS ( δ DLS ), as given in Table 1, based on the infill typologies.

Fig. 4: (a) Definition of the bilinear curve for C j =1.0; (b) Variation of the bilinear curve for different values of C j (Hak et al., 2018).

For each seismic-resistant direction of the structural configuration (e.g., longitudinal and transverse), given the distribution of infill panels and the required properties to characterize each infill type, and knowing the average simplified stiffness parameter of the infills ( K I,j ) and the structural stiffness ( K S,j ) for each storey j ( j = 1... n s , where n s is the number of storeys), it is possible to define the stiffness-density parameter C j for each floor, which represents the average ratio of infill stiffness to structural stiffness along the building height, as indicated in Equation (2): = , , (2) The interstorey drift at ground floor is conservatively assumed to be equal to that of the bare structure, leading to a C j value equal to zero at that level. The values of the simplified stiffness parameter of the infills ( K I,j ) and the structural stiffness ( K S,j ) for each storey j can be simply evaluated following the procedure by Hak et al. (2018). This approach has been included in an informative Annex of prEC8-1-2 (2025). In addition to the simplified method define above, the Guidelines also include a procedure for the evaluation of in plane drift demand considering an explicit modelling of infill walls, which can be simplified by using a macro-model based on a single concentric diagonal strut. The Guidelines define the criteria to calculate the secant stiffness of the struts to be considered for the drift evaluation at different limit states. It is important to clarify that, for the determination of internal forces and the verification of structural elements at the Ultimate Limit State (SD), analyses must always be performed also on the bare frame. This is because, at ultimate