Seismic Reliability of Masonry Structures Strengthened with FRP Materials

The problem of the seismic reliability assessment of old masonry wall systems strengthened with fiber reinforced polymers (FRP) is of great practical concern. Recently, the use of these high strength composite materials, like Carbon or Glass fiber composites, has considerably increased in the field of structural repair. The applications in the field of architectural heritage are mainly intended to prevent the loss of the construction (with its frescoes, ornaments, sculptures), as a result of exceptional actions like earthquake. However, the application of FRP external reinforcements with elastic-brittle behaviour on low ductility structural elements like masonry walls induces a further reduction of the componential ductility, moving the system toward an ideal elastic-brittle behaviour. This involves a careful examination of the brittle components “bundle effect” in order to evaluate the change in the reliability of the structure as a whole. In this study, a great number of existing ordinary masonry buildings located in Italy are considered and the wall organizations are examined in detail. The selected buildings are sorted within different architectural typologies in order to have a representative sample of structural systems, show structural regularity along the height, and count typically one or two rigid floor levels allowing for proportional load sharing among the structural walls in the elastic range. The ultimate strength and displacement of wall systems is evaluated by means of a displacement driven non linear analysis (pushover analysis). Then, the structural strengthening with elastic brittle behavior like FRP strips is considered, and the increased shear strength of each masonry wall is provided, computing finally the building force-displacement relationship up to failure. The statistical analysis of the population’s performance points out with strong evidence the conflicting effect of strength and brittleness of FRP materials in enhancing the seismic reliability of complex low strength stone and masonry structures. As this preliminary study shows, the effective safety increment of the repaired system is seriously limited by its reduced ductility. The presented results seem to predict that very small reliability increments can be produced even applying huge FRP reinforcement ratios, so that for a class of buildings characterized by very low ductility, the strengthening with FRP is simply unfeasible.