Structural Performance of Prefabricated Timber-Concrete Composite Floor Constructed Using Open Web Truss Joist Made of LVL Paraserianthes Falctaria

This paper investigates an open web truss joist (OWTJ) made of laminated veneer lumber (LVL) Paraserianthes falctaria as a prefabricated timber-concrete composite floor system. The four-point bending test showed that structural per-formances of the OWTJ, conventional composite floor (CCF), and prefabricated composite floor (PCF) were similar. Although composite action was not developed as no lateral deformation was observed at the shear connectors, in-stalling a concrete slab above the OWTJ can slightly increase the ductility factor of the composite floor. Furthermore, a finite element model was developed, and the model proved to be suitable for simulating the structural performance of the composite floor.


Introduction
The use of green construction materials has increased along with the trend of green building construction in recent decades. Wood from well-managed forests is the most sustainable construction materials and has been used for various purposes since 2500 BC [1] [2] [3]. Timber has been used as a construction material due to its high strength-to-weight ratio, which is essential for buildings in active seismic zones; better sound and thermal insulation; availability especially The timber-concrete composite concept has been widely used in the floor system since many years ago. This concept is based on the composite action of timber in the tension zone and concrete in the compression zone connected through shear connectors. This kind of construction offers several advantages, as it is lighter compared with concrete beams and possesses higher timber durability than timber bridges because the timber joist does not experience any direct contact with rain and wind [6]. Previous research focused mainly on the evaluation of the system performance using different timber products as components, such as glulam [7] [8], solid timber [9], laminated veneer lumber (LVL) [10], [11] [12], cross-laminated timber (CLT) [13], and built-up joists. To connect the timber to the concrete part, the following are commonly used: adhesives [8], screws [7] [13] [14], punched metal plates [9], notched connections [10], steel mesh connections [11], or a combination of these connections (hybrid systems) [12]. In about 45% of previous studies, metal dowel-type fastener connections were used [15].
To increase the bending stiffness and reduce the self-weight of the joists, the solid wood joist is replaced with various types of built-up section joists, such as the I-shaped joist [16], box-shaped joist [16], and open web truss joist (OWTJ) (see Figure 1). The OWTJ is potentially the most promising regarding the load-bearing capacity compared with other built-up sections. In the OWTJ, the bending and shear forces developed in the floor are transformed into truss action [17]. The use of concrete materials in the timber-concrete composite floor is an innovative way to obtain a structure with a high bearing capacity and stiffness. In addition, to ease the material transportation and construction process, prefabricated elements can be applied in this system. The use of prefabricated elements in the structures is popular since this method provides numerous advantages such as quick and easy installation, low production cost, no-waste materials, and improved quality [18].
Constructing residential buildings with high sustainability is necessary to support sustainable development. However, despite the advancing technology in

Materials
The wood species Paraserianthes falctaria, known as Sengon timber in Indonesia, comes from is a fast-growing tree widely spread in Indonesia that is generally harvested within 5 -7 years of planting [20]. LVL produced from Paraserianthes falctaria has better mechanical properties compared with the solid Paraserianthes falctaria wood, as summarized in Table 1. This LVL product was made from 2 to 3 mm-thick veneers, glued together using a urea-formaldehyde adhesive, and compressed at 0.6 -0.7 MPa and a temperature of 105˚C -110˚C.
This LVL product has a great potential to be used as structural materials in shear walls, floor systems, and OWTJs [21].
The LVL Paraserianthes falctaria was cut into a certain dimension and arranged to form an OWTJ system, as shown in Figure 2. Each truss node was glued using adhesive and compressed with a pressure of 3.5 MPa for 45 minutes.  Three kinds of produced specimens were tested in this research, denoted as timber joist (TJS), conventional composite floor (CCF), and prefabricated composite floor (PCF). The length of the OWTJ was limited to 2100 mm due to the testing facility limitation. The bearing capacities of these specimens were examined through a four-point bending test. As shown in Figure 2, the specimens for TJS consisted of two OWTJs, to resist the load, while the test specimens for CCF and PCF consisted of two OWTJs and a 60 mm-thick concrete slab with a design compressive strength of 15 MPa at the top. The concrete slab and OWTJ were connected using 5"-5/16 lag screw with a bending yield strength (Fyb) of 568 MPa, as performed in a previous study [22]. The lag screws were inserted perpendicular to the LVL grain. For the PCF specimens, the concrete slab was connected to the OWTJ after 28 days of curing. Lead-holes of 10 mm in diameter were made, and the lag screw was fastened through these holes. To fill the gap between the screw and the hole, the epoxy resin Sikadur 52ID was injected after the lag screws installation.

Test Setup
The composite floor was examined in a four-point bending test, as presented in Figure 3. The load was applied using a 100 kN hydraulic jack. The load was distributed using steel beams located above the composite floor. Three linear variable displacement transducers (LVDTs), one at mid-span and the other two at loading points set at 550 mm from the supports, measured the vertical displacement during the test. The interlayer slips between the concrete slab and OWTJ were also measured using LVDT 4. Each LVDT had a measurement length of up to 50 mm with an accuracy of 0.01 mm. Open Journal of Civil Engineering

Gamma Method
The gamma (γ) method is a design method that is commonly used to analyze structures with multiple contributing elements. This method is used to calculate the bearing capacity of a timber-concrete composite floor connected by a mechanical fastener. This method is based on the linear elastic theory [23]. The effective bending stiffness of the composite floor, EI ef can be determined using where i refers to the number of elements in the composite floor, E is the modulus of elasticity (MOE), A is the cross-sectional area, and a is the distance from the center of each layer to the center of the composite section. In Equation (1) In Equation (2), s is the shear connector spacing, and l is the length of the joist. The lateral shear connector stiffness, K, between the LVL of Paraserianthes falctaria and the concrete varied from 2.03 kN/mm to 6.30 kN/mm depending on the concrete compressive strength, penetration depth, and the insertion angle of the lag screw axis [24].

Finite Element Model
In the finite element method, especially in the nonlinear analysis, the method of weighted residual is often used. It is an approximate technique for solving boundary value problems that utilizes trial functions to satisfy the existing boundary conditions [25]. In this research, ABAQUS software was selected to predict the structural performance of the composite floor due to its easiness in defining the behavior of each material. Figure 4 shows the finite element model of the composite floor. In ABAQUS, OWTJ, plywood sheet, and concrete slab were assumed as 3D solid elements.
Connector elements were used to represent the lag screws, in order to connect OWTJ with plywood sheeting or a concrete slab. The stiffness values used for the connectors were obtained from previous studies [24]; the behavior could be classified as linear elastic. The load-controlled four-point bending test was performed applying the load in the specific surface area located 700 mm away from the support.   The OWTJ shape was non-prismatic and complex. The tetrahedron mesh (C3D4) type was selected, as shown in Figure 6, to obtain a better simulation   Figure 7, because the deformation of the first compression test was not properly recorded.

Results
Based on calculation using the γ method, the composite action between concrete and OWTJ was found to be low, only 0.032. Almost no composite action was developed by the construction system. In addition, the maximum load capacity obtained from this method was 69.43 kN. Figure 8 shows the     Structures [35]. Besides the strength requirement, a structure has to meet the serviceability requirement, which is related to the maximum displacement occurring at the service load. Based on the Indonesian Standard SNI 2002, the maximum displacement in the structure at the service load was limited to L/360 [36]. The length of the composite floor was 2100 mm, and this means that the displacement in the composite floor at the serviceability load level must not exceed 5.81 mm. Figure 8 shows that the maximum mid-span displacements of the TJS, CCF, and PCF at service load were 2.58 mm, 2.55 mm, and 2.50 mm, respectively. The ratios of the bearing capacity at service condition to the minimum design load of TJS, CCF, and PCF were 4.74, 4.20, and 4.41, respectively.
The displacements at the service load of TJS and PCF were 56% lower than the displacement limit. Moreover, the displacement at the service load of CCF was 57% lower than the displacement limit. This indicates that the proposed composite floor meets the minimum bearing capacity requirement at the service load, with a sufficient safety factor both in terms of strength and serviceability. However, the bearing capacities at service condition would decrease for a longer span and in the presence of connections in the OWTJ members.
Structure ductility is the ability of a structure to experience large deformation without rupture before failure. This is an important aspect for structures, especially for structures located in seismic zones, as it can be a basis for warning occupants before a structure collapse [37]. The ductility factor was defined by di- accordance with the equivalent energy elastic-plastic method [38]. Here, ultimate displacement represents the displacement at failure or at 80% of the peak load. Table 3 summarizes the ductility factor of each specimen.
After reaching the ultimate load capacity, each specimen experienced a different type of failure. As shown in Figure 9, the joint of the TJS specimen failed.
The TJS specimen collapsed when the load applied in the structure reached 47.78 kN. The failure of the structure was caused by the glueline shear failure at the node at the top edge and in the middle of the OWTJ.
After the CCF specimen reached its ultimate bearing strength, the diagonal members of OWTJ near the support experienced lateral buckling, resulting in delamination on the LVL member, as shown in Figure 10. In this condition, the capacity of the structure decreased as the displacement increased. The concrete slab did not show damaged when the test was terminated.    OWTJ specimen is shown in Figure 12. Comparing the interlayer slip values between the CCF and PCF specimens, the slips were almost the same at the initial stage of the loading. The epoxy resin that was used to remove the hole clearance the lag screws installation maintained a good bond with the concrete slab.
As a result, the structural performance of PCF was similar to that of CCF both in terms of initial stiffness, interlayer slip, and ultimate strength. However, after the load reached 15 kN, the interlayer slip on the PCF specimen became greater than that on the CCF specimen.
In both the CCF and PCF specimens, the concrete slab around the lag screws was removed after the test to observe the shear connector damage. As shown in Figure 13, the lag screw was still straight; no irreversible lateral deformation occurred. This indicates that the shear connector was still in its elastic condition.

Floor Structure
As stated earlier, for a built-up timber structure, the γ method can be used to   where the load applied in the concrete is distributed directly to the OWTJ, causing damage to the LVL member. The lag screws did not experience any yielding, as evidenced by the very small lateral deformation.

The Pattern of Composite Floor Failure
The experimental result did not provide any specific pattern of composite floor failure. However, one of the diagonal OWTJ members located near the supports of the CCF and PCF specimens experienced buckling failure, followed by delamination in the LVL member; moreover, the concrete near the shear connectors  Figure 11. For further application, predrilling holes in the concrete slab is not recommended. The use of removable steel pipes to create holes during the concreting work is suggested in order to avoid the existence of micro-cracks near the holes. Figure 14 compares the load-displacement curves obtained from the laboratory test and FE model. The displacement was measured at the mid-span, since this location experienced the highest deformation. As shown in Figure 14, the curve of the FE model was close to that of the laboratory test result. This figure also indicates that the introduction of a plywood sheet in the FE model had no significant effect on the FEM result. Therefore, an FEM that only consists of the OWTJ is appropriate for further design and application. Furthermore, for the proposed composite floor, this model is also suitable due to the reasons below.

The Comparison between the Finite Element Model and the Experimental Result
1) The degree of composite action between the concrete slab and the OWTJ was only 0.032. The concrete slab and OWTJ will not act as a single unit structure but as two separate structural elements.
2) The lag screws did not experience irreversible lateral deformation (see Figure 13). This indicates that there is no high load transfer mechanism in the mechanical joint.
3) Simplifying the concrete slabs as a distributed load acting on the OWTJ will accelerate the simulation process.
A parametric study was conducted to evaluate the structural performance of the OWTJ. Several models for the OWTJ with various span lengths were evaluated using FE simulation. The ultimate capacity of the structure was evaluated to determine two criteria, which are allowable deflection at service load and maximum compression stress in joist members. The capacity of the structure at service condition was then taken as 40% of its ultimate capacity. From Figure  15, it can be concluded that the capacity of the structure will decrease with the  increase of span length. In addition, the capacity of the structure will be governed by the deflection limit of the structure when the span is longer than 5 m.

Conclusion
This paper presents a numerical and experimental study on the use of the OWTJ in a composite floor system. The proposed composite floor consists of an OWTJ made of LVL Paraserianthes falctaria and a concrete slab connected by lag screws to allow prefabrication. Three specimens, i.e., OWTJ (TJS), CCF, and PCF, were assessed using a four-point bending test. From the experimental work in the laboratory, these three specimens showed similar capacities and structural stiffness values. The composite action between the OWTJ and the concrete slab was extremely small since the difference between the shear connection stiffness and the bending stiffness of the LVL compared with bending stiffness of the concrete (MOE) was not well balanced. After the test, no irreversible lateral deformation was found in the lag screws. Nevertheless, positioning a concrete slab above the OWTJ can increase the ductility factor of the composite floor. From the numerical work, the developed FE model is suitable to simulate the structural performance of the composite floor. The concrete slab above the composite floor can be simplified as a distributed load acting on the OWTJ. For further development, the proposed composite flooring system can be modified by applying different types of connectors and wood materials.