Seismic retrofit of masonry-to-timber connections in historical constructions

Tese de Doutoramento em Engenharia Civil.Proper structural connections play an important role in ensuring seismic loads distribution and developing global damage mechanisms of structures. In unreinforced masonry buildings, effective connections between masonry walls and timber floors or walls through the use of anchors can prevent the occurrence of out-of-plane mechanisms and promote “box-like” behavior. This thesis aimed at studying and developing seismic retrofit solutions for connections between masonry walls and timber structural elements, such as floors and timber framed walls, for historical unreinforced masonry buildings built in Lisbon, from the late 19th century to the beginning of the 20th century. However, similarities concerning constructive details of connections found on unreinforced masonry buildings across different countries, bring the possibility of expanding the solutions under study to other contexts. The tasks carried out focused on characterizing the tensile response of the connections, through an experimental campaign followed by analytical development of the results. Quasi-static monotonic and cyclic tests were carried out on pairs of injection anchors, to be applied on masonry-to-timber elements connections, and on wall-to-floor connections retrofitted with a system of steel tie rod with anchor plate. The pairs of injection anchors failed by combined cone-bond failure, with strength capacity and shape of the envelopes being highly influenced by the boundary conditions of the masonry wall. Hysteretic loops’ shape is mainly influenced by bond slip at the grout/masonry interface, causing strength and stiffness degradation, and pinching. Unstrengthened wall-to-floor specimens’ failure mechanism was nails pullout, while for the strengthened ones was failure of the bolted connection between steel angle and timber joist. The hysteretic behavior of the latter shared similarities with the one of the injection anchors, but it was controlled by slip at the bolted connection. There were considerable increases in maximum pullout force and dissipated energy, from unstrengthened to strengthened wall-to-floor connections, and ductility was kept high as well. From the experimental results, strength prediction formulae were studied and adapted to better fit the results obtained and idealized curves were developed based on force-displacement backbone curves. Existing behavior models provided good approximations to the experimental results but further validation needs to be implemented. The trilinear curves provided better approximations to the backbone curves, and ASCE/SEI 41-06 approach revealed to be very conservative. Retrofit design and acceptance criteria regarding displacements, forces and behavior factors were proposed.Ligações estruturais eficientes assumem um papel crítico na transmissão de solicitações sísmicas e no desenvolvimento de mecanismos globais de colapso, em detrimento de modos de rotura locais como derrubamento parcial ou total de paredes para fora do plano. Estes mecanismos para fora do plano das paredes são particularmente recorrentes e nefastos em edifícios antigos de alvenaria não armada, sendo necessário o desenvolvimento de soluções de reforço sísmico de ligações capazes de promover um comportamento de “caixa” de todo o edifício. Esta tese teve como objetivo principal colmatar esta necessidade e, para tal, focou-se em ligações entre paredes de alvenaria e pavimentos e paredes de madeira, existentes em edifícios contruídos em Lisboa, depois do terramoto de 1755 até ao início do século XX. Ensaios de arrancamento quase-estáticos monotónicos e cíclicos foram realizados em provetes constituídos por paredes de alvenaria e mangas injetadas e em provetes representativos de ligações piso-paredes, sem reforço e reforçados com tirantes e placas de ancoragem. Os pares de mangas injetadas romperam por combinação de cone de alvenaria com corte na interface alvenaria/grout, mas com diferentes contribuições dos mesmos, conforme a sua localização na parede. As forças de arrancamento e a forma das envolventes foram muito influenciadas pelas condições fronteiras da parede. No entanto, em todos os ensaios os ciclos histeréticos foram governados por escorregamento na interface alvenaria/grout, sendo caracterizados por perdas de força e rigidez entre ciclos, assim como mudanças de rigidez em carga e descarga. O mesmo se verificou para os ensaios realizados nos provetes piso-parede, sendo no entanto, o escorregamento na ligação aparafusada entre cantoneira e viga de madeira o responsável pelo comportamento histerético observado. O mecanismo de rotura verificado nos provetes pisoparede não reforçados foi o arrancamento dos pregos, enquanto nos provetes reforçados foi a rotura da ligação aparafusada. Houve um aumento da capacidade em força e da energia dissipada nos reforçados, sendo possível manter uma ductilidade elevada na ligação. Modelos de comportamento e respetivas expressões para a estimativa de forças máximas foram aplicados aos resultados experimentais, assim como foram desenvolvidas curvas idealizadas com base nas envolventes força-deslocamento. As expressões existentes permitem boas aproximações às forças obtidas experimentalmente, mas necessitam ser melhoradas. As envolventes ASCE/SEI 41-06 são bastante conservativas e as curvas trilineares constituem melhores aproximações às envolventes experimentais. Foram também propostos critérios de aceitação com base em deslocamentos e forças, assim como fatores de comportamento.FP7-ENV-2009-1-244123-NIKER of the 7th Framework Program of the European Commission

Bond behaviour of twisted stainless steel bars in mortar joints

The use of twisted stainless steel bars has proven to be effective in scenarios where lateral loads (e. g. earthquakes and winds storms) can cause partial or complete out-of-plane collapse of masonry wall’s outer leaves or separation of wood diaphragms from masonry walls. The particular application of these bars as a dry system in mortar joints, without any binder, brings additional advantages in terms of cost, installation time and weather restrictions. An experimental campaign composed by 60 pull-out tests aimed at characterizing the bond behaviour of twisted stainless steel bars in mortar joints. The influence on bond behaviour of two diameters of the helibar (ϕ8 mm and ϕ10 mm), three different anchorage lengths (8ϕhb, 12ϕhb, and 20ϕhb), and two diameters of the pre-drilled holes (ϕhb–2 mm and ϕhb–4 mm) was studied. Bond strength increased for the tighter pre-drilled hole but decreased for the higher twisted steel bar diameter. Good correlations between bond strength and anchorage length were found for the less tight pre-drilled holes

Experimental research on masonry wall and timber elements connection

Studies developed for the past years and recent earthquakes have brought to light the importance that efficient connections between structural elements can have in the global behaviour of the structure. Although out-of-plane mechanisms of masonry walls are considered to be local, the reality is that their occurrence can lead to total collapse of the structure. The scarcity of data, both at experimental and numerical levels, introduces an urgent need to characterize the response of connections to seismic actions and understand their impact on the overall behaviour of the structure. Therefore, a total of 15 quasi-static monotonic and cyclic pull-out tests were performed on representative wall-to-floor (timber pavement beams) and wall-to-timber framed wall connections, unstrengthened and strengthened, in order to assess their performance and allow their characterization. The analysis contemplates parameters like: failure mode, hysteretic curve, strength degradation and total energy. The study of these parameters promotes better understanding of this type of connection and also the development of design recommendations for the strengthening. As further development, the experimental data will allow calibration of representative numerical models, enabling parametric studies of material properties and formulation of backbone curves

Design parameters for retrofitted masonry to timber connections

Proper structural connections play an important role in ensuring seismic loads distribution and developing global damage mechanisms of structures. In unreinforced masonry buildings, positive connections between masonry walls and timber floors or walls through the use of anchors can prevent the occurrence of out-of-plane mechanisms and promote box-behavior. Therefore, this paper aims at developing structural modeling parameters and acceptance criteria that allow the design of anchored connections for historical URM buildings from the late 19th century, in Portugal. An experimental campaign was carried out, where quasi-static monotonic and cyclic pullout tests were carried out on strengthened wall-to-floor connections and wall-to-timber framed connections. Both retrofitting solutions rely on anchoring the timber floor or framed wall to the masonry wall, through the use of steel tie-rods with anchor plates or injection anchors, respectively. From these tests, it was possible to study their hysteretic behavior and failure modes, as well as quantify the maximum pullout capacity, the ductility, the energy dissipation and other parameters. This information was the base to establish multilinear backbone curves and design parameters for each type of behavior observed experimentally. Experiments performed in strengthened wall-to-floor connections with two wall thicknesses (0,4 m and 0,6 m) and in wall-to-timber framed wall connections with injection anchors at the top of a wall demonstrated high ductility and were classified as deformation-controlled actions. Being governed by shear slip enabled them to obtain large displacements with small strength loss. For the injection anchors, the applicability of strength prediction formulas based on different failure models was studied. The adapted ACI 530-05 model for cone breakout was the one that better predicted the experimental values obtained for the tests performed at the top of the wall. Bond failure models were highly dependent on the bond strength of the grout/masonry interface and provided reasonable approximation to the results. Further use requires the determination of accurate grout/masonry interface bond strength. Future work includes simplification of backbone curves and development of hysteretic rules

Flexural out-of-plane retroftting technique for masonry walls in historical constructions

The paper presents a recent technology for the out-of-plane flexural strengthening of historic masonry walls. The method is based on the forming of an irregular net following the joint texture of the wall, which would be able to work in tension. The applied net can be hidden by repointing the joints. This way the technology does not cause visual alteration to walls without rendering, respecting their authenticity. An ongoing research in Portugal is aiming to increase the workability and efficiency of the technique by developing a new anchor element for the application of the net. The discussion includes the introduction of the method and partial results of the research from the University of Minho, Portugal

Developing a seismic retrofitting solution for wall-to-floor connections of URM buildings with wood diaphragms

Out-of-plane collapse of masonry walls in unreinforced masonry (URM) buildings has been identified as a major local failure mechanism affecting the global stability of a structure. Continuity between structural elements maintains proper load paths during seismic action, discouraging the formation of out-of-plane mechanisms. This is a key factor for an engineer to consider when trying to fully utilize the in-plane capacity of a masonry wall. This paper focuses on the seismic retrofit of wall-to-floor connections of URM buildings with wood diaphragm floors. The solution consists of a tie rod bolted to a timber floor joist and anchored to the masonry wall using an anchor plate. Several cyclic pull-out tests were carried out to characterize the behaviour of the strengthened connection so that an ‘engineered’ strengthening can be developed. The resultant behaviour has the contribution of different failures from yielding of the steel components to masonry cone breakout. The strengthen connection presents a higher increase in tensile capacity, displacements, and consequently energy dissipation

Experimental study on the seismic behavior of masonry wall-to-floor connections

The global structural performance of masonry buildings, under earthquake loading, is affected by the efficiency of wall-to-floor connections, since they assure the continuity of the energy path and prevent the occurrence of most of the local collapse mechanisms. In fact, out-of-plane behaviour of masonry walls observed in recent seismic events showed the critical importance of proper connections in historical buildings. A review of current literature yields little in terms of experimental and numerical data on the subject. Thus, there is an urgent need to study the behaviour of these connections. The present paper presents a series of tests carried out to characterize the wall-to-floor connections. Different specimens were constructed in laboratory to represent connections found in ‘Gaioleiro’ and Late ‘Pombalino’ buildings in downtown Lisbon. Pull-out tests of wall-to-floor connections were carried out on unstrengthened and strengthened specimens in order to study failure modes, maximum pullout forces, and corresponding displacements. These parameters allow better understanding of this type of connection and also the development of design recommendations for the strengthening

Caracterização experimental do reforço de ligações parede-piso

O desempenho sísmico de edifícios tradicionais de alvenaria é geralmente afetado pela eficiência das ligações piso-parede, uma vez que estas podem assegurar a continuidade estrutural e impedir a maioria dos colapsos locais. De facto, o comportamento para fora do plano das paredes de alvenaria observado em sismos recentes evidencia a importância crítica que as ligações entre os elementos verticais e horizontais assumem em edifícios históricos. A literatura científica neste domínio é muito escassa, justificando a necessidade de investir recursos na análise, experimental e numérica, do comportamento das ligações piso-parede. Este artigo apresenta uma série de ensaios experimentais sobre a caracterização do comportamento de ligações piso-parede em construções tradicionais de alvenaria. Foram construídos diversos provetes em laboratório à escala real, representativos dos edifícios pombalinos tardios e gaioleiros da baixa de Lisboa. O artigo discute os resultados experimentais das ligações reforçadas ensaiadas, com destaque para as cargas e deslocamentos máximos, bem como os modos de rotura associados. Com base nos resultados obtidos, é apresentada ainda uma idealização bilinear das envolventes dos ensaios, como primeira aproximação à utilização prática dos resultados

Ponte pedonal espiada com tabuleiro de madeira modular pré-esforçado

Tese de mestrado integrado. Engenharia Civil (especialização em Estruturas). Faculdade de Engenharia. Universidade do Porto. 200

Comportamento experimental de ligações estruturais madeira-alvenaria

Na sequência de vários sismos ocorridos recentemente – L’Aquila 2009 e Christchurch 2011 – e de estudos desenvolvidos nos últimos anos, o reconhecimento da importância de ligações entre elementos estruturais tem vindo a crescer, e tem-se demonstrado que estas são um ponto crítico no comportamento global da estrutura. Ligações apropriadas, piso-parede e parede-parede, contribuem para a mobilização da resposta combinada de ambos os elementos estruturais, em detrimento da formação de mecanismos locais de dano para fora do plano. A escassez de dados, quer a nível experimental, quer numérico, introduz a necessidade concreta de caracterizar a sua resposta a ações sísmicas e compreender o seu impacto no comportamento global da estrutura. Desta forma, foi realizada uma série de ensaios de arrancamento quase-estáticos, monotónicos e cíclicos, em ligações pavimento de madeira-parede de alvenaria (piso-parede) e frontal-parede de alvenaria (frontal-parede). Devido à variedade de ligações existentes, foram reproduzidas em laboratório as ligações estruturais mais desfavoráveis, semelhantes às encontradas em edifícios de alvenaria construídos após o terramoto de 1755, em Lisboa. Dos 15 ensaios de arrancamento realizados até à data, 8 são relativos a ligações piso-parede e 7 relativos a ligações frontal-parede. Nos ensaios de arrancamento de piso-parede foram considerados provetes não-reforçados e reforçados, sendo o reforço composto por uma cantoneira fixa à viga de pavimento e ancorada à parede através de uma solução atirantada. Para a ligação frontal-parede, o reforço aplicado consiste numa placa Gusset a estabelecer a ligação entre o frontal e o sistema de mangas injetadas na parede de alvenaria. Esta exaustiva campanha experimental permitiu caracterizar modos de rotura, forças máximas de arrancamento, comportamento histerético, degradação de resistência e energia total dissipada. Com base nesta informação, é possível estabelecer critérios de apoio ao dimensionamento de reforço de ligações à ação sísmica, explorar pormenores de execução em obra e criar condições para desenvolvimento futuro de modelos numéricos capazes de descrever os fenómenos observados experimentalmente