Timber-concrete Composite Structures Research Articles

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.

Experimental investigation of a prefabricated timber-concrete composite floor structure: Notched-slab approach

This study presents the experimental performance of a high ductility and energy dissipation capacity connector named notched-slab fastener, which is developed for timber-concrete composite floor structures. Notched-slab fastener is unique because the notch was left in the concrete slab instead of a timber beam for the first time. In this way, cross-section loss in the timber beam is prevented. This is one of the most important benefits of the proposed connector. Experimental performance of the purposed connector was investigated with two test methods: push-out test and bending test. For comparison purposes, a notched fastener was also produced and tested as reference sample. Test results showed that notched-slab fastener has stable load carrying behavior under push-out loading with high-energy dissipation and no sudden strength degradation. In addition, notched slab design provided moment arm to the fastener and the damage is concentrated in the connector instead of concrete.

TIMBER-CONCRETE COMPOSITE RIBBED SLABS WITH HIGH-PERFORMANCE FIBRE-CONCRETE

Composite of such renewable material as timber and the most popular man-made material as concrete offers many benefits. Such of them are high load-bearing capacity with low dead load and increased structural bending stiffness. Higher specific strength of high-performance concrete in comparison with ordinary concrete ensures more efficient use of the material. Addition of fibres can reduce the fragility and autogenous shrinkage cracks of high-performance concrete and makes it possible to design thinner layers of concrete for timber-concrete composite structures. Ribbed slabs as solution for the floor slabs, allows to reduce material consumption and to integrate engineering communications into the structures. The current study focuses on determining the effect of the use of high-performance fibre reinforced concrete for timber-concrete composite ribbed slabs with adhesive connection between layers, as the most effective connection type for composite action. The effect of the use of high-performance fibre reinforced concrete is determined by comparison of mid-span displacements of the ribbed slabs numerical models. Three-dimensional finite element models of timber and ordinary concrete composite ribbed slab and high-performance fibre reinforced concrete with additional longitudinal reinforcement ribbed slab are validated by experiment data. Developed numerical models makes it possible to predict the dependence of applied load on mid-span displacement in three-point bending with sufficient precision. Obtained results showed, that replacement of ordinary concrete layer by high-performance fibre reinforced concrete in timber-concrete composite ribbed slab with adhesive connection up to 1.68 times decrease vertical mid-span displacements.

Experimental and Nonlinear Analytical Studies on Prefabricated Timber–Concrete Composite Structures with Crossed Inclined Coach Screw Connections

AbstractIn this study, shearing tests on connections and bending tests on prefabricated timber–concrete composite (PTCC) beams were performed to evaluate the feasibility of using crossed inclined c...

Modelling short-term monotonic response of timber–concrete composite structures

Timber–concrete composite is an efficient hybrid construction material that exploits the advantageous properties of timber and concrete. The use of shear connectors enables the two dissimilar materials to act together, resulting in an increase in global stiffness as well as load-carrying capacity. As this composite material is becoming increasingly more popular in the construction industry, there is a need to develop analysis tools that have general applicability to timber–concrete composite systems with variations in loading schemes, specimen configurations, materials, and types of shear connectors. One such tool, a generic two-dimensional nonlinear finite element model, is proposed in this paper; it is verified through numerical simulations of six experiment series carried out by other researchers. Good agreement between experimentally observed behaviour and numerical simulations was generally obtained.

An Analytical Method to Predict the Structural Behavior of Timber-Concrete Structures With Brittle-to-Ductile Shear Connector Laws

Timber-Concrete Composite (TCC) structures allow taking synergistic advantage of the properties of both materials to optimize the overall performances in terms of lightness, slenderness, acoustic insulation, vibrational behaviour and environmental footprint. In the last years, ductile shear connectors have been developed to allow the structural ductility of TCC structures. Considering the limitations of current design methods, this work aims at developing a closed-form solution for accurately predicting the nonlinear structural response of a TCC structure directly from the materials' property and the shear law of ductile connectors. In particular, we have assumed a generalized shear law based on 3 parameters which allow considering shear law from a pure elasto-plastic to pure brittle behaviour.After a short introduction, Section 2 briefly presents the basics of the well-estabilished elastic theory for a 2-layer composite beam with a linear shear law in terms of horizontal shear vs. slip (Vh-s) law; Section 2.3 extends the semi-analytical method proposed by Bažant and Vitek for composite structures with a generalized shear law; Sections 3 extends the previous method by developing a new closed-form analytical solution for predicting the TCC structural response for a generalized elasto-plastic Vh-s shear law described by an initial linear response up to Vmax followed by a plastic plateau at a constant load, which can range from 0 to Vmax; Section 4 compares the model results with the ones of existing methods and FEM analysis. Furthermore, a parametric analysis is carried out to investigate the model sensitivity to the connector parameters; Finally, Section 5 presents a simple point-by-point design procedure of which prediction accuracy of the ultimate moment, deflection and slip was statistically assessed for a large range of possible TCC structures against FEM analysis. Eventually, the effect of concrete cracking is also considered and a correction factor is proposed for engineering purpose.

Shear stiffness of inclined screws in timber–concrete composite beam with timber board interlayer

The timber board interlayer is applied as the formwork for the pouring of concrete slab in various practical applications of timber–concrete composite structures, with the rehabilitation of timber buildings, in particular. At present, there are few studies performed to study the shear stiffness of inclined screws in timber–concrete composite beams with timber board interlayer. In this article, eight groups of shear tests were carried out to study the shear stiffness of inclined screws in timber–concrete composite beams with timber board interlayer. The key parameters included the embedment depth of the screw connector into timber, screw diameter, the thickness of concrete slab, and concrete strength. As indicated by the test results, the shear stiffness of the inclined screws was improved as the embedment depth of screw into timber and screw diameter increased. When the embedded depth of screw into concrete remained unchanged, the thickness of concrete slab and concrete strength exhibited no significant impact on the shear stiffness of inclined crossing screws. On the basis of the theory of a beam on a two-dimensional elastic foundation, the calculation method for predicting the shear stiffness of inclined screw in timber–concrete composite beams with interlayer was proposed. The comparisons demonstrated that the shear stiffness of inclined screw can be well predicted using the calculation method.

Load carrying capacity of inclined crossing screws in glulam-concrete composite beam with an interlayer

In many practical applications of timber-concrete composite (TCC) structures, especially in rehabilitation of timber buildings, the floorboard interlayer is utilized as formwork for pouring concrete slab. So far, few researches have been conducted to study the shear performance of inclined crossing screws in TCC beam with an interlayer. In this paper, seven groups of shear tests were conducted to study the shear performance of inclined crossing screws in glulam-concrete composite beam with an interlayer. The analyses focused on the effects of embedded length of screw into glulam, thickness of the concrete slab and screw diameter on the failure mechanism, the load-slip behavior and the load carrying capacity. Two typical failure mechanisms were observed: cone failure of the concrete layer around shear-compression screw and pull-out failure of shear-tension loaded screw. The test results demonstrated that with the increase of screw diameter and embedded length of screw into glulam, the load carrying capacity of the inclined crossing screws was enhanced. According to theoretical studies, in timber-concrete composite beams with interlayer, the calculation method for predicting load carrying capacity of inclined crossing screws was proposed, and significantly the predicted results agreed well with the experimental results.

Engineering ductile notch connections for composite floors made of laminated timber and high or ultra-high performance fiber reinforced concrete

Different kinds of ductile connectors have been lately developed for enhancing the structural ductility of Timber-Concrete Composite (TCC) structures. In particular, ductile notch connections can be designed by favoring the local compression failure of wood fibers. This work aims at further developing economic and ductile notch connector by considering different floor systems made of Glulam Laminated Timber (GLT) beam or Cross Laminated Timber (GLT or CLT) slab connected with a High Performance Concrete (HPC) slab or a Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) slab.Firstly, the geometry of the notch connector was suitably designed for favoring a ductile hierarchy of collapse modes. Then, a wide campaign of push-out tests was carried out to characterize the shear behaviour of 14 connection configurations by varying the notch geometry, the concrete type and the possible presence of acoustic insulation. Finally, based on the experimentally identified connection shear law, an example of design is presented for a TCC slab of 9 m span. The insulation layer was found to reduce the connection stiffness, but to increases the structural stiffness thanks to the enhanced lever arm of the composite action. For plastically designed TCC structures, the connection ductility allows increasing the structural ductility for both GLT-(U)HPC and for CLT-(U)UPC floor systems.

Frictional behaviour of timber-concrete contact pairs

While friction plays an important role in many timber-concrete composite structures, it has been studied only to a limited extent and current knowledge is not comprehensive enough for rigorous modelling of e.g.timber-concrete connections. The purpose of this study is to fill part of this gap by investigating timber to concrete friction under cyclic loading experimentally. The tests were conducted with small-scale double-sided shear specimens. The materials included surface treated and untreated edge-faces of coniferous laminated veneer lumber and birch plywood combined with high-strength concrete grout that was cast directly on the timber parts. The initial shear strengths are evaluated by the Mohr-Coulomb criterion, and the static and kinetic friction coefficients are determined for each load cycle. Based on the results, influence of the cumulative displacement, the wood product, the grain orientations and the surface treatment on the initial shear strength and the friction coefficients are analysed. The tests also showed interlocking that is likely related to the matching surfaces created by prior de-bonding. As this interlocking affects the results, it was also investigated.

Experimental and numerical investigations on timber-concrete connections with inclined screws

In new and existing buildings, it is important that floors are sufficiently strong and stiff in plane. In comparison with pure timber floor systems, timber–concrete composite structures are often used as floor solutions to better combine acoustic separation with both greater load-carrying capacity and increased stiffness, also resulting in significant improvement of the serviceability vibrational comfort. The choice of a structurally effective yet inexpensive shear connection between the concrete topping and the timber joist is crucial to make the composite floor a viable solution that can compete with reinforced concrete and steel floors. The use of inclined screws to connect the timber beam to the concrete topping represents a possible solution to maximize the slip modulus of the connection and, at the same time, to keep the construction cost within acceptable values. In this paper, the mechanical behavior of such a type of timber to concrete connection is explored via experimental pushout tests and numerical investigations. The specimens tested were made of an inner timber block connected to two outer concrete slabs by means of two 8 mm diameter screws per side. To reproduce the timber flooring used in real practice as permanent formwork for the placement of the concrete topping, an OSB layer was introduced between the timber block and the concrete slabs of the specimens. Experimental results were statistically assessed to compute the mean slip moduli and the characteristic values of the shear strength. A mathematical model of an inclined elastic beam on an elastic foundation capable to provide the slip modulus of the system was developed. The dependency of the slip modulus upon the screw inclination and the interlayer flooring thickness was also investigated. The accuracy of the theoretical results was assessed through comparisons with experimental results.

Fostering a cast-in-place steel–UHPFRC connector for ductile timber–concrete composite structures: parametric study of the shear behaviour and design considerations

Timber–concrete composite (TCC) structures provide a breakthrough solution for multi-storey buildings. Besides the importance of the connection stiffness, recent research has focused on ductile connectors. This work aims at developing an economic new cast-in-place connector to guarantee the suitable stiffness and structural ductility. A new composite connector has been developed by combining an ultra-high performance fiber reinforced concrete (UHPFRC) shell with an embedded inner steel rod. The former governs the connection stiffness, while the latter governs the connection strength and ductility. To realize a cast-in-place construction method, firstly a wide experimental investigation on the connection shear behaviour with different geometry was carried out. Secondly, the results were analyzed using Winkler’s model, which allowed to define a simplified formula. Finally, a design example of the ductile connectors is presented with emphasis on the importance of a smart design of the connection system for avoiding a brittle structural failure.

Literature Review on Timber–Concrete Composite Structures in Fire

AbstractThe present paper summarizes recent research on timber–concrete composite (TCC) systems exposed to fire conditions. Experimental work on these systems in fire conditions is not as extensive...

Optical fiber sensors implementation for monitoring the early-age behavior of full-scale Timber-Concrete Composite slabs

The study of the early-age behavior of Timber-Concrete Composite (TCC) structures is of great interest as it provides valuable information for manufacturing specification development, quality control, and optimization of the formwork design. In this study, the results of the continuous monitoring of the short-term behavior of TCC slabs using Brillouin Distributed Optical Fiber Sensors (DOFS) are reported. Two TCC slabs with 8.5 m of length were monitored. The composite elements are constituted of Cross-Laminated Timber (CLT) connected to a High-Performance Concrete (HPC) slab. During a monitoring period of about 30 days, the early-age temperature/strain variation in the fresh concrete and in the CLT slab was measured in great details by DOFS. From the presented results, the significant influence of the curing conditions on the early-age shrinkage was highlighted. It was also observed that creep and the daily hygrometric variations of environment affect considerably the composite action between the timber and the concrete. In addition, it was experimentally demonstrated that such mechanisms generate considerable structural changes in the composite elements even before their entry into service.

MONITORING AND PROTECTION OF TIMBER-CONCRETE BRIDGES

The majority of timber bridges for ordinary road traffic are often made as timber-concrete composite structures. Concrete slab is something like as umbrella for timber. The reason for problems in timber-concrete bridges is nearly always poor detailing for durability and neglected maintenance. Mainly all aspects of timber protection and maintenance should therefore be considered even during the planning phase of bridge. Paper deals with evaluation of timber properties, system of opto-fiber sensors for monitoring and diagnostics of mechanical stress of timber-concrete bridges, and degradation of timber due to UV radiation, atmospheric conditions and biological agents after application of photocatalytic materials on surface.