Static cyclic response of masonry walls retrofitted with fiber-reinforced polymers

The behavior of seven one-half scale masonry specimens before and after retrofitting using fiber-reinforced polymer (FRP) is investigated. Four walls were built using one-half scale hollow clay masonry units and weak mortar to simulate walls built in central Europe in the mid-20th century. Three walls were first tested as unreinforced masonry walls; then, the seismically damaged specimens were retrofitted using FRPs. The fourth wall was directly upgraded after construction using FRP., Each specimen was retrofitted on the entire surface of a single side. All the specimens were tested under constant gravity load and incrementally increasing in-plane loading cycles. The tested specimens had two effective moment/shear ratio, namely, 0.5 and 0.7. The key parameter was the amount of FRP axial rigidity, which is defined as the amount of FRP reinforcement ratio times its E modulus. The single-side retrofitting/upgrading significantly improved the lateral strength, stiffness, and energy dissipation of the test specimens. The increase in the lateral strength was proportional to the amount of FRP axial rigidity. However, using high amount of FRP axial rigidity led to very brittle failure. Finally, simple existing analytical models estimated the ultimate lateral strengths of the test specimens reasonably well.

Shear strength of URM walls retrofitted using FRP

Marc Badoux, Mohamed Elgawady, Pierino Lestuzzi

This paper compares different models currently used to calculate the shear strength of unreinforced masonry (URM) walls retrofitted using fiber reinforced polymers (URM-FRP). The shear strengths of six recently tested URM-FRP walls were compared to shear strengths predicted by the models herein. Four of these specimens were tested under constant gravity load and incrementally increasing in-plane loading cycles. The other two specimens were tested on a uniaxial earthquake simulator. The specimens were subjected to synthetic earthquake motions with increasing intensity. Each specimen was retrofitted on the entire surface of a single side using FRP with different axial rigidities. One of the shear strength models compared in this study has been recently developed by the authors: The model was explicitly developed to predict the shear strength of unreinforced masonry walls retrofitted using FRP. The model idealized masonry, epoxy, and FRP in a URM-FRP as. different layers of isotropic homogeneous elastic materials. Then, using principles of the theory of elasticity, the governing differential equation of the system is formulated and linearly solved. Then, the material nonlinearity was implemented via a step-by-step degradation in the layer stiffness; after each step the equations were resolved linearly. In most cases, failure occurred in either the masonry or the epoxy and in no case did FRP reach its ultimate load. Comparisons between the different shear models showed that the authors' model is more conservative than the other existing models. In addition, for a small FRP axial rigidity, the difference between the models was insignificant. However, with increasing FRP axial rigidity the differences between the models became more significant. This paper highlighted the advantages and disadvantages of each model. It was found that the authors' model offered several advantages over the other available models. However, the authors' model also has its own disadvantages and limitations. One of these limitations is that it does not explicitly take into consideration the out-of-plane normal stresses. Finally, additional experimental verification of the authors' model is recommended. (c) 2006 Elsevier Ltd. All rights reserved.

Elsevier2006

Seismic in-plane behavior of URM walls upgraded with composites

Mohamed Elgawady

Existing unreinforced masonry (URM) buildings, many of which have historical and cultural importance, constitute a significant portion of existing buildings around the world. Recent earthquakes have shown the vulnerability of such URM buildings. This thesis investigates the in-plane seismic behavior of URM walls retrofitted using composites. The thesis includes an extensive dynamic and static cyclic tests followed with development of an analytical model. For the dynamic tests, five half-scale single wythe URM walls were built using either strong or weak mortar and half-scale hollow clay brick units. These five walls were dynamically tested as reference specimens. Then, these reference specimens were retrofitted on single side only using composites and retested. As consequence a total of eleven specimens were tested on the earthquake simulator at ETHZ. For the static cyclic tests, five half-scale single wythe URM walls were built using weak mortar and half-scale hollow clay brick units. Of them, three specimens were tested as reference specimens. Then, two specimens of these three reference specimens were retrofitted using composites and tested again. The third reference specimen was retrofitted using post-tensioning and tested; then, the post-tension forces were released and the specimen was retrofitted using composites and retested. Finally, two virgin specimens were retrofitted directly after construction and tested. As consequence a total of nine specimens were tested in the Structural Laboratory at EPFL. For analytical models, an innovative shear model is developed. In addition, a simple flexural model is developed. For shear analysis, masonry, epoxy, and composites in a URM wall retrofitted using composites (URM-FRP) were idealized as different layers with isotropic homogenous elastic materials. Then, using principles of theory of elasticity the governing differential equation of the system is formulated. A double Fourier sine series was used as the solution for the differential equations. The solution can be used to model the linear shear behavior of URM-FRP. To take into consideration material nonlinearity, step-by-step stiffness degradation has been implemented in a computer program. For flexural analysis, a simple model using linear elastic approach with the well-known assumptions of Navier-Bernoulli and Whitney's equivalent stress block is developed. The experimental work shows that the retrofitting technique improved the lateral resistance of the URM walls by a factor ranged from 1.3 to 5.9 depending on the applied normal force, the reinforcement ratio, and mode of failure. However, improvement in lateral drift was less significant. Moreover, no uneven response was observed during tests due to single sided retrofitting. Several phenomena and relationships have been correctly determined by the model. These phenomena and relationships are originally observed in the literature during tests on reinforced concrete beams that were retrofitted using composites. This includes the relationship between strains in FRP and reinforcement ratio as well as the interaction between masonry lateral resistance and FRP contribution to the lateral resistance of URM-FRP. In addition, effects of epoxy ductility and allowable shear stresses as well as masonry ductility and allowable shear stresses have been studied. Such development is of interest to the structural engineering community and material producers. Regarding flexural analysis, the simple model leads to unconservative designs. Correlation analysis of the test data show that the ratio between the experimental lateral resistance to the estimated flexural lateral resistance is proportional to reinforcement mechanical ratio times the square of the effective moment/shear ratio up to a certain limit. Within the limits of experimental testing, a correlation factor is proposed.

EPFL2004

Quasi-static cyclic tests of two prefabricated, reinforced masonry walls

Katrin Beyer, Alessandro Dazio, Michael Markus Rupf

In the second half of the 20th century, the majority of residential buildings in Switzerland have been built with unreinforced brick masonry walls and reinforced concrete floors. Following a re-evaluation of the seismic hazard in Switzerland, a country of moderate seismicity, the seismic design spectra have increased in the last revision (2003) of the Swiss building code. As a consequence, it has become very difficult to justify the use of unreinforced masonry walls as sufficient seismic resistance elements. In many new residential buildings, structural walls are constructed as reinforced concrete walls instead of unreinforced masonry walls although the latter provide better thermal and insulation properties at a lower cost. Reinforced masonry systems, which could constitute a viable alternative to reinforced concrete walls, are rarely used within Switzerland. The objective of this research project was therefore to develop a reinforced masonry system unlike most reinforced masonry systems that are available on the international market today. It was decided to develop a prefabricated wall system that would require less construction time and allow an easier introduction to the Swiss market. The most critical construction detail is the connection between the prefabricated wall and the foundation. In a preliminary study two different connection details were developed and their functionality tested by means of quasi-static cyclic tests of full-scale wall elements. This paper summarises the details of the system, the test setup, the instrumentation and the test results. The tests were able to show that the proposed systems achieve similar displacement ductilities as ductile reinforced concrete walls. Finally, recommendations for further improvement of the suggested prefabricated reinforced masonry wall system are made.