PSI - Issue 78

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

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Fig. 5: Shear force acting on the column due to the horizontal component of the infill thrust.

6. Out-of-Plane Resistance Verifications of infills The effects of seismic action for out-of-plane verifications on infill walls can be determined by applying a horizontal force to the element, defined as follows: = ⋅ (5) where F a is the horizontal out-of-plane seismic force acting at the centre of mass of the infill panel, W a is the weight of the infill, t w , L w , and h w are the thickness, length, and height of the infill, respectively, S a is the maximum out-of-plane acceleration normalized with respect to the gravitational acceleration g , acting on the panel during the earthquake, and q a is the behaviour factor of the panel. In the absence of more specific assessments, q a can be assumed equal to 2, as stated in the Commentary to the Italian Building Code (NTC 2018). However, for slender/weak infills (e.g., those belonging to Group 4), it is conservatively recommended to adopt a lower value of q a , in the range of 1.3 to 1.5. Since out-of-plane safety checks are usually performed in terms of lateral forces per unit area, it is necessary to divide the seismic force F a by the frontal area of the panel, in order to obtain the force per unit area acting out-of plane on the infill wall, equal to w a = F a /( L w h w ), where h w is the panel’s height. The Guidelines provide various expressions for the evaluation of S a , including those found in the Italian Commentary of the NTC2018 (equations C7.2.3 and C7.2.11), as well as the formulation proposed in the latest draft of prEC8-1-2, which is based on a m ethod developed by Vukobratović and Fajfar (2024). The Guidelines also propose additional criteria for the evaluation of out-of-plane action, including a specific approach for masonry infills developed by Di Domenico et al. (2021). Verifications are conducted comparing the acting force w a with the resisting force R, β . If it is assumed that the infill is restrained both at the top and bottom, the resistance R can be evaluated using a vertical arching model, assuming perfect bond with ideal rigid beams and the formation of a horizontal crack at mid-height, as shown in Fig. 6(a). If rigid body out-of-plane movement of the panel is prevented, the vertical arching resistance is calculated using the following expression, in line with the one proposed in EC6-1-1 (2022): = 0.8 (0.9 −∆ ) ℎ 2 (6) where ∆ is the out-of-plane deflection at mid-height of the arch and f d is the design vertical compressive strength of the masonry. Although EC6 currently allows the deflection of the arch to be neglected for h w /t w ≤ 20 ( ∆ = 0), it is always recommended to adopt a value of ∆ at least equal to t w /10. To account for in-plane damage of the infill, the final out-of-plane resisting load per unit area R, β can be calculated as the product of the out-of-plane resistance w R of the undamaged panel and the reduction factor β R (value ≤ 1), which accounts for the decrease in out-of-plane resistance due to any prior in-plane damage. Based on the experimental results obtained, possible approximations of the out-of-plane strength reduction factor β R as a function of in-plane drift demands δ w at SD for the j-th storey can be represented by a multi-linear reduction, illustrated in Fig. 6(b), where the reduction factors are also provided in tabular form as a function of the masonry typologies, in accordance with prEC8-1-2 (see Morandi et al. 2013, 2021, 2025b).