Structural Analysis of Historic Construction – D’Ayala & Fodde (eds) © 2008Taylor & Francis Group, London, ISBN 978-0-415-46872-5 Experimental and numerical study on the application of the flat-jack tests to masonry walls M. Acito, L. Binda & G. Cardani Structural Engineering Department, Politecnico of Milan, Milan, Italy J. Guedes & L. Miranda Department of Civil Engineering, Faculty of Engineering, University of Porto, Portugal ABSTRACT: Double flat-jack tests were performed on a brick masonry wall using a series of LVDT’s to monitor the distribution of displacements in the wall above the top flat-jack. The results allowed estimating: (i) the way the stress induced by the flat-jacks is distributed in the wall; (ii) the main direction of the joints openings/cracks that in fact extend to the sides of the flat-jack. The analysis of these results shows that actually a volume of wall larger than the one above the top flat-jack is directly involved in the test. A numerical simulation of the flat-jack test using a finite element code was also performed. 1 INTRODUCTION The double flat-jacks test is used to detect the stress- strain behaviour of a masonry wall. The experimental research carried out by the authors had the aim of studying the strain diffusion in the masonry, when the double flat-jack is performed. The research was part of an experimental work per- formed on two masonry walls of the Museum of Sci- ence and Technology “Leonardo da Vinci” in Milan, in the building of the “Cavallerizze”, the riding-stables built by Napoleon’sArmy (Fig. 1). The diagnostic investigation was aimed to the mor- phological and structural characterization of the Cav- allerizze masonry walls, carried out by the DIS and supported by the Ministry of Cultural Heritage. The flat-jack test was calibrated for masonry by P.P. Rossi (1982) starting with the use of a single flat-jack for the local state of stress determination. Furthermore the double flat-jack test was developed in order to detect the stress-strain behaviour under compression stress. A large number of applications of the flat-jack test is reported in literature (Binda et al. 1983, Ronca et al. 1997, Binda et al. 1999, Binda et al. 2004, Binda et al. 2007). The test was codified by ASTM C1196-1991 and recommended by RILEM LUM.D.2-1990; now it is also recommended by the new Italian Seismic Code since 2003, as part of the on site investigation on existing masonry buildings. The test with a single flat-jack, is based on the stress relaxation due to a cut made in a compressed masonry Figure 1. Milan: part of the museum with the riding-stables “Cavallerizze”. (Fig. 2a). A jack is then inserted into the cut and oil is pumped in it until the original geometry is recovered. The state of stress is calculated from the pressure given to the jack. In the double flat-jacks tests, a second cut is released in another bedding joint, parallel to the first one and a second jack is inserted, at a distance of about 40 to 50 cm from (Fig. 2b). The two jacks delimit a wall specimen (WS) of appreciable size to which a uni-axial compression stress can be applied. Displace- ments transducers (LVDTs) applied to the WS face provide information on vertical and lateral displace- ments. Loading cycles can be performed at increasing stress levels in order to determine the deformability modulus of the masonry in its loading and unloading phases. It is important to compare these last results to the stress level measured with the single flat-jack, 875