1. Introduction
As a green and renewable building material, timber has the characteristics of light weight, high strength, and easy processing. Using steel–timber composite (STC) floors instead of conventional steel–concrete composite (SCC) floors can effectively reduce energy consumption and carbon emissions throughout the life cycle of the structures [
1]. Compared with that of conventional SCC beams, the STC beams can decrease structural weight, seismic response, and cross-section of structural elements significantly [
2]. The STC beams consist of an upper timber slab connected to the bottom steel beam with shear connectors. The shear connectors are mainly responsible for transmitting the shear force between the slab and the beam, and meanwhile, preventing vertical uplift between two materials. Several types of shear connectors were developed for STC floor system including dowel-type connectors (e.g., screws and bolts), dowels and adhesive composite connection, and bolted connectors embedded in grout pockets [
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14]. The existing research on STC shear connections mainly focuses on the conventional dowel-type connection. Hassanieh et al. [
3,
4] through push-out test studied the shear performance of steel-CLT (cross-laminated timber) and steel-LVL (laminated veneer lumber) connections with diverse types of connectors, including coach screw, dog screw, bolt, and screw and adhesive composite connection. Moreover, the behavior as well as composite efficiency of STC beams with the above-mentioned shear connections were analyzed by four-point bending tests [
5,
6]. Loss et al. [
7,
8,
9] designed and tested different connections for steel–timber hybrid prefabricated systems, and it was reported that one of which was ideal solution for STC floor, ensuring a high load-bearing capacity and slip modulus, as well as ductility. Wang et al. [
10] proposed the inclined self-tapping screws for STC joints to address problems of construction inconvenience and buried depth of screws. Ataei et al. [
11] investigated the cyclic behavior of screw and bolt connectors reporting that the STC joints exhibited high ductility and energy dissipating capacity. Chiniforush et al. [
12] studied the long-term performance of steel-CLT composite connections through push-out test and established a long-term rheological model that considered their predictions of the slip.
Although typical dowel-type connectors for STC beams (i.e., bolts and screws) demonstrated an ideal load–slip response, most of them need predrilling at both the steel beam flanges and timber slabs before connecting, which requires higher construction accuracy and is considerably inconvenient for installation. Accordingly, the grouted stud connectors (GSC) shear connections consisting of two main parts, the welded shear studs, and its surrounding grout were proposed for connecting the timber slab and the steel girder in STC beams. The studs are welded directly to the beam flange without predrilling, then grooving in CLT panel at the locations corresponding to the studs, and finally filling the groove with cement grout to form the GSC shear connections. Using GSC shear connections can effectively reduce the overhead work required to install conventional fasteners to construct STC floors and compensate for the lack of construction accuracy. Furthermore, Hassanieh et al. [
6,
13,
14] proved the effectiveness of the STC joints with bolt connectors embedded in grout pockets (BCGP) through experimental and numerical studies. The results indicated that the BCGP connections presented better stiffness, bearing capacity, ductility, and composite efficiency compared to that of the conventional bolted or screwed STC connections, and the long-term behavior was also proved to be superior [
12]. However, compared to that of shear studs, the BCGP connections might not be the most effective connecting methods for STC system because of the predrilling for the assembly of bolts; thus, the studs were expected to be an alternative, which were proved to be of outstanding load-bearing capacity and stiffness, as well as of convenient construction by abundant studies [
15,
16,
17,
18] and practical engineering. Consequently, the GSC shear connections for STC beams were exploratorily proposed in this study. FE modeling on the GSC shear connections was conducted using ABAQUS to investigate the shear performance of the connections; further, the influences of the stud diameter, stud strength, grain directions of timber, configurations, angles of grouting groove, and thickness of CLT panel on the load–slip response, peak load capacity, and stiffness were also studied. The results of this study can provide some references for the design of STC connections.
4. Calculations of Shear Capacity
To some degree, the shear force transmission between the upper slab and the bottom beam in STC system is quite similar to that of the stud connectors in SCC beams. Therefore, the available calculation formulas for the shear capacity of stud in SCC beams were adopted to predict the shear capacity of GSC connections herein.
In the
Standard for Design of Steel Structures (GB 50017-2017) [
22], the shear capacity of an individual stud connector is calculated as:
where,
is the shear capacity (N) for individual stud;
and
are the modulus of elasticity and compressive strength of concrete (MPa);
is the cross-sectional area of the shank of stud (mm
2); and
is the tensile strength of stud (MPa).
According to the
Code for Design of Steel and Concrete Composite Bridges (GB50917-2013) [
31], the shear capacity of a single stud connector should take the smaller value in Equation (6):
where,
is the shear capacity (N);
is the cross-sectional area of the shank of stud (mm
2);
and
are the modulus of elasticity of concrete and steel (MPa);
is the cubic compressive strength of concrete (MPa);
is the design value of axial compressive strength of concrete (MPa);
is the tensile strength of stud (MPa); and
is the reduced coefficient of group effect.
The design shear resistance of a stud in Eurocode 4 [
32] is reported to be taken as the minimum value in the following two formulas:
where,
is the shear capacity (N);
for
;
is the diameter of the shank of the stud (mm);
is the overall nominal height of the stud;
is the modulus of elasticity of concrete (MPa);
is the characteristic cylinder compressive strength of the concrete (MPa);
is the cross-sectional area of the shank of stud (mm
2);
is the ultimate tensile strength of stud (MPa);
is the partial factor; and the recommended value is 1.25.
The
Specification for Structural Steel Buildings (ANSI/AISC 360-16) [
33] presents that the shear strength of one stud should be determined as follows:
where
is the shear capacity (N);
is the cross-sectional area of the stud shank (mm
2);
is the specified compressive strength of concrete (MPa);
is the modulus of elasticity of concrete (MPa);
is the specified minimum tensile strength of stud (MPa);
= 0.85; and
= 0.75.
Ding et al. [
26] established a calculation formula for the shear capacity of an individual stud based on the push-out test results of stud connectors in worldwide, expressed as Equation (9):
where
is the shear capacity (N);
is the diameter of the stud shank (mm);
is cubic compressive strength of concrete (MPa); and
is the yield strength of stud (MPa).
Zhou et al. [
34] proposed a formula for calculating the shear capacity of one stud by regression analysis of the push-out test data of 233 stud connectors in worldwide, as shown in Equation (10):
where
is the shear capacity (N);
is the cross-sectional area of the shank of the stud (mm
2);
is the modulus of elasticity of concrete (MPa); and
is prism compressive strength of concrete (MPa).
Zhang [
30] made a regression analysis of the push-out test data of 80 stud connectors and proposed a calculation model for the shear capacity of a single stud connectors as follows:
where
is the shear capacity (N);
is the cross-sectional area of the shank of the stud (mm
2);
and
are the modulus of elasticity of concrete and steel (MPa);
is the cubic compressive strength of concrete (MPa); and
is the ultimate tensile strength of stud (MPa).
Wang [
35], suggested the following formula for calculating the shear capacity of one stud based on linear regression analysis of the push-out test values of stud connectors:
where
is the shear capacity (N);
is the cross-sectional area of the shank of the stud (mm
2);
and
are the modulus of elasticity of concrete and steel (MPa);
is the cubic compressive strength of concrete (MPa); and
is the yield strength of stud (MPa).
The concrete strength in all the above calculation modes is directly substituted by the corresponding strength of grout in the pockets.
Figure 19 shows the comparison between the predicted shear capacity of the GSC connections from the abovementioned calculation formulas and the FE simulated results. Generally, the calculation modes in the design codes (i.e., Eurocode 4, ANSI/AISC 360-16, GB 50017, and GB 50917) conservatively estimate the shear capacity of the GSC connections compared to that of the FE simulated results. Further, the shear capacity predicted by the formulas (Ding et al., Zhang, Zhou et al., and Wang) is generally in good agreement with the simulation results; in particular, the formulas from Ding et al. and Zhang [
26,
30] show a difference of less than 12% compared to the simulation values. Thus, the Ding et al. and Zhang calculation modes are suggested for predicting the shear capacity of the GSC connections for STC systems.
5. Group Effect
Since the GSC shear connections in STC beams are probably composed of a group of studs, the group effect of the stud group is deserved to be discussed. Theoretically, the stud group has a discounted shear capacity compared to the sum of the shear capacity of individual stud, which can be illustrated by a reduce factor. To obtain this reduce factor, The influence of group effect on the shear properties of the GSC connections was evaluated by FE modeling.
The GSC connections with 2–5 rows of studs with a row spacing of 48 mm (3d) were designed and compared to the GSC connection with a single row of studs; the stud diameter is 16mm.
Figure 20 shows the comparison of load–slip responses of individual stud between the connections with 1–5 rows of studs.
Table 7 shows the averaged shear capacity, initial stiffness, and pre-peak stiffness of the GSC connections with different rows of studs. Generally, the shear capacity, initial stiffness, and pre-peak stiffness decrease gradually as the stud row increased from one to five, confirming the influence of group effect in the average shear strength and stiffness of individual stud.
Such a performance degradation can be described by the concept of effective row number (
), which is obtained by reducing the rows of studs (
). In this paper, the reduced coefficient of stud rows (
) was obtained by regression analysis of the simulation results (
Figure 21), and
. When evaluating the peak load capacity, the group effect can be expressed as:
For initial stiffness and pre-peak stiffness, the group effect can be described as: