Collapse prediction and creep effects

The recent collapse of famous historical constructions attributed mainly to the time-dependent behaviour of masonry has driven the attention of the technical community over this issue. Numerical analyses in which units and mortar are individually represented have proven to be of great interest to understand the phenomena at the level of the masonry constituents. Nevertheless, before analysing the influence of long-term effects, it is important that numerical models are able to adequately reproduce the behaviour under short-term compression as it provides a solid basis to correctly capture the response under sustained stresses. Reproduction of short-term behaviour remains, however, unresolved in literature. A contribution is given in the present paper by considering a standard continuum model and a discrete particle model to represent units and mortar. The particle model has showed clear advantages. In addition, the results of an experimental investigation on the creep behaviour of regular ancient masonry including both short-term and long-term creep tests are provided together with a careful discussion of the results

Masonry compression : a numerical investigation at the meso-level

The analysis of masonry assemblages under compression using detailed modelling strategies in which units and mortar are modelled separately is a challenging task. Sophisticated standard non-linear continuum models, based on plasticity and cracking, are widely available to represent the masonry components but such models overestimate the experimental strength of masonry prisms under compression. Alternative modelling approaches are therefore needed. This paper focuses on the discussion and detailed analysis of a particle model consisting in a phenomenological discontinuum approach to represent the micro-structure of units and mortar. The micro-structure attributed to the masonry components is composed by linear elastic particles of polygonal shape separated by non-linear interface elements. All the inelastic phenomena occur in the interfaces and the process of fracturing consists of progressive bond-breakage. Clear advantages have been shown by the particle model, when compared to standard continuum models.Portuguese Foundation for Science and Technology - SFRH/BD/5002/2001

Validation of analytical and continuum numerical methods for estimating the compressive strength of masonry

The advances in computational mechanics witnessed in the last decades have made available a large variety of numerical tools. Sophisticated non-linear models are now standard in several finite element based programs. This paper addresses the ability of continuum numerical methods, based on plasticity and cracking, as well as on analytical methods to provide reliable estimations of masonry compressive strength. In addition, a discussion on the load transfer between masonry components is presented and special attention is given to the numerical failure patterns. The results found overestimate the experimental strength and peak strain. Alternative modelling approaches that represent the micro-structure of masonry components are therefore needed

Masonry micro-modelling adopting a discontinuous framework

Several continuous and discontinuous micro-modelling approaches for masonry assemblages are currently under investigation by the authors. The aim is to find a model that provides a suitable description of the material behaviour under compression. Firstly, the behaviour under short-term loading is being considered and, in a later stage, also long-term loading will be addressed. The present paper illustrates the recent advances in the research, focused on the assessment and application of a proposed discontinuous model to the simulation of uniaxial compression tests of masonry prisms. In this model, a fictitious micro-structure composed by linear elastic particles separated by non-linear interfaces is adopted to model units and mortar. The main results obtained and their critical discussion are given in the paper

Testing and modelling of masonry creep and damage in uniaxial compression

The problems related to the analysis of ancient constructions are gigantic due to the difficulties in characterizing the geometry, the materials, the sequence of construction, the existing damage and the building processes.A difficult aspect in the repair and strengthening of existing structures is the long term loading of masonry in compression. Safety assessment has been greatly influenced by the collapse of monumental buildings in the last decade, where creep behaviour and creep-fatigue interaction proved to be of relevance for massive masonry walls. Experimental tests on masonry creep have been recently carried out, together with numerical implementations. Both testing and numerical modelling of multi-leaf masonry walls need still more research.Experimental testing on the creep of regular ancient masonry together with numerical modelling is currently under investigation and is briefly addressed in the paper. In addition, a discussion of the load transfer between masonry components is given. This represents a contribution for understanding creep behaviour and load transfer mechanisms in multi-leaf walls.- (undefined

Alvenaria em compressão: comportamento à rotura e efeitos diferidos

Tese de doutoramento em Engenharia Civil.The work presented in this thesis has been developed at the Department of Civil Engineering of University of Minho, Portugal, and at the Department of Structural Engineering of Politecnico di Milano, Italy. Recent sudden collapses of famous historical buildings, as the Pavia Civic Tower in 1989 or the Noto Cathedral in 1996, has drawn the attention of researchers to the compressive behaviour and time-dependent effects of heavily stressed masonry structures, with an emphasis in multiple-leaf pillars and walls, as this typology is frequently found in historical centres. The objective of this research is to contribute to the present state of knowledge in these fields. For masonry under compression, a validation of simple analytical methods and nonlinear continuum simulations based in plasticity and cracking has been performed. In fact, sophisticated non-linear models are now standard in several finite element based programs but the ability of such models to predict the compressive strength of masonry based on the properties of the constituents, units and mortar, is still an insufficiently debated issue. In the present study, the results obtained using non-linear continuum models and simple analytical methods, based on elastic considerations, have been compared with experimental results available in literature. A clear overestimation of the experimental strength by both numerical models and analytical methods was found, except by the empirical formulas provided by the European and North-American codes, which underestimated the experimental strength. Alternative modelling approaches that account for the discrete nature of masonry components are therefore of interest, in order to provide reliable estimations of masonry compressive strength. A detailed analysis of a particle model consisting in a phenomenological discontinuum approach to represent the micro-structure of units and mortar was therefore addressed. The micro-structure attributed to masonry components is composed by linear elastic particles of polygonal shape separated by non-linear interface elements. All the inelastic phenomena occur in the interfaces and the process of fracturing consists of progressive bond-breakage. Clear advantages have been shown by the particle model, when compared to continuum models.O trabalho apresentado nesta tese foi desenvolvido no Departamento de Engenharia Civil da Universidade do Minho e no Departamento de Engenharia Estrutural do Politecnico di Milano, Itália. O colapso súbito de algumas construções históricas famosas ocorrido recentemente, como por exemplo a Torre Cívica de Pavia em 1989 e a Catedral de Noto em 1996, despertou o interesse da comunidade científica sobre o comportamento à compressão e efeitos diferidos em estruturas de alvenaria sob estados de compressão muito elevados, nomeadamente em paredes e pilares de alvenaria composta, dada a importante presença deste tipo de elementos em centros históricos. Este trabalho tem como objectivo contribuir para o presente estado de conhecimento nestas áreas. No caso da alvenaria submetida a esforços de compressão, procedeu-se à validação de métodos analíticos simplificados e de modelos não lineares contínuos, baseados em plasticidade e fendilhação. De facto, modelos não-lineares sofisticados são hoje correntes em diversos programas de elementos finitos. No entanto, a capacidade desses modelos em estimar correctamente a resistência à compressão da alvenaria, com base nas propriedades dos componentes, não se encontra ainda devidamente analisada. Neste trabalho, os resultados obtidos utilizando métodos numéricos do contínuo não-linear e métodos analíticos simplificados, baseados em hipóteses do domínio da elasticidade, foram comparados com resultados experimentais disponíveis na bibliografia. Constatou-se que os resultados numéricos sobrestimavam claramente os resultados experimentais, exceptuando os resultados obtidos de acordo com os regulamentos europeu e norte-americano, que os subestimavam. Desta forma, abordagens numéricas alternativas que considerem a natureza discreta dos componentes da alvenaria revestem-se de grande importância para uma correcta previsão da resistência à compressão da alvenaria. Realizou-se, assim, uma análise detalhada de um modelo de partículas consistindo numa abordagem fenomenológica e discreta para representar a micro-estrutura das unidades e da argamassa. A micro-estrutura atribuída aos componentes da alvenaria é constituída por partículas de forma poligonal e comportamento elástico linear, separadas por interfaces com comportamento não-linear. Todos os fenómenos inelásticos ocorrem nas interfaces e o processo de fractura consiste na progressiva rotura da ligação entre partículas. Claras vantagens foram observadas pelo modelo de partículas quando comparado com o modelo contínuo. Nos referidos casos de colapso da Torre Cívica de Pavia e da Catedral de Noto, o comportamento diferido de paredes e pilares de alvenaria sob elevadas cargas verticais foi apontado como uma possível principal causa de colapso. Neste estudo, foi realizada uma investigação experimental em 25 provetes de alvenaria antiga, que inclui ensaios monotónicos, ensaios de fluência de curto-prazo e ensaios de fluência de longo-prazo. Os resultados obtidos e a sua interpretação cuidada são apresentados neste trabalho. Paredes e pilares de alvenaria composta são uma tipologia frequentemente observada em construções históricas, nomeadamente na Catedral de Noto. No entanto, prever o comportamento à compressão de paredes compostas de alvenaria representa um importante desafio, dada a influência de um elevado número de factores como as propriedades mecânicas dos panos, a dimensão relativa dos panos e a forma como os panos estão ligados entre si. Neste estudo, novos resultados experimentais em provetes de três panos de grandes dimensões, ensaiados ao corte e à compressão, são apresentados em conjunto com uma interpretação numérica dos resultados. É apresentada, ainda, uma discussão sobre cálculos simplificados para avaliação expedita de paredes existentes.Fundação para a Ciência e a Tecnologia (FCT) - Projecto (SFRH/BD/5002/2001)

Modelling of masonry creep and damage

The problems related to the analysis of ancient constructions are gigantic due to the difficulties in characterizing the geometry, the materials, the sequence of construction, the existing damage and the building processes. A difficult aspect in the repair and strengthening of existing structures is the long-term behaviour of masonry. Safety assessment has been greatly influenced by the collapse of monumental buildings in the last decade, where creep behaviour and creep-fatigue interaction proved to be of relevance for massive masonry walls. Experimental testing on the creep of regular and rubble ancient masonry together with numerical modelling is currently under investigation by the authors. In a first stage, the research is primarily focused on the monotonic behaviour. This represents a contribution for understanding creep behaviour, providing insight into the basic phenomena that occur in masonry upon increasing loading. The results obtained using a continuous model to represent the masonry components largely overestimated the experimental strength and peak strain. Alternative modelling approaches seem, thus, to be needed and a discontinuous model in which a fictitious micro-structure was given to the components was considered. Clear advantages were shown by this last model. In the present paper, the results obtained using these models are briefly reviewed. The results from the ongoing simulations on the long-term behaviour of masonry assemblages will be presented in the oral communication.(undefined

Testing and modelling of multiple-leaf masonry walls under shear and compression

Predicting the behaviour of multiple-leaf masonry walls is a challenging issue, given the influence of a wide range of factors as the mechanical properties of the leaves, their dimensions and the way they are connected to each other. In the present paper, experimental results in large specimens are carefully reviewed together with numerical interpretation of the shear and compressive behaviour of multiple-leaf walls. Simplified calculations for practical assessment of existing walls are also addressed.MURST – Cof. 2000, 2002

A contribution for the understanding of load-transfer mechanisms in multi-leaf masonry walls : testing and modelling

Predicting the behaviour of multiple-leaf masonry walls is a challenging issue, given the influence of a wide range of factors as the mechanical properties of the leaves, the leaves dimensions and the way the leaves are connected to each other. In the present paper, novel experimental results in large three-leaf wallets subjected to shear and compression are introduced together with a careful numerical interpretation. Two types of collar joints (with and without shear keys) and two types of stone (weak limestone and strong sandstone) are considered in the tests. The influence of the boundary conditions on the numerical response is thoroughly investigated and good agreement with the experimental results is found. Moreover, a discussion on simplified calculations for practical assessment of existing walls is addressed.Portuguese Foundation for Science and Technology - SFRH/BD/5002/2001.MURST - cofin. 2000/2002

Strengthening of three-leaf stone masonry walls: an experimental research

The paper summarizes the results of an experimental research carried out on three-leaf masonry walls of typical granite stone constructions from the North of Portugal. The research aimed at studying the behaviour under compression of this wall typology, as well as the improvements introduced by common strengthening techniques applied for the structural rehabilitation of masonry heritage buildings. Ten masonry specimens were tested, plain or strengthened by transversal tying of the external leaves, with GFRP bars, or/and by injection of the inner leaf, with a lime-based grout. The results obtained showed that these strengthening techniques were successful in increasing the compressive strength of the walls and in improving their behaviour under compressive loads.The authors would like to thank the technical staff of the Structural Laboratory of University of Minho for the help provided. Acknowledgements are also due to the companies Fradical, Mapei and Augusto de Oliveira Ferreira for providing raw materials and workmanship. Finally, the funding provided by the Portuguese Science and Technology Foundation, through the POCI/ECM/58987/2004 project, is gratefully acknowledged