In the province of Groningen (NL), where human-induced earthquakes take place due to gas extraction, a large part of the building stock is composed of brick masonry walls and timber diaphragms. In this framework, timber-masonry connections play a crucial role in the global seismic response of the buildings, but their properties and structural behaviour have not been investigated yet for the Dutch context. This work describes the experimental campaign conducted at Delft University of Technology to characterize as-built and strengthened timber-masonry connections. The joints were tested under either quasi-static monotonic, cyclic or dynamic loading, to analyse the effect of an induced earthquake signal on the connections’ response in terms of strength, stiffness and damage evolution. The obtained test results provided more insight into the capacity and properties of existing connections, and useful knowledge on the effectiveness of the tested retrofitting methods.

@en

Existing masonry buildings are very frequently part of the architectural context for several countries all over the world. These constructions often feature masonry walls as vertical structural elements, and timber floors and roofs as horizontal components. The often poor characteristics of masonry, along with the in-plane flexibility of the floors and their frequently weak connections to the walls, make such buildings very vulnerable against seismic actions, as proved by the destructive consequences of several earthquakes in the last decades. The improvement of seismic capacity of existing masonry buildings is still an open research topic, with several retrofitting techniques for masonry walls and timber floors being proposed and tested. An acknowledged way of increasing the structural performance of a masonry building under an earthquake consists of the development of the so-called box behaviour, enabling the construction to react as a whole to the ground motion. To pursue the box behaviour, the main adopted retrofitting methods are linked to in-plane stiffening of the timber floors, and seismic strengthening of the connections. In this context, several (nonlinear) analysis method for masonry buildings (e.g. the pushover analysis) assume that rigid floors are present, and out-of-plane failure mechanisms of masonry are prevented. Therefore, also in the context of numerical modelling, the in-plane response of the diaphragms is generally not taken into account in detail, since they are only considered as linear elastic orthotropic membranes or stiff elements. However, past seismic events demonstrated that an excessive stiffening of the diaphragms can also be detrimental. This triggered the study of several lighter, moderately stiff strengthening methods for timber floors, referred to various architectural frameworks. Since existing floors proved to be excessively flexible, but also too stiff diaphragms could not be recommendable, the overarching research question of this dissertation arises: “How is it possible to predict the global seismic behaviour of existing and retrofitted masonry buildings, and to optimize it by quantifying the influence of strengthening interventions on timber floors and timber-masonry connections?” The research question is answered starting from the specific situation of the Groningen area, in the northern part of the Netherlands, where human-induced earthquakes caused by gas extraction take place. The local building stock is composed for more than 50% of low-rise masonry constructions with timber floors and roofs, none of which was designed or realized with seismic events in mind, since earthquakes were absent until recently. Up to now, these events have not caused extensive structural damage, because their intensity was from low to moderate, but according to probabilistic studies, more intense earthquakes might also occur. For this reason, a seismic characterization of local timber and masonry structural components was firstly necessary: this dissertation focused in particular on the testing and modelling of as-built and retrofitted timber diaphragms and timber-masonry connections. As a first step, because it was not possible to test a large number of whole structural components on site, a replication method based on material properties was defined. This ensured that the specimens constructed in laboratory could be representative for the actual structural components in practice. With regard to timber diaphragms, in-plane quasi-static reversed-cyclic tests were performed on five as-built samples, which showed an approximately linear, and very flexible response. For these diaphragms, a retrofitting technique enhancing not only strength and stiffness, but also energy dissipation of the diaphragms, was designed. This dissipative contribution of the floors can be relevant, because it can potentially (greatly) dampen the seismic shear forces on masonry walls, and this characteristic can be even more important for the Groningen area, where low-quality masonry and very slender piers are often present. The strengthening technique for the floors consisted of an overlay of plywood panels screwed along their perimeter to the existing sheathing: the retrofitted diaphragms exhibited a great enhancement in seismic properties, with relevant increase in strength, stiffness, and energy dissipation. The great potential of the developed retrofitting technique could not be limited to an experimental characterization: an analytical model was also necessary to enable the design of the strengthening for other contexts or floor configurations. Hence, starting from the analytical formulation of the load-slip behaviour of the single screws connecting planks and plywood panels, the global in-plane response of the floors was derived, including their characteristic pinching behaviour. This analytical model had also another important function, because it was the basis for an advanced numerical implementation of the seismic response of timber diaphragms in finite element software. This enabled to account in detail for the (dissipative) in-plane behaviour of the diaphragms, so that the seismic response of existing masonry buildings could be optimized with a well-designed retrofitting of the floors. Yet, this energy dissipation can only be activated by means of the in-plane deflection of the diaphragms: to avoid out-of-plane collapses of masonry walls, this deflection has not to be excessive, but at the same time also effectively strengthened timber-masonry connections are needed. Therefore, an experimental characterization of two as-built and five strengthened timber-masonry connections was conducted. The joints were tested under monotonic, cyclic, and also high-frequency dynamic loading, by subjecting them to an induced Groningen seismic signal. Seven replicates per joint type were built and tested, and analytical models for evaluating strength and stiffness of the joints were derived, useful for design purposes or as input for numerical models. Finally, in order to study the possible optimization of seismic capacity of existing masonry buildings, it is also necessary to define proper criteria for an optimal retrofitting. The current seismic design framework is extensively based on peak ground acceleration, which cannot, however, take into account factors such as load duration and quantification of structural damage. These parameters can play a crucial role for the Groningen region, because of the transient nature of the local earthquakes, featuring short, high-frequency and sudden signals, if compared to the longer and more damaging tectonic earthquakes. Therefore, an energy-based approach was adopted, which allows to predict and quantify the hysteretic energy provided by a building as a function of its period and the load duration of the earthquake. This approach opened up the opportunity to quantify structural damage in terms of number of cycles on the system: the role of timber diaphragms becomes, then, even more relevant, because with an optimized retrofitting the floors are only moderately stiff, thus the period of a building would be higher than that of the same structure featuring stiff diaphragms. Furthermore, the possibility to include load duration enables a characterization of the seismic capacity independent of the context or the earthquake type. Thus, to prove the beneficial, dissipative effect of the optimized retrofitting of floors, as well as the effectiveness of the adopted modelling strategies, numerical time-history analyses were performed on three case-study buildings. The first two were typical Dutch constructions, subjected to induced earthquakes, while the third was a country house from the Italian context, to which tectonic earthquakes were imparted. This additional building was included because it enabled to demonstrate that the developed retrofitting and modelling principles, along with the energy-based characterization of seismic capacity, can be generalized to other context besides the reference Dutch one. The results from the analyses show that excessively flexible floors cause, as expected, out-of-plane collapses in masonry walls, while excessively stiff floors limit the energy dissipation to masonry piers only, thus reducing the seismic capacity of the building. On the contrary, an optimized retrofitting is able to retrieve the global base shear of the building, and at the same time its maximum displacement capacity within masonry drift limits. The optimal strengthening also corresponds to the maximum hysteretic energy that can be provided by the structure. Furthermore, the period of the building is also increased compared to stiff floor configurations, meaning that the structure is subjected to a lower number of cycles, besides benefitting from the additional damping effect activated by dissipative diaphragms. This dissipative contribution can be brought into play provided that an effective strengthening of timber-masonry joints is realized. The beneficial, dissipative effect of well-retrofitted, optimized timber floors was quantified in terms of an equivalent hysteretic damping ratio of 15% (additional to the dissipation already provided by masonry walls), and of an increased behaviour factor (q) range for masonry structures: from the usual values of q = 1.5 ÷ 2.5 to q = 2.5 ÷ 3.5 in presence of dissipative diaphragms. This research study can contribute to a more efficient seismic retrofitting of existing buildings, enabling preservation of the architectural heritage and more dissipative, earthquake-safe masonry structures.@en

Historical or existing buildings are often composed of brick or stone masonry walls, and timber floors and roofs. When these buildings are subjected to earthquakes, the interaction among such structural components is essential to avoid collapse or excessive damage to the constructions. In this framework, a crucial role is played by the connections between horizontal and vertical structural elements: this is especially true when considering existing buildings not designed or realized taking into account seismic actions, because no measures against these events are present. An example of such a situation is noticeable in the province of Groningen, in the northern part of the Netherlands, where human-induced earthquakes take place due to gas extraction. Given the absence of seismic events until recently, a characterization of the existing buildings is necessary, and strengthening measures have to be analysed, in order to make them earthquake-safe. This work describes the experimental campaign conducted at Delft University of Technology to assess the seismic behaviour of as-built and strengthened timber-masonry connections. Results show that existing joints are not suitable to withstand earthquakes, while the proposed strengthened configurations can increase not only strength and stiffness, but also energy dissipation of the connections.@en

Timber diaphragms in existing buildings are often too flexible in their plane, and can thus potentially cause out-of-plane collapses of walls during earthquakes. A very efficient retrofitting method to increase their in-plane stiffness and energy dissipation is the overlay of plywood panels. However, the usual characterization of the floors by means of a general equivalent shear stiffness cannot account for their nonlinearity and dissipative properties. Therefore, in this work, an analytical model is formulated to describe the in-plane response of timber diaphragms strengthened with plywood panels screwed along their perimeter to the existing sheathing. The proposed formulation starts from the definition of the load-slip equation for a single screw connecting a plank and a plywood panel. The whole floor’s response is then derived, with the prediction of both backbone curve and pinching cycles. From the comparison between the response of tested full-scale diaphragms and the analytically calculated one, it can be concluded that the proposed model accurately predicts the in-plane behaviour and dissipative properties of timber floors retrofitted with plywood panels.@en

The inadequate seismic performance of existing masonry buildings is often linked to the excessively low in-plane stiffness of timber diaphragms and the poor quality of their connections to the walls. However, relevant past studies and seismic events have also shown that rigid diaphragms could be detrimental for existing buildings and do not necessarily lead to an increase in their seismic performance. Therefore, this work explores the opportunity of optimizing the retrofitting of existing timber floors by means of a dissipative strengthening option, consisting of a plywood panel overlay. On the basis of past experimental tests and previously formulated analytical and numerical models for simulating the in-plane response of these retrofitted diaphragms, in this work nonlinear incremental dynamic analyses were performed on three case–study buildings. For each structure three configurations were analyzed: an as-built one, one having floors retrofitted with concrete slabs and one having dissipative diaphragms strengthened with plywood panels. The results showed that the additional beneficial hysteretic energy dissipation of the optimized diaphragms is relevant and can largely increase the seismic performance of the analyzed buildings, while rigid floors only localize the dissipation in the walls. These outcomes can contribute to an efficient seismic retrofitting of existing masonry buildings, demonstrating once more the great potential of wood-based techniques in comparison to the use of reinforced concrete for creating rigid diaphragms.@en

In the region of Groningen (NL), human-induced earthquakes initiated by gas extraction are causing structural damage. In that area, the building stock is mainly composed of unrein- forced masonry (URM) buildings with light and flexible timber floors and roofs. Thus, an ex- perimental campaign was arranged for assessing the in-plane response of these diaphragms, and a retrofitting method was developed, consisting of an overlay of plywood panels screwed to the existing sheathing around their perimeter. This light, reversible stiffening measure showed a great increase in the in-plane strength, stiffness, and energy dissipation of the floors. Subsequently, an analytical model was developed, showing very good agreement with experimental results, and enabling the design of retrofitting interventions with this technique. Starting from the formulated model, a user-supplied subroutine was implemented in the finite element software DIANA FEA, allowing to represent the in-plane response of the diaphragms, including their energy dissipation. Finally, the impact of this retrofitting intervention on a case study of an existing building was evaluated by means of nonlinear time-history analyses. The results of numerical analyses show that the user-supplied subroutine accurately describes the in-plane behaviour of the retrofitted timber floors. Besides, the proposed retrofitting tech- nique greatly increases the global seismic performance of the building, compared with both its as-built configuration and to stiffer and less reversible strengthening measures.@en

In-plane behavior of timber diaphragms is usually characterized by means of an equivalent shear stiffness. However, this value depends on how the stiffness of the floors is evaluated from the experimental tests. Although an increasing number of research studies have provided a deeper insight into the seismic characterization of as-built and retrofitted timber diaphragms, the use of different standards or assumptions have led to inhomogeneous and not comparable results. With a focus on light, reversible, wood-based strengthening techniques applied to existing diaphragms, this study proposes a uniform and simple method based on the calculation of the secant stiffness of the floors at reference drifts. By means of this procedure, relevant research studies from the literature were compared, and homogeneous, indicative values of equivalent shear stiffness were proposed for each considered strengthening technique. These results can contribute to a more aware and reliable use, design, and linear modeling of wood-based retrofitting solutions for existing timber diaphragms.@en

In the field of seismic retrofitting, a common intervention to improve box-like behaviour in an existing building is the strengthening and stiffening of existing timber floors and roofs. However, these retrofitting methods should be carefully applied, because they change the static scheme and the buildings' response to earthquakes. Strengthening solutions with high reversibility and light weight have therefore to be preferred, and in this context the overlay of plywood panels on existing floors can improve their characteristics in terms of strength and stiffness, but also enhance their energy dissipation, already at a very limited deflection. This strengthening technique was adopted and tested within an experimental campaign aimed at assessing the seismic response of timber diaphragms with typical characteristics from the Groningen area, located in the northern part of the Netherlands. In that region, human-induced earthquakes take place due to gas extraction and the existing buildings are not suitable to safely withstand these seismic events. This paper presents a summary of the results of the experimental campaign on as-built and retrofitted timber diaphragms, and evaluates the beneficial damping properties of floors strengthened with plywood panels, connected only to the underlying planks and not directly to the joists. The results are compared with the data available in literature and provide new reference values for the coming version of the Dutch seismic standard.

@en

In this work, the relationship for a socket-type connection in wooden foundation piles is investigated, and a trilinear moment-rotation diagram was determined by means of experiment. Numerical and analytical models confirmed that the material properties for compression perpendicular to the grain are governing for the connections’ strength and stiffness properties. This was similar to the response found in the experiment, where ductile behaviour was observed, which can be explained from the plasticization after the compression strength perpendicular was reached. Although because of the specific configuration (confined in a circular tube) the found strength perpendicular to the grain cannot be assigned to other configurations, a good estimation of the stiffness of the connection can be made with FEM models, assuming an MOE90 of 70 N/mm2 for a fully saturated pile. With a conservative assumption for the strength perpendicular to the grain the influence of the connection in a pile-soil model can be investigated also for other configurations than the tested one. A procedure to study the influence of tangential (usually horizontal) soil displacements on timber piles with concrete extensions piles on top is proposed. The outcome of a specific situation from practice with high horizontal loads showed that the bending capacity of the timber pile will in most cases be governing and not the investigated socket connection.@en

Traditional timber floors cannot normally withstand horizontal seismic loads without large deformations. This may lead to a corresponding out-of-plane collapse of masonry walls in existing buildings. This situation is even more critical in the Netherlands, around the city of Groningen, where human-induced earthquakes started to take place. Since no seismic events have been experienced until recently, none of the existing buildings was designed with seismic events in mind, with no exception for the timber floors: therefore, it was necessary to characterize their in-plane response. To obtain representative results, firstly floor and roof samples were extracted from existing buildings. The relevant material properties were determined, together with the plank-joist connections behaviour. Replicas were then built with new material and tested to confirm the similarity in response compared to extracted samples. Based on these results, full-scale replicated diaphragms were constructed, and tested quasi-static reversed-cyclic in their plane, either parallel or perpendicular to the joists. Besides characterizing as-built diaphragms, a simple strengthening technique with plywood panels was applied as well, improving their in-plane response in terms of strength, stiffness and energy dissipation, as test results confirm. This study is concluded with an analytical characterization of the diaphragms’ in-plane response, for as-built and strengthened configurations.

@en

This document presents the interpretation of the test results obtained within the whole 2019 experimental campaign on timber-masonry connections, in as-built and strengthened configurations tested under monotonic, cyclic and dynamic loading. In this document, starting from those results, failure mechanisms are analysed, which could occur in each type of tested connections. Furthermore, the main properties of them, such as strength, stiffness and damping are determined.@en

This document presents the complete overview on the test results obtained in the 2019 experimental campaign on timber-masonry connections, in as-built and strengthened configurations. The tests have been performed in two phases, the first phase in spring 2019 and the second phase in summer 2019. The aim of this test campaign was to characterize the seismic response of joist-wall connections, therefore monotonic, cyclic and high-frequency dynamic tests were scheduled. In the present report, after a general introduction, a detailed overview on the adopted samples and on the testing methods will be presented, followed by discussion of test results and some general conclusions. The conducted experimental campaign refers to common typologies of two as-built timber-masonry connections, with five proposed retrofitting solutions. The presence of the as-built tested connections and the field of application of the strengthened versions are quite wide: the percentage of unreinforced masonry buildings in the Groningen gas field area can be quantified as 77% of the existing building stock.@en

In this report an experimental pilot study on the seismic response of timber-masonry connections is presented@en

This report presents the materials, experiment setups and testing procedures directed at determining the back-bone curves that describe the plank-joist connections of samples extracted from existing unreinforced masonry houses. The last section contains an overview of the results obtained for the different specimens tested. The use of the outcome of these experiments are twofold: - They provide input for FEM models to predict the mechanical behaviour of full size floors (diaphragms) made of joists with planks nailed to them. - They provide input for the selection of materials to build up replicas of diaphragms. For the plank-joist connection a distinction is made between nailed and screwed connections. The testing started with nailed connections, which is common use in pre-war houses. The loading is applied in three ways: loading the plank perpendicular to the joist and parallel to the joist, as well as rotation of the plank relative to the joist. All specimens were tested under cyclic loading, according to a prescribed loading program.@en

The in-plane strength and stiffness as well as the hysteretic behaviour of timber diaphragms is investigated by performing quasi-static cyclic tests. In this report the behaviour of traditional timber floors and roof configurations was studied representing a typical detached masonry house. The configurations are based on a case study from a detached Groningen house that was demolished. Characteristics of the demolished house were: - A detached house, built in 1920 - At ground storey double wythe clay masonry, at first storey single leaf gables of clay masonry - Timber floors built-up with joists and planks nailed on top of the joists. - Timber roof with vertical rafters spanning from wall plate to top beam, small horizontal purlins on rafters with vertical planks nailed to the purlins. Based on the extracted samples the configurations and material properties could be determined. Based on these findings timber diaphragms (floors and roof) were replicated, built from new timber material and fasteners, with the same material properties as found in the extracted samples. Replicated specimens of approximately 4.0 m by 2.75 m were prepared and tested. After the testing of the replicated specimens representing the situation in practice (non-strengthened), the specimens were strengthened with plywood panels screwed on the timber planks.@en

This report presents the material properties of timber and fasteners specimen extracted from existing unreinforced masonry houses.@en