Multi-leaf stone masonry walls are highly vulnerable under seismic actions and prone to brittle collapse. The scarce connection among the leaves, the presence of voids in the inner core, the inhomogeneity of materials and the irregularities in the arrangement of stones and mortar are the main causes of weakness for this typology (Giuffrè et al. 1993; Binda et al. 1997). Failure occurs at low level of horizontal loads, by out-of-plane expulsion of material, which may involve the outer leaves in a various extent, or even the whole wall.

Interventions are aimed at improving the collaboration among the leaves, to provide a monolithic behaviour in the wall section, by injections or transverse ties (Vintzileou et al. 1995; Valluzzi et al. 2004). Transverse ties are able to counteract directly the out-of-plane forces, thus limiting the relative movements between the outer leaves (Valluzzi et al. 2004; Mazzon 2010).

Although the problem of multi-leaf irregular stone masonry walls has been tackled by several researchers, dynamic tests reproducing the effect of seismic actions on single stone masonry walls are very limited in literature (Elmenshawi et al. 2010). As a matter of facts, research is mainly focussed on dynamic behaviour of reduced scale buildings (Tomaževic et al. 1989; Benedetti et al. 1998; Mazzon et al. 2009) or, in case of single walls, they manly refer to brickworks (ABK 1981; Meisl et al. 2007) or to the use of concrete blocks (Simsir et al. 2004).

To investigate the dynamic and mechanical behaviour of three-leaf rubble stone masonry, an experimental campaign based on shaking table tests on full-size single walls was performed. The masonry was tested under out-of-plane actions in plain and strengthened conditions, considering injections and transverse steel ties, also used in combination. Two panels having dimensions (hxbxt) 260x130x50 cm³ were tested for each condition. The external leaves of the wall were made of irregular ashlars bonded by mortar, whereas the inner core was composed by small pieces of stones mixed with mortar poured in non-continuous strata, to provide a weak core and no connection among the layers.

Shaking table tests were performed at the ENEA Laboratory in Casaccia (Rome) using a test rig designed on purpose. According to the response spectra compatible with the design spectrum of actual codes, the Montenegro earthquake was selected.

Results showed that the brittle out-of-plane behavior of three-leaf rubble masonry walls can be effectively improved by transverse tying and grout injections. In fact, the collapse of plain walls involved only a single leaf. Transverse tying and/or injections modify significantly the failure mechanism: the use of steel ties improved the monolithic behaviour of the element, limiting the separation of layers and the area in which the damages occurred. Furthermore, the use of hydraulic lime based grout enhanced the overall performance, since a monolithic behaviour could be achieved even up to the collapse.

Finally, the use of a combined intervention, that is transverse steel ties and grout injections, allowed the failure to be avoided.

REFERENCES

ABK, 1981. Methodology for mitigation of seismic hazards in existing unreinforced masonry buildings: wall testing, out-of-plane. ABK Topical Report 04. ABK.

Benedetti, D., Carydis, P., Pezzoli, P., 1998. Shaking table tests on 24 simple masonry buildings, Earthquake Engineering & Structural Dynamics, 27(1), 67–90.

Binda, L., Modena, C., Baronio, G., Abbaneo, S., 1997. Repair and investigation techniques for stone masonry walls. Construction and Building Materials, 11(3), 133–142.

Elmenshawi, A., Sorour, M., Mufti, A., Jaeger, L.G., Shrive, N., 2010. Damping mechanisms and damping ratios in vibrating unreinforced stone masonry. Engineering Structures, 32, 3269-3278.

Giuffrè, A., Baggio, C. and Carocci, C., 1993. Sicurezza e conservazione dei centri storici, Laterza, Bari.

Mazzon, N., 2010. Influence of grout injection on the dynamic behaviour of stone masonry buildings. Ph.D. Thesis, University of Padova, Italy.

Meisl, C., Elwood, K. and Ventura, C., 2007. Shake table tests on the out-of-plane response of unreinforced masonry walls. Canadian Journal of Civil Engineering, 11(34), 1381–1392.

Simsir, C. C., Aschheim, M. A. and Abrams, D. P., 2004. Out-of-plane dynamic response of unreinforced masonry bearing walls attached to flexible diaphragms. 13^th World Conference on Earthquake Engineering, Vancouver, Canada.

Tomaževic, M., Modena, C., Petcovic, C. and Velechovsky, T., 1989. Shaking table study of an unreinforced-masonry building model with an internal cross-shaped wall - test results. Technical report, Report ZRMK. Ljubljana, Slovenia.

Valluzzi, M. R., da Porto, F., Modena, C., 2004. Behavior and modeling of strengthened three-leaf stone masonry walls. Materials and Structures, 37(3), 184–192.

Vintzileou, E. and Tassios, T. P., 1995. Three-leaf stone masonry strengthened by injecting cement grouts. Journal of Structural Engineering, 121(5), 848–856.