Introduction

The preservation of the architectural heritage presents one of the important challenges in civil engineering due to the complexity of the geometry of the structures, the variability of the materials used and the loading history of the buildings. Structures of historic buildings because of their nature and history present a number of typical aspects that limit the application of modern codes and building standards. The knowledge of behaviour of historic masonry under seismic action is fundamental to preserve the architectural heritage. During an earthquake, masonry walls are subjected to shear loads that often carry out to the ruin the whole building. The shear strength of unreinforced masonry (URM) was analysed in the last decades by experimental and theoretical works (Benjamin & Williams 1958; Hendry & Sinha 1969, 1971; Turnesek & Cacovic 1971; Yokel & Fattal 1976; Tomazevic 1977; Bernardini et al.1980; Hamid & Drysdale 1980; Calvi et al. 1985). On the base of theoretical and experimental analysis, the shear criterion for URM known as Coulomb-type failure criterion is adopted (EC6-ENV 1996). The shear failure widely, is characterised by joint failure as a function of the bond strength linked to the frictional resistance at brick-mortar interface and the compressive stress normal to the bed joints. Although many experimental results are available on shear strength of masonry as yet only few data are available for historic unrein-forced masonry walls (HURM) (Capozucca & Sinha 2004,2005,2007). In fact, shear behaviour of masonry has been investigated extensively for modern masonry. Moreover, between the different types of HURM, a scarcity of data is on multiple-leaf walls with two or three leaves. Multiple-leaf masonry is usual in historic towns and the most common cause of damage during an eartquake is actually due to a ruin of the buildings with this type of wall masonry (Pina-Henriques J. et al. 2004). In this paper, cyclic shear tests on HURM wall models with multiple-leaf masonry are shown and experimental results are discussed to investigate cracking patterns, failure mechanisms and the ultimate shear strength. Experimental results have been compared with those obtained from a non linear theoretical analysis carried out modelling multiple-leaf masonry wall with tie & truss finite elements.

2 EXPERIMENTAL MODEL 2.1 HURM wall specimens

To have the meaningful results, it was essential to have similar materials as used in the HURM. Luckily, a few full-scale solid bricks became available during the renovation of a 18th century Italian building, hence test specimens were built in 1/3rd scale utilizing these. The dimensions of the model bricks were 100 x 50 x 17 mm obtained from sawing the full-scale bricks. The average compressive strength ofthe model bricks was 34.3 N/mm2. 1:1:5 (cement: lime: sand) mortar was used for the construction of specimens.

The model was built with full masonry for flanges and multiple-leaf masonry for web (Fig. 1). The section is double T shape as shown in Figures 2(a) and (b); bricks in plane in flanges and on the list in the web (Fig. 2(a)) with intervals of bricks in plane as ties between two leaves (Fig. 2(b)). 1:5 (lime: sand) mortar has been placed between two masonry leaves of web.

A series of preliminary tests were done on small wallet specimens of HURM to obtain the compressive

Figure 3. Set-up of shear test and instruments.

Figure 2. Double T shape wall section of HURM model with (a) multiple-leaf masonry and (b) full masonry courses.

Table 1. Mechanical values of HURM by compression tests

Strength

Direction

Average values

young's modulus

of test

(N/mm2)

fy

Normal to

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