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Determination of Behaviour and Strength of Non-Seismically Designed R/C Frames with Masonry Infill

Reinforced-concrete frames with masonry infill (“framed-masonry”) are composite structural element commonly used in Southern Europe. Its application started in Europe after the World War 2 and remained, up to date, as the most common bearing system in low and medium-high structures built in seismic areas. Field observations after recent strong earthquakes (Kocaeli 1999., Wenchuan 2008., Christchurch 2011.) showed that masonry infill had positive and negative influence on the seismic behaviour of infilled frames. The presence of masonry infill significantly changed structural characteristics of frame buildings by increasing its stiffness and strength, especially in the range of small deformations. It also reduced drift capacity of the system, while on the other hand increased base-shear response which could lead to serious damage and creation of the new failure mechanisms ultimately leading to structural collapse. Modern seismic codes usually treat masonry infill as non-structural element and provide only recommendations related to reinforcement detailing in the frames and distribution of masonry infill in plan and elevation of the structure. They enable collapse prevention of the newly constructed buildings, but do not prevent infill’s damage and do not use the infill’s positive effect. Thus, structural reparations after the earthquake could be very costly. Evaluation of the infill’s contribution to the structural behaviour during earthquakes is in the design phase and in the evaluation phase completely neglected. Non-seismically designed frames (frames that are not designed according to modern or any seismic codes), represent a special case that needs particular attention. They represent most buildings of the present building stock in Southern Europe. These buildings have some typical deficiencies among which are: use of smooth reinforcing bars, inadequate anchorage of beam longitudinal reinforcement in the column, lack of joint transverse reinforcement, inadequate amount and spacing of transverse reinforcement in the column, etc. Presence of masonry infill modifies and magnifies the shear demand on the frame columns by shortening distance between plastic hinges, thus potentially leading to the creation of the short column effect. In that case, unbraced length of column defines strength and drift capacity of the system. The main question that we would like to answer within the scope of our work is: „Does the unbraced length of column depends on the internal friction angle of masonry infill?“. In order to answer that question we divided our investigation in experimental and numerical parts. Experimental part has two stages in which we want to valorise the influence of different parameters of the masonry and geometrical properties of frame columns to the behaviour of the system. The contribution of individual components to drift, strength and failure mode would be observed. The masonry infill parameters are: strength and stiffness of the masonry infill and the internal friction angle. Both of them cloud be obtained from the so-called RILEM tests (compressive test and direct shear test of masonry infill). The influence of geometrical properties of the frame columns along with different masonry infill parameters would be observed on the ten scaled “framed-masonry” models that would be tested under constant vertical and cyclic lateral loads. Some of the tests have been finished and some are underway. All scaled models represent one-storey one-bay specimen having constant frame aspect ratio of 1, 33. They are loaded with a constant vertical force of about 30% of the compressive strength of the concrete applied at the columns ends, and are subjected to horizontal in-plane cyclic loading up to the failure. The aim of this experimental research part is to quantify contribution of the mentioned parameters and to create database that will be used for calibration of numerical models. The subsequent numerical modelling will examine the contribution of parameters that have not been investigated experimentally, such as: different height to length ratio, presence of neighbouring bays, additional stories, etc. Final results of the overall research, complemented with additional results within the bigger research project, would enable the “framed-masonry” structures to be properly evaluated for their seismic capacity and vulnerability.

Non-Seismically Designed R/C Frames; Masonry Infill; strength; stiffness; vulnerability; experimental analysis

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