Timber-concrete Composite Structures Research Articles

Experimental study of failure of glulam-concrete composite beams

AbstractThis paper is dedicated to the memory of Dr. Miklós Iványi, who instilled in the authors an appreciation for experimental investigations, which are foundational to understanding material and structural behavior. Timber-concrete composite structures are increasingly adopted for new buildings due to their favorable sustainability parameters and the increased availability of cross laminated timber. For larger spans, however, solid timber floors lead to higher timber volumes and the use of glulam beams may become necessary for a more efficient use of wood. This paper presents laboratory tests of glulam-concrete composite beams and is the first in a series of two papers on investigating the associated failure mechanisms. Three full-scale glulam-concrete beam specimens were studied. The glulam and concrete are monolithically interconnected using a continuous layer of adhesive. Shear reinforcement was added to the glulam beams to allow for failure mode control. Static load tests to failure were conducted along with acoustic emission monitoring to track the progression of the failure. The results indicate that the shear reinforcement of the glulam layer affects the load capacity of the composite beam through shifting the failure from a shear to a tension failure mode. Similar glulam-concrete beams can enable larger span applications for buildings and bridges while maintaining an attractive sustainability performance.

Verification of a Simplified Design Method for Timber–Concrete Composite Structures with Metal Web Timber Joists

This study presents a comprehensive analysis of a simplified design methodology for timber–concrete composite roof and floor structures employing metal web beams, also known as posi-joisted beams, easi-joist, or open web joists, validated through both laboratory experiments and finite element (FE) method analyses. The proposed method integrates the transformed section method and the γ-method, as outlined in Annex B of EN1995-1-1 for mechanically jointed beams. The investigation focuses on roof and floor structures featuring posi-joisted beams, oriented strand board (OSB) sheets connected by screws, and a layer of concrete bonded to the OSB sheets using epoxy glue and granite chips. Two groups, each consisting of four specimens, were prepared for the laboratory experiments. Each specimen comprised two posi-joisted beams, 1390 mm long, connected by OSB/3 boards measuring 400 mm in width and 18 mm in thickness. The beams had a cross-sectional depth of 253 mm, corresponding to beams of grade PS10, with top and bottom chords made from solid timber (95 mm × 65 mm). Bracing members with cross-sections of 100 mm × 45 mm were used to join the bottom chords of the beams. A layer of self-levelling mass SakretBAM, 50 mm thick, was bonded to the OSB/3 boards using SicaDur 31 epoxy glue and granite chips (16–32 mm). The specimens underwent three-point bending tests under static loads, and FE modelling, conducted using Ansys R2 2022 software, was employed for both experimental groups. A comparative analysis of results obtained from the simplified design method, FE simulations, and experimental data revealed that the simplified method accurately predicted maximum vertical displacements of the roof fragment, including posi-joisted beams, with precision up to 11.6% and 23.10% in the presence and absence of a concrete layer, respectively. The deviation between normal stresses in the chords of the beams obtained through the simplified method and FE modelling was found to be 7.69%. These findings demonstrate the effectiveness and reliability of the proposed design methodology for timber–concrete composite roofs with posi-joisted beams.

Experimental Analysis of Mechanical Behavior of Timber-Concrete Composite Beams with Different Connecting Systems

Timber-concrete composite structures are innovative structural systems which have become the subject of extensive research and practical usage, primarily due to their attractive mechanical properties. This article deals with the experimental procedure and the analysis of the mechanical behavior of two different series of timber-concrete composite beams with the same span and geometry of cross-sections. In the first BF-series, the screws were used as a connecting system between the timber and concrete parts, whereas in the BN-series the combination of notches and screws, as a more complex system, was used for the same purpose. Both series were exposed to loading up to a failure by means of the standard four-point bending test. The mechanical behavior of the BF and BN-series beams was analyzed by a comparative analysis referring to: the correlation of the failure loading and the deflection, mechanisms of failure, the strain development across the height of mid-span’s and support’s cross-sections, the horizontal displacement in the timber-concrete interlayer at the support zones, the value of shear stresses and the calculated values of the effective bending stiffness of the beams. The differences in bearing capacity between both series of beams were negligible (about 5%), the effective bending stiffness of BF beams is lower for 32.86% compared to the BN-series and the average value of deflections in BF-series beams is twice as high than in the BN-series. The BN-series beams showed better mechanical behavior in aspects of development of shear stresses in support zones, exhibiting lower shear stress values with an average of 40%.

Design and analysis of timber-concrete-based civil structures and its applications: A brief review

Abstract Builders, designers, and the research community are becoming interested in incorporating timber and concrete based composite structures because they effectively integrate the structural qualities of timber and concrete. The structure’s stiffness, ductility, and load capacity are all affected by the quality of the timber connections used in construction. However, timber-concrete-based structures are limited due to a lack of design knowledge and the brittle failure behaviour of timber under shear or tensile loading. Experimental, numerical, and analytical methods have been proposed in the literature, and the key parameters influencing the performance of timber-concrete structures have been discussed. This study addresses the current challenges in designing timber-concrete connections and their failure modes and suggests simple performance-based analytical models that determine the failure mode. It looks at some of the best numerical design methods used in the past and tries to determine the best way to use timber as a possible way to use safe design principles for timber–concrete composite structures in the future.

Novel Use of Scanning Methods to Investigate the Performance of Screw Connections in Timber-Concrete Composite Structures

This paper investigates the shear force capacity, stiffness, and effective length of the connection screws in timber-concrete composite structures. Ten samples (six hardwood and four softwood) were fabricated with the connection screws installed at different angles through the interface. The shear force capacities and the global stiffness characteristics of the connections were determined directly from double shear tests. The local characteristics of the screw connections were investigated by scanning the final residual screw shapes at the end of the tests for softwood specimens. Using the 2D digital scans of the screws, the screw curvatures were determined. From the curvatures, the local distribution of moment along the screw embedded within the concrete at the conclusion of the test was estimated. The distance of the plastic hinge in the screw within the concrete from the interface between the concrete and timber (the effective length) was obtained from the maximum bending moment location calculated via this image scanning method. Empirical equations of effective screw length were developed from the test data and applied in a shear force capacity model for softwood. These new equations of effective length of inclined screws in connections predicted the shear force capacity of the connection better on the softwood specimens. In hardwood specimens, the screw failed in snapping. An equation of shear force capacity was developed based on the influence of the inclination angle of the screw with the reduction factors and can predict the shear capacity of the connection in hardwood specimens.

Simulation of Load–Slip Capacity of Timber–Concrete Connections with Dowel-Type Fasteners

Quality assessment of stiffness and load-carrying capacity of composite connections is of great importance when it comes to designing timber–concrete composite structures. The new European regulation intended explicitly for timber–concrete structures has made a significant contribution to this field, considering that until today there was no adequate design standard. Due to the proposed general expressions for determining the stiffness and load-carrying capacity of composite connections made with dowel-type fasteners, which are incapable of describing most of the commonly applied fasteners, engineering, and scientific practice remained deprived of a quality assessment of the essential mechanical properties of the connection. In order to overcome this problem, this paper proposes a numerical model of the connection suitable for determining the whole load–slip curve, allowing it to estimate the stiffness and load-carrying capacity of the connection. The model was developed by considering the non-linear behavior of timber and fasteners, which is determined through simple experimental tests. For the numerical model validation, experimental tests were carried out at the level of the applied materials and on the models of the composite connection. Through numerical simulations, analysis of obtained results, and comparison with experimental values, it can be confirmed that it is possible to simulate the pronounced non-linear behavior of the timber–concrete connection using the proposed model. The estimated values of stiffness and load-carrying capacity are in agreement with the conducted experimental testing. At the same time, the deviations are much less than the ones obtained from recommendations given by the new regulation. Additionally, apart from evaluating the value and the simulation of the complete curve, it is possible to determine local effects, such as the crushing depth in timber and concrete, the fastener’s rotation, and the participation of forces in the final capacity of the connection.

Optimizing composite floors for sustainability and efficiency: Cross laminated timber, concrete types, and ductile notch connectors with enhanced shape

Timber-concrete composite (TCC) structures allow designing efficient building floors in terms of lightness, slenderness, and acoustic insulation. The late development of ductile notch connectors has further advanced TCC structures. This work aims to further optimize the shape of ductile notch connections and to optimally design TCC floors with minimum embodied carbon, floor thickness, self-weight and cost.Firstly, an experimental campaign was carried out to characterize the shear behavior of ductile notch connectors with different shapes, which varied from rectangular to trapezoidal to avoid the use of screws, and concrete types, which varied from normal concrete to ultra-high performance fiber reinforced concrete. For the sake of comparison, a glued connection was also considered. Then, a multi-criteria optimization method was performed to optimally design TCC floor with the developed notch connectors and concrete types. The latest design recommendations for TCC floors developed in Canada were considered. A Non-Linear Finite Element Analysis (NLFEA) was carried out for validation. The presented results push the current design limits in terms of embodied carbon and volume of construction materials towards climate-positive floors.

Flexural behaviour of timber-concrete composite floor systems linearly supported at two edges

Timber-concrete composite (TCC) structures consist of thin concrete slabs connected by stud or bonding to glulam beams. They emerged as a valuable system with light-weight elements characterised by high strength and stiffness. However, the structural efficiency of real-scale TCC floors with an adhesive-based connection systems requires further investigation to confirm the promising results of small-scale experimental studies.To contribute to this investigation, this study conducts real-scale experiments on five timber-concrete composite floor specimens that are linearly supported at two edges. The flexural behaviour of the system is assessed using a four-point bending test, and its strength and displacement capacity are also tested. The study also provides insights on the strain profile and relative displacement of the system’s cross sections.The experimental results indicate that the composite structure has adequate mechanical behaviour that is characterised by a linear response up to failure, without relative displacement at the concrete–timber interface surface. The study confirms the potential of TCC floors as prefabricated construction elements, demonstrating that the system complies with standard performance requirements.

Experimental and theoretical investigation on shear performances of glued-in perforated steel plate connections for prefabricated timber–concrete composite beams

Glued-in perforated steel plate (GIPSP) connections demonstrate significant shear strength and high slip modulus. Consequently, they indicate substantial potential for application in timber–concrete composite (TCC) structures according to the emerging tendencies in high-storey and large-span buildings. However, the application pattern in prefabricated TCC structures and the theoretical analysis of the shear performances of GIPSP connections are highly deficient. This hinders the application of this type of shear connection. In this study, the shear performances of GIPSP connections were evaluated using push-out tests. Ten groups of push-out specimens with different steel plate numbers, steel plate lengths, and concrete slab types were tested. The concrete slab types investigated in the experiments included a prefabricated concrete slab and cast-in-situ concrete slab. The experimental results were discussed in terms of the failure mode, load-carrying capacity, and slip modulus. The theoretical models for the load-carrying capacity related to the associate failure mode were discussed based on an analysis of the failure mechanisms. In addition, design proposals with regard to the load-carrying capacity and slip modulus of the GIPSP connection were presented. The research results can provide design guidance for TCC beams using GIPSP connections and prefabricated concrete slabs.

Shear Behaviors of Steel-Plate Connections for Timber-Concrete Composite Beams with Prefabricated Concrete Slabs

To promote the development of timber-concrete composite (TCC) structures, it is necessary to propose the assembly-type connections with high assembly efficiency and shear performances. This article presented the experimental results of the innovative steel-plate connections for TCC beams using prefabricated concrete slabs. The steel-plate connections consisted of the screws and the steel-plates. The steel-plates were partly embedded in the concrete slabs. The concrete slabs and the timber beams were connected by screws through the steel-plates. The parameters researched in this article included screw number, angle steel as the reinforcement for anchoring, and shallow notches on the timber surface to restrict the slip of the steel-plates. Experimental results were discussed in terms of failure modes, ultimate bearing capacities, and slip moduli. It was found that increasing the number of screws could lead to the obvious improvement on the ultimate bearing capacities and the slip moduli at the ultimate state; and the angle steel as the reinforcement showed the slight influence on the ultimate bearing capacities and the slip moduli. The application of the shallow notch can greatly improve the ultimate bearing capacities and the slip moduli. The calculation models for the ultimate bearing capacities and the slip moduli of the steel-plate connections with and without shallow notches were proposed, which showed good accuracy compared with the experimental results.

Experimental analysis of timber-concrete composite behaviour with synthetic fibres

With the growing importance of the principles of sustainable construction, the use of load-bearing timber-concrete composite structures is becoming increasingly popular. Timber-concrete composite offers wider possibilities for the use of timber in construction, especially for large-span structures. The most significant benefit from combining these materials can be obtained by providing a rigid connection between the timber and concrete layers, which can be obtained by the adhesive timber-to-concrete connection produced by the proposed stone chips method. A sustainable solution involves the abandonment of steel longitudinal reinforcement. The use of such a solution in practice is often associated with fears of a fragile collapse. Therefore, the issue of how to increase the safety factor of the proposed material is topical now. The experimental investigation is made to determine the effect of synthetic fibre use on timber-concrete composite behaviour by testing a series of timber-concrete composite specimens with and without fibres in the concrete layer. The obtained results show that adding 0.5 % of synthetic macro fibres allows to abandon the use of longitudinal steel reinforcement and prevents the formation of large cracks in concrete and the disintegration of the concrete layer in case of collapse.

Experimental and numerical analysis on timber-concrete connections with glued reinforcing bars

Shear connectors have a great influence on the mechanical behavior of timber-concrete composite structures, interfering with the stiffness and strength of the structural elements. This paper presents experimental and numerical studies about the mechanical behavior of timber-concrete composite structures by evaluating two types of connectors: one formed by an inclined rebar and another formed by the association of an inclined rebar with a triangular notch. The rebars used were ribbed-type with yield strength equal to 500 MPa, and their fixation on timber was made by gluing with epoxy resin. The experimental determination of the strength, the slip modulus and the failure modes were made by means of push-out tests. Subsequently, a numerical evaluation using ABAQUS was carried out in order to develop a three-dimensional numerical model that represented these types of connections, considering the nonlinearities of the materials, the orthotropy of timber and the interaction between the components. It was observed that the notch connection showed greater strength and stiffness when compared to the connection without notch. The comparison between the numerical and experimental results showed that the numerical model was able to predict, with good approximation, the failure load and the slip modulus of the connection without notch, while the stiffness of the numerical model proved to be higher than that observed in the experimental analysis for the notch connection.

Non-Destructive Quality Control of the Adhesive Rigid Timber-to-Concrete Connection in TCC Structures

One of the limitations of using glued connections in practice is related to the need for connection quality control. Still, the need for the non-destructive quality control of finished products to determine the compliance of the developed structure with the designed one still exists. Considering the small amount of research on timber–concrete composites with glued connections, there is a lack of research on non-destructive methods for the quality control of rigid connections in timber–concrete composite structures. During the literature analysis, no information was found on the possibilities of testing the quality of the rigid timber-to-concrete connection. Therefore, two well-known methods—operational modal analysis and ultrasonic testing—were tested to verify the possibilities of applying these methods in determining defects in the rigid glued connection between the concrete and timber layers in the timber–concrete composite structures. A series of small-scale specimens produced by the stone chips method with and without artificially made defects in the timber-to-concrete adhesive connection was tested by both methods. Operational modal analysis shows significant changes in mode shape, frequency values, and spectral density diagrams. Despite the sufficiently large reflection of the ultrasonic signal on the timber and concrete boundary, the transmitted signal is sufficient to perform local ultrasonic tests for detecting defects in the adhesive connection. Thus, it is concluded that the principles of both methods can be applied in practice, and further research is needed to develop testing technology.

Behavior of horizontal steel plate + studs connectors in a glulam-UHPC composite system: Experiment and analysis

Compared with conventional timber–concrete composite structures, glued laminated timber (glulam)–ultrahigh–performance concrete (UHPC) composite (GUCC) structures can remarkably reduce the deadweight and have small long-term deflection, good durability, and improved span capacity. Thus, this work designed a horizontal steel plate + stud (HS) connector with high stiffness and high shear capacity to ensure a reliable connection in the designed glulam–UHPC composite system and then investigated the structural behavior of the HS connector in the GUCC. In total, fifteen push–out specimens (in five groups with three replicates of each group) were fabricated and conducted a static load test. Effect of size of steel plate, the height-diameter ratio of stud, size of slab, and slab type (UHPC or NC) on the load-slip rules, shear stiffness, failure mode, and the shear capacity of HS connectors in GUCC were evaluated experimentally. The experimental results show that the average slip modulus and shear capacity of the HS connection ranges from 134.9 to 165.6 kN/mm and from 167.6 to 200.3 kN respectively. The primary failure mode of the structure comprises the shear failure of the glulam along the grain and the studs. Lastly, the empirical formula for calculating the load–slip relationship and the design formula for the shear capacity of the HS connection of GUCC systems are proposed. A comparison between the theoretical and experimental results reveals that the developed method has reasonable accuracy and can be used to guide the design of GUCC structures.

Review of connections for timber-concrete composite structures under fire

A timber-concrete composite structure has the advantages of energy saving, environmental protection, and low carbon, and has wide application prospects. However, the effects of fire on timber-concrete composite structures are complicated. It is important to study the fire performance of connections and their influencing factors for the promotion and application of timber-concrete composite structures. This paper summarizes the research progress of connections for timber-concrete composite structures under fire. Firstly, research on the performance of connections in timber-concrete composite structures under fire is introduced, including screwed connections, notched, and grooved connections, and steel truss plate connections. Secondly, the calculation methods focused on connections of timber-concrete composite structures under fire are introduced. Finally, the main points of modeling timber-concrete composite structures under fire are also briefly introduced.

Energy-based time equivalent method for evaluating the fire resistance of timber–concrete composite structures exposed to travelling fires

Timber–concrete composite (TCC) structure system is most frequently used in large-span structures. Recently, a performance-based fire safety design method named improved travelling fires methodology (iTFM), which is suitable for long-span structures, has been developed. In this article, an energy-based time equivalent method (EBTEM) is proposed, while its effectiveness is experimentally verified. The proposed method can be used to evaluate the fire resistance of the TCC structures subjected to iTFM fires. The results indicated that the damage degree at different spatial positions of the TCC structure under the same fire size was different, and the location with the least and most serious damages occurs towards the beginning and middle of the fire path, respectively. The results show that this difference in damage degree mainly depends on the fire size, and the difference becomes less pronounced as the fire size increases. Concomitantly, it can be obtained that the damage degree under the different fire sizes was different, and the smaller the fire size, the greater the damage degree. Hence, when the EBTEM is adopted to design the TCC structures exposed to iTFM fires, the setting of fire protection target is largely determined by the actual fire size.

Experimental and finite element studies of prefabricated timber-concrete composite structures with glued perforated steel plate connections

To evaluate the feasibility of adopting glued perforated steel plate (GSP) connections in prefabricated timber-concrete composite (PTCC) structures, two series of tests were conducted in this study: push-out tests on prefabricated GSP (PGSP) connections and bending tests on PTCC beams. The same experimental programmes on cast-in-situ timber-concrete composite (TCC) structures with this type of connection were also performed. For validation, the three-dimensional finite element (FE) models considering the orthotropic elastic–plastic damage behavior of the timber, damage plasticity characteristics of the concrete, fracture of the adhesive, and the nonlinear shear-slip behavior of the connections were developed. The test results demonstrated that the glued perforated steel plate connections have excellent slip stiffness and shearing capacity but low ductility due to the brittle fracture of the adhesive. Owing to the high anchorage performance of the grouting cements and low shrinkage behavior of the dry construction method, the connection stiffness and load-carrying capacity of PGSP are higher than those of GSP. The PTCC beams showed better bending performance than the cast-in-situ TCC beams as a result of the higher slip stiffness of PGSP than that of GSP. Additionally, the FE numerical results are in good agreement with the test results in terms of the failure modes, shear-slip behavior of the connections, and bending performance of the composite beams.

Shear performances of hybrid notch-screw connections for timber-concrete composite structures

This paper presents the push-out experimental results of hybrid notch-screw (HNS) connections for timber-concrete composite structures. A total of 7 groups of specimens were designed and tested. The experimental parameters included the loading constraint conditions (i.e., the test specimens were loaded either in local compression or in uniform compression), shapes of notches in the wood, screw number in notch, notch width, and the inclusion of a self-tapping screw reinforcement for timber or not. The experimental results were discussed in terms of failure modes, ultimate strength, slip moduli, and ductility. The yield strengths and ductility factors were determined based on the load-slip curves according to existing standards. The experimental results showed that both the shear timber width and the self-tapping screw reinforcement played important roles in terms of the ultimate strengths, ductility, and deformability. Rectangular notched connections with screw reinforcements displayed timber shear failure coupled with brittle failure. With the trapezoidal notch, the ductility of the connections improved, coupled with a decrease in the slip modulus. The self-tapping screw reinforcement for shear timber could greatly improve the ductility performance of the HNS connections. The slip modulus models for the connection with vertical deep notches were provided, which were in agreement with the experimental results.

Nonlinear finite element analysis on timber-concrete composite beams

To support the parametric analysis of timber-concrete composite (TCC) beams and the structural analysis of TCC frame systems, the computational effectiveness of various dimensional finite element (FE) models for TCC beams were evaluated in this study, including the one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D). It was found that the 1D frame FE model achieved by the fibre beam elements and the 2D model formed by the layered shell elements show apparently higher computational efficiency than the 3D FE model with solid elements. The FE models all exhibit good consistence with the test results in terms of the macroscopic bending performance of the TCC beams. Owe to the developing of the ABAQUS user subroutines in this study, the 3D model established in the ABAQUS/Explicit module could almost perfectly present the failure modes of the TCC beams, including the fracture of timber, failure of end connections, and cracks on concrete slab, and show higher accuracy of the numerical results than the 1D and 2D models. Additionally, the FE numerical interfacial strain and slip distributions are slightly larger than the test ones, due to various shortcomings of the definitions of the connections in the FE models. Generally, the FE models were validated to be feasible sufficiently for further studies on TCC structures.

Effect of cast-in-place concrete application on moisture distribution in timber-concrete composite floors with notched connections, investigated via finite element simulations

Timber-concrete composite (TCC) structures are an efficient way to combine the advantages of cross-laminated timber (CLT) and concrete plates. By cutting notches into the timber part and applying the concrete on top, efficient shear connections can be formed, eliminating the need for additional use of any type of fasteners. However, fresh concrete releases moisture after application, which is absorbed by the highly hygroscopic wood and can lead to a critical reduction in mechanical properties or to problematic situations due to a difference in expansion behavior. Therefore, a separating foil is usually applied between the two materials, which represents an additional time and cost effort and can also negatively influence the connection properties or make the use of notch-only connections impossible. Thus, we investigate numerically what effects the exclusion of such a foil has on the moisture distribution in the CLT plate. Further, the moisture propagation after a fictitious installation on site is analyzed by applying realistic indoor climates to the open wood surface on the bottom of the CLT plate for a period of two years. In addition, the numerical model allows us to study the effect of local sealings of the most critical wooden part, the end-grain surfaces in the notch region. We were able to confirm that, especially in the unsealed case, locally high moisture contents can occur in the critical region next to the notch, where the highest shear stresses are also to be expected. However, by fully sealing the end-grain surfaces in these regions, the moisture levels and thus the risk of failure could be reduced efficiently. The use of such detailed moisture simulations, where moisture uptake due to bleeding of fresh concrete has been calibrated based on experiments, allows the long-term moisture behavior of such critical situations to be studied and effective solutions to be developed.