Abstract
Non-structural masonry infills in existing reinforced concrete (RC) frame structures are known to affect their seismic behaviour significantly with potential detrimental effects. Increasing experimental evidence is available for the use of composite materials, such as fibre-reinforced polymers (FRP) and textile reinforced mortars (TRM), for in-plane retrofitting of brittle masonry infills. In order to apply such strengthening solutions in practice, adequate analytical models for predicting the behaviour are needed, however the large variation in infill properties already makes modelling the behaviour of non-retrofitted infills a challenge. Based on existing experimental and numerical studies on retrofitted infills, a macro-model is proposed, comprising a tie to account for the tensile strength of the composites materials, but also an increased compressive strut width due to the composite materials improving connection of the infill to the frame. After compiling a database of experimental data for composite retrofitted specimens tested in the literature, empirical equations for tie and strut strength was obtained. These equations constitute the first unified approach for FRP- and TRM-strengthened infills and were verified against the largest database of experimental results to date. The strut model was calibrated for the increase in strut width in terms of experimentally obtained stiffness increase, while the tie model was determined based on the remaining increase in strength. The empirical equations were shown to achieve a relatively high correlation with experimental results and to represent the mechanics of tested specimens well in terms of observed damage, hence indicating their potential for use as design-oriented equations for composite strengthened infills.
6. Conclusions
A study on a unified design-oriented macro-modelling approach for masonry-infilled RC frames retrofitted with composite materials was presented. It was shown that a variety of materials and layouts have been tested experimentally and yielded satisfactory strengthening results. Damage to the infills can be reduced significantly and larger lateral forces can be sustained. In terms of modelling the retrofitted infills, a new expression for compressive strut width increase due to a better connection between frame and infill was proposed and applied together with a tensile tie.
Using a database of experimental results gathered for this study, empirical equations for the strut and tie model were determined. First, an empirical equation was proposed for the strut width increase based on the increase in secant stiffness for experimentally tested specimens. Based on this new strut width equation, a tensile tie was defined based on the remaining strength increase for the experimental specimens. The equations for normalised strut and tie strength were defined in terms of the effective mechanical reinforcement ratio. The proposed model was found to reproduce experimental findings well in terms of the increase in strength. For the empirical equations relating to the stiffness of retrofitted infills, however, the fit was significantly worse.
