DAMAGE ASSESSMENT OF MASONRY INFILLED FRAMES

There are many different parameters that affect the behavior of masonry-infilled reinforced concrete frames under earthquake loadings and it is difficult to predict. Selection of the parameters, that have the greatest impact on the response system, was carried out on the EDIF database of 113 onestorey onebay infilled frame tests processed by statistical sensitivity analysis. Those parameters were height to length ratio, longitudinal reinforcement ratio of column and compressive strengths of concrete and masonry infill. A simplified and reliable analytical model, based on the equivalent diagonal and validated on different experimental tests, has been adopted for extensive studies on seismic vulnerability of infilled frame buildings. We investigated the seismic response and the influence of the masonry infills on the global structural response in terms of inter-storey drift ratio and damage states by incremental dynamic analysis. Results are presented in order to determine the relationship between infill’s type and damage state for low-rise infilled framed buildings. INTRODUCTION The buildings with reinforced-concrete frames infilled with masonry (“framed-masonry”) under earthquake excitation behave neither as frames nor as confined masonry walls. Their behaviour depends on many different parameters and on the expected drift. The masonry infill wall within the reinforced-concrete frame increases system stiffness and strength, at small and medium drifts. It also could cause irregularities, in both geometrical and mechanical distribution of infills, and could lead to undesirable failure mechanisms that compromise the bearing capacity of the structure. Due to the incomplete understanding of such composite structures there is a lack of code guidance. That limits the engineer’s ability to design these structures taking into account both, the beneficial and detrimental properties. In order to increase the understanding of the „framed-masonry“structural elements we have collected the experimental database of infilled frames –EDIF. It has been organized in a way that could be used for the performance evaluation and for calibration of the numerical model. Due to the high variability of mechanical properties of masonry and their uncertainty, their properties were divided into three groups that represented a wide number of possible situations that could happen in practice. A lower bound was represented by weak infill, the upper bound with strong infill and medium infill panel had a range of compressive strengths between them. Further, we parametrically investigated the seismic response “framed-masonry” structures, with the numerical model that was simple enough in the computation and elaboration of results, and sufficiently refined for adequate simulation of the main behavioural characteristics. Such reliable and calibrated numerical model was used for the parametric analysis and damage assessment of the “framed-masonry” structures. 1 Senior research assistant, Faculty of Civil Engineering, Osijek, Croatia, tkalman@gfos.hr 2Full professor, Faculty of Civil Engineering, Osijek, Croatia, sigmund@gfos.hr 2 On the basis of EDIF database and parametric analysis done with the calibrated numerical model a practical displacement-based damage evaluation methodology has been suggested. EXPERIMENTAL DATABASE OF INFILLED FRAMES The collected experimental database (Experimental Database of Infilled Frames – EDIF) contains data of 113 published tests of the basic ‘‘framed-masonry’’ composite system. It contains all available input and output data systematized in a way applicable for performance evaluation. Input data were selected considering their availability and uniformity. Primary geometrical properties of the frames were span length (L) and story height (H). Dimensions of the frame elements and longitudinal reinforcement in the columns were taken as secondary parameters expressed as moments of inertia and reinforcement ratios. All parameters were expressed as dimensionless in all possible cases (a= height to length ratio, b= ratio of moments of inertia of beam to column, g= ratio of column width to the thickness of masonry infill, rb = longitudinal reinforcement ratio of beam and rc = longitudinal reinforcement ratio of column). The material properties of the frames consisted of the concrete compressive strength (fck) and its modulus of elasticity (Ec) as well as steel yield strength (fy). The properties of the masonry infill were compressive strength (fk) modulus of elasticity (Ei)and thickness of the masonry infill wall (t). The applied lateral loading sequence in the tests was either static–monotonic, cyclic or seismic excitation and the chosen parameters in the EDIF database were horizontal loading(monotonic or cyclic) and axial vertical load (N) applied to the columns. For representing the behavior of the framed-masonry composite measured resistance envelope curve was replaced by bilinear curve obtained by the equal energy rule (Figure 1.). Two points on the idealized bilinear primary curve, important for description of the system behavior, could be obtained from the EDIF database by using the Artificial Neural Network algorithms trained on it, that is yield (Vy and dy) and capping point (Vu and du) as well as the Failure mode (FM). Figure1. Bilinear approximation of the primary curve in the EDIF database A statistical sensitivity analysis, based on the forward stepwise method, was used for elimination of the unimportant parameters in the learning process of the ANN (Kalman Šipoš at all, 2013). Forward stepwise method assesses the change in the mean squared error of the network by sequentially adding input neurons to the neural network (rebuilding the neural network at each step). According to the results of the ANN, the most important input parameters were: a= height to length ratio, rc = longitudinal reinforcement ratio of column and compressive strengths of concrete (fck) and masonry infill (fk). CALIBRATION AND VALIDATION OF THE NUMERICAL MODEL The behaviour of the “framed-masonry” structural elements has been analysed using the existing nonlinear element models used for modelling the frame and infill-panel elements. Its calibration was done on three samples taken from the EDIF database, and its validation was performed on the model of multi-storey multi-bay “framed-masonry” building structures tested in the European dy, Vy yield point du, Vu ultimate point 0 0,1 0,2 0,3 0,4 0,5 0,6 0 0,1 0,2 0,3 0,4 0,5 0,6 0 0,2 0,4 0,6 0,8 1 1,2 B a s e s h e a r r a ti o H /V Drift (%) Bare frame Infilled frame with hollow masonry units Bilinear aproximation Phase 2 Phase 3 P h a s e 1 T. Kalman Šipoš and V. Sigmund 3 laboratories. It has been proven that calibrated model could be applied with sufficient reliability, for the parametric analysis of the real “framed-masonry” buildings. All numerical analyses were performed by the Seismostruct v6.5 (2013). RC frame elements were modeled as force-based elements with concentrated plastic hinges at the ends. The nonlinear confined concrete model and the nonlinear model for reinforcing steel were used. The masonry infill wall was modeled as the infill panel model with calibrated parameters of hysteretic behavior. The initial diagonal width, w1, was determined according to proposal of Stafford Smith and Carter (1969). It is based on the parameter λh which presents a measure of the relative stiffness of the frame to infill. Reduced area of the compressed diagonal depends on the stiffness coefficient λh, according to recommendations of Decanini (1986). Corresponding deformations were determined according to the limit states: start of reduction of the initial area Ams1 corresponds to deformation at the end of linear elastic behaviour (εm/3) while Ams2 secondary area is reached at 70% of the maximum compressive stress and the associated strain corresponds to the 1.5×εm (Figure 2).