Structural efficiency of full-scale timber–concrete composite beams strengthened with fiberglass reinforced polymer
Introduction
One of the ideal precepts of structural building designs regards applying the appropriate material in the most appropriate location, considering its physical and mechanical characteristics. The timber–concrete composite (TCC) structures are clearly in line with this principle, because in the composite system the reinforced concrete slabs are usually fully compressed, while timber beams are predominantly solicited by normal tensile stresses [1]. Thus, the combination of these materials enables to achieve a structurally efficient, stiff yet lightweight cross-section.
Several studies are currently researching TCC structures [2], [3], [4], [5], [6], however its use dates back to the early twentieth century. There are reports that composite structures were used before the First World War in England [7]. In 1914, Redpath Brown and Company began a series of tests with steel–concrete composite structures. Between 1922 and 1939 several buildings and bridges were built using this system, especially due to the shortage of steel at that time [8].
Successive advances have been observed in this area in recent years. Yeoh et al. [9] present the state of the art concerning TCC structures, covering different aspects related to the connection system, the influence of concrete properties, short- and long-term behavior, design approaches, and numerical modeling. Recently, several Brazilian researchers have also investigated the subject [10], [11], [12]. However, it was in the 1970s that investigations into the structural behavior of TCC systems were initiated in Brazil [13], leading to the construction of various Brazilian bridges [14].
The convenient rationalization of timber used in buildings, as well as the necessary reduction of Portland cement, exemplifies strong arguments to enhance the development of TCC systems. In this context, this construction technique has been successfully used in the flooring systems of residential, industrial and sports buildings, with potential applications in structural repairs of historical works, as well as in the construction of new buildings. A concrete slab can be supported on a wooden deck [15], which facilitates the construction process. Fig. 1(a) and (b) illustrate the application of the system in the construction of a housing project located in São Paulo, Brazil.
Additionally, some advantages of TCC systems can be highlighted:
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Significant increased stiffness and strength compared with the independent use of the materials [9], [15].
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Increased vibrational damping – which means that verifications of limit states due to vibrations are more easily met.
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Improved sound insulation – the increase in floor mass, compared with traditional wooden floor, is advantageous to reduce airborne sound transmission.
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Advantageous under fire conditions – the top layer of concrete is an effective barrier against the spread of fire.
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Flexible under diaphragm actions, which can significantly alter the seismic response of a building [16].
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Construction agility, cost competitive and conducive to prefabrication [17].
There needs to be a bonding element between the materials for the composite system to perform properly, this ensures the transfer of horizontal shear efforts and also prevents vertical separation between the parts. The connection system can be obtained by means of nails, screws, metal plates, metal rings, HSB connectors [18], pins (or hooks) achieved from reinforcing steel bars, notches plus vertical dowels [19] or adhesives. The connectors are usually positioned along the beam, according to the shear stress distribution [9], [20], and largely affect the behavior of the composite system. The proper spacing indication between the connectors is critical in order to maximize the load-carrying capacity of the composite system [21].
The slip between the concrete slab and the timber beams causes the partial composition effect of the cross-section [22], [23]. Under bending, the Bernoulli hypothesis is not valid for the entire cross-section, however the sections can still be considered flat when the materials are analyzed separately. The shrinkage and creep of the materials involved are responsible for increasing the slip deformation, which is then responsible for increasing the vertical displacement of the composite system [24]. Another concern regards the behavior of TCC structures when exposed to fire [25].
An alternative to analyzing the behavior of TCC beams consists in the development of computational modeling, often based on the Finite Element Method (FEM). These models are subjected to calibration, comparing the responses with the values obtained experimentally. However, there is a lack of experimental results on full-scale composite floors to validate numerical methods and analytical formulas, especially when the analysis involves the time-dependent behavior of composite floors [1], [26].
The structural performance of TCC structures is encouraging. Davids [27] points out that when compared to timber–concrete beams considering the materials separately, that is without any interaction effect, the timber–concrete interaction results in a bending strength increase of least 40% and stiffness increase of 200% or more. In practice, however, these gains can be quite different, because they depend on the behavior of the connection system utilized and they are also subordinate to the time-dependent behavior of materials that constitute the composite system, among other properties [4], [20], [21].
Nevertheless, adding fiber reinforced polymer (FRP) to the beams opens new perspectives for the design of timber structures [28], [29]. Concrete and sawn timber composite beams reinforced with fiberglass (GFRP), and screw-based connection system, were tested and the results are in Brody et al. [30]. The results demonstrate that the actual stiffness of the composite beam is of approximately 67% of the theoretical stiffness of the beam (considering the total composition of materials, that is, no slip at the interface of the materials).
Weaver [31] evaluated the performance of fiberglass-reinforced glulam beams, considering the partial contribution of the associated reinforced concrete slab. In this study, the connectors were subjected to loads varying between 2,000,000 and 2,500,000 cycles. Yeoh et al. [23] also performed studies on TCC beams under cyclic loading to simulate fatigue on bridges.
Premrov and Dobrila [28] analyzed the behavior of four TCC beams reinforced with CFRP (Carbon Fiber Reinforced Polymer) strips of 1.2 mm in thickness and 150 mm in width. These specimens were subjected to three-point bending tests, with the load applied at the middle of the 4.5 m span. Based on the tests, it was shown that the composite beams reinforced with carbon fibers had the first failure in the tensile region of the wood, and its average value reached was 130.5 kN.
Due to the great potential of TCC structures as a sustainable and efficient solution, our paper presents the experimental evaluation results of TCC beams with and without fiberglass reinforcement, as a contribution to this state of the art construction technique.
High costs, long manufacturing time and other difficulties inherent in the handling of large structural components justify the production of only two specimens of each type of beam discussed in this paper. The need for a preliminary investigation of the behavior of materials and adhesives used during the tests also corroborates the number of repetitions in the methodological procedures adopted in this research.
Section snippets
Characterization of materials and the connection system
The production of the glued laminated timber beams included Lyptus wood – as known in Brazil – corresponding to the hybrids Eucalyptus grandis and Eucalyptus urophylla. For manufacturing of the glulam beams was acquired one lot containing 2.3 m3 of Lyptus wood. Table 1 shows the mean values of the physical and mechanical properties of the material, corrected at 12% for moisture content, the number of tested specimens and the corresponding coefficients of variation (cov). All tests followed the
Production of timber–concrete composite beams
The glued laminated timber beams were produced with lengths of 5.4 m. The entire production was accompanied by the main author of this work, with the support of technicians from the Laboratory of Wood and Timber Structures (LaMEM), São Carlos School of Engineering – University of São Paulo, Brazil. The beam heights were based on the height/span relation used in Weaver [31].
In order to demonstrate increasing in stiffness provided by the composition of timber beams with concrete slabs were
Experimental setup
Following the requirements of ASTM D 198-05a [37], with the test scheme shown in Fig. 9, the eight glulam beams produced were subjected to static bending tests in order to determine their moduli of elasticity (MOE). This standard was chosen to carry out the tests required for analyzes relating to this research because it is a standard that covers the evaluation of lumber members with structural sizes.
According to ASTM D 198-05a [37], the structural members should be subjected to a bending moment
Results from short-term tests
All beams were subjected to an initial loading and unloading cycle; in the second cycle they were loaded until failure, and the lower dial gauges (Fig. 9) were removed at around 90 kN to preserve the integrity of the equipment.
Fig. 12 shows the maximum slips in the timber–concrete interface during the beam V6 testing. The other composite beams tested showed sliding in the timber–concrete interface of the same order of magnitude as the values recorded in Fig. 12. Because of the similarity in the
Conclusion
Because of the significant increase in stiffness and desired increased durability – for structures exposed to weathering – the use of TCC structures has been propagated and is the subject of ongoing research. This system is highly dependent on the properties of the associated materials, as well as on the stiffness of the connecting system used. Furthermore, in high load situations, large spans or retrofit of old buildings, the application of synthetic fiber reinforcement on the most tensioned
Acknowledgements
The experimental works reported in this paper were conducted in the Structures Laboratory of São Carlos School of Engineering – University of São Paulo, Brazil. The financial support to this research by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) is gratefully acknowledged.
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A linear finite element for timber–concrete layered beams with interlayer slip
2023, Composite StructuresStrengthening of old timber floor joists with cross-laminated timber panels and tempered glass strips
2021, Construction and Building MaterialsCitation Excerpt :If the thickness of CLT panels is limited due to the requirement to maintain the original floor height, or if lower parts of timber floor joists are damaged or have low tensile strength, additional transparent elements made of structural glass, especially tempered glass with its high strength and stiffness (compared to timber), could be used to additionally strengthen the timber floor joists in a visually unobtrusive way. In terms of adding reinforcements to both the top and bottom of the timber beams, similar procedures but with different reinforcements have already been presented: in [10], three-point bending tests were carried out on timber beams strengthened with concrete slabs in the compressive and CFRP strips in the tensile zone; in [11], timber planks in the compressive zone and CFRP strips in the tensile zone were used to strengthen timber floor beams; in [12], glulam beams were strengthened with concrete slabs in the compressive zone and GFRP strips in the tensile zone. In [13], the idea of strengthening timber floors with CLT panels and screwed glass strips was introduced in detail and an analytical structural analysis, based on the gamma method from Eurocode 5 [14], of an exemplary composite floor was carried out to show possible advantages in terms of load-bearing capacity and stiffness.
Experimental and analytical investigation on flexural behaviour of glulam-concrete composite beams with interlayer
2021, Journal of Building EngineeringCitation Excerpt :The results demonstrated that the installation of an interlayer at the timber-concrete interface resulted in a decline in the shear stiffness of inclined screw connectors. So far, many researches have been performed on the structural behaviour of TCC beams [21–33]. Persaud et al. [23] conducted three-point bending tests on ten full-scale glulam-concrete composite beams with vertical screw connectors.
Experimental study on seismic performance of glulam-concrete composite beam-to-column joints
2020, Composite StructuresCitation Excerpt :Furthermore, the shear connector can transfer interface shear force between timber and RC slab to provide reliable connection, thus increase the stiffness and bearing capacity compared with the traditional timber floors [5,6]. Long-term and short-term mechanical behavior of composite beams and shear performance of connectors have been studied over the past few years [7–12]. Although the research on TCC beams under static loading test showed promising results, the seismic performance of composite joints, in particular, still requires special attention.
Experimental study of moment sharing in multi-joist timber-concrete composite floors from zero load up to failure
2019, Construction and Building MaterialsCitation Excerpt :Moreover the engineered timbers used previously have typically been softwood glulam, commonly spruce, with other studies using hardwoods [9,18], cross-laminated timber (CLT) [19] or laminated veneer lumber (LVL) [20]. The slabs have largely been of normal concrete, with a small number of studies using lightweight [21], cork-aggregate [22] concrete, limecrete [23], and fibreglass-strengthened concrete [24]. The most common timber-concrete shear connectors have been dowels, rods, or inclined screws affixed into the timber, with concrete cast around them [3,5,9,25–27].