Micro-Modeling Options for Masonry

Introduction

Masonry is one of the oldest building materials. It is composed of masonry units (i.e. brick, blocks) bonded with or without mortar. Despite its simplicity of construction, the mechanical behavior of masonry remains a challenge. Masonry is a heterogeneous anisotropic material where mortar joints act as a plane of weakness. The failure mechanism of the material includes tensile failure of units and joints, shear failure of joints and compressive failure of the composite. At low levels of stress, masonry behaves as a linear elastic material. Its behavior becomes increasingly non-linear as the load applied on it increases and cracks develop and propagate. Cracking in masonry structures may be induced by deformation in bending/shear or volumetric changes of the component bricks, blocks, or mortar arising from natural expansion or shrinkage, temperature change, corrosion, or associated reactions (Cook & Pegam 1993; Sarhosis et al. 2015). Experience demonstrates that many masonry constructions have collapsed in the past. In Europe, there are several examples demonstrating this, like the ones related to the earthquake of Lisbon in 1755, where several monuments and hundreds of constructions were heritage and collapsed. Fatigue and strength degradation, accumulated damage due to traffic, wind and temperature loads, soil settlements and the lack of structural understanding of the original constructors are some of the factors that contribute to the deterioration as well as the continuous degradation of masonry structures. The loss is even more dominant when damage occurs at historic and cultural structures where damage is most of the time non-reversible. Recent examples of losses of the cultural and architectural damage can be found, for example, in Italy: Campanile of St. Marcus in Venice (total collapse in 1902 after being repeatedly struck by lightning), Civic Tower of Pavia (total collapse in 1989 with hardly any warning) and Cathedral of Noto (collapse of the dome in 1996). Famous examples of historical constructions in risk due to soil settlements are the Cathedral of Mexico City and the Tower of Pisa, and constructions in risk due to a deficient structural conception are the Cathedral of Pavia and the Cathedral of Florence (Lourenço, 2002).

Research is needed to be able to understand the behaviour of masonry construction, exhibiting highly non-linear characteristics. In particular it is important to understand the pre- and post-cracking behaviour to inform decisions concerning the maintenance needs, management of safety risks, assessment of levels of safety and the need for repair or strengthening. As experimental research is prohibitively expensive, it is fundamentally important to have available a computational model that can be used to predict the in-service and near-collapse behaviour with sufficient reliability. Once such a model has been established, it can be used to investigate a range of complex problems and scenarios that would not, otherwise, be possible.

A review of the current strategies for modelling structural masonry will be given with an emphasis on those considered to be appropriate for modelling low bond strength or dry joint masonry. The different available approaches are considered and compared with regard to their ability to define the initial state of the structure, realism in simulation, computer efficiency and data availability for their application to model masonry structures.