engleski

PERFORMANCE BASED EVALUATION AND DESIGN OF REINFORCED CONCRETE FRAMES WITH STRONG MASONRY INFILL

Within a scope of the Croatian project "Seismic design of infilled frames" the contribution of various types of masonry infill, commonly used in Croatia, to the behaviour of RC frames has been investigated. As a part of the project, one story RC infilled frames were designed according to the EC8 and built in a scale 1:2.5 according to the complete similarity rules. They were subsequently infilled with high strength hollow clay brick blocks, without additional shear connection to the frame. One additional RC frame has been built for the comparison reasons. The specimens were tested under constant vertical and cyclic lateral loads applied in a direction along the plane. They were instrumented to monitor the applied loads at each loading point, displacements at the frame top and bottom, elongation of the diagonals on the frame and on the infill and deformations of the critical frame portions. The experimental results were analysed in the form of the failure types, shear deformations, hysteresis loops and load- displacement envelope curves. One of the goals of this paper is to compare the experimental results with the results of analytical and numerical modelling. As an analytical model of the infilled frame’s ultimate capacity an already well known expressions from the scientific literature in the form of bilinear capacity curve were used. In numerical calculations a substitute diagonal macro model approach for modelling of the infill wall was used by means of Seismostruct computer program. Guided by the results of experiments and using well known analytical behaviour models, computer models of the reinforced concrete masonry infilled frames were calibrated. All three approaches corresponded well and they all showed that infill walls have a beneficial effect on the structural response, provided that they do not cause shear failures of columns. The possibility of applying such calibrated models for analysis of the behaviour of infilled frames with different geometrical and mechanical characteristics was evaluated. In order to implement these results in common engineering practice and guided by the results of the experiments and numerical analysis, a substitute equivalent design and evaluation methodology, based on linear calculation of reinforced concrete frame with infill is proposed. According to the methodology, reinforced concrete frame with infill wall is designed and analysed as a system ("frame + wall”). Numerically, it is based on modelling the infill wall as compression members (diagonal strut) that connect the opposite corners of the reinforced concrete frame. Based on model calibration, an expression for the substitute diagonal member’s width is proposed. Also, the behaviour factor is corrected according to the observed equivalent damping coefficients determined by the experiments. Seismic performance based approach is introduced by means of the evaluation of the expected behaviour of models (demand vs. capacity), using the N2 method, i.e., determining the expected nonlinear drifts. The results corresponded well and showed that taking into account the infill wall is fully justified.

reinforced concrete frame; masonry infill; experimental test; analytical model; numerical model; results comparison; performance based design methodology

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