Mechanical behaviour of timber-concrete joints

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(1)Mechanical behaviour of timber-concrete joints. A.M.P.G. Dias.

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(3) Mechanical behaviour of timber-concrete joints. Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 4 april 2005 te 15:30 uur door Alfredo Manuel Pereira Geraldes DIAS Engenheiro Civil, Universidade de Coimbra, geboren te Sertã, Portugal.

(4) Dit proefschrift is goedgekeurd door de promotoren: Prof.ir. F.S.K. Bijlaard Prof.dr.ir. S.M.R. Lopes Toegevoegd promotor: Dr.ir. J.W.G. Van de Kuilen. Samenstelling promotiecommissie: Rector Magnificus, Prof.ir. F.S.K. Bijlaard, Prof.dr.ir. S.M.R. Lopes, Dr.ir. J.W.G. Van de Kuilen, Prof.dr.ir. J.C. Walraven, Prof.dr. A. Ceccotti, Prof.dr.ir. L.M.C. Simões, Prof.dr.ir. H.M.P. Cruz,. voorzitter Technische Universiteit Delft, promotor University of Coimbra, promotor Technische Universiteit Delft, toegevoegd promotor Technische Universiteit Delft University of Venice University of Coimbra University of Coimbra. ISBN 90-9019214-X. Copyright © 2005 by A.M.P.G. Dias.

(5) Table of contents. Table of contents Table of contents. v. Summary. ix. 1 Aims and organization. 1. 1.1 Aim of the study. 1. 1.2 Organisation of the thesis. 2. 2 Literature review. 5. 2.1 Historic background. 5. 2.2 Calculation models for timber-concrete composite structures. 7. 2.3 Timber-concrete joints. 8. 3 Joint behaviour and composite action. 19. 3.1 Models to analyse composite structures. 19. 3.2 Experimental assessment of the joint mechanical properties. 23. 3.3 Importance of the joint in the composite structures. 26. 3.3.1 Introduction 3.3.2 Influence of the joint stiffness in the behaviour of composite structures 3.3.3 Influence of the joint strength in the behaviour of composite structures 3.3.4 Influence of the joint elastic non-linearity in the behaviour of the composite structures 3.3.5 Influence of the joint plastic deformation capacity in the behaviour of the composite structures 3.3.6 Influence of the joint creep coefficient in the behaviour of the composite structures. 4 Experimental programme and results. 26 28 31 33 34 38. 41. 4.1 Introduction. 41. 4.2 Short-term shear tests. 42. 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8. Introduction Preliminary tests Main test programme Test configuration Materials Test set-up and procedure Shear test results for the dowel type fastener joints Discussion of test results for the dowel type fastener joints. 42 42 45 47 49 50 52 59. v.

(6) Mechanical behaviour of timber-concrete joints. 4.2.9 Shear test results for the notched joints 4.2.10 Discussion of test results for the notched joints 4.2.11 Conclusions 4.3 Embedment tests 4.3.1 Test specimens and test set-up 4.3.2 Test results 4.3.3 Discussion of the results 4.4 Long-term shear tests 4.4.1 Test specimens and test set-up 4.4.2 Test results 4.4.3 Discussion of the results. 5 Analytical models for timber joints. 72 72 74 76 76 76 80 82. 85. 5.1 Introduction. 85. 5.2 Strength models for timber-timber and steel-timber joints. 86. 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5. Introduction Timber-timber joints Steel-timber joints Steel-timber joints with gap between timber and steel Models from Eurocode 5. 5.3 Strength models for timber joints with interlayer 5.3.1 Introduction 5.3.2 Steel-timber joints with interlayer 5.3.3 Timber-timber joints with interlayer 5.4 Stiffness models for timber-concrete joints 5.4.1 5.4.2 5.4.3 5.4.4. Introduction Beam on elastic foundation Wilkinson’s models Beam on the elastic foundation for timber-concrete joints with gap between timber and concrete 5.4.5 Models from Eurocode 5. 5.5 Global load-slip models for timber-concrete joints 5.5.1 Introduction 5.5.2 Predictive models 5.5.3 Descriptive models. 86 86 88 89 90 90 90 92 94 96 96 96 101 101 103 103 103 104 106. 5.6 Models for embedding strength and foundation modulus of timber. 109. 5.7 Models for the long-term behaviour of timber-timber joints. 110. 6 FEM models for timber joints. 113. 6.1 Introduction. 113. 6.2 Modelling of the embedment tests. 113. 6.2.1 6.2.2 6.2.3 6.2.4. vi. 66 68 71. Introduction Test specimen configuration and mesh definition Boundary conditions, contact and friction Material properties. 113 114 116 118.

(7) Table of contents. 6.2.5 Results. 121. 6.3 Modelling of timber-concrete shear tests with 2D FEM models. 121. 6.4 Modelling of timber-concrete shear tests with 3D FEM models. 127. 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5. Introduction Mesh definition Material properties Loads application, supports, symmetry constraints and contact Results. 7 Comparison between simulations and tests. 127 129 132 133 135. 137. 7.1 Introduction. 137. 7.2 Embedment tests. 138. 7.2.1 Analytical models for embedding strength and foundation modulus 7.2.2 FEM models to simulate the embedment tests 7.2.3 Conclusions 7.3 Analytical models and short-term shear tests 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.3.6. Introduction Strength models for timber-concrete joints Stiffness models for timber-concrete joints Strength and stiffness models for joints with interlayer Foschi’s and Jaspart’s models applied to timber-concrete joints Conclusions. 7.4 FEM 2D models and short-term shear tests 7.4.1 Discussion of the numerical results and comparison with experimental results 7.4.2 Conclusions 7.5 FEM 3D models and short-term shear tests 7.5.1 Discussion of the numerical results and comparison with experimental results 7.5.2 Parametric studies 7.5.3 Conclusions 7.6 Long-term tests. 8 Conclusions and recommendations. 138 140 152 153 153 153 156 159 161 169 170 170 176 177 177 189 192 193. 201. 8.1 Conclusions. 201. 8.2 Recommendations. 204. References. 207. Annex 1 – Shear tests with dowel type fastener. 213. Annex 1.1. 8mm test series. 213. Annex 1.2. 10mmA test series. 222. Annex 1.3. HSC test series. 231. Annex 1.4. MP test series. 240. vii.

(8) Mechanical behaviour of timber-concrete joints. Annex 1.5. C test series. 249. Annex 1.6. LWAC test series. 258. Annex 1.7. 10mmB test series. 267. Annex 1.8. INT test series. 272. Annex 2 – Shear tests with notched joints. 277. Annex 2.1. dvwN test series. 277. Annex 2.2. dvwNI test series. 282. Samenvatting. 287. Resumo. 289. Acknowledgements. 291. Curriculum vitae. 293. viii.

(9) Summary. Summary This study is focused on the mechanical behaviour of timber-to-concrete joints. The research deals with the short-term behaviour as well as the long-term behaviour. The work begins with a bibliographic review focused in three main topics: historic background of timber-concrete composite structures, calculation methods used for timber-concrete composite structures and timber-to-concrete joint methods. After the bibliographic review was accomplished, the importance of the properties of the joints to the global mechanical behaviour of composite timber-concrete structures was analysed. Among other conclusions, this analysis led to the conclusion that often the slip modulus is the joint property with highest influence on the mechanical behaviour of the composite structures. This is followed by an analysis of the behaviour of the joints, using the results from tests and numerical simulations. Three different types of tests were performed to evaluate the joints and the material properties: short-term shear tests, long-term shear tests, and complementary tests. Two different types of joints were tested: timberconcrete joints made with dowel type fasteners and timber-to-concrete joints made with notches. Eight test series were performed with dowel type fasteners and three with notched joints. The differences in the test series were in the material properties (timber and concrete), in the diameter of the fastener and in two test series was used a 20mm interlayer. The results of the tests were analysed and compared with results obtained in other researches with similar joints. It was concluded that all the material properties have a significant influence in the ultimate load carrying capacity of the joints, however, the slip modulus is only significantly influenced by the material properties of timber. The numerical simulations were performed using two types of models: analytical models and Finite Element Models (FEM). The analytical models consist of: equations based on the European yield model, equations obtained from the differential equation of the beam on elastic foundation and descriptive equations. These equations were obtained in the literature and were adapted to the particular case of timber-concrete joints. They are used either to predict the value of certain mechanical properties (load carrying capacity, slip modulus, embedding strength and foundation modulus) or to simulate the whole load-displacement behaviour. FEM models are proposed to simulate the embedment tests on timber and on the shear short-term tests with dowel type fasteners. The models for the embedment tests are 3D models with non-linear behaviour of timber and steel. The simulations of the short-term shear tests used two different models: 2D models and 3D models. The 2D models assumed elastic behaviour for concrete and elastic-plastic behaviour for timber and steel. The fastener was directly modelled using a beam element, while the foundations on timber and on concrete were modelled indirectly using spring elements connected to the beam, which modelled the fastener. The 3D models used non-linear behaviour for the three materials: timber, steel, and concrete. The three components of the joint ix.

(10) Mechanical behaviour of timber-concrete joints. (timber member, concrete member and the fastener) were modelled with their actual geometric properties. The interactions between each other were modelled by using contact elements with friction. The results of the analytical models were used to evaluate what are the best models to predict the values of the different joint properties and the level of accuracy that is possible to achieve with them. The FEM models were used to analyse important phenomena of the mechanical behaviour of the joints that are usually difficult to evaluate by tests as, for instance, the friction and the stress distributions. Generally the predictions are good. However, it was found that they could be improved if some changes were made to the models. These changes consist mainly of taking into consideration some effects (e.g. compression strength of concrete, deformations on concrete) that are now disregarded but, when considered in the models (analytical and FEM models), improve the accuracy of the predictions. Finally, the most significant conclusions from this work regarding: - the importance of the properties of the joints and the methods used to determine them, - the experimental behaviour of timber-concrete joints made with dowel type fasteners and notched joints, - the analytical models available to predict the properties of timber concrete joints, - the potential of FEM models to simulate the load-slip behaviour of timber-concrete joints, - the long term behaviour of timber concrete joints, are presented and discussed. As a consequence of these conclusions, a number of recommendations to improve the methods and models available to predict the mechanical behaviour of timber-concrete joints are presented. It is also proposed another type of recommendations to extend and complement the work presented here.. x.

(11) Aims and organisation. Chapter 1. 1 Aims and organisation 1.1 Aim of the study The mechanical performance of timber-concrete structures is highly influenced by the quality of the connection between timber and concrete. However, the knowledge on the mechanical properties of timber-concrete joints is still very limited. The work described here intends to give a contribution for a better knowledge on this subject. The study of the mechanical behaviour of the timber-concrete joints includes both the short and the long-term effects. The most important joint properties with regard to the composite behaviour of timberconcrete structures are identified. The procedures usually used to determine these properties are based on standard rules for timber-timber joints. This study evaluates whether these procedures are appropriate for timber-concrete joints, and proposes changes if necessary. In this study, various types of joints, using different materials, are studied by means of an experimental programme complemented with analytical and numerical models. In a first phase, the results are analysed using analytical models. This analysis evaluates the ability of the models to predict the mechanical behaviour of the joints. Whenever necessary, changes are proposed to these models in order to improve the accuracy of their predictions. This analysis is complemented with the use of Finite Element Method (FEM) models developed to simulate the behaviour of the joints. This type of model allows a detailed analysis of the phenomena that influence the mechanical behaviour of the joints and that are usually difficult to evaluate from tests. Such analysis helps to clarify the relationship between the physical, geometric and material properties of the joint and their mechanical behaviour. The results of this analysis are also used to improve the analytical models to predict the value of the most important mechanical properties of the joint referred to before.. 1.

(12) Mechanical behaviour of timber-concrete joints. 1.2 Organisation of the thesis This thesis is composed of 8 chapters. These chapters are connected between themselves, and those relationships are schematically represented in Figure 1-1. The thesis begins with a literature review about timber-concrete joints and timber-concrete composite structures. An historic introduction on the use of timber-concrete structures is presented first. The objective of this review is to show how and why this type of structures was developed and used in practice. Different practical situations where the system is presently used in practice are also identified. After that, a brief overview is made of the models currently used for the analysis of composite timber-concrete structures. The literature review ends with the presentation of the joints methods to connect timber and concrete. The objective is not only to present various possibilities in terms of joints systems, but also to give an idea of the mechanical performance of these joints. Chapter 3 begins with a description of the models used to analyse the short-term and long-term behaviour of the composite structures. After that, the test method indicated in EN 26891 to determine the properties of the timber-timber joints is presented and discussed. The discussion is focused on the applicability of the method in the particular case of composite timber-concrete structures. The final part of the chapter is reserved for the analysis and evaluation of the importance of the joint properties on the behaviour of timber-concrete composite structures. The analysis shows why and how those properties are important. The contribution of the experimental part of this study is significant. A large number of tests were made to evaluate the short and long-term mechanical behaviour of the joints and to determine the properties of the used materials. Two different types of joints were tested, timber-concrete joints, made with dowel type fasteners, and timber-concrete notched joints. Short-term and long-term tests were performed on both types of joints. A total of 11 different joint configurations were studied (3 with notched joints, and 8 with dowel type fasteners). In Chapter 4 the description of these tests is followed by the presentation of the experimental results determined in accordance with EN 26891, together with a discussion and comparison with results obtained in other researches for similar conditions. The contribution of the numerical part is based on predictions obtained with models developed to simulate the mechanical behaviour of timber-concrete joints. This contribution is divided in two parts: the analytical models presented in Chapter 5 and the FEM models presented in Chapter 6. The analytical models are mostly closed form equations for the beam on elastic foundation and equations based on the European yield theory collected in the literature. The objective of these models is to predict the slip modulus, the ultimate load carrying capacity and the load-slip behaviour of joints made with dowel type fasteners. A number of models to predict the embedding strength and foundation modulus of timber are also presented. In Chapter 5, together with the formulations, the assumptions that support them and brief discussions about the possibilities and limitations of each one are presented. FEM models were developed to simulate the short-term shear tests and the embedment tests. The shear tests were modelled using 2D and 3D non-linear models. The embedment tests were modelled with 3D non-linear models. The description of each model starts with a literature review which presents FEM models developed earlier for similar joints. This is followed by a detailed description of the model properties, 2.

(13) Aims and organisation. Chapter 1. concerning the mesh definition, the input and the output of the models, as well as all the numerical procedures used in the simulations. For each one of the models, are also discussed the phenomena considered and the phenomena discarded and, in the last case, the error expected. The experimental results obtained in Chapter 4 and the results obtained with the models from Chapters 5 and 6 are compared in Chapter 7. From that comparison the errors of the simulation and predictions are evaluated. Based on these results, the ability of the different models to simulate the behaviour of the joints or to predict the values of the joint properties is discussed. A detailed analysis of the numerical results and the results of the parametric studies, performed with the 3D FEM models, are also presented and discussed. Finally, Chapter 8 presents the final conclusions and recommendations. These include a summary with the most significant conclusions from all the conclusions presented in the earlier chapters. This is followed by recommendations for changes in the procedures and models used to determine the joints properties, and recommendations for future development in some of the topics analysed in this study.. 3.

(14) Mechanical behaviour of timber-concrete joints. Chapter 1 Introduction. Chapter 2 Literature review. Chapter 3 Importance of joint. Chapter 4 Shear and embedment tests. Chapter 6. Chapter 5 Analytical models. FEM models of shear and embedment tests. Chapter 7 Comparison between models and tests. Chapter 8 Final conclusions and recommendations Figure 1-1. Scheme of the relationship between the various chapters.. 4.

(15) Literature review. Chapter 2. 2 Literature review 2.1 Historic background Timber-concrete composite structures appeared due to the shortage of steel between the two world wars of last century as referred to by Van der Linden (1999). The author mentions patents for timber-concrete composite systems, the first one with nails and steel braces, and the second one from the German Patent Bureau where the joint between timber and concrete was made with Z and I profiles. There are not many reports on developments of the technique up to the 70’s, after which the researches in this area increased with the development of new jointing technologies and calculation models. This technique is used either for new construction or renovation and strengthening of existing timber structures. Poštulka (1997) mentions more than 10,000m2 of timber floors renovated with this technique. Similar application is described by Godycki, Pawlica and Kleszczewski (Rilem TC111 CST, 1992) which mention the renovation of more than 1000m2 in Lodz. In both cases the joint was made with nails. Poštulka also mentions one of the first applications in historic buildings in Bratislava already in 1960. At the time, the costs of renovation were less than half of the costs necessary to build a new floor. Other examples of renovation in historic buildings are given by Turrini and Piazza (Rilem TC111 CST, 1992) which describe applications in Italian historic buildings where steel dowels were inserted in an oversized hole and then glued to the timber, after which a concrete layer was poured on the floor. Blasi and Ceccotti (Rilem TC111 CST, 1992) describe another application using corrugated metal sheets as lost formwork (Convent of Santa Caterina in Forli, Italy). The renovation and strengthening of timber floors are a natural application of this technique, however, it has also been used as a solution for new residential and public building floors, bridges and prefabricated floors and walls. Examples of new residential floor systems are given by Natterer et al. (1998) which describe the application of a post-stressed dowel combined with concrete notches on multi-storey buildings in Switzerland using nail-laminated slabs and log slabs. An application using a similar 5.

(16) Mechanical behaviour of timber-concrete joints. system in the floor of a multiple family dwelling is mentioned by Kuhlmann and Schänzlin (2001). Examples of applications on prefabricated structures are given by Poutanen (Rilem TC111 CST, 1992) who describes wall and floor prefabricated elements where the connections between timber and concrete were assured using nailplates. Another example presented in the same publication describes a prefabricated system, this time using steel U clamps as joints. It is also revealed that there were nine producers of these systems in the world and that in 1990 the production had been more than 400,000m2. More recently, Toratti and Kevarinmäki (2001) presented a new prefabricated system developed by VTT Building Technology in Finland to be used in multi-storey buildings. The composite elements were produced from timber trusses connected to the concrete slab by nailplates. Another application for timber-concrete composite systems are the bridge decks which have already been built for a long time. Nolan (2002) refers to the fact that the Oregon State Highway Department in the USA built this type of bridges at least as early as 1932. The same work refers also to a number of bridges in Australia that were inspected after several years of use. These bridges were built from 1949 to 1980, and had total spans with lengths between 6 and 37.3m. The evaluation carried out shown that even with low maintenance they could be in service for relative long periods if specific aspects, such as the exposition from timber to the water or the contact from timber with the soil, are correctly considered in the design and during the building process. Nauta (1984) describes a number of bridges built in New Zealand since 1970 using two different systems: the first one consists of glue laminated beams connected with a concrete slab by glued shear blocks of timber combined with mechanical fasteners, the second system consisted of nail-laminated slab connected to the concrete slab using notches and triangular steel plates (Figure 2-3). The first system has been used for bridges with spans from 10.8 to 24.5m. The second system was used on a timber bridge with a total span of 6m. Other bridges using similar systems are described in the literature, such as for example one with 9.1m span built in California (Cook, 1977) or another one mentioned in the Rilem TC111 CST Report (1992) with a 4.88m span. Another possibility to build bridges is the use of timber logs as structural elements. This solution constitutes an interesting solution, especially in forest roads were the log elements are available at low costs. Timber logs have been used for many years as a solution for timber decks. This solution had, however, a number of disadvantages as the limited load capacity or a small durability. The combination of the logs deck with a concrete slab can improve significantly the performance of this type of bridges. The composite solution results not only in stronger and stiffer solutions but also the concrete layer is a very important protection against moisture. A good example of this is presented by Natterer et al. (1998), which describes a timber-concrete bridge in Switzerland with a total span of 13m on which the timber logs were connected to the concrete slab by the use of post-stressed dowels combined with notches. Similar solutions are described by Yttrup and Nolan (2002), however, they used simple dowels instead of post-stressed dowels. Recent publications of the Nordic Timber Council (1999, 2002) present 5 bridges recently built in Finland using composite timber-concrete systems. The first one was built in 1997 for road traffic with a span of 15m. The structural system is composed of three glued laminated beams with variable heights connected to a concrete slab with 6.

(17) Literature review. Chapter 2. glued-in rods of 16mm diameter. Similar bridges were built in Oulu and near Pori. The first one had two side spans of 13m and a central span of 16m, and the second one had two spans of 11.6 and 10.8m. A completely different system was used in 2000 to build another timber-concrete bridge with a span of 19m. The main load carrying element consists of a king-post truss, the post members are timber-concrete composite elements (timber round logs) connected using glued in rods. The biggest of these bridges (Vihantasalmi Bridge) was built in 1999 at a distance of 180 km from Helsinki using a mixture of steel, timber and concrete. It has 3 central spans of 42m each and two side members of 21m (Figure 2-1).. Figure 2-1. Vihantasalmi Bridge in Finland. The increased use of this type of structures in practice resulted in an increase in the number of researches about several different aspects of the technique. Those researches were focused either on the proposal and testing of new applications or on the study of different aspects of these systems that are important for practical applications.. 2.2 Calculation models for timber-concrete composite structures Timber-concrete composite structures are the combination of two materials which are expected to interact resulting in a composite structure. Usually that interaction is only partial, increasing the difficulty of the analysis. These difficulties are also increased by phenomena such as the non-linear behaviour of the materials on short and long-term. Most of the models were developed for composite structures with partial composite behaviour in general and then applied for timber-concrete composite systems. Werner (1992) presented a linear model taking into consideration the slip between timber and concrete based on previous work by Newmark et al. (1951) and Möhler (1956) based on 7.

(18) Mechanical behaviour of timber-concrete joints. the differential equations of equilibrium. The model is simple and easy to use, and for that reason it became widely used. This model is presented in the Annex of Eurocode 5 (2003) to analyse the behaviour of the composite structures. However, it has limitations related with the geometry of the elements, the loads applied and the mechanical behaviour of the materials. The elements must have constant sections on the whole length as well as constant joint properties. Besides the load applied must have a sinusoidal distribution in order to have exact solutions and the materials and joints are considered to behave linear elastic. Other models have been developed to overcome these limitations. Natterer and Hoeft (1987) presented models derived from the general differential equation to consider different loading conditions, as for example, point loads, point bending loads or distributed loads varying linearly. The model considers different effective widths of the concrete slab for the different types of loads applied. Girhammar and Gopu (1991) presented a model that included axial forces acting together with bending moments, and second-order effects (P-∆). This approach is useful especially for columns or timber-concrete walls where axial forces are likely to occur. Usually, the analytical methods available are not able to consider non-linear effects and can lead to non-conservative results under certain conditions as it will be analysed in detail later. One solution to overcome the problem is the use of other methods, such as the Finite Element Method that has been tried by different authors, e.g. Zakaria and Ghazali (1989), Ahmadi and Saka (1993), Van der Linden (1999). In most of the cases the non-linear behaviour of the materials or of the joints was the main reason to use this method. A different approach, using the Finite Differences Method, was tried by Timmerman and Meierhofer (1992) to model timber-concrete beams with different joint properties along the longitudinal length and different types of loads.. 2.3 Timber-concrete joints In the first timber-concrete composite structures the connection systems used were copied from timber constructions. Murthy (1984) and Van der Linden (1999) referred to the application of adapted timber-timber joints as the nails or railroad spikes in the first composite structures documented. There are many other joints traditionally used in timber structures that have been adapted to be used in timber-concrete structures. Among those joints, nails are probably the timber fastener more often used in timberconcrete applications. As a consequence they have also been tested in different research works. Stevanovic (1989), for example, presented a method based on the experimental results to consider the mechanical behaviour of nailed joints in the analysis of the composite structures. Ahmadi and Saka (1993) tested different types of nails in shear and bending tests. One of the objectives was the selection of the best nail type to be used in timber-concrete residential floors. Gutkowski and Tser-Ming (1996) performed also shear and bending tests in the composite elements using nails with different properties, as for example the penetration length and the diameter. One of the objectives of the research was the evaluation of the influence from these parameters in the mechanical behaviour of the joints.. 8.

(19) Literature review. Chapter 2. Other types of dowel type fasteners, such as screws, dowels or bolts, have also been used, but they have in most of the cases the disadvantage of requiring pre-drilling. Bolts and screws were tried, for example by Murthy (1984) in the construction of a composite structure for a staircase. The dowels are also commonly used, Gelfi and Giuriani (1999), performed 12 tests using dowels, including 12mm (6 test specimens) and 16mm dowels (6 test specimens). The penetration depth of the dowel in the timber varied, with two test specimens with 3d (diameter), two with 4d, and two with 6d, in both test series. In the test series with 16mm diameter dowels there was either a gap or a timber interlayer with a thickness of 22mm between timber and concrete. Takač (1996) tested timber-concrete joints by means of “one-side bulldog type dowels” which consisted of a tooth plate combined with a dowel. Joints made with nailplates have also been used (Poutanen, 1987, Van der Linden, 1999). In that case, part of the plate is fixed on the surface of the beam, while the other part is inside the concrete slab. Most of these joints, however, allow relatively large slip between timber and concrete resulting usually in low composite actions and consequently in inefficient structures. This problem is particularly sensible for the simplest joints as the dowel type fasteners, therefore many researchers have undertaken studies to develop new joints specifically for timber-concrete with the objective of obtaining more efficient connecting systems. One of the first patented timber-concrete composite systems date from the beginning of the last century. It consisted of nails combined with steel braces (Van der Linden, 1999). Pincus (1970) used epoxy resin and could completely eliminate the slip, however certain problems with the joint were not solved, e.g. the one resulting from the moisture variations, the long-term behaviour or impact loading. Murthy (1984) tried horizontal mechanical shear joints. They were made with mild steel rods that passed through the timber beams, the top of which was slotted in concrete (see Figure 2-2a). It is mentioned that the joint was able to avoid almost completely the timber-concrete slip, particularly for loads up to half of the maximum load. This efficiency was attributed not only to the enclosed bearing of the joint but also to a significant frictional resistance between surfaces of timber and concrete. One attempt to improve the mechanical behaviour of dowels was made by using them combined with epoxy resin. A pre-drilling was done slightly larger than the nominal diameter of the dowel. The hole was then partially filled with epoxy resin, after which the dowel was inserted (Figure 2-2b). One of the advantages of the system resulted from the improved strength and stiffness of the material surrounding the dowel. At the same time any gap that could exist around the dowel was eliminated (Rilem TC111 CST, 1992). A similar strategy was followed in a joint system presented by Messina (Rilem TC111 CST, 1992), but this time V and C steel sections were glued to the top surface of a timber beam (Figure 2-2c). They were first positioned by means of screws, but the forces were transmitted to the timber by the glue. These types of joints are used either without or with interlayer. In the last case holes were made on the interlayer to glue the fasteners directly to the timber beam.. 9.

(20) Mechanical behaviour of timber-concrete joints. Top view Cross section. Cross section. Top view Side view a) Horizontal steel bars. b) Glued steel bars. Side view c) Glued steel profiles. Figure 2-2. Horizontal steel bars, glued-in steel bars, and steel profiles glued to the timber beams. Nauta (1984) described bridge decks where the joints were made by means of notches combined with steel plates and screw spikes (Figure 2-3a). The function of the later was to avoid any possible uplift of the concrete member relatively to the timber member. SFS Provis AG developed one of the first steel fasteners produced specifically for timber-concrete structures (Spirig, 1985). The fastener was produced of high strength steel and had two heads (Figure 2-3b). The upper head was used to screw the fastener and afterwards anchor it to the concrete member, the lower head would be on the surface of the timber member and thus fix the length of the screw in concrete and in timber. Küng (1987) tested coach screws in different arrangements. It was found that driving them with a 60º degree to the horizontal (see Figure 2-3c) could double their load carrying capacity and at the same time increase the slip modulus. This enhanced performance was thought to result from the positioning of the fastener since with such arrangement they are also loaded on tension and compression instead of only on shear. Top view (timber). Side view. Cross Section. a) Notches and steel plates. b) SFS screw. c) Inclined screws. Figure 2-3. Notches combined with steel plates, SFS screw, and inclined screws. Poutanen (1987) presented an innovative system based on conventional nailplates. Part of the nailplates was partially pressed into the side face of the timber beams while the other part remained above them. A thin steel sheet was then placed through the. 10.

(21) Literature review. Chapter 2. nailplates (see Figure 2-4a) to be used as formwork and reinforcement. Finally, the concrete was cast against this steel sheet. One of the strongest and stiffest joints known was presented by Ceccotti and Cocan (1990). It consists of a “T” steel profile with the web inserted and glued to the timber beam by epoxy resin (see Figure 2-4b). The flange of the “T” has an iron lattice welded to it, to be cast with concrete. The shear forces are transmitted from the lattice to the T profile and from this to the timber beam. The high stiffness of the joint results not only from the continuity of the connection along the beam length but also from the high stiffness of the connection timber-steel that eliminates any slip. Another system was proposed in France, in Paris Quest (1988), in which the joint consists of steel tubes inserted 40mm in the timber (see Figure 2-4c). The complete characteristics of this system including design, acoustic, thermal, fire resistance and durability are given in that document.. Cross section. Cross section. Top view. Cross section. Side view. Side view a) Nailplate. b) Tubes. c) T profiles. Figure 2-4. Nailplate, tubes, and T profiles glued to the timber. A prefabricated timber-concrete element was produced in Sweden to be used in floors and walls. The joint system used consisted of steel wire (the material used to produce nails) with a shape of a “U” with both ends inserted into the timber (Rilem TC111 CST, 1992) (see Figure 2-5a). One of the most efficient systems to transmit the shear forces is through a contact surface in compression. It combines relatively low costs with good load carrying capacities and very high stiffness, e.g. notched joints. Many different notched joints have been developed, with different dimensions and shapes that are normally used in combination with steel fasteners in order to avoid the possibility of uplifting. The simplest solution consists of grooves on timber or timber blocks glued to the main timber elements. The first attempts were made already in the 70’s in new Zealand Central Laboratories (Rilem TC111 CST, 1992) where timber blocks were glued to the beams resulting in a block shape (see Figure 2-5b). Yttrup (1996) proposed another system combining notches on the horizontal surface of the timber beam with notches on the vertical surface to prevent the uplifting. This solution proved to be very strong and stiff but requires that part of the timber beam is inside the concrete element.. 11.

(22) Mechanical behaviour of timber-concrete joints. Side view. Cross section a) U profile steel wire. Side view. Side view. Cross section b) Timber blocks. Cross section c) Horizontal and . vertical notches. Figure 2-5. Steel wire with U profile, timber blocks glued to the timber beams, and horizontal notches combined with vertical notches. Van der Linden (1999) tested a notched joint suitable for plate-like composite structures. It consists of a circular notch, with 125mm diameter, drilled in a laminated veneer lumber (LVL). Due to the large diameter of the notch this joint is only suitable for plates or timber beams with a minimum width of 160mm. Ballerini et al. (2002) made tests with notches drilled on the upper surface of timber. They had different configurations and dimensions varying: the depth of the notch, the angle of the notch, and size of the timber area (representing the end distance or the notch spacing). To prevent uplifting, screws were used, which position also varied. In spite of its efficiency to transmit the shear forces, notched joints are usually not able to prevent the uplifting that can occur in the composite beams (except for the joints with vertical notches combined with horizontal notches presented in Figure 2-5c). For that reason they are usually used combined with other steel fasteners. Applications have been presented by different authors, for example Godycki or Kessel (Rilem TC111 CST, 1992) (Figure 2-6a). Natterer (1990) tried to optimize the use of fasteners combined with notches. He presented a notched joint combined with a threaded dowel that was tensioned after the concrete had hardened. This arrangement not only prevents any uplifting of the concrete, but also forced the contact between timber and concrete increasing the friction resistance.. Side view a) Notches combined with steel fasteners. Side view b) Notches combined with post-tensioned bolts. Figure 2-6. Notches combined with steel fasteners, and notches combined with post-tensioned dowels. 12.

(23) Literature review. Chapter 2. Werner (1992) tried a different approach using concrete reinforcement bars surrounded by notches filled with concrete. The use of the notch increases the shear strength but specially the stiffness. The proportions of the dowel and notch diameter are such that both transmit shear forces instead of the traditional arrangement, where the fastener only prevents uplifting and the notch transmits the shear forces (Figure 2-7a). Erler (1992) was also able to almost eliminate the slip between timber and concrete, using a special concrete. The concrete was based on polymers (a resin) filled with sand. The adhesion between the polymers and timber was enough to transmit the shear forces without any other joint. Insa Hilti developed a fastener specifically for timber-concrete joints (Mungwa et al., 1998) (named here Hilti 1). The fastener was composed by two main parts: one that remains in concrete and other that is driven into the timber. The part on concrete is a hollow cylindrical shell with a 27mm diameter and 38mm depth that merges into another hollow cylinder with 20mm diameter and 35mm depth which bottom is dented to improve its connection to timber. The fastener was developed to improve the mechanical performance obtained from the traditional joints and reduce the time/cost of application. Another continuous system was proposed by Bathon and Graf (2000). It consists of a steel mesh which has half height glued on a longitudinal drill open on the top of the timber beam, while the other half is poured with concrete (Figure 2-7b). In spite of its high stiffness it showed also a certain amount of plastic deformation capacity. Another joint specifically developed for timber-concrete was presented by Sonda (2001). It consists of a stud to cast in the concrete connected to toothed plate which was driven into the wood by 2 screws (Figure 2-7c).. Side view. Side view. Top view. Top view. a) Circular notch combined with dowels. b) Steel mesh. c)Tecnaria. Figure 2-7. Circular notch combined with dowel, steel mesh, and Tecnaria joint. Grosse et al. (2001) proposed different joints to connect timber-concrete composite structures composed by a solid layer of timber under a concrete slab. The joints were. 13.

(24) Mechanical behaviour of timber-concrete joints. made either from flat-steel-locks, punched steel panels or concrete cams (see Figure 2-8a, b). They showed good mechanical properties, however, their use is limited to the systems with a concrete deck connected to a massive timber deck. Recently Said et al. (2002) presented another fastener developed by Hilti (named here Hilti 2). Its upper part (concrete) has a conical shape to increase the plastic deformation capacity before failure, and the bottom part consists of a cylinder which was inserted in holes on timber. The adhesion timber fastener was ensured using epoxy glue (Figure 2-8c).. Cross section. Side view. a) Concrete cams. Cross section. Side view. Side view. b) Flat steel locks. c) Hilti 2. Figure 2-8. Joints for layered systems and Hilti joint 2. One of the most efficient joints to connect timber and concrete is gluing. This system can lead to a perfect interaction between the two materials at reasonable prices. There are, however, still problems to be studied, for instance, the very brittle failure and the long-term behaviour of the joint. The development of new adhesives and gluing techniques resulted in new attempts to implement it in practice. Examples of this are the research works performed to test different adhesives and gluing techniques by Negrão et al. (2004) or to study the long-term behaviour by Brunner and Gerber (2002). There are other connection systems used in timber-concrete composite structures not presented here, however, the joint systems presented are expected to be representative from the timber-concrete composite connections in general. Mechanical performance of the different joint types. Usually, the ultimate load and slip modulus (Ks in EN 26891 and Kser in Eurocode 5) are considered to describe the whole load-slip behaviour. For that reason it would be interesting to know the values of these properties for the joints presented above with significantly different load-slip behaviours. These properties can not be quantified exactly for each one of the different joints since they are highly dependent on different aspects involved as, for example, the materials or the test configuration. Nevertheless Table 2-1 gives the results obtained in the literature, they are merely indicative and do not intend to be a reference for that type of joint. The values obtained for one joint were smeared in the longitudinal direction and adjusted to a timber beam with a width of 100mm in transverse direction when the joint was continuous in that direction. The maximum slip value reached for each joint is also presented.. 14.

(25) Literature review. Chapter 2. Table 2-1. Smeared ultimate load and slip modulus for the different joint types Joint. Fmax1. Ks2. δu3. (N/mm). (N/mm/mm). (mm). Reference. Nails 6mm 271 68 15 Dias (1999) Screws 469 49 15 Dias et al. (2004a) Dowels 10mm 226 152 15 Dias et al. (2003) Dowel epoxy 10mm 250 STEP 2-E13 (1995) Nailplate 180 183 10 Van der Linden (1999) SFS-45º 305 405 5 Van der Linden (1999) Hilti 1 296 5144 15 Mungwa et al. (1998) Hilti 2 3004,6 6004,6 >3 Said et al. (2002) Tecnaria 333 128 13 Sonda (2001) Steel mesh 372 1385 >4 Bathon and Clouston (2004) Flat steel locks 1186 2436 >4 Grosse et al. (2001) Dowel with notch 317 494 15 Van der Linden (1999) Notch with postKuhlmann, U. and 525 123 tensioned bolt Michelfelder, B. (2004) 6 6 Notch – Hole in timber 413 663 <1 Ballerini et al. (2002) Glued joints 647 <1 Negrão et al. (2004) 1 – Smeared ultimate strength 2 – Smeared slip modulus 3 – Ultimate slip 4 – Values obtained from graph presented in the paper 5 – Characteristic value 6 – Minimum spacing considered as the spacing used in the shear tests. In order to complement the information given in Table 2-1, Figure 2-9 presents typical load-slip curves for a number of these joints, they allow a direct and easier comparison between the different joint systems.. 15.

(26) Mechanical behaviour of timber-concrete joints. FORCE 100%. Glued joints Notched joints Circular notch with dowel Axially loaded screws. Nailplates. Dowel type fasteners. SLIP 15mm. Figure 2-9. Typical load-slip behaviour for different types of joints. From the load-slip graphs it is clear that the mechanical behaviour of glued or contact joints is completely different from the mechanical behaviour of joints with dowel type fasteners. The first are characterized by high strengths and stiffness with an almost linear behaviour up to the failure which occurs for low values of deformations. On the other hand, the joints made with dowel type fasteners have much smaller strengths and stiffness but much higher plastic deformation capacities. The mechanical properties of joints such as the nailplates or the loaded axially screws are between the glued joints and dowel type fasteners either in terms of ultimate load capacity or stiffness. The long-term behaviour of timber-concrete joints has also been studied in different research programmes. Amadio et al. (2001a) presented results of long-term push-out tests on a special connection system (Tecnaria) using normal concrete and lightweight aggregate concrete. These tests were performed under constant and variable climatic conditions. The constant climatic conditions were characterized by a relative humidity of 70% and a temperature of 24ºC while the variable climatic conditions were characterized by weekly cycles of alternate relative humidity of 50% and 70%. On part of the test specimens, one unloading cycle was also applied. The creep coefficient after 75 days ranged from 0.33 to 0.60. Van der Linden (1999) derived also values for the creep coefficient for 4 different types of timber-concrete joints for periods of loading of 1200 and 18 250 days (aprox. 50 years). These results, however, were not obtained directly from shear tests but derived based on measurements in composite beam long-term tests associated with analytical models and Finite Element Models. The creep coefficients derived varied between 1.4 up to 2.4 after 1200 days and from 2.0 up to 3.4 after 18.250 days (50 years) for the different joints considered. 16.

(27) Literature review. Chapter 2. Higher amount of information is available for timber-timber joints because they were studied in detail by many authors, e.g. Van de Kuilen (1999). On this study the creep effects on different types of timber joints were studied for constant and variable climatic conditions. In variable climatic conditions creep coefficients may reach values of up to 8 after 13 years.. 17.

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(29) Joint behaviour and composite action. Chapter 3. 3 Joint behaviour and composite action 3.1 Models to analyse composite structures Short-term behaviour. The calculations of stresses and deformations in timber-concrete composite structures are usually performed on the basis of linear-elastic behaviour for all the materials and joints, as indicated in Eurocode 5 part 1 (2003). On the basis of these assumptions, the linear models referred to earlier are used, leading to accurate results for most practical applications, decreasing the need for more complex and time consuming non-linear models. This has been already shown by a number of authors, e.g., Dias (1999) and Van der Linden (1999), who compared experimental results with the predictions from the model. Based on this, the analysis of the composite timber-concrete elements is carried out here using that model. In Eurocode 5 the following assumptions are presented: - the beams are simply supported with a span l; - the individual parts are connected to each other by mechanical fasteners with a slip modulus Kt-c; - the connection between timber and concrete is considered to behave in a linear elastic fashion, and its stiffness is constant or varies uniformly with the shear force; - the load is acting in the z-direction, giving a moment M=M(x) which varies sinusoidally or parabolically, and a shear force V=V(x).. 19.

(30) Mechanical behaviour of timber-concrete joints. bc. σc (Eq.3-6) σm,c(Eq.3-7) 0,5hc. hc. ac at. ht. +. =. 0,5ht. σt (Eq.3-6) σm,t (Eq.3-7). bt. Figure 3-1. Parameters of the cross section used on the equations of the composite element and the stress distribution for bending. The calculations are based on an effective bending stiffness calculated using the geometric, and material properties of the elements as well as stiffness and strength of the joint. The effective bending stiffness is given by: ( EI )ef = Ec I c + γ t − c Ec Ac ac2 + Et I t + Et At at2. (3-1). where: At = bt ht ; Ac = bc hc. It =. bt ht3 b h3 ; Ic = c c 12 12. γ t −c. at =. (3-2). ⎡ π 2 Ec Ac sc ⎤ = ⎢1 + ⎥ Kt −cl 2 ⎦ ⎣. (3-3). −1. γ t −c Ec Ac(hc + ht ) ; 2(γ t −c Ec Ac + Et At ). (3-4). ac =. ht + hc − at 2. (3-5). in which: Kt-c is joint stiffness (Ks) for the serviceability limit states calculations, and Ku, for the ultimate limit states calculations with K u = 23 K s , Ks is the slip modulus of the joint determined in accordance with EN 26891 (1991),. 20.

(31) Joint behaviour and composite action. Chapter 3. Ec and Et are the mean values of the elasticity modulus of concrete and timber, respectively, s is the spacing of the fasteners along the beam axis, l is the span of the beam. The normal stresses are given by :. γ t −c Et at M ( x ). σt =. ( EI )ef. σ m,i =. ;. 0,5 Et ht M ( x ) ; ( EI )ef. σc =. γ t −c Ec ac M ( x ). σ m,i =. ( EI )ef 0 ,5 Ec hc M ( x ) ( EI )ef. (3-6). (3-7). The maximum shear stresses in the timber beam are given by:. τ t ,max =. 0,5Et bt ht2 V ( x) bt ( EI )ef. (3-8). The load in the fastener is given by:. F=. γ t −c Ec Ac ac s ( EI )ef. V ( x). (3-9). Considering a uniform distributed load applied to the structure, the midspan displacement is given by:. ωms =. 5 pl 4 384( EI )ef. (3-10). where: p is the load applied on the structure by length unit,. ωms is the midspan displacement. Here, only the final equations of the model are given, the full derivation can be found, for example, in STEP lecture B11 STEP (1995).. 21.

(32) Mechanical behaviour of timber-concrete joints. Long-term behaviour. The long-term effects are an important issue of the mechanical analysis of timberconcrete structures. When the composite structures are subjected to long-term loads, the time dependent behaviour of timber, concrete and joints results in extra deformations and new stress distributions. In many situations the long-term conditions are significantly more severe for the structure and can not be neglected. The model usually followed for the creep calculations is based on the model used for the short-term analysis. This method has been used by different authors, for example, Natterer and Hoeft (1987) or Van der Linden (1999). Ceccotti and Cocan (1990) proposed a number of adaptations in order to improve the accuracy of the results. A completely different approach was proposed by Mungwa and Kenmou (1993a, 1993b) who used a Finite Differences Method. Van der Linden (1999) and Amadio et al. (2001b) used Finite Element Models to predict the long-term behaviour of timberconcrete composite beams. The results obtained with different methods were compared with experimental results from tests on timber-concrete beams by Van der Linden (1999). It was found that the method usually followed was not only the simplest but produced also the best results. The method predicts the long-term behaviour based on long-term values of the elastic properties of the composite structure. These long-term properties are obtained by dividing the short-term properties by the corresponding creep coefficient, as shown in Equations 3-11 to 3-13.. Ec ,t =. Ec ,0 1 + ϕc. (3-11). Et ,t =. Et ,0 1 + ϕt. (3-12). K t −c ,t =. K s ,0 1+ ϕ f. (3-13). where : Et,0, Ec,0, are the short-term elasticity modulus of timber and concrete, Et,t, Ec,t, are the long-term elasticity modulus of timber and concrete, Kt-c,0, Kt-c,t are the short and long-term stiffness of the joint,. ϕt, ϕc, ϕf are the creep coefficients of timber, concrete and joint, respectively. In order to obtain the long term prediction these values are applied to Equations 3-1 to 3-10 instead of the short-term values.. 22.

(33) Joint behaviour and composite action. Chapter 3. 3.2 Experimental assessment of the joint mechanical properties The experimental assessment of strength and deformation properties of timber-concrete joints is usually done according to the European Standard EN 26891 (1991). This standard sets out the rules and principles for the determination of the strength and deformation properties of timber-timber joints made with mechanical fasteners. However, since there is no specific standard for timber-concrete joints, this procedure is normally used. In this standard all the parameters of the load procedure are defined based on an initial estimate of the maximum load (Fest). This estimate is obtained from the experience, from calculations or from results obtained in preliminary tests and is maintained for all the tests, being changed only if during the tests the mean value of the maximum load deviates more than 20% from Fest. The test is performed with load control up to 70% of the maximum estimated load and, from that point, with displacement control. The test ends when the maximum load is reached or when the slip is 15mm. Note that, in accordance with the specifications, the total duration of the test ought to be between a minimum of 10 min and a maximum of 15 min. In Figure 3-2 the time-load curve for the whole test is given. Force Fmax. 0.7Fest. 0.4Fest. 0.1Fest. δ04. δ01 120 150 240. Time (s) 450. 600-900. Figure 3-2. Load-time curve for tests according to EN 26891. The slip measurements ought to be recorded for at least the points presented in Figure 3-2, while the ultimate load needs to be recorded at 15mm or before that if a maximum load is reached. Based on these results, different properties need to be calculated, as, for example, the maximum load (Fmax), and the slip modulus (Ks). The maximum load (Fmax) provides information about the load capacity of the joint, while the slip modulus provides information about the load-slip behaviour of the joint at an elastic stage (stiffness). The ultimate load is obtained directly from the load-slip curve, while the slip modulus is obtained from δ01 (slip measured when 10% of the estimated load is. 23.

(34) Mechanical behaviour of timber-concrete joints. applied), δ04 (slip measured when 40% of the estimated load is applied) and Fest in accordance with Equation 3-14.. Ks =. 0.4 Fest 4 3 (δ 04 − δ 01 ). (3-14). The slip modulus of the joint is then determined on the basis of the estimated ultimate load of the joint and the slip in two points. In the serviceability limit states, the stiffness of the joint is considered the slip modulus Ks. In the ultimate limit states, the joint stiffness is considered as 2/3 of Ks (Eurocode 5). Discussion of the calculation method. According to the standard, the test procedure and the calculations are related to the estimated load Fest, and only indirectly linked with the maximum load Fmax, because the estimated load should not differ by more than 20% from the mean value of the maximum load measured in the tests. If they do, the load settings must be changed and the calculations adjusted to the new estimated load. This procedure would always be exact if the behaviour of the joint was perfectly linear-elastic at least up to 40% of the ultimate load. This is almost the case with glued or notched joints, for instance, but joints made with steel fasteners show a significant non-linear behaviour from the early beginning of the tests. In the latter case, the estimated load as well as the points where the slip is estimated has a significant influence on the determination of the slip modulus. In practice, the differences are identified using 2 different methods, when there is the possibility of having different slip modulus for the same joint depending on the Fest, and when there is the possibility of having higher slip modulus for more flexible joints due to the joints behaviour after the 0.4Fest. Here a load-slip curve from a timber-concrete joint made with a lag screw is considered (Dias et al., 2004a) and the slip modulus calculated in three different situations: with the Fest exactly equal to the ultimate load Fmax, equal to the inferior limit allowed by the standard (Fest = 0.8 Fmax), and finally, equal to the superior limit (Fest = 1.2 Fmax). Figure 3-3 gives the load-slip curve as well as those corresponding to the slip modulus calculated with the various ultimate loads.. 24.

(35) Joint behaviour and composite action. Chapter 3. 60. Force - F (kN). 50 40 30 Load slip curve Ks - 0.8Fmax Ks - Fmax Ks - 1.2Fmax 2/3Ks - Fmax. 20 10 0 0. 3. 6 9 Slip - δ (mm). 12. 15. Figure 3-3. Stiffness from a timber-concrete joint with a lag screw calculated using various values of Fest and the ultimate slip calculated as indicated in Eurocode 5.. Table 3-1. Slip modulus for different values of Fest. Fest (kN). 0.4Fest (kN). Ks (kN/mm). 0.8Fmax. 47. 28. 15. Fmax. 59. 24. 10. 1.2Fmax. 71. 19. 7. The results from Figure 3-5 and Table 3-1 clearly show that the influence of the estimated ultimate load on the slip modulus is significant. The value obtained for 0.8Fmax is more than double the value obtained for 1.2Fmax. It can be concluded therefore that the differences obtained for the value of the joint slip modulus are higher than 100% even if the tests are performed according to the standard, solely because of a different load estimation. Nevertheless, it should be noted that, if the load estimation is higher, the speed of the test is also higher and in those circumstances it is probable that the deformations for the same load level will be smaller. In any case, it is not likely that an increase of 40% in the speed of the load will decrease the deformation by 50%. The second situation considered has a fictitious joint with exactly the same load-slip behaviour up to 0.4Fmax of the load but with horizontal yielding after that. The slip modulus was then calculated for the two joints (Fest = Fmax). The value of the slip moduli obtained were 10 and 22kN/mm for the joint with the normal yielding and for the joint with horizontal yielding, respectively.. 25.

(36) Mechanical behaviour of timber-concrete joints. 60. Force - F (kN). 50 40 30 20 Load-slip curve - Normal yielding Load-slip curve - Horizontal yielding Ks - Normal yielding Ks - Horizontal yielding. 10 0 0. 3. 6. 9. 12. 15. Slip - δ (mm). Figure 3-4. Load-slip curves and stiffness for joints with constant and increasing load after the yielding. In order to achieve the same slip modulus for the fictitious load-slip curve (horizontal yielding), the deformations should be multiplied by a factor 2 (Figure 3-4). These results show that joints with similar load-slip behaviour can have very different slip moduli, or joints with the same slip modulus can have completely different load-slip behaviours. It might be argued that the joint is less stiff but the loads allowed are higher. This is true, but the difference is only between the 0.4Fmax of the weakest joint and 0.4Fmax of the strongest joint, because in up to 0.4Fmax of the weakest joint the behaviour is exactly the same for both joints. On the other hand, the consideration of Ku as 2/3Ks can lead to load-slip behaviours that do not represent the actual behaviours of the joint in the structure. In the example presented in Figure 3-3, the actual deformations on the joint for the maximum load were 15mm, almost twice of the expected 8mm. This problem is caused by the difference in the stiffness before and after yielding. The slip modulus Ku is based on the stiffness before the yielding but the slip in failure is highly influenced by the stiffness after the yielding.. 3.3 Importance of the joint in the composite structures 3.3.1 Introduction An efficient composite solution is only obtained when an effective joint is used to connect timber and concrete. An effective joint is a joint with enough strength to transmit the shear loads developed in the interface and stiff enough to limit the slip between timber and concrete. This point has been referred to by various authors as, for example, Van der Linden (1999) and Ceccotti (2002). Two extreme situations of a simple supported beam are shown in Figure 3-5: no connection (materials acting completely independent) and a perfect connection between the materials (full composite action).. 26.

(37) Joint behaviour and composite action. Chapter 3. Stress distribution Deflection. Slip. Figure 3-5. Deformation and stress distribution with and without composite action. In the first case, there is slip between timber and concrete resulting in two materials resisting independently to the bending loads and consequently to compression and tension stresses. In the second case, both materials are forced to act together resulting mostly in tension stresses in timber element and compression stresses in concrete. This stress distribution results also in different strains resulting in much lower deformations in the composite element. For these reasons the mechanical behaviour of the joint has a significant importance in the behaviour of the composite structures. It has a direct influence in the stresses as well as in the deformations of the structure. This section is focused on the most important properties of joints as regarding their influence in the composite structure. The assumptions made here were oriented for current applications with small to medium spans (up to around 10m) and small to medium distributed loads (up to around 10kN/m2) causing essentially bending. For other types of structures, as, for instance, bridges, where the point loads are significant and the spans usually higher than 10m, the analysis made here may not be always valid. The analysis of the importance of the joint properties will be focused on three different aspects: linear elastic behaviour, non-linear behaviour, and long-term behaviour. Ideally, the analysis of timber-concrete composite structures should be based on the actual load-slip curve, however, the behaviour of timber-concrete joints is usually represented by a limited number of properties obtained from the load-slip curve, expected to be representative of the actual behaviour. The properties with influence in a linear elastic analysis can be easily identified from the linear elastic model normally used (see section 3.1). The model uses two properties of the joint, the stiffness through the slip modulus, and the strength through the ultimate load capacity of the joint. The slip modulus is necessary to determine the effective bending stiffness (EI)ef (Equations 3-1, 3-4 and 3-5). This parameter is the basis for the calculation of all the stresses (Equations 3-6, 3-7 and 3-8), shear forces (Equation 3-9) and deformations (Equation 3-10), thus the slip modulus has indirectly influence on all the mechanical stresses and deformations applied on the structure. On the other hand, the ultimate strength is necessary to verify the shear load capacity of the fasteners 27.

(38) Mechanical behaviour of timber-concrete joints. (Equation 3-9). Considering these two properties, the behaviour of the joints is represented by a bilinear load-slip curve defined by the slip modulus and the ultimate load capacity. In a non-linear analysis, the non-linear phenomena need to be considered. In that case, the behaviour of the joints can not be represented only by those two properties. Additional properties need to be considered in order to represent the non-linear behaviour of the joints. From the several properties that influence the joint behaviour, two will be further analysed: the elastic non-linearity and the plastic deformation capacity of the joint. The analytical model presented earlier to analyse the long-term behaviour of timberconcrete composite structures requires three different long-term properties, the creep coefficients of timber, concrete and joint. Therefore, the long-term influence of the joint behaviour on the composite structure is made through the creep coefficient ϕf. At longterm, the failure stresses of timber tend to be smaller and thus the failure loads tend also to decrease the so called load duration effects. Therefore, there are other properties not taken into consideration in the model used but that need to be considered, as, for example, the long-term carrying capacity of the joint.. 3.3.2 Influence of the joint stiffness in the behaviour of composite structures As mentioned earlier, if the analysis of the composite structures is made using the elastic models from Eurocode 5, the stiffness of the joint is considered through its slip modulus/spacing. The property is necessary to calculate the value of γt-c (Equation 3-4) used in Equation 3-1 to determine the value (EI)ef (effective bending stiffness). The level of composite action in the composite structure is zero without any fastener or full with an infinitely rigid connection. Theoretically, the ratio between the values of these two effective stiffnesses can be calculated and the maximum ratio possible to obtain is four. This value occurs, however, only for certain combinations of geometric and material properties, this analysis was addressed by Van der Linden (1999). This leads directly to a maximum decrease of the deformations of 75%, which may be important since the deformations often governs the design for this type of structures. Additionally, the increased bending stiffness is also advantageous for the stress distribution for the reasons pointed earlier. From the equations presented, it becomes evident that the level of composite action increases with the increase of the slip modulus. This increase, however, it is not linear. It tends asymptotically to the maximum for infinite values of slip modulus and to the minimum for zero values of the slip modulus (Figure 3-6).. 28.

(39) Joint behaviour and composite action. Chapter 3. 4.5 4 EIef * 1012 (Nmm 2). 3.5 3 2.5 2 1.5. span 9 m. 1. span 6 m span 3 m. 0.5 0 1. 10. 100 K (N/mm/mm). 1000. 10000. Figure 3-6. Relationship between the bending stiffness of the composite structure and the slip modulus of the joint on a logarithmic scale (Van der Linden, 1999). From the Figure is clear that above certain levels the increase of the slip modulus becomes useless due to the small effect it has on the bending stiffness of the composite structure. On the other hand, below certain values of the slip modulus the composite action becomes negligible and thus is not worth to use composite systems. In order to evaluate the importance of the slip modulus, an analysis was made to evaluate the slip modulus above which the composite action obtained in the structure was high (close from perfect composite action), and to evaluate the slip modulus below which the same composite action was low, almost negligible. Different material and geometrical conditions were considered in order to have results representative for a wide number of practical applications. The evaluation of the effectiveness of the composite action was based on the value of the effective bending stiffness, (EI)ef (Equation 3-1). The comparison was made using the coefficient of composite action (γS) calculated as a relation between the minimum, maximum and partial bending stiffness according to Equation 3-15.. γS =. EI comp − EI min EI max − EI min. (3-15). where: EI max is the bending stiffness for a perfect composite action,. EI comp is the bending stiffness for the partial composite action, EI min is the bending stiffness for a zero composite action. The coefficient of composite action varies from 0, when there is no composite action, up to 1, when the composite action is perfect. In these calculations, high composite action was considered ensured when the composite coefficient was higher than 0.95, while the 29.

(40) Mechanical behaviour of timber-concrete joints. low composite action was considered when the composite coefficient was lower than 0.1. To perform the calculations, a number of relations and values for geometric and material properties were necessary, and for that reason certain assumptions had to be made. In the first place, the relation between timber width, concrete width, timber elasticity modulus and concrete elasticity modulus was assumed to be given by Equation 3-16 (CP). In order to cover a wide range of conditions, the value of that parameter was considered with different values: 30, 24, 18, 12, 6 and 3.. CP =. n × bc bt. (3-16). where:. n=. Ec Et. bc is the width of the concrete member, bt is the width of the timber member, Ec and Et are the mean values of the elasticity modulus of concrete and timber, respectively. The parameter n was assumed to be always 3 since it is a realistic value. Nevertheless, if the value is different, this does not mean that the calculations made here are not valid, but only that relations between the other parameters will be slightly different. For the values considered here CP=30 means that if the timber beam has a width of 100mm the concrete slab has a width of 1000mm. On the other hand, for CP=3 timber beam width of 100mm corresponds to a concrete width of 100mm, resulting in a timber layer under a concrete slab. In this analysis it was also assumed that the ratio between the maximum and minimum bending stiffness EImax / EImin was always 4. Finally it was also assumed that the width of the timber beam was always equal to its height divided by 2.5, in order to optimize the use of the timber section. Using these assumptions it was possible to determine all the parameters except the height of the timber beam. This was calculated in order to meet a certain design requirement for the composite beam. The requirement chosen was the maximum midspan displacement allowed. In order to have realistic conditions, the displacement was considered to be l/600 in a situation of perfect composite action of the composite beam for a load of 4kN/m2 uniformly distributed in the whole concrete slab. It was not necessary to define the beam span because the coefficient of composite action is independent of it, provided that the design requirement, tension/compression or deformations, is kept constant (here l/600). In the parametric calculations, the value of the smeared slip modulus was calculated to obtain coefficients of composite action from 0.1 and 0.95 for each value of CP. The. 30.

(41) Joint behaviour and composite action. Chapter 3. values obtained were adjusted to the same width (100mm) of the timber beam (Figure 3-7). 12 coeff. comp. action = 0.95. 1000. 10. 800. 8. Notch. 600. 6. Dowel with notch. 400 coeff. comp. action = 0.10 200. 4. SFS - Screw. 2. Nailplate. 0. Smeared slip modulus γ S =0.10 (N/mm/mm). Smeared slip modulus γ S = 0.95 (N/mm/mm). 1200. 0 0. 6. 12. C P (-). 18. 24. 30. Figure 3-7. Smeared slip modulus required for a high and a low composite action. The results show that the importance of the slip modulus increases with the increase of the relation nbc/bw (CP). It is also clear from the figure that only using continuous joint systems, as for instance the steel mesh glued to timber (Bathon and Clouston, 2004), it is possible to achieve a high composite action for certain combinations of geometric and material properties. In any case, a coefficient of composite action of 0.95 is only reached using joints with high stiffness (slip modulus higher than 400 N/mm/mm). Such values of slip modulus are only possible with the use of glued systems or notches. On the other hand, a value of composite action higher than 0.1 is possible to achieve with any type of fastener. The value of slip moduli required for this is between 2 and 7 N/mm/mm, and the lowest value presented on Table 2-1 is 49 for the lag screws. In order to evaluate the level of composite action that would result from the use of this joint, extra calculations were made. For each value of the parameter CP the coefficient of composite action was calculated using the slip modulus of the lag screw. The values of γS obtained varied between 0.44 and 0.86 for CP values of 30 and 3, respectively. These results show that even the most flexible joints can lead to reasonable composite action. Moreover, they show that generally the composite action is at least 50% even with the use of the most flexible joints available. These facts clearly show that slip modulus is an important property in the mechanical behaviour of timber-concrete floors. For this reason, the decision about what joint type to use should consider it since the final behaviour of the composite structure clearly depends on the joints behaviour through their slip moduli.. 3.3.3 Influence of the joint strength in the behaviour of composite structures On the linear elastic models for timber-concrete composite beams, the strength of the joint is necessary for the transmission of the shear forces. Therefore, in normal conditions it does not influence the deformations nor the stress distribution directly,. 31.