INFLUENCE OF PANEL-FRAME INTERFACE BOND ON THE STRUCTURAL BEHAVIOUR OF INFILLED FRAMES

Abstract

For multistoreyed buildings a rigid frame usually combined with reinforced concrete or masonry shear walls is the most com-monly employed and efficient structural system. In the traditional design of rigid frames, the beneficial effect of masonry infilled walls, which are necessary for the subdivision of internal space or external cladding of the building, is ignored., the infill being treated as non-structural element. Behaviour of such buildings during past earthquakes has, however, shown that the frame and the infill do interact and their composite behaviour may be quite different from that of the frame alone. The infill increases the stiffness of the bare frame in a significant way. Many researchers have investigated this problem, and have come out with different structural idealization ranging from very simple equivalent strut approach to more exact application of the finite element method for the analysis. The simpler methods like equivalent strut method have been shown to predict the behaviour of the infill-frame struc-ture more accurately after the infill has separated from the frame at the corners along tension diagonal. The finite element method on the other hand is able to simulate the behaviour of the frame-infill structures if suitable idealisation of the interface behaviour is done in the analysis. The interface characteristics for masonry infills in rein-forced concrete frames have not been properly defined yet. The present study is an attempt to study the behaviour of masonry infill-reinforced concrete frames by suitable idealisation of the bonding medium. Effect of two types of bondings at the interfce has been studied. They are (i) cement paste. (ii) epoxy resin-sand mortar. Botl, experimental and analytical ,investigations have been carried out. In the analytical study, the infill and the frame have been idealized by two dimensional parabolic type elements. Interface (iv) ) between the frame and the infill has been idealised by six noded interface element. A suitable computer program EAIF has been devel-oped to facilitate computer aided analysis of the infilled frame using the proposed finite element formulation. The infilled frame has also been analysed by the equivalent strut method. For experimental investigations of the effect of the interface bond between the frame and the infill, a two storey single bay R .C. frame with masonry infills using two different types of interface bonding materials i .e . cement paste and epoxy resin-sand mortar, has been constructed and tested under vertical and lateral loads. Interface shear stiffness has also been determined experimentally by conducting direct shear tests on 310x310x250 mm size couplet type specimens. The study shows that the proposed finite element method formulation with inter-face elements having experimentally determined characteristics, predicts the behaviour of the infilled frames quite accurately. By comparison, suitable recommendations have also been made as regards the choice of dimensions of the equivalent struts for the panels for an accurate assessment of stiffness behaviour, of the structure. The capability of the masonry infills to share lateral loads with the frames has also been established