Masonry Buildings Under Seismic Actions

Abstract

This last chapter deals with the study of the seismic behaviour of historic masonry buildings. Starting point of the chapter is the remark that traditional masonry buildings have not been built to offer any resistance to horizontal actions. This is why most of the seismic damage occurs in old historic centres, as well as why there is currently such a great demand to determine the most suitable means to reinforce them. The first sections of the chapter are focused to point out that, contrariwise to steel or reinforced concrete structures, that can oppose the seismic action by using their ductility, masonry constructions don’t dissipate energy during their deformation, even if accompanied by cracks. If properly reinforced, to avoid early local failures, masonry constructions have the sole resource to escape the seismic action exhibiting rocking without failure, under alternate seismic action. A constant acceleration impulse, of a suitable duration, can represent the seismic action. A masonry pier wall, the basic resistant element of a masonry building, overturns under an acceleration impulse A _o of suitable duration t _o that turns out to be quite larger than the limit acceleration A _L producing the statical collapse. The magnitude of the so-called reduced strength factor q = A_o/A _L —the ratio between the above accelerations—can measure the actual capacity of the construction to follow the alternate seismic action exhibit rocking without overturning, over the whole duration of the quake. Due to the actual quite low values of this so defined reduced strength factor q, as shown in the chapter, the seismic protection of historic masonry constructions requires design criteria where strength has to be dominant. A first focal point is thus the analysis of the chain of transmission of the seismic forces along the resistant structure of the construction. The weak rings of this chain are thus pointed out: they are due to the natural lack of connection among the various components of the building structure. Suitable primary reinforcements, discussed in the chapter, have to be inserted in the structure to ensure that early local failures cannot occur. The out-plane and the in-plane strength of masonry walls is then evaluated, with new elaborations and inclusions. All the results presented have been obtained in the framework of the Limit Analysis of masonry structures, according to the approach followed in the book. Numerical examples and comparisons with Code prescriptions are given.