PSI - Issue 78

Gaetano Della Corte et al. / Procedia Structural Integrity 78 (2026) 199–206

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Another typical problem in predicting the response of connections is the inclusion of prying forces, i.e. the contact forces developed at the edge of bent plates. Prying forces can develop depending on the relative stiffness of the plate in bending and the anchors in tension. EN 1993-1-8 provides criteria to assess whether prying forces develop. In the case of prying forces developing in the elastic range of connection response, the code suggests considering their effect by means of a fictious reduction of both the stiffness of the anchors in tension and the plate in bending, separately and with different reduction factors. For the plastic range of response, the code assumes that the plate completely detaches from the support on the tension side, whichever is the plastic mechanism. The latter assumption is perhaps oversimplifying the model, because plastic mechanisms involving the plate in bending could lead to developing prying forces also in the plastic range. as shown by the experimental tests described in Della Corte and Landolfo (2017). The approach followed in this study for including prying forces in the model, while keeping the simplicity objective, is slightly different from the current EN 1993-1-8 and it is depicted in Fig. 2b. A spring is added onto the tension side of the connection, with a negative stiffness (- � ) such that the corresponding reactive force simulates a compression force on the tension side. The relationship between the prying force and the connection rotation can be found by simply relating the displacement at the location of the spring simulating the prying force to the rigid bar rotation. However, following EN 1993-1-8, it is assumed that a single equivalent spring can be utilized at the anchor tension spring location, with a stiffness given by Eq. (3), where k b is the tension stiffness of the anchor bolts, k p is the bending stiffness of the plate, and k PF is the stiffness due to the prying forces. � =�1 � ⁄ +1 � ⁄ � −1 − � (3) There are no simple methods available to evaluate the stiffness due to the prying forces, k PF . Numerical results from finite element model analysis of a sample of connections, which are described in the following sections, have indicated that the ratio between � and the combined stiffness due to the plate in bending and the anchor in tension is relatively constant. Therefore, a relative stiffness parameter, ρ , is introduced, as defined by Eq. (4). � �1 � ⁄ +1 � ⁄ � −1 � (4) Consequently, the total stiffness of the equivalent tension spring is evaluated by means of Eq. (5). � =�1 � ⁄ +1 � ⁄ � −1 (1− ) (5)

a)

b)

Fig. 2. (a) Simplified scheme for the compression springs; (b) scheme to account for prying forces.

2.4. The plastic response

In the case of unstiffened connections, the center of compression forces is fixed at the center of the compression flange of the connected steel member, whichever is the response phase of the connection, i.e. also for the plastic