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Composite connections between CLT slab and steel beam: Experiments and empirical models
Journal of Constructional Steel Research 138 (2017) 823–836
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Journal of Constructional Steel Research
Composite connections between CLT slab and steel beam: Experiments and empirical models A. Hassanieh ⁎, H.R. Valipour, M.A. Bradford Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, UNSW Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 25 April 2017 Received in revised form 22 August 2017 Accepted 4 September 2017 Available online xxxx Keywords: Bolted connectors Cross laminated timber (CLT) Grouted connection Push-out
a b s t r a c t A composite floor comprising of steel beams connected to cross laminated timber (CLT) panels is a sustainable alternative to composite floors with reinforced concrete slab. The structural performance of the steel-CLT floor depends strongly on the load-slip behaviour (particularly the stiffness and strength) of the shear connection between the steel beams and the CLT slabs. In this paper, the short-term behaviour of steel-CLT joints is studied by laboratory push-out experiments conducted on CLT-steel-CLT specimens, being analogous to familiar push tests on steel-concrete specimens. In addition to coach screws and bolts, a high-performance steel-CLT composite connection comprising of high-strength bolted shear connectors embedded in pockets of grout is proposed and tested. The push-out experiments are used to determine the load-slip, failure mode, stiffness and ultimate strength/capacity of the steel-CLT composite joints and to calibrate an empirical formula for the load-slip response of the CLT-to-steel connections. Furthermore, simple empirical formulae for the stiffness and ultimate capacity of the CLT-to-steel composite connections are proposed. Crown Copyright © 2017 Published by Elsevier Ltd. All rights reserved.
1. Introduction The application of the engineering wood products (EWPs), especially cross laminated timber (CLT), in building construction has been growing rapidly over the past decade. Having lower embodied energy and less environmental ramifications, and with a considerably higher strength to weight ratio, timber has many favourable attributes in construction when compared to steel and concrete alone. These advantages, along with the possibility for full prefabrication of timber structures, make EWPs a promising alternative to conventional steel and reinforced concrete [1]. Moreover, EWPs can be used in conjunction with steel and reinforced concrete to develop hybrid structures with improved sustainability and structural performance [2–4]. For example, timberconcrete composite system with improved short- and long-term behaviour, acoustic performance and fire rating have been developed by connecting reinforced concrete slabs to sawn and engineered wood (i.e. laminated veneer lumber, glued laminated timber) beams [5–7]. Moreover, the possible application of cross laminated timber (CLT) wall panels as lateral load resisting systems in steel structures has been investigated experimentally and numerically [8,9]. In a recent study, Dickof et al. [8] utilised CLT-shear walls in a moment resisting steel frame. Similarly, the structural performance of the steel frames with infill wood shear walls was investigated by He et al. [9]. In addition, Bhat et al. [10] conducted monotonic and cyclic tests on a hybrid system comprising of steel beams embedded in CLT wall. However, little ⁎ Corresponding author. E-mail address: [email protected] (A. Hassanieh).
https://doi.org/10.1016/j.jcsr.2017.09.002 0143-974X/Crown Copyright © 2017 Published by Elsevier Ltd. All rights reserved.
research has been devoted to development of sustainable steel-timber hybrid floors in which horizontally laid timber panels (connected to steel beams) replace conventional reinforced concrete slabs [4,11]. The development of composite action in a steel-timber composite (STC) system, and the feasibility of such STC beams and flooring systems for large scale construction, has been investigated in previous studies [4,11–13]. The essential role of steel-to-timber connections for the efficient and economic design of STC systems has been demonstrated through push-out experiments on steel-laminated veneer lumber (LVL) and steel-CLT composite joints [13,14] and four-point bending of full scale steel-LVL beams [12]. Dowel-type connectors are typically used in hybrid timber structures. Accordingly, most of the studies on hybrid STC system have focused on screw, dowel and bolted connections. For example, Asiz et al. [15] investigated the behaviour of lag-screws and self-tapping screws in CLT-steel connections in parallel and perpendicular to the grain directions. More recently, different arrangements of screws, glue and a variety of steel-CLT connections have been proposed and tested under cyclic loading conditions by Loss et al. [4,11]. However, the results of push-out and four-point bending experiments on STC members with coach screw and bolted shear connectors have shown that the conventional mechanical shear connectors such as coach screws manufactured from mild steel can only provide medium composite efficiency [12–14]. Accordingly, the effect of the yield strength of dowel shear connectors on the structural performance of steel-CLT composite joints requires further investigation. Furthermore, there is a need for developing innovative STC connections with elevated levels of strength, stiffness, ductility and composite efficiency.
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Apart from conventional dowel-type connectors, the application of grouted connections in timber structures has been investigated recently and promising results have been reported [16–19]. The primary application of the grouted connections in timber structures was mainly limited to anchorage zones where the load from timber elements or structures needs to be transmitted to the foundation or supporting walls through anchor rods or bolts [16]. Recently, Kaestner et al. [17]. used grouted connections in timber-concrete composite bridge decks, while Schober et al. [18] proposed and studied grouted joints for timber trusses. Furthermore, corner timber beam-to-column joints were fabricated from reinforced concrete and tested in the laboratory by Negrão et al. [19]. In this paper, a high-performance steel-CLT composite connection comprising of bolted shear connectors (manufactured from mild or high-strength steel) embedded in pockets of grout is proposed. The structural behaviour of this new connection is studied by push-out experiments and the stiffness, ultimate strength and ductility of the STC joints with grouted pockets is compared with STC joints with screws and bolts. Moreover, push-out are performed on STC specimens with coach screws and dog screws to evaluate the influence of the yield strength of mechanical connectors on the structural performance of the steel-CLT composite members. Lastly, empirical formulae for the loadslip, stiffness and strength of the steel-CLT composite connections with dog screw connectors and with bolts embedded in the pockets of grout are proposed.
2.2.2. Steel profile In the fabrication of specimens, a hot-rolled 310UB40.4 steel profile complying with AS/NZS 3679.1 [22] requirement was used. The steel profile was made of Grade 300PLUS steel, and its nominal mechanical properties and dimensions are provided in Table 3. 2.2.3. Mechanical fasteners The dog screws had a nominal ultimate strength of 500 MPa (Grade 5.8 steel) and the hexagonal coach screws (Grade 4.6 steel) had a nominal yield and ultimate strength of 240 MPa and 400 MPa, respectively. The conventional and high-strength hexagonal bolts, used in composite connections with pockets of grout, were made of Grades 4.6 and 8.8 steel. The bolts and screws used as shear connectors comply with the requirements of AS1110.1 [23] and AS1112.1 [24], and AS/NZS 1393 [25], respectively. The mechanical fasteners used as shear connectors and stress-strain relationship of the Grade 4.6, 5.8 and 8.8 steel obtained from uniaxial tension tests (as per AS 4291.1 [26]) on fasteners are shown in Fig. 4, and the details and dimensions of the mechanical fasteners are provided in Table 4.
2. Experimental program
2.2.4. Cementitious grout A high early strength cement-based grout was used in the BGP group of specimens to fill the pockets around the bolted shear connectors. Uniaxial compression tests were conducted using cube samples of grout according to the specifications of AS 1478.2:2005 [27], and the tensile and compressive strengths at different ages, the setting time and other mechanical properties of the grout are given in Table 5.
2.1. Push-out specimens
2.3. Experimental setup, fabrication and instrumentation
Laboratory push-out tests on three major groups (and 10 subgroups) of steel-CLT connections have been performed. The STC shear connectors were classified into three categories, i.e. coach screws (CS), dog screws (DS) and bolted grout pocket (BGP) and three identical specimens of each type of STC joint were fabricated and tested. The geometry and specifics of the push-out specimens are given in Table 1 and Fig. 1, and the set up adopted for the push-out experiments is shown in Fig. 2. The primary variables considered in the experimental program were the diameter of screws and bolts, loading direction with respect to the timber grains, and the yield strength of the fasteners.
2.3.1. Connections with coach screws In this group of connections, hexagonal coach screws are used to connect the flanges of the steel profile to 400 mm long and 400 mm wide CLT panels (Fig. 1). Two screws having a centre to centre spacing of 80 mm, on each side of the push out specimen were used (Fig. 1). The 80 mm spacing of screws in a row satisfies the minimum requirement of 4d = 80 mm (d being diameter of the fastener) specified in BS EN 1995-1-1 [28]. In addition to the flanges of the steel profile, the CLT panels were predrilled to facilitate installation of the coach screws. The holes predrilled in the CLT were 2 mm smaller and the holes predrilled in the flanges of the profiles were 0.2 mm larger than the coach screw diameter.
2.2. Material properties 2.2.1. Cross laminated timber (CLT) The CLT panels were made from machine graded C24 [20] Norwegian spruce timber. The CLT panels were manufactured from five layers as shown in Fig. 3, with the overall thickness of the CLT panels being 120 mm. The elastic moduli and strength of the CLT panels are provided in Table 2. The mean moisture content of the panels measured by oven dry testing according to AS/NZS2098.1 [21] was 12% and the mean density of the panels was 490 kg/m3. Table 1 Tested connections details. Connection type
Dimension (mm) (Diameter × length)
Fasteners steel grade
Grain direction
Coach screw
12 mm × 100 mm 16 mm × 100 mm 20 mm × 100 mm 16 mm × 125 mm 19 mm × 135 mm 12 mm × 130 mm
4.6 4.6 4.6 5.8 5.8 8.8
Perpendicular
16 mm × 130 mm
4.6 8.8 8.8 8.8
Dog screw Grout pocket
20 mm × 130 mm
Parallel Pocket Size 60 mm × 135 mm 80 mm × 135 mm 80 mm × 135 mm 60 mm × 135 mm 80 mm × 135 mm
Parallel
2.3.2. Connections with dog screws In this group of connections, dog screws are used to connect the flanges of the steel profile to 600 mm long and 400 mm wide CLT panels (Fig. 1). Dog screws are used mainly for connecting railway tracks to timber sleepers. Due to the special shape of their threads (Fig. 4c), cracking and crushing of the timber during installation of the screws are minimised. In the fabrication of the push out specimens with dog screw shear connectors, a configuration and procedure similar to that of the specimens with the coach screw shear connectors was used. 2.3.3. Connections with bolts embedded in grout pockets (BGP) The bolt shear connectors embedded in the pockets of cementitious grout is an innovative STC connection that can facilitate the construction process by minimising overhead works while preserving the possibility for prefabrication and deconstruction of STC floors and beams. In addition, it is hypothesised that the bolts embedded in grout pockets (BGP) can have higher strength and stiffness than common dowel-type mechanical fasteners, due to the higher elastic modulus and compressive/crushing strength of the cementitious grout compared to Spruce wood (or CLT panels). In the BGP connections, the horizontal shear force required for the development of composite action is transferred to a larger surface of CLT (i.e. the wall of the
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Fig. 1. Geometry, cross-section and details of push-out specimens with (a) bolted grout pocket, (b) dog screw and (c) coach screw connections.
pockets) that in turn postpones the localised damage and crushing of the timber. In the first step for fabrication of the BGP connections, rectangular holes were cut off the CLT panels (Fig. 5a) and the holes were sealed by water-resistant adhesive tapes to prevent moisture ingress into the timber panels (Fig. 5b). The bolt shear connectors were
then installed on the steel flanges and post-tensioned to 30% of the yield strength of the bolts by a torque wrench. In the last step, the holes were filled with the flowable high-strength cementitious grout (Fig. 5c). The length of the pockets was 135 mm (less than the flange width of 165 mm) to ensure that no formwork at the soffit of the pocket is
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CLT Panel
CLT Panel
CLT Panel
Steel Beam
500 kN Actuator
Fig. 2. Outline of push-out test setup.
required. To investigate the effect of pocket width, specimens with pocket widths of 60 mm and 80 mm were also fabricated. The bolt shear connectors were inserted through the holes predrilled (with dimeters equal to the actual bolt size, i.e. 12, 16 and 20 mm) in the flanges of the profile. The bolt shear connectors in BGP specimens were fastened by top and bottom nuts (double nuts) as shown in Fig. 5b. In the final stage of the fabrication following the bolt tightening, the pockets were filled with the high-strength cementitious grout (Fig. 5c). 2.3.4. Setup and instrumentation Push-out testing was conducted on CLT-steel-CLT specimens with a symmetric configuration, owing to ease of fabrication, providing a uniform load distribution and minimising unwanted friction at the CLT-steel flange interface. The relative displacements of the CLT panels with respect to the steel profile were measured by four linear variable displacement transducers (LVDTs) mounted on the four corners of the STC joints (Fig. 6). The maximum stroke of the LVDTs was limited to 100 mm. 2.4. Loading protocol The STC joints were loaded according to the loading protocol given in BS EN26891 [29] (Fig. 7). First, the load was ramped up from 0 to
40% of the estimated ultimate capacity (fest), it remained steady at this level for 30 s and then was decreased to 10% of fest. The reloading (second loading) stage started after 30 s and continued to 70% of fest using a load control procedure, where it was switched to displacement control with a rate of 2 mm/min. The second stage of loading continued until failure of the STC joint. 3. Discussion of tests results In this section, the designation CSxx, DSxx and BGPxx is adopted with respect to xx, being the size/diameter of the coach screws (CS), dog screws (DS) and bolts embedded in the grout pocket BGP), respectively. 3.1. Modes of failure 3.1.1. Connections with coach screws (CS) Direction of loading in the push-out specimens with coach screw connections was perpendicular to the timber grain in the outer lamellas of the CLT. In these specimens, a relatively large crushing zone around the fasteners was observed (Fig. 8). The extent/size of the crushed zone correlates directly with the size of the screws; in specimens with 20 mm coach screws, the first layer of the panel crushed completely (Fig. 8c). The failure modes in dowel-type fasteners with timber connections can be classified into three categories [12,28,30]; viz. Modes I, II and III. In failure Mode I, timber crushing occurs only, without any plastic deformation in the fasteners (Fig. 9a). However, in failure Modes II and III, one and two plastic hinges occur within the fasteners,
Table 2 Mechanical properties of CLT (in MPa).
Fig. 3. Schematic outline of CLT panel, orientation of planks, configuration and thickness of lamellas.
Bending (fb)
Tension parallel to grain (ft)
Shear in beams (fs)
Compression parallel to grain (fc)
Compression perpendicular to grain (fp)
Elastic modulus (E)
Modulus of rigidity (G)
24
16.5
4.6
24
2.7
12,000
690
A. Hassanieh et al. / Journal of Constructional Steel Research 138 (2017) 823–836 Table 3 Mechanical and geometrical properties of 310UB40.4 steel profile.
Table 4 Nominal mechanical properties and dimensions of the fasteners.
Section depth Flange d width bf
Flange thickness tf
Web Yield Ultimate thickness strength strength tw fy fu
Elastic modulus Es
304 mm
10.2 mm
6.1 mm
205 GPa
165 mm
320 MPa
450 MPa
respectively. Furthermore, partial crushing of the timber around the dowel connectors and in the outer lamellas of CLT may occur in failure modes II and III (Fig. 9b and c). With regard to this classification, it was observed that failure Mode III was dominant in 12 mm coach screws (Fig. 8d), whereas failure Mode II became dominant in the 16 and 20 mm coach screw shear connectors (Fig. 8e and f). 3.1.2. Connections with dog screws (DS) Direction of loading in the push-out specimens with dog screw connections was parallel to the timber grain in the outer lamellas of the CLT,
Bolt 12mm - 8.8 Bolt 16mm – 4.6
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Fastener
Diameter (mm)
Length (mm)
Yield strength (MPa)
Ultimate strength (MPa)
Coach screw Dog screw Bolt
16 19 16 20
100 135 150 110
240 400 640
400 500 800
and the size of the crushing zone near the dog screw connectors was small in contrast to specimens with coach screws in which the outer layers of timber panels were loaded in perpendicular to the grain direction. The width over which the crushing of the timber occurred was between 1.5d and 2d approximately (d being the fastener diameter) [12] as shown in Fig. 10a and b. The deformation of the 16 mm and 19 mm dog screws was demonstrative of a failure Mode II with only one plastic hinge in the fasteners (Fig. 10c and d).
Coach Screw 12mm
Coach Screw 16mm
Bolt 16mm - 8.8 Coach Screw 20mm Bolt 20mm - 8.8
(a)
(b)
Dog Screw 16mm
Dog Screw 19mm
(c)
(d)
Fig. 4. Outline of (a) bolts, (b) coach screws and (c) dog screws used as shear connectors, and (d) stress-strain relationship of shear connectors obtained from uniaxial tension test.
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Table 5 Mechanical properties of the cementitious grout. Compressive strength (MPa) Tensile strength (MPa) a
2 ha
1 Daya
3 Days
7 Days
28 Days
30
42
44.6
52.4
57.2
3.0
Flexural strength (MPa)
6.5
Setting time (min)
30–40
500 kN actuator
According to the manufacturer product description.
LVDTs 3.1.3. Connections with bolts embedded in the grout pocket (BGP) The BGP connections had a mixed mode of failure associated with crushing of the CLT panels, cracking and crushing of the grout, development of a plastic hinge in the bolt shear connectors and bending and
135 mm
60/80 mm
Fig. 6. Instrumentation of push-out STC specimens.
(a) Bolted shear connectors
Sealing Tape
plastic deformation in the flange of the steel profiles (Figs. 11 and 12). In spite of cracking and crushing of the grout and crushing of the CLT panels, no evidence of damage and/or excessive deformation was seen in the bolts in BGP20 connections (Fig. 11). However, after visual inspection of the BGP20 specimens, it was evident that the flanges of the steel profile had undergone bending and plastic deformation. In BGP16 with Grade 8.8 and 4.6 bolts, a plastic hinge and excessive plastic deformation occurred in the bolt shear connectors (Fig. 12). Similarly, in the BGP12 connections, a plastic hinge and excessive deformations were observed in the 12 mm bolt shear connectors, but the size of the crushing zone in the BGP12 specimens was smaller than the crushing zones in specimens BGP20 and BGP16. 3.2. Load-slip behaviour
(b)
Fresh grout
(c) Fig. 5. Fabrication of BGP connections (a) cutting and sealing pocket, (b) tightening bolts and (c) pouring fresh grout.
3.2.1. Connections with coach screws (CS) The load-slip response of the STC connections (loaded perpendicular to the grain of outer lamellas of the panels) with coach screw connectors is shown in Fig. 13. In addition, the yield strength Py, ultimate load carrying capacity Pu, and stiffness ks,0.4 and ks,0.6 of the connections are given in Table. The values of ks,0.4 were evaluated by considering the stiffness of the connection between 10% and 40% of Pu, whereas ks,0.6 is the stiffness calculated between 10% and 60% of Pu. In this paper, ks,0.4 and ks,0.6 are considered as being initial and pre-peak stiffness of the connections respectively. It is observable that the initial stiffness ks,0.4 of the steelCLT composite connections with coach screws is independent of the screw diameter/size, but the pre-peak stiffness ks,0.6 depends on the size of screws and it increases with an increase of the size of the screws. The load versus slip response of the joints with coach screw shear connectors loaded parallel and perpendicular to the grain of the outer lamellas of the panels are compared in Fig. 14. In addition to the load versus slip response shown in Fig. 14, the ultimate strength/capacity Pu, and stiffnesses ks,0.4 and ks,0.6 of the STC connections in which CLT panels are loaded parallel to the grain obtained from the tests of
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Fig. 7. Loading procedure adopted for push-out tests as per BS EN 26891 [29].
Crushing zone
Plastic hinge Densification of grains
(a)
(d) 16 mm coach screw
Crushing zone
Plastic hinge Densification of grains
(b)
(e) 20 mm coach screw
Crushing zone
Densification of grains Plastic hinge
(c)
(f)
Fig. 8. Failure modes in steel-CLT specimens with coach screw connectors, (a, b, c) crushing in CLT panels (outer lamellas loaded perpendicular to the grain) for specimens with 12, 16 and 20 mm coach screws, respectively and failure modes of fasteners associated with formation of (d) two plastic hinges in 12 mm screws, (e) one plastic hinge in 16 mm screw and (f) one plastic hinge in 20 mm screw.
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and perpendicular to the grain is much smaller than other steel-timber composite joints such as steel-LVL composite joints, because of the grain altering at 90° for different lamellas of CLT panels [13,14]. Contrary to the connection initial stiffness ks,0.4, the ultimate capacity Pu and pre-peak stiffness ks,0.6 of the joints with coach screws is almost proportional to the size of the coach screw (Fig. 15). The nearlinear relationship between the size of the coach screw shear connectors and pre-peak stiffness ks,0.6 and ultimate load capacity Pu of the steelCLT joints is illustrated by lines of best fit shown in Fig. 15a and b respectively. Furthermore, the negligible influence of the diameter of the coach screws on the initial stiffness ks,0.4 of steel-CLT composite joints is evident from the best line fit (i.e. ks,0.4 = 0.017d + 6.23) shown in Fig. 15a that has a small correlation coefficient of 0.0175 between ks,0.4 and d.
Fig. 9. Failure mode of dowel-type fasteners in timber connections (a) mode I, (b) mode II and (c) mode III.
Hassanieh et al. [14] are provided in Table 6. It is seen that the ultimate capacity of STC joints loaded perpendicular to the grain of the outer lamellas is greater than or equal to that of joints loaded parallel to the grain of the outer layers, and this larger load capacity (particularly for larger screw sizes, 16 and 20 mm) can be attributed to potential wood densification in the perpendicular to the grain direction. However, the stiffness of steel-CLT connections loaded perpendicular to the grain is smaller than that of specimens loaded parallel to the grain of the outer layers of CLT panels, and this smaller stiffness is due to the significantly lower elastic modulus of the timber in the perpendicular to the grain direction compared with parallel to the grain. It is noteworthy that the difference in the behaviour of steel-CLT connections loaded parallel
3.2.2. Connections with dog screws (DS) The load versus slip behaviour of the connections DS16 and DS19 with dog screw connectors loaded parallel to the grain of the outer layers of the timber panels is shown in Fig. 16. Furthermore, the initial stiffness ks,0.4, pre-peak stiffness ks,0.6 and ultimate load carrying capacity Pu of the double dog screw connections are provided in Table. The initial and pre-peak stiffness of dog screw connections has a negative correlation with the size of the dog screws, but the ultimate load carrying capacity of the connections has a positive correlation with the size of the dog screws. The ultimate capacity of DS19 is around 26% more than the ultimate capacity of connection DS16. The load-slip behaviour s of steel-CLT joints DS16 and CS16 (loaded parallel to the grain of the outer layers) are compared in Fig. 17, while the load versus slip responses of joints DS19 and CS20 are compared in Fig. 18. It can be seen that the ultimate capacity of DS16 (54.4 kN) is about 10% higher than that of CS16 (49.9 kN) (Fig. 17), because of the higher yield and ultimate strengths of DS16 (having Grade 5.8 steel) compared to CS16 (with Grade 4.6 steel). In terms stiffness (Tables 6 and 7), the initial stiffness ks,0.4 of DS16 (29.65 kN/mm) is about 33% higher than CS16 (22.23 kN/mm) and the pre-peak
Crushing zone
Plastic hinge
16 mm dog screw
(a)
(c) Crushing zone
Plastic hinge
19 mm dog screw
(b)
(d)
Fig. 10. Failure modes in steel-CLT specimens with dog screw connectors, (a, b) crushing in CLT panels (outer lamellas loaded parallel to the grain) for specimens with 16 and 20 mm dog screws, respectively and failure modes of fasteners associated with formation of one plastic hinge in (d) 16 mm and (e) 19 mm screw (failure mode II).
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Cracking of grout
(a)
(b) Deformation of flange
Timber crushing zone
(c)
(d)
Fig. 11. Failure of BGP20 connections (a) no visible damage/deformation in the bolt shear connectors, (b) crushing and cracking of grout, (c) timber crushing and (d) plastic deformation in the flange of the steel profile.
stiffness ks,0.6 of DS16 (11.66 kN/mm) is about 42% higher than CS16 (8.21 kN/mm). However, the difference between the load-slip curves, pre-peak stiffness and ultimate capacity of the DS19 and CS20 joints is negligible (Tables 6 and 7). This is because the structural behaviour of the steel-CLT joints with large mechanical fasteners is governed primarily by the behaviour of the timber panels (e.g. timber crushing) rather than the strength and stiffness of the fasteners.
3.2.3. Connections with bolts embedded in the grout pocket (BGP) The initial stiffness ks,0.4, pre-peak stiffness ks,0.6 and ultimate strength/capacity Pu of the BGP joints are given in Table 8. In this table, the designation BGPxx y.y W × L is adopted with respect to the bolt size xx (i.e. 12, 16, 20 mm), Grade of the steel y.y (i.e. 4.6 or 8.8), width W (i.e. 60 mm or 80 mm) and length L (i.e. 135 mm) of the grouted pockets.
Grout crushing
Plastic hinge
(a)
(b)
Grout crushing
Plastic hinge
(c)
(d)
Fig. 12. BGP16-8.8 connection failure (a) grout crushing (b) bolt plastic deformation and BGP16-4.6 connection failure (c) grout crushing and (d) bolt plastic deformation.
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A. Hassanieh et al. / Journal of Constructional Steel Research 138 (2017) 823–836 Table 6 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints with two coach screw shear connectors. Coach Screw
CS12 CS16 CS20 a
Fig. 13. Load versus slip of steel-CLT connections with CLT panels loaded perpendicular to the grain of outer lamellas (two coach screw shear connectors).
Loaded perpendicular to graina
Loaded parallel to graina [14]
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
6.29 6.81 6.43
2.14 6.11 6.24
37.5 53.6 74.9
18.69 22.23 10.37
9.52 8.21 6.11
35.8 49.9 67.5
Loaded with respect to orientation of the grain in the outer lamellas of CLT panel.
The load-slip behaviour of the BGP steel-CLT joints with varied sizes of bolt connectors (12, 16 and 20 mm) and two sizes of grouted pockets (60 mm × 135 mm and 80 mm × 135 mm) are shown in Figs. 19 and 20 respectively. Both BGP20 connections (irrespective of the size of the grouted pocket) exhibit similar load versus slip and ductility characteristics up to the ultimate/peak load (Fig. 20), but the BGP20 connection with a 60 mm wide pocket (a smaller pocket of grout) shows a steeper post-peak softening branch due to severe crushing of the grout in the smaller pockets. The load versus slip behaviour of BGP16 connections with 16 mm bolts manufactured from 8.8 and 4.6 steel grades is shown in Fig. 21. Both connections exhibit identical initial and pre-peak stiffnesses. In terms of peak (ultimate) capacity, increasing the nominal yield strength of the bolt connectors (embedded in the grout pocket) from 240 MPa (Grade 4.6) to 640 MPa (Grade 8.8) increases the ultimate strength of the BGP16 connections by 26% (Table 8). These observations demonstrate that strength and stiffness of the steel bolt shear connectors embedded in cementitious grout (or concrete) depends largely on mechanical properties (particularly strength) of the cementitious
k 0.4, s
0.0175d
k 0.6, s
6.23
0.51d 3.37
(a)
Pu
4.68d 19.5
(b) Fig. 14. Comparison of load versus slip of steel-CLT connections with CLT panels loaded perpendicular and/or parallel to the grain of outer lamellas, for two (a) 12 mm, (b) 16 mm and (c) 20 mm coach screws.
Fig. 15. Relationship between coach screw diameter and (a) stiffness and (b) peak load capacity of steel-CLT joints with double coach screws (loading perpendicular to the grain of outer lamellas of CLT panels).
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Table 7 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints (CLT panels loaded in the direction parallel to the grain of outer layers) with two dog screw shear connectors. Dog screw
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
DS16 DS19
29.65 17.16
11.66 6.23
54.4 68.6
Table 8 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints (CLT panels loaded in the direction parallel to the grain of outer layers) with two bolts embedded in the grout pocket (BGP). Fig. 16. Load versus slip response of steel-CLT connections loaded parallel to the grain of outer lamellas of CLT panels, with two (a) 16 mm and (b) 19 mm dog screw connectors.
Pu=54.4 kN
Bolted Grout Pocket
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
BGP12 8.8 60 mm × 135 mm BGP16 4.6 80 mm × 135 mm BGP16 8.8 80 mm × 135 mm BGP20 8.8 60 mm × 135 mm BGP20 8.8 80 mm × 135 mm
39.53 58.68 55.12 64.57 77.19
29.76 45.26 45.77 53.68 63.08
82.9 103.4 129.9 153.3 159.7
Regarding the experimental results provided in Tables 6 to 8 and Figs. 13 to 21, it can be concluded that stiffness and strength of the bolt shear connectors in the pocket of grout is significantly higher than that of coach screws and dog screws.
Pu=49.9 kN
3.3. Analytical model of connections
Fig. 17. Comparisons of load versus slip response of steel-CLT joints with 16 mm dog screw and with 16 mm coach screw connectors.
grout (or concrete). A linear regression was conducted on the experimental stiffness and ultimate strength/capacity of the BGP connections versus the diameter of the embedded bolt shear connectors and the results are shown in Fig. 22. Accordingly, the formulas proposed for calculating the initial stiffness ks,0.4 (in kN/mm), pre-peak stiffness ks,0.6 (in kN/mm) and ultimate strength/capacity Pu (in kN) of the BGP connections are given by. k0:4;s ¼ 4:71d − 18;
ð1aÞ
k0:6;s ¼ 4:19d − 21
ð1bÞ
and P u ¼ 9:64d − 30;
ð2Þ
where d is the diameter (in mm) of bolt shear connector.
Fig. 18. Comparisons of load versus slip response of the steel-CLT joints with 19 mm dog screw and with 20 mm coach screw shear connectors.
In this section, non-linear regression on the load-slip results (Figs. 13 to 21) are carried out to develop empirical models for predicting the shear force (f) versus slip (s) relationship for the steel-CLT composite connections with coach screws, dog screws and bolts in pockets of grout. The proposed load-slip function (Fig. 23) has seven parameters which are estimated by non-linear regression [13,14]. The adopted shear force (f) versus slip (s) function is defined as. k0 − kp s kp þ ks s þ f ¼n h h in2 on1 − ks s; in1 on11 n 2 1 þ k0 − kp fs 1 þ kp þ ks f −s f 0
1
ð3Þ
0
where f denotes the shear force, s denotes the slip, k0, kp and ks are the initial, pre-peak and post-peak stiffness, respectively, f0 is the first reference shear force corresponding to the pre-peak branch and f1 is the second reference shear force corresponding to the post-peak branch, and n1 and n2 are parameters that control the curvature of the first and second curves, respectively [13]. For each group of the STC connections, the seven input parameters providing the best fit to the mean of the experimental load-slip curves were obtained from method of least squares with trust-region and the estimated input parameters are given in Table 9. The analytical loadslip curves correlate very well with and experimental results as shown
Fig. 19. Load versus slip response of BGP steel-CLT joints with different bolt sizes.
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Fig. 20. Effect of grouted pocket size on load versus slip response of BGP20 steel-CLT joint. Fig. 23. Proposed analytical model for load-slip behaviour of steel-timber composite joints [13,14].
in Figs. 24 and 25 and the good correlation is also is evident from the Rsquare values given in Table 9. 4. Conclusions
Fig. 21. Effect of yield strength of bolt connectors embedded in grout pockets on load versus slip behaviour of BGP16 steel-CLT joints.
k0.4, s
The load-slip behaviour of steel-CLT composite connections has been investigated through experimental push-out tests. The effect of shear connection type (conventional mechanical fasteners versus shear connectors embedded in grouted pockets), the effect of the size and yield strength of the fasteners (coach screws, dog screws and bolts) and effect of the loading direction (i.e. parallel or perpendicular to the grain of outer lamellas) on the stiffness, strength and failure modes of steel-CLT connections were investigated. Finally, analytical models for load-slip behaviour of all types of steel-CLT connections have been proposed and calibrated against laboratory test results. The main conclusions of the experimental study are the following.
(a)
• The loading direction (i.e. parallel or perpendicular to grain of outer lamellas) have a significant influence on the initial stiffness, but minor influence on the ultimate strength of the steel-CLT joints with coach screws. The initial stiffness of the joints loaded perpendicular to grain is less than initial stiffness of the joints loaded parallel to the grain. • The pre-peak stiffness and ultimate strength of the steel-CLT connections with dog screw (Grade 5.8 steel) shear connectors are slightly higher than those for connections with coach screw (Grade 4.6 steel) shear connectors. Accordingly, the yield strength of mechanical shear connectors has minor effect on the stiffness and strength of the steel-CLT composite connections. This is particularly the case for mechanical connectors with larger diameters (d ≥ 16 mm), because the
Pu
Table 9 Input parameters for the analytical load-slip model of the steel-CLT composite connections.
4.71d 18
k0.6, s
4.19d 21
9.64d 30
Connection Coach screws
Dog screws Bolt in grout pocket
(b) Fig. 22. Relationship between diameter of bolt shear connectors and (a) stiffness and (b) peak load capacity of BGP steel-CLT joints.
S12 S16 S20 DS16 DS19 BGP12 8.8 BGP16 4.6 BGP16 8.8 BGP20 60,135 BGP20 80,135
f0
f1
k0
kp
ks
n1
n2
R2
80 53 150 46 30 88 103 88 89
200 300 180 192 59 95 500 135 156
72.5 13.5 12.3 303.4 150.9 62.8 53 58 65
0 0 2.6 0.8 2.8 2.0 −0.2 10.0 14.5
2.2 4.2 0.4 1.2 −0.3 0.6 6.43 0.95 0.38
0.35 1.25 0.52 0.6 0.85 1.24 4.35 3.25 3.30
5.6 10.0 5.9 1.2 5.0 4.0 5.0 6.0 5.0
0.9799 0.9965 0.9957 0.9945 0.9993 0.9987 0.9971 0.9901 0.9983
20
0.12
4.80 2.0
100 161 66.8
0.9994
A. Hassanieh et al. / Journal of Constructional Steel Research 138 (2017) 823–836
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Fig. 24. Correlation between analytical model and mean of experimental load-slip response for steel-CLT composite joints with (a) coach screw connections in CLT panels loaded perpendicular to the grain and (b) dog screws connections in CLT panels loaded parallel to the grain.
•
•
•
•
structural behaviour of joints with large mechanical connectors is governed mainly by non-linear localised effects (such as timber crushing) rather than the strength of the connectors. Steel-CLT connections with bolt connectors embedded in grouted pockets (BGP) have significantly higher initial stiffnesses, pre-peak stiffnesses and peak load capacities compared to joints with conventional coach screw and dog screw connectors. The size (width) of grouted pockets has little or no influence on the pre-peak load-slip behaviour of the joints with BGP shear connectors. However, joints with smaller grouted pockets are slightly more brittle, as evidenced by the steeper slope of their post-peak behaviour. The yield strength of bolt shear connectors embedded in grout pocket has negligible influence on the stiffness of joints with BGP connections, but increasing the yield strength of bolt shear connectors increases the ultimate strength/capacity of the joints with BGP connections. Within the range of the relevant parameters, a relationship for the load-slip behaviour of the steel-CLT joints in analytical form is possible using non-linear regression. This relationship was proposed in the paper.
Acknowledgement The work in this paper was funded by a Discovery Project (DP160104092) awarded to the second and third authors by the Australian Research Council. This support is acknowledged with thanks.
Fig. 25. Correlation between analytical model and mean of experimental load-slip response for steel-CLT composite joints with BGP connections (a) BGP20, (b) BGP16 and (c) BGP 12.
References [1] A. Hassanieh, H.R. Valipour, M.A. Bradford, Development of steel-timber composite system for large scale construction, WCTE 2016 - World Conf Timber Eng 2016, pp. 4290–4299. [2] F. Wanninger, A. Frangi, Experimental and analytical analysis of a post-tensioned timber connection under gravity loads, Eng. Struct. 70 (2014) 117–129, https:// doi.org/10.1016/j.engstruct.2014.03.042. [3] M.P. Newcombe, S. Pampanin, A.H. Buchanan, A. Palermo, Section analysis and cyclic behavior of post-tensioned jointed ductile connections for multi-story timber buildings, J. Earthq. Eng. 12 (2008) 83–110, https://doi.org/10.1080/13632460801925632. [4] C. Loss, M. Piazza, R. Zandonini, Connections for steel–timber hybrid prefabricated buildings. Part II: innovative modular structures, Constr. Build. Mater. (2015) https://doi.org/10.1016/j.conbuildmat.2015.12.001. [5] R. Gutkowski, K. Brown, A. Shigidi, J. Natterer, Laboratory tests of composite woodconcrete beams, Constr. Build. Mater. 22 (6) (2008) 1059–1066. [6] N. Khorsandnia, H.R. Valipour, K. Crews, Experimental and analytical investigation of short-term behaviour of LVL-concrete composite connections and beams, Constr. Build. Mater. 37 (2012) 229–238. [7] D. Yeoh, M. Fragiacomo, M. De Franceschi, A.H. Buchanan, Experimental tests of notched and plate connectors for LVL-concrete composite beams, J. Struct. Eng. ASCE 137 (2) (2011) 261–269.
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[8] C. Dickof, S.F. Stiemer, M.A. Bezabeh, S. Tesfamariam, CLT-steel hybrid system: ductility and overstrength values based on static pushover analysis, J. Perform. Constr. Facil. 28:A4014012 (1–12) (2014) https://doi.org/10.1061/(ASCE)CF.19435509.0000614. [9] M. He, Z. Li, F. Lam, R. Ma, Z. Ma, Experimental investigation on lateral performance of timber-steel hybrid shear wall systems, J. Struct. Eng. ASCE 140 (2014) 4014029, https://doi.org/10.1061/(asce)st.1943-541x.0000855. [10] P. Bhat, M.R. Azim, M. Popovski, T. Tannert, Experimental and numerical investigation of novel steel-timber-hybrid system, WCTE 2014 - World Conf Timber Eng Proc, 2014. [11] C. Loss, M. Piazza, R. Zandonini, Connections for steel–timber hybrid prefabricated buildings. Part I: experimental tests, Constr. Build. Mater. (2015) https://doi.org/ 10.1016/j.conbuildmat.2015.12.002. [12] A. Hassanieh, H.R. Valipour, M.A. Bradford, Experimental and numerical study of steel-timber composite (STC) beams, J. Constr. Steel Res. 122 (2016) 367–378, https://doi.org/10.1016/j.jcsr.2016.04.005. [13] A. Hassanieh, H.R. Valipour, M.A. Bradford, Experimental and analytical behaviour of steel-timber composite connections, Constr. Build. Mater. 118 (2016) 63–75, https://doi.org/10.1016/j.conbuildmat.2016.05.052. [14] A. Hassanieh, H.R. Valipour, M.A. Bradford, Load-slip behaviour of steel-cross laminated timber (CLT) composite connections, J. Constr. Steel Res. 122 (2016) 110–121. [15] A. Asiz, I. Smith, Connection system of massive timber elements used in horizontal slabs of hybrid tall buildings, J. Struct. Eng. ASCE 137 (2011) 1390–1393, https:// doi.org/10.1061/(ASCE)ST.1943-541X.0000363. [16] S. Moreira, L.F. Ramos, D.V. Oliveira, P.B. Lourenço, Experimental behavior of masonry wall-to-timber elements connections strengthened with injection anchors, Eng. Struct. 81 (2014) 98–109, https://doi.org/10.1016/j.engstruct.2014.09.034. [17] M. Kaestner, K. Rautenstrauch, Efficient shear transfer in timber-concrete composite bridges by means of grouting with polymer mortar, WCTE 2016 - World Conf on Timber Eng, 4249–58, 2016.
[18] K. Schober, W. Becker, J. Weber, Grouted joints in timber engineering, World Conf on Timber Eng 2016, pp. 4282–4289. [19] J.H. Negrão, L.D. Brito, A.G. Dias, C.C. Júnior, F.R. Lahr, Numerical and experimental study of small-scale moment-resistant reinforced concrete joints for timber frames, Constr. Build. Mater. 118 (2016) 89–103, https://doi.org/10.1016/j.conbuildmat. 2016.05.036. [20] British Standards Institute, BS EN 338: Structural timber - Strength Classes, 2009. [21] AS/NZS 2098.1, 2016 Methods of Test for Veneer and Plywood - Moisture Content of Veneer and Plywood, Standards Australia, 2016. [22] Standards Australia, AS/NZS 3679.1:2010 Hot Rolled Bars and Sections, Standards Australia, 2012. [23] Standards Australia, AS 1110.1-2000 ISO Metric Hexagon Bolts and Screws-product Grades A and B, 2000. [24] Standards Australia, AS 1112.1-2000 ISO Metric Hexagon Nuts, 2000. [25] Standards Australia, AS/NZS 1393:1996 Coach Screws-Metric Series With ISO Hexagon HeadsStandards Australia 1996. [26] Australian Standard, AS 4291.1:2015 Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel Paet 1: Bolts, Screws And Studs, Aust. Stand, 2015 https://doi.org/10.1017/CBO9781107415324.004. [27] AS 1478.2, 2005 Chemical Admixtures for Concrete, Mortar and Grout - Methods of Sampling and Testing Admixtures for Concrete, Mortar and Grout, Standards Australia, 2005. [28] Eurocode 5 (EC 5), Design of timber structures- Part 1-1: General-common rules and rules for buildings. BS EN 1995-1-1, 2004. [29] EN-26891, CEN, Timber Structures, Joints Made With Mechanical Fasteners, General Principles for the Determination of Strength and Deformation Characteristics, BSI: European Committee for Standardization, Brussels (Belgium), 1991. [30] P. Aune, M. Patton-Mallory, Lateral Load-bearing Capacity of Nailed Joints Based on the Yield Theory: Experimental Verification. Res Pap 470, United States Department of Agriculture, Forest Service, Forest Products Laboratory, 1986 1–29.
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Composite connections between CLT slab and steel beam: Experiments and empirical models A. Hassanieh ⁎, H.R. Valipour, M.A. Bradford Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, UNSW Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 25 April 2017 Received in revised form 22 August 2017 Accepted 4 September 2017 Available online xxxx Keywords: Bolted connectors Cross laminated timber (CLT) Grouted connection Push-out
a b s t r a c t A composite floor comprising of steel beams connected to cross laminated timber (CLT) panels is a sustainable alternative to composite floors with reinforced concrete slab. The structural performance of the steel-CLT floor depends strongly on the load-slip behaviour (particularly the stiffness and strength) of the shear connection between the steel beams and the CLT slabs. In this paper, the short-term behaviour of steel-CLT joints is studied by laboratory push-out experiments conducted on CLT-steel-CLT specimens, being analogous to familiar push tests on steel-concrete specimens. In addition to coach screws and bolts, a high-performance steel-CLT composite connection comprising of high-strength bolted shear connectors embedded in pockets of grout is proposed and tested. The push-out experiments are used to determine the load-slip, failure mode, stiffness and ultimate strength/capacity of the steel-CLT composite joints and to calibrate an empirical formula for the load-slip response of the CLT-to-steel connections. Furthermore, simple empirical formulae for the stiffness and ultimate capacity of the CLT-to-steel composite connections are proposed. Crown Copyright © 2017 Published by Elsevier Ltd. All rights reserved.
1. Introduction The application of the engineering wood products (EWPs), especially cross laminated timber (CLT), in building construction has been growing rapidly over the past decade. Having lower embodied energy and less environmental ramifications, and with a considerably higher strength to weight ratio, timber has many favourable attributes in construction when compared to steel and concrete alone. These advantages, along with the possibility for full prefabrication of timber structures, make EWPs a promising alternative to conventional steel and reinforced concrete [1]. Moreover, EWPs can be used in conjunction with steel and reinforced concrete to develop hybrid structures with improved sustainability and structural performance [2–4]. For example, timberconcrete composite system with improved short- and long-term behaviour, acoustic performance and fire rating have been developed by connecting reinforced concrete slabs to sawn and engineered wood (i.e. laminated veneer lumber, glued laminated timber) beams [5–7]. Moreover, the possible application of cross laminated timber (CLT) wall panels as lateral load resisting systems in steel structures has been investigated experimentally and numerically [8,9]. In a recent study, Dickof et al. [8] utilised CLT-shear walls in a moment resisting steel frame. Similarly, the structural performance of the steel frames with infill wood shear walls was investigated by He et al. [9]. In addition, Bhat et al. [10] conducted monotonic and cyclic tests on a hybrid system comprising of steel beams embedded in CLT wall. However, little ⁎ Corresponding author. E-mail address: [email protected] (A. Hassanieh).
https://doi.org/10.1016/j.jcsr.2017.09.002 0143-974X/Crown Copyright © 2017 Published by Elsevier Ltd. All rights reserved.
research has been devoted to development of sustainable steel-timber hybrid floors in which horizontally laid timber panels (connected to steel beams) replace conventional reinforced concrete slabs [4,11]. The development of composite action in a steel-timber composite (STC) system, and the feasibility of such STC beams and flooring systems for large scale construction, has been investigated in previous studies [4,11–13]. The essential role of steel-to-timber connections for the efficient and economic design of STC systems has been demonstrated through push-out experiments on steel-laminated veneer lumber (LVL) and steel-CLT composite joints [13,14] and four-point bending of full scale steel-LVL beams [12]. Dowel-type connectors are typically used in hybrid timber structures. Accordingly, most of the studies on hybrid STC system have focused on screw, dowel and bolted connections. For example, Asiz et al. [15] investigated the behaviour of lag-screws and self-tapping screws in CLT-steel connections in parallel and perpendicular to the grain directions. More recently, different arrangements of screws, glue and a variety of steel-CLT connections have been proposed and tested under cyclic loading conditions by Loss et al. [4,11]. However, the results of push-out and four-point bending experiments on STC members with coach screw and bolted shear connectors have shown that the conventional mechanical shear connectors such as coach screws manufactured from mild steel can only provide medium composite efficiency [12–14]. Accordingly, the effect of the yield strength of dowel shear connectors on the structural performance of steel-CLT composite joints requires further investigation. Furthermore, there is a need for developing innovative STC connections with elevated levels of strength, stiffness, ductility and composite efficiency.
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Apart from conventional dowel-type connectors, the application of grouted connections in timber structures has been investigated recently and promising results have been reported [16–19]. The primary application of the grouted connections in timber structures was mainly limited to anchorage zones where the load from timber elements or structures needs to be transmitted to the foundation or supporting walls through anchor rods or bolts [16]. Recently, Kaestner et al. [17]. used grouted connections in timber-concrete composite bridge decks, while Schober et al. [18] proposed and studied grouted joints for timber trusses. Furthermore, corner timber beam-to-column joints were fabricated from reinforced concrete and tested in the laboratory by Negrão et al. [19]. In this paper, a high-performance steel-CLT composite connection comprising of bolted shear connectors (manufactured from mild or high-strength steel) embedded in pockets of grout is proposed. The structural behaviour of this new connection is studied by push-out experiments and the stiffness, ultimate strength and ductility of the STC joints with grouted pockets is compared with STC joints with screws and bolts. Moreover, push-out are performed on STC specimens with coach screws and dog screws to evaluate the influence of the yield strength of mechanical connectors on the structural performance of the steel-CLT composite members. Lastly, empirical formulae for the loadslip, stiffness and strength of the steel-CLT composite connections with dog screw connectors and with bolts embedded in the pockets of grout are proposed.
2.2.2. Steel profile In the fabrication of specimens, a hot-rolled 310UB40.4 steel profile complying with AS/NZS 3679.1 [22] requirement was used. The steel profile was made of Grade 300PLUS steel, and its nominal mechanical properties and dimensions are provided in Table 3. 2.2.3. Mechanical fasteners The dog screws had a nominal ultimate strength of 500 MPa (Grade 5.8 steel) and the hexagonal coach screws (Grade 4.6 steel) had a nominal yield and ultimate strength of 240 MPa and 400 MPa, respectively. The conventional and high-strength hexagonal bolts, used in composite connections with pockets of grout, were made of Grades 4.6 and 8.8 steel. The bolts and screws used as shear connectors comply with the requirements of AS1110.1 [23] and AS1112.1 [24], and AS/NZS 1393 [25], respectively. The mechanical fasteners used as shear connectors and stress-strain relationship of the Grade 4.6, 5.8 and 8.8 steel obtained from uniaxial tension tests (as per AS 4291.1 [26]) on fasteners are shown in Fig. 4, and the details and dimensions of the mechanical fasteners are provided in Table 4.
2. Experimental program
2.2.4. Cementitious grout A high early strength cement-based grout was used in the BGP group of specimens to fill the pockets around the bolted shear connectors. Uniaxial compression tests were conducted using cube samples of grout according to the specifications of AS 1478.2:2005 [27], and the tensile and compressive strengths at different ages, the setting time and other mechanical properties of the grout are given in Table 5.
2.1. Push-out specimens
2.3. Experimental setup, fabrication and instrumentation
Laboratory push-out tests on three major groups (and 10 subgroups) of steel-CLT connections have been performed. The STC shear connectors were classified into three categories, i.e. coach screws (CS), dog screws (DS) and bolted grout pocket (BGP) and three identical specimens of each type of STC joint were fabricated and tested. The geometry and specifics of the push-out specimens are given in Table 1 and Fig. 1, and the set up adopted for the push-out experiments is shown in Fig. 2. The primary variables considered in the experimental program were the diameter of screws and bolts, loading direction with respect to the timber grains, and the yield strength of the fasteners.
2.3.1. Connections with coach screws In this group of connections, hexagonal coach screws are used to connect the flanges of the steel profile to 400 mm long and 400 mm wide CLT panels (Fig. 1). Two screws having a centre to centre spacing of 80 mm, on each side of the push out specimen were used (Fig. 1). The 80 mm spacing of screws in a row satisfies the minimum requirement of 4d = 80 mm (d being diameter of the fastener) specified in BS EN 1995-1-1 [28]. In addition to the flanges of the steel profile, the CLT panels were predrilled to facilitate installation of the coach screws. The holes predrilled in the CLT were 2 mm smaller and the holes predrilled in the flanges of the profiles were 0.2 mm larger than the coach screw diameter.
2.2. Material properties 2.2.1. Cross laminated timber (CLT) The CLT panels were made from machine graded C24 [20] Norwegian spruce timber. The CLT panels were manufactured from five layers as shown in Fig. 3, with the overall thickness of the CLT panels being 120 mm. The elastic moduli and strength of the CLT panels are provided in Table 2. The mean moisture content of the panels measured by oven dry testing according to AS/NZS2098.1 [21] was 12% and the mean density of the panels was 490 kg/m3. Table 1 Tested connections details. Connection type
Dimension (mm) (Diameter × length)
Fasteners steel grade
Grain direction
Coach screw
12 mm × 100 mm 16 mm × 100 mm 20 mm × 100 mm 16 mm × 125 mm 19 mm × 135 mm 12 mm × 130 mm
4.6 4.6 4.6 5.8 5.8 8.8
Perpendicular
16 mm × 130 mm
4.6 8.8 8.8 8.8
Dog screw Grout pocket
20 mm × 130 mm
Parallel Pocket Size 60 mm × 135 mm 80 mm × 135 mm 80 mm × 135 mm 60 mm × 135 mm 80 mm × 135 mm
Parallel
2.3.2. Connections with dog screws In this group of connections, dog screws are used to connect the flanges of the steel profile to 600 mm long and 400 mm wide CLT panels (Fig. 1). Dog screws are used mainly for connecting railway tracks to timber sleepers. Due to the special shape of their threads (Fig. 4c), cracking and crushing of the timber during installation of the screws are minimised. In the fabrication of the push out specimens with dog screw shear connectors, a configuration and procedure similar to that of the specimens with the coach screw shear connectors was used. 2.3.3. Connections with bolts embedded in grout pockets (BGP) The bolt shear connectors embedded in the pockets of cementitious grout is an innovative STC connection that can facilitate the construction process by minimising overhead works while preserving the possibility for prefabrication and deconstruction of STC floors and beams. In addition, it is hypothesised that the bolts embedded in grout pockets (BGP) can have higher strength and stiffness than common dowel-type mechanical fasteners, due to the higher elastic modulus and compressive/crushing strength of the cementitious grout compared to Spruce wood (or CLT panels). In the BGP connections, the horizontal shear force required for the development of composite action is transferred to a larger surface of CLT (i.e. the wall of the
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825
Fig. 1. Geometry, cross-section and details of push-out specimens with (a) bolted grout pocket, (b) dog screw and (c) coach screw connections.
pockets) that in turn postpones the localised damage and crushing of the timber. In the first step for fabrication of the BGP connections, rectangular holes were cut off the CLT panels (Fig. 5a) and the holes were sealed by water-resistant adhesive tapes to prevent moisture ingress into the timber panels (Fig. 5b). The bolt shear connectors were
then installed on the steel flanges and post-tensioned to 30% of the yield strength of the bolts by a torque wrench. In the last step, the holes were filled with the flowable high-strength cementitious grout (Fig. 5c). The length of the pockets was 135 mm (less than the flange width of 165 mm) to ensure that no formwork at the soffit of the pocket is
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CLT Panel
CLT Panel
CLT Panel
Steel Beam
500 kN Actuator
Fig. 2. Outline of push-out test setup.
required. To investigate the effect of pocket width, specimens with pocket widths of 60 mm and 80 mm were also fabricated. The bolt shear connectors were inserted through the holes predrilled (with dimeters equal to the actual bolt size, i.e. 12, 16 and 20 mm) in the flanges of the profile. The bolt shear connectors in BGP specimens were fastened by top and bottom nuts (double nuts) as shown in Fig. 5b. In the final stage of the fabrication following the bolt tightening, the pockets were filled with the high-strength cementitious grout (Fig. 5c). 2.3.4. Setup and instrumentation Push-out testing was conducted on CLT-steel-CLT specimens with a symmetric configuration, owing to ease of fabrication, providing a uniform load distribution and minimising unwanted friction at the CLT-steel flange interface. The relative displacements of the CLT panels with respect to the steel profile were measured by four linear variable displacement transducers (LVDTs) mounted on the four corners of the STC joints (Fig. 6). The maximum stroke of the LVDTs was limited to 100 mm. 2.4. Loading protocol The STC joints were loaded according to the loading protocol given in BS EN26891 [29] (Fig. 7). First, the load was ramped up from 0 to
40% of the estimated ultimate capacity (fest), it remained steady at this level for 30 s and then was decreased to 10% of fest. The reloading (second loading) stage started after 30 s and continued to 70% of fest using a load control procedure, where it was switched to displacement control with a rate of 2 mm/min. The second stage of loading continued until failure of the STC joint. 3. Discussion of tests results In this section, the designation CSxx, DSxx and BGPxx is adopted with respect to xx, being the size/diameter of the coach screws (CS), dog screws (DS) and bolts embedded in the grout pocket BGP), respectively. 3.1. Modes of failure 3.1.1. Connections with coach screws (CS) Direction of loading in the push-out specimens with coach screw connections was perpendicular to the timber grain in the outer lamellas of the CLT. In these specimens, a relatively large crushing zone around the fasteners was observed (Fig. 8). The extent/size of the crushed zone correlates directly with the size of the screws; in specimens with 20 mm coach screws, the first layer of the panel crushed completely (Fig. 8c). The failure modes in dowel-type fasteners with timber connections can be classified into three categories [12,28,30]; viz. Modes I, II and III. In failure Mode I, timber crushing occurs only, without any plastic deformation in the fasteners (Fig. 9a). However, in failure Modes II and III, one and two plastic hinges occur within the fasteners,
Table 2 Mechanical properties of CLT (in MPa).
Fig. 3. Schematic outline of CLT panel, orientation of planks, configuration and thickness of lamellas.
Bending (fb)
Tension parallel to grain (ft)
Shear in beams (fs)
Compression parallel to grain (fc)
Compression perpendicular to grain (fp)
Elastic modulus (E)
Modulus of rigidity (G)
24
16.5
4.6
24
2.7
12,000
690
A. Hassanieh et al. / Journal of Constructional Steel Research 138 (2017) 823–836 Table 3 Mechanical and geometrical properties of 310UB40.4 steel profile.
Table 4 Nominal mechanical properties and dimensions of the fasteners.
Section depth Flange d width bf
Flange thickness tf
Web Yield Ultimate thickness strength strength tw fy fu
Elastic modulus Es
304 mm
10.2 mm
6.1 mm
205 GPa
165 mm
320 MPa
450 MPa
respectively. Furthermore, partial crushing of the timber around the dowel connectors and in the outer lamellas of CLT may occur in failure modes II and III (Fig. 9b and c). With regard to this classification, it was observed that failure Mode III was dominant in 12 mm coach screws (Fig. 8d), whereas failure Mode II became dominant in the 16 and 20 mm coach screw shear connectors (Fig. 8e and f). 3.1.2. Connections with dog screws (DS) Direction of loading in the push-out specimens with dog screw connections was parallel to the timber grain in the outer lamellas of the CLT,
Bolt 12mm - 8.8 Bolt 16mm – 4.6
827
Fastener
Diameter (mm)
Length (mm)
Yield strength (MPa)
Ultimate strength (MPa)
Coach screw Dog screw Bolt
16 19 16 20
100 135 150 110
240 400 640
400 500 800
and the size of the crushing zone near the dog screw connectors was small in contrast to specimens with coach screws in which the outer layers of timber panels were loaded in perpendicular to the grain direction. The width over which the crushing of the timber occurred was between 1.5d and 2d approximately (d being the fastener diameter) [12] as shown in Fig. 10a and b. The deformation of the 16 mm and 19 mm dog screws was demonstrative of a failure Mode II with only one plastic hinge in the fasteners (Fig. 10c and d).
Coach Screw 12mm
Coach Screw 16mm
Bolt 16mm - 8.8 Coach Screw 20mm Bolt 20mm - 8.8
(a)
(b)
Dog Screw 16mm
Dog Screw 19mm
(c)
(d)
Fig. 4. Outline of (a) bolts, (b) coach screws and (c) dog screws used as shear connectors, and (d) stress-strain relationship of shear connectors obtained from uniaxial tension test.
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Table 5 Mechanical properties of the cementitious grout. Compressive strength (MPa) Tensile strength (MPa) a
2 ha
1 Daya
3 Days
7 Days
28 Days
30
42
44.6
52.4
57.2
3.0
Flexural strength (MPa)
6.5
Setting time (min)
30–40
500 kN actuator
According to the manufacturer product description.
LVDTs 3.1.3. Connections with bolts embedded in the grout pocket (BGP) The BGP connections had a mixed mode of failure associated with crushing of the CLT panels, cracking and crushing of the grout, development of a plastic hinge in the bolt shear connectors and bending and
135 mm
60/80 mm
Fig. 6. Instrumentation of push-out STC specimens.
(a) Bolted shear connectors
Sealing Tape
plastic deformation in the flange of the steel profiles (Figs. 11 and 12). In spite of cracking and crushing of the grout and crushing of the CLT panels, no evidence of damage and/or excessive deformation was seen in the bolts in BGP20 connections (Fig. 11). However, after visual inspection of the BGP20 specimens, it was evident that the flanges of the steel profile had undergone bending and plastic deformation. In BGP16 with Grade 8.8 and 4.6 bolts, a plastic hinge and excessive plastic deformation occurred in the bolt shear connectors (Fig. 12). Similarly, in the BGP12 connections, a plastic hinge and excessive deformations were observed in the 12 mm bolt shear connectors, but the size of the crushing zone in the BGP12 specimens was smaller than the crushing zones in specimens BGP20 and BGP16. 3.2. Load-slip behaviour
(b)
Fresh grout
(c) Fig. 5. Fabrication of BGP connections (a) cutting and sealing pocket, (b) tightening bolts and (c) pouring fresh grout.
3.2.1. Connections with coach screws (CS) The load-slip response of the STC connections (loaded perpendicular to the grain of outer lamellas of the panels) with coach screw connectors is shown in Fig. 13. In addition, the yield strength Py, ultimate load carrying capacity Pu, and stiffness ks,0.4 and ks,0.6 of the connections are given in Table. The values of ks,0.4 were evaluated by considering the stiffness of the connection between 10% and 40% of Pu, whereas ks,0.6 is the stiffness calculated between 10% and 60% of Pu. In this paper, ks,0.4 and ks,0.6 are considered as being initial and pre-peak stiffness of the connections respectively. It is observable that the initial stiffness ks,0.4 of the steelCLT composite connections with coach screws is independent of the screw diameter/size, but the pre-peak stiffness ks,0.6 depends on the size of screws and it increases with an increase of the size of the screws. The load versus slip response of the joints with coach screw shear connectors loaded parallel and perpendicular to the grain of the outer lamellas of the panels are compared in Fig. 14. In addition to the load versus slip response shown in Fig. 14, the ultimate strength/capacity Pu, and stiffnesses ks,0.4 and ks,0.6 of the STC connections in which CLT panels are loaded parallel to the grain obtained from the tests of
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Fig. 7. Loading procedure adopted for push-out tests as per BS EN 26891 [29].
Crushing zone
Plastic hinge Densification of grains
(a)
(d) 16 mm coach screw
Crushing zone
Plastic hinge Densification of grains
(b)
(e) 20 mm coach screw
Crushing zone
Densification of grains Plastic hinge
(c)
(f)
Fig. 8. Failure modes in steel-CLT specimens with coach screw connectors, (a, b, c) crushing in CLT panels (outer lamellas loaded perpendicular to the grain) for specimens with 12, 16 and 20 mm coach screws, respectively and failure modes of fasteners associated with formation of (d) two plastic hinges in 12 mm screws, (e) one plastic hinge in 16 mm screw and (f) one plastic hinge in 20 mm screw.
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and perpendicular to the grain is much smaller than other steel-timber composite joints such as steel-LVL composite joints, because of the grain altering at 90° for different lamellas of CLT panels [13,14]. Contrary to the connection initial stiffness ks,0.4, the ultimate capacity Pu and pre-peak stiffness ks,0.6 of the joints with coach screws is almost proportional to the size of the coach screw (Fig. 15). The nearlinear relationship between the size of the coach screw shear connectors and pre-peak stiffness ks,0.6 and ultimate load capacity Pu of the steelCLT joints is illustrated by lines of best fit shown in Fig. 15a and b respectively. Furthermore, the negligible influence of the diameter of the coach screws on the initial stiffness ks,0.4 of steel-CLT composite joints is evident from the best line fit (i.e. ks,0.4 = 0.017d + 6.23) shown in Fig. 15a that has a small correlation coefficient of 0.0175 between ks,0.4 and d.
Fig. 9. Failure mode of dowel-type fasteners in timber connections (a) mode I, (b) mode II and (c) mode III.
Hassanieh et al. [14] are provided in Table 6. It is seen that the ultimate capacity of STC joints loaded perpendicular to the grain of the outer lamellas is greater than or equal to that of joints loaded parallel to the grain of the outer layers, and this larger load capacity (particularly for larger screw sizes, 16 and 20 mm) can be attributed to potential wood densification in the perpendicular to the grain direction. However, the stiffness of steel-CLT connections loaded perpendicular to the grain is smaller than that of specimens loaded parallel to the grain of the outer layers of CLT panels, and this smaller stiffness is due to the significantly lower elastic modulus of the timber in the perpendicular to the grain direction compared with parallel to the grain. It is noteworthy that the difference in the behaviour of steel-CLT connections loaded parallel
3.2.2. Connections with dog screws (DS) The load versus slip behaviour of the connections DS16 and DS19 with dog screw connectors loaded parallel to the grain of the outer layers of the timber panels is shown in Fig. 16. Furthermore, the initial stiffness ks,0.4, pre-peak stiffness ks,0.6 and ultimate load carrying capacity Pu of the double dog screw connections are provided in Table. The initial and pre-peak stiffness of dog screw connections has a negative correlation with the size of the dog screws, but the ultimate load carrying capacity of the connections has a positive correlation with the size of the dog screws. The ultimate capacity of DS19 is around 26% more than the ultimate capacity of connection DS16. The load-slip behaviour s of steel-CLT joints DS16 and CS16 (loaded parallel to the grain of the outer layers) are compared in Fig. 17, while the load versus slip responses of joints DS19 and CS20 are compared in Fig. 18. It can be seen that the ultimate capacity of DS16 (54.4 kN) is about 10% higher than that of CS16 (49.9 kN) (Fig. 17), because of the higher yield and ultimate strengths of DS16 (having Grade 5.8 steel) compared to CS16 (with Grade 4.6 steel). In terms stiffness (Tables 6 and 7), the initial stiffness ks,0.4 of DS16 (29.65 kN/mm) is about 33% higher than CS16 (22.23 kN/mm) and the pre-peak
Crushing zone
Plastic hinge
16 mm dog screw
(a)
(c) Crushing zone
Plastic hinge
19 mm dog screw
(b)
(d)
Fig. 10. Failure modes in steel-CLT specimens with dog screw connectors, (a, b) crushing in CLT panels (outer lamellas loaded parallel to the grain) for specimens with 16 and 20 mm dog screws, respectively and failure modes of fasteners associated with formation of one plastic hinge in (d) 16 mm and (e) 19 mm screw (failure mode II).
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Cracking of grout
(a)
(b) Deformation of flange
Timber crushing zone
(c)
(d)
Fig. 11. Failure of BGP20 connections (a) no visible damage/deformation in the bolt shear connectors, (b) crushing and cracking of grout, (c) timber crushing and (d) plastic deformation in the flange of the steel profile.
stiffness ks,0.6 of DS16 (11.66 kN/mm) is about 42% higher than CS16 (8.21 kN/mm). However, the difference between the load-slip curves, pre-peak stiffness and ultimate capacity of the DS19 and CS20 joints is negligible (Tables 6 and 7). This is because the structural behaviour of the steel-CLT joints with large mechanical fasteners is governed primarily by the behaviour of the timber panels (e.g. timber crushing) rather than the strength and stiffness of the fasteners.
3.2.3. Connections with bolts embedded in the grout pocket (BGP) The initial stiffness ks,0.4, pre-peak stiffness ks,0.6 and ultimate strength/capacity Pu of the BGP joints are given in Table 8. In this table, the designation BGPxx y.y W × L is adopted with respect to the bolt size xx (i.e. 12, 16, 20 mm), Grade of the steel y.y (i.e. 4.6 or 8.8), width W (i.e. 60 mm or 80 mm) and length L (i.e. 135 mm) of the grouted pockets.
Grout crushing
Plastic hinge
(a)
(b)
Grout crushing
Plastic hinge
(c)
(d)
Fig. 12. BGP16-8.8 connection failure (a) grout crushing (b) bolt plastic deformation and BGP16-4.6 connection failure (c) grout crushing and (d) bolt plastic deformation.
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A. Hassanieh et al. / Journal of Constructional Steel Research 138 (2017) 823–836 Table 6 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints with two coach screw shear connectors. Coach Screw
CS12 CS16 CS20 a
Fig. 13. Load versus slip of steel-CLT connections with CLT panels loaded perpendicular to the grain of outer lamellas (two coach screw shear connectors).
Loaded perpendicular to graina
Loaded parallel to graina [14]
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
6.29 6.81 6.43
2.14 6.11 6.24
37.5 53.6 74.9
18.69 22.23 10.37
9.52 8.21 6.11
35.8 49.9 67.5
Loaded with respect to orientation of the grain in the outer lamellas of CLT panel.
The load-slip behaviour of the BGP steel-CLT joints with varied sizes of bolt connectors (12, 16 and 20 mm) and two sizes of grouted pockets (60 mm × 135 mm and 80 mm × 135 mm) are shown in Figs. 19 and 20 respectively. Both BGP20 connections (irrespective of the size of the grouted pocket) exhibit similar load versus slip and ductility characteristics up to the ultimate/peak load (Fig. 20), but the BGP20 connection with a 60 mm wide pocket (a smaller pocket of grout) shows a steeper post-peak softening branch due to severe crushing of the grout in the smaller pockets. The load versus slip behaviour of BGP16 connections with 16 mm bolts manufactured from 8.8 and 4.6 steel grades is shown in Fig. 21. Both connections exhibit identical initial and pre-peak stiffnesses. In terms of peak (ultimate) capacity, increasing the nominal yield strength of the bolt connectors (embedded in the grout pocket) from 240 MPa (Grade 4.6) to 640 MPa (Grade 8.8) increases the ultimate strength of the BGP16 connections by 26% (Table 8). These observations demonstrate that strength and stiffness of the steel bolt shear connectors embedded in cementitious grout (or concrete) depends largely on mechanical properties (particularly strength) of the cementitious
k 0.4, s
0.0175d
k 0.6, s
6.23
0.51d 3.37
(a)
Pu
4.68d 19.5
(b) Fig. 14. Comparison of load versus slip of steel-CLT connections with CLT panels loaded perpendicular and/or parallel to the grain of outer lamellas, for two (a) 12 mm, (b) 16 mm and (c) 20 mm coach screws.
Fig. 15. Relationship between coach screw diameter and (a) stiffness and (b) peak load capacity of steel-CLT joints with double coach screws (loading perpendicular to the grain of outer lamellas of CLT panels).
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Table 7 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints (CLT panels loaded in the direction parallel to the grain of outer layers) with two dog screw shear connectors. Dog screw
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
DS16 DS19
29.65 17.16
11.66 6.23
54.4 68.6
Table 8 Mean of peak load capacity and slip moduli ks,0.4 and ks,0.6 for steel-CLT joints (CLT panels loaded in the direction parallel to the grain of outer layers) with two bolts embedded in the grout pocket (BGP). Fig. 16. Load versus slip response of steel-CLT connections loaded parallel to the grain of outer lamellas of CLT panels, with two (a) 16 mm and (b) 19 mm dog screw connectors.
Pu=54.4 kN
Bolted Grout Pocket
ks,0.4 (kN/mm)
ks,0.6 (kN/mm)
Pu (kN)
BGP12 8.8 60 mm × 135 mm BGP16 4.6 80 mm × 135 mm BGP16 8.8 80 mm × 135 mm BGP20 8.8 60 mm × 135 mm BGP20 8.8 80 mm × 135 mm
39.53 58.68 55.12 64.57 77.19
29.76 45.26 45.77 53.68 63.08
82.9 103.4 129.9 153.3 159.7
Regarding the experimental results provided in Tables 6 to 8 and Figs. 13 to 21, it can be concluded that stiffness and strength of the bolt shear connectors in the pocket of grout is significantly higher than that of coach screws and dog screws.
Pu=49.9 kN
3.3. Analytical model of connections
Fig. 17. Comparisons of load versus slip response of steel-CLT joints with 16 mm dog screw and with 16 mm coach screw connectors.
grout (or concrete). A linear regression was conducted on the experimental stiffness and ultimate strength/capacity of the BGP connections versus the diameter of the embedded bolt shear connectors and the results are shown in Fig. 22. Accordingly, the formulas proposed for calculating the initial stiffness ks,0.4 (in kN/mm), pre-peak stiffness ks,0.6 (in kN/mm) and ultimate strength/capacity Pu (in kN) of the BGP connections are given by. k0:4;s ¼ 4:71d − 18;
ð1aÞ
k0:6;s ¼ 4:19d − 21
ð1bÞ
and P u ¼ 9:64d − 30;
ð2Þ
where d is the diameter (in mm) of bolt shear connector.
Fig. 18. Comparisons of load versus slip response of the steel-CLT joints with 19 mm dog screw and with 20 mm coach screw shear connectors.
In this section, non-linear regression on the load-slip results (Figs. 13 to 21) are carried out to develop empirical models for predicting the shear force (f) versus slip (s) relationship for the steel-CLT composite connections with coach screws, dog screws and bolts in pockets of grout. The proposed load-slip function (Fig. 23) has seven parameters which are estimated by non-linear regression [13,14]. The adopted shear force (f) versus slip (s) function is defined as. k0 − kp s kp þ ks s þ f ¼n h h in2 on1 − ks s; in1 on11 n 2 1 þ k0 − kp fs 1 þ kp þ ks f −s f 0
1
ð3Þ
0
where f denotes the shear force, s denotes the slip, k0, kp and ks are the initial, pre-peak and post-peak stiffness, respectively, f0 is the first reference shear force corresponding to the pre-peak branch and f1 is the second reference shear force corresponding to the post-peak branch, and n1 and n2 are parameters that control the curvature of the first and second curves, respectively [13]. For each group of the STC connections, the seven input parameters providing the best fit to the mean of the experimental load-slip curves were obtained from method of least squares with trust-region and the estimated input parameters are given in Table 9. The analytical loadslip curves correlate very well with and experimental results as shown
Fig. 19. Load versus slip response of BGP steel-CLT joints with different bolt sizes.
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Fig. 20. Effect of grouted pocket size on load versus slip response of BGP20 steel-CLT joint. Fig. 23. Proposed analytical model for load-slip behaviour of steel-timber composite joints [13,14].
in Figs. 24 and 25 and the good correlation is also is evident from the Rsquare values given in Table 9. 4. Conclusions
Fig. 21. Effect of yield strength of bolt connectors embedded in grout pockets on load versus slip behaviour of BGP16 steel-CLT joints.
k0.4, s
The load-slip behaviour of steel-CLT composite connections has been investigated through experimental push-out tests. The effect of shear connection type (conventional mechanical fasteners versus shear connectors embedded in grouted pockets), the effect of the size and yield strength of the fasteners (coach screws, dog screws and bolts) and effect of the loading direction (i.e. parallel or perpendicular to the grain of outer lamellas) on the stiffness, strength and failure modes of steel-CLT connections were investigated. Finally, analytical models for load-slip behaviour of all types of steel-CLT connections have been proposed and calibrated against laboratory test results. The main conclusions of the experimental study are the following.
(a)
• The loading direction (i.e. parallel or perpendicular to grain of outer lamellas) have a significant influence on the initial stiffness, but minor influence on the ultimate strength of the steel-CLT joints with coach screws. The initial stiffness of the joints loaded perpendicular to grain is less than initial stiffness of the joints loaded parallel to the grain. • The pre-peak stiffness and ultimate strength of the steel-CLT connections with dog screw (Grade 5.8 steel) shear connectors are slightly higher than those for connections with coach screw (Grade 4.6 steel) shear connectors. Accordingly, the yield strength of mechanical shear connectors has minor effect on the stiffness and strength of the steel-CLT composite connections. This is particularly the case for mechanical connectors with larger diameters (d ≥ 16 mm), because the
Pu
Table 9 Input parameters for the analytical load-slip model of the steel-CLT composite connections.
4.71d 18
k0.6, s
4.19d 21
9.64d 30
Connection Coach screws
Dog screws Bolt in grout pocket
(b) Fig. 22. Relationship between diameter of bolt shear connectors and (a) stiffness and (b) peak load capacity of BGP steel-CLT joints.
S12 S16 S20 DS16 DS19 BGP12 8.8 BGP16 4.6 BGP16 8.8 BGP20 60,135 BGP20 80,135
f0
f1
k0
kp
ks
n1
n2
R2
80 53 150 46 30 88 103 88 89
200 300 180 192 59 95 500 135 156
72.5 13.5 12.3 303.4 150.9 62.8 53 58 65
0 0 2.6 0.8 2.8 2.0 −0.2 10.0 14.5
2.2 4.2 0.4 1.2 −0.3 0.6 6.43 0.95 0.38
0.35 1.25 0.52 0.6 0.85 1.24 4.35 3.25 3.30
5.6 10.0 5.9 1.2 5.0 4.0 5.0 6.0 5.0
0.9799 0.9965 0.9957 0.9945 0.9993 0.9987 0.9971 0.9901 0.9983
20
0.12
4.80 2.0
100 161 66.8
0.9994
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Fig. 24. Correlation between analytical model and mean of experimental load-slip response for steel-CLT composite joints with (a) coach screw connections in CLT panels loaded perpendicular to the grain and (b) dog screws connections in CLT panels loaded parallel to the grain.
•
•
•
•
structural behaviour of joints with large mechanical connectors is governed mainly by non-linear localised effects (such as timber crushing) rather than the strength of the connectors. Steel-CLT connections with bolt connectors embedded in grouted pockets (BGP) have significantly higher initial stiffnesses, pre-peak stiffnesses and peak load capacities compared to joints with conventional coach screw and dog screw connectors. The size (width) of grouted pockets has little or no influence on the pre-peak load-slip behaviour of the joints with BGP shear connectors. However, joints with smaller grouted pockets are slightly more brittle, as evidenced by the steeper slope of their post-peak behaviour. The yield strength of bolt shear connectors embedded in grout pocket has negligible influence on the stiffness of joints with BGP connections, but increasing the yield strength of bolt shear connectors increases the ultimate strength/capacity of the joints with BGP connections. Within the range of the relevant parameters, a relationship for the load-slip behaviour of the steel-CLT joints in analytical form is possible using non-linear regression. This relationship was proposed in the paper.
Acknowledgement The work in this paper was funded by a Discovery Project (DP160104092) awarded to the second and third authors by the Australian Research Council. This support is acknowledged with thanks.
Fig. 25. Correlation between analytical model and mean of experimental load-slip response for steel-CLT composite joints with BGP connections (a) BGP20, (b) BGP16 and (c) BGP 12.
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[18] K. Schober, W. Becker, J. Weber, Grouted joints in timber engineering, World Conf on Timber Eng 2016, pp. 4282–4289. [19] J.H. Negrão, L.D. Brito, A.G. Dias, C.C. Júnior, F.R. Lahr, Numerical and experimental study of small-scale moment-resistant reinforced concrete joints for timber frames, Constr. Build. Mater. 118 (2016) 89–103, https://doi.org/10.1016/j.conbuildmat. 2016.05.036. [20] British Standards Institute, BS EN 338: Structural timber - Strength Classes, 2009. [21] AS/NZS 2098.1, 2016 Methods of Test for Veneer and Plywood - Moisture Content of Veneer and Plywood, Standards Australia, 2016. [22] Standards Australia, AS/NZS 3679.1:2010 Hot Rolled Bars and Sections, Standards Australia, 2012. [23] Standards Australia, AS 1110.1-2000 ISO Metric Hexagon Bolts and Screws-product Grades A and B, 2000. [24] Standards Australia, AS 1112.1-2000 ISO Metric Hexagon Nuts, 2000. [25] Standards Australia, AS/NZS 1393:1996 Coach Screws-Metric Series With ISO Hexagon HeadsStandards Australia 1996. [26] Australian Standard, AS 4291.1:2015 Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel Paet 1: Bolts, Screws And Studs, Aust. Stand, 2015 https://doi.org/10.1017/CBO9781107415324.004. [27] AS 1478.2, 2005 Chemical Admixtures for Concrete, Mortar and Grout - Methods of Sampling and Testing Admixtures for Concrete, Mortar and Grout, Standards Australia, 2005. [28] Eurocode 5 (EC 5), Design of timber structures- Part 1-1: General-common rules and rules for buildings. BS EN 1995-1-1, 2004. [29] EN-26891, CEN, Timber Structures, Joints Made With Mechanical Fasteners, General Principles for the Determination of Strength and Deformation Characteristics, BSI: European Committee for Standardization, Brussels (Belgium), 1991. [30] P. Aune, M. Patton-Mallory, Lateral Load-bearing Capacity of Nailed Joints Based on the Yield Theory: Experimental Verification. Res Pap 470, United States Department of Agriculture, Forest Service, Forest Products Laboratory, 1986 1–29.