Keywords: masonry buildings, seismic vulnerability, strengthening techniques.

Old load-bearing masonry buildings exist all around the world, with special significance in urban historical city centres. These buildings, besides their patrimonial, cultural and architectural heritage value, frequently present a high level of degradation, urging for the need of conservation and strengthening actions. Recently, the consciousness of the public opinion became aware of to this need through the creation of safeguarding and preservation policies for the architecturally valued buildings and urban aggregates. The inoperativeness of the responsible agents and the lack of strategies and policies in the last half of the twentieth century in this domain drove the built urban stock to a situation of deep degradation in a great number of historical centres [1]. A great percentage of the built urban stock of the historical city centre of Coimbra is constituted by buildings dating from the seventeenth to the mid twentieth century, most of them built without any earthquake resistant design (no specific construction rules). In areas prone to seismic action (Central and Southern Portugal), the need to take the preventive measures of structural strengthening to minimise the damage, or avoid losses of incalculable value is surely a priority. Such measures require the previous evaluation of the expected seismic response through modelling representative buildings of this type of construction.

In this paper a study is presented of an aggregate of four typical masonry buildings representative of the constructive typology of the old masonry buildings in Coimbra, Portugal. In this study numerical analyses were performed of four buildings and the efficiency of three typical strengthening techniques was tested. Finally, the seismic behaviour is discussed, structural fragilities are identified and the efficiency of the strengthening techniques studied concerning cost analysis. To understand the behaviour of the old constructions, an aggregate of four buildings was modelled with a finite element tool. The results of these models will aid in the identification of fragile areas of the buildings and in the vulnerability evaluation of the aggregate. With this numerical analysis it is intended: i) to estimate the natural frequencies and vibration mode shapes, for the original structure and for different strengthening solutions; and, ii) to understand the global seismic response of the structure through global parameters in terms of top-displacements and drifts. Rehabilitation and structural interventions to improve the seismic behaviour of traditional masonry buildings should respect the original building materials and construction techniques [2]. The numerical model developed was also oriented in the sense of evaluating suitable strengthening solutions. Three strengthening solutions were modelled intending to reduce the building's seismic vulnerability namely: floor stiffening, tie-rods and masonry consolidation.

The high number and dimensions of openings at ground floor highly influence the wall façades deformation and stress concentration for earthquake acting in the longitudinal direction. Inter-storey drifts are rather high at ground level, which can originate a soft-storey mechanism. Openings enlargement or suppression of masonry walls at ground floor, for example, to install commercial spaces or garages is an inadequate practice in old buildings that should not be overlooked. The asymmetry of the total area of openings between the front and rear façades induces a global torsion of the group of buildings studied. However, it is recognised that the global behaviour of the overall aggregate, where the buildings are included, attenuates the torsional effects mentioned. The masonry walls are very vulnerable to the out-of-plane deformations. The connection to the floor timbered structures with orthogonal walls are important measures to reduce its vulnerability to the out-of-plane collapse mechanisms, which are particularly important for higher floor levels, as observed in the displacement profiles.

1

Vicente, R.S.; Varum, H.; Silva, J.A.R., "Seismic vulnerability assessment of buildings in the old city centre of Coimbra", International Conference 250th Anniversary of the 1755 Lisbon Earthquake, 1-4 November 2005.

2

Giuffrè, A., Editore Laterza & Figli Spa, Sicurezza e Conservazione dei Centri Storici, Il caso Ortigia, Rome-Bari, 2000 [in Italian].

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