Doctoral Dissertations

Bassam Jamal

Date of Award

5-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Richard M. Bennett

Committee Members

Edwin G. Burdette, Joseph W. Forrey, David Goodpasture

Abstract

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 a 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 comers 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. When the frame columns differed, the average properties were used to estimate the relative stiffness used in calculating the diagonal strut effective width. However, the exact frame columns were still used in the strut model. Therefore, the infill behavior depended on the load direction. An average value could be used in estimating the structural stiffness, and thereafter, the natural frequency. 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. The effective diagonal strut width was divided by a constant, √2, for hinged connection frames. The secant stiffness model used the same equation and constant. However, the strut used a secant modulus, 0.662 of the infill Young's elastic modulus. The equivalent strut model successfully reflected the positive influence that vertical loads had on the infilled frame stiffness. The ultimate load model used the same equation and a penalty factor, 0.387, to reduce the infill ultimate strength due to the infill imperfect plasticity. The model ultimate load was the smaller lateral load which caused the failure of the windward column or the diagonal strut. Vertical loads did not effect the ultimate load when the infill was weak or under moderate vertical loads when the infill was relatively strong. Heavy gravity loads decreased the infilled frame ultimate strength 0f strong infills. 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.

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