Infilled frames: Influence of vertical load on the equivalent diagonal strut model

The influence of masonry infills on framed structures behaviour is a central topic in the seismic design procedures and in the hazard evaluation of existing buildings. Many models use equivalent strut elements in order to represent the infill but among the several parameters influencing the interaction between frame and infill the level of vertical loads is hardly considered. Nevertheless, neglecting this effect may produce inaccuracy because the axial deformations of the loaded columns can produce non-negligible variation in the contact region between infill and surrounding frame, influencing the seismic response of the infilled frame. It can easily been observed that, when this regions extends, the infill behaviour switches from that of a strut element to the one of a plate-shell. An equivalent diagonal pin-jointed strut model, able to represent the stiffening effect of the infill in presence of vertical loads, is given in this paper. By a numerical experimentation based on a FEM discretization of the frame-infill system, the lateral stiffness of some infilled frames is evaluated; then the ideal cross-section of the strut equivalent to the infill is obtained for different levels of vertical loads by imposing the equivalence between the frame containing the infill and the frame containing the diagonal strut. This way a correlation available in the literature between a parameter depending on the characteristics of the infilled frame and the equivalent strut width is generalized here to consider the vertical load presence. This correlation is provided in an analytical approximated form of immediate use in the practical applications