Plastic and elastic behavior of infilled frames subjected to in-plane loading

Elastic and plastic finite element studies were conducted to evaluate the in-plane behavior of concrete masonry infilled steel frames. ABAQUS was used to develop the finite element model and perform a parametric analysis. The model was verified by comparing the results with the experimental program series carried out at the University of New Brunswick, Canada. The initial stiffness could be matched using an elastic model with an frame-infill interface element. The ultimate load and secant stiffness, at approximately 50% of the ultimate, could be matched using a plastic model. The ABAQUS plastic model used a concrete model for the infill and an elastic-plastic material for the steel frame. A softened interface was used to account for the localized mortar crushing that tends to occur at the corners of infilled frames. The parametric study results indicated that an appropriate equivalent strut could closely match the infilled frame behavior. The model had the same frame beam and columns. The masonry infill was replaced by an equivalent diagonal strut. The strut had the same infill net thickness and Young's modulus. The model connections were hinged regardless of the frame's actual connections. The study included one and multiple bay infilled frames. For the latter case, the properties, area and moment of inertia of each middle column were equally divided between the two adjacent one bay infilled frames. Infilled frames under gravity loads were also examined.; Stafford-Smith and Carter's (1969) formula to estimate the diagonal strut effective width for rigidly connected frames matched the initial stiffness. Constants were added to the equation for secant stiffness and ultimate load models. The effective diagonal strut width was divided by a constant for hinged connection frames. The model ultimate load was the smaller lateral load which caused the failure of the windward column or the diagonal strut. The study found that the ultimate load of a multiple bay infilled frame was the sum of the ultimate strength of each individual one bay infilled frame.