Construction and Building Materials 118 (2016) 63–75
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Experimental and analytical behaviour of steel-timber composite
connections
A. Hassanieh, H.R. Valipour ⇑, M.A. Bradford
Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, UNSW Australia, UNSW Sydney, NSW 2052, Australia
h i g h l i g h t s
A composite system comprising steel beam and LVL timber slab is proposed.
Load-slip behaviour of steel-LVL composite joints is obtained from push-out tests.
Efficiency of screw and bolt dowels in steel-LVL composite joints is studied.
Empirical model for load-slip behaviour of steel-LVL composite is proposed.
a r t i c l e
i n f o
Article history:
Received 14 October 2015
Received in revised form 21 April 2016
Accepted 3 May 2016
Available online 12 May 2016
Keywords:
Hybrid structure
Load-slip
Shear-connectors
Steel-timber composite (STC)
Timber
a b s t r a c t
The mechanical characteristics of steel-timber composite (STC) connections play an essential role in the
safe and economical design of hybrid STC structures and floor systems. Accordingly, this study investigates the load-slip behaviour of lap Laminated Veneer Lumber (LVL) timber-steel plate composite joints.
Push-out tests on four different types of STC lap joints connected by coach screws (with and without a
reinforcing nail plate), high-strength bolts and a combination of glued and screwed joints are reported,
and the load-slip behaviour and failure modes of the connections are characterised. The use of a nail plate
is found to be effective in reinforcing the timber and in increasing the stiffness of STC lap joints with
coach screws, but they did not produce a significant improvement of their strength. A non-linear regression is carried out and an empirical load-slip formulation for STC lap joints with coach screws and high
strength bolted connectors is proposed in analytical form. Furthermore, simple formulae for the strength
and stiffness of LVL-steel lap connections with coach screws are proposed.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
The application of timber and engineered wood products such
as Laminated Veneer Lumber (LVL), Glued laminated timber
(Glulam) and Cross Laminated Timber (CLT/X-lam) have increased
dramatically in the construction industry, owing to their lower
carbon footprint, relatively high strength and stiffness, lower
self-weight and the faster installation of prefabricated structural
components made of engineered timber [1]. There are also significant advantages in using timber in conjunction with contemporary
construction materials (i.e. concrete and steel), and over the past
two decades considerable research has been devoted to numerical
analysis and experimental study, as well as development of simplified design methods and provisions for timber-timber and timberconcrete composite structures [2–5]. However, little attention has
⇑ Corresponding author.
E-mail address: h.valipour@unsw.edu.au (H.R. Valipour).
http://dx.doi.org/10.1016/j.conbuildmat.2016.05.052
0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
been paid to the application of hybrid steel-timber composite
systems, and existing design provisions and data on steel-timber
composite structures are lacking and they are limited mainly to
the use of steel plates and/or sheets for strengthening timber
elements or for composite flitch beams [6–8].
The steel and timber components of a structural system can be
hybridised at the structure level in different ways, such as timber
floors with steel beams or steel-timber composite (STC) floors, steel
beams with timber columns and timber/timber-concrete composite (TCC) floors with steel columns. For example, Xu et al. [9]
recently conducted cyclic tests on doweled steel beam-to-Glulam
column subassemblies and used a non-linear finite element model
to predict the global and local responses of the subassemblies that
were tested. The results of the laboratory tests and finite element
simulations revealed reasonable ductility and favourable performance of the hybrid steel-timer moment-resisting connections that
were proposed by Xu et al. [9] under cyclic load. However,
developing and characterising the behaviour of practicable simple
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A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
connections are able to transfer the forces from the timber to the
steel and vice versa both reliably and efficiently is still the most significant challenge for the development of hybrid steel-timber structures, and it requires systematic and thorough investigation.
Lap joints with dowel connectors (nails, screws and bolts) are
among the most common type of timber-to-timber and timberto-steel plate connections. Accordingly, several experiments on
different wood species and laminated timbers have been conducted to determine directly the stiffness, strength and failures
mode of dowel-type joints or to link the behaviour of doweled timber connections to the embedding strength and/or compressive
strength of the timber in directions parallel or perpendicular to
the grain [10–14]. In addition to the timber strength and size of
the fasteners, the loaded edge distance as well as type of fastener
(e.g. bolt or lag, coach screw, self-tapping screw) were found to
have significant influences on the failure mode and the ultimate
strength of doweled connections [13,15]. Several studies on LVL
and CLT components with different screw connections have been
carried out [2,16–20], and simplified models and design provisions
with respect to the type of dowel connectors have been developed
[5,21,22].
The failure mode of dowel-type connections is usually associated with crushing of the timber (close to the surface of the timber
element), as a result of the low elastic modulus of timber and localisation of the strain around the penetrations. Because of this, different reinforcing strategies have been proposed to prevent the
stress concentrations around the holes and to improve the stiffness
and strength of doweled connections and timber beams having
penetrations [23]. For example, glass fibres or CFRP laminates have
been used to reinforce around the holes and to improve the
strength and ductility of timber beams and connections under static and dynamic loading conditions [24–26], and punched metal
plates (nail plates) have been used to enhance the performance
of semi-rigid timber joints or to prevent the brittle failure mode
of the connections [27,28]. In addition, the application of glue
and adhesives in conjunction with mechanical fasteners has been
investigated in order to improve the strength and stiffness of doweled lap connections, as well as timber-timber and timber-concrete
composite joints with dowel connectors [29–31].
Amongst different hybrid steel-timber systems, STC floors
(cross banded LVL or CLT floor slabs connected compositely to steel
beams) have the advantage of being lightweight compared to conventional steel-concrete composite floors and, because of this, STC
floors can improve the speed and reduce the cost of construction
significantly. From a structural engineering perspective, the application of timber in constructing floors (STC floors) can significantly
reduce the self-weight of the structure that in turn leads to smaller
sizes for the beams, columns and foundations. Furthermore, lowering the self-weight of the structure significantly reduces the inertial forces induced by seismic actions and also facilitates
construction of tall buildings on soft and/or problematic soils.
Accordingly, this paper investigates the short-term mechanical
behaviour of STC joints in which a cross-banded LVL panel is connected to the flange of a steel girder using coach screws, bolts and/
or glue (Fig. 1a). The load-slip response and failure modes of the
STC joints are obtained from push-out tests and empirical
load-slip formulae for STC lap joints with coach screws and highstrength bolts are proposed and calibrated against the experimental data. These load-slip models can be used for the non-linear
analysis of steel-timber composite beams as well as hybrid
steel-timber frames in which parallel continuous LVL beams are
connected to the steel column flanges by using screws or bolts
(Fig. 1b). It is noteworthy that while the fire performance of timber
floors is deemed a major concern that requires thorough investigation, however, such a study of a STC system is beyond the scope of
this paper.
Fig. 1. Schematic outline of a hybrid steel-timber (a) floor with steel beams and
cross-banded LVL slabs and (b) semi-rigid frame with parallel continuous LVL
girders connected to the flange of the steel column.
2. Experimental program
2.1. Specimen details
The push-out specimens were categorised into six different groups (Fig. 2) with
respect to the type of connection (i.e. bolts and screws with and without glue) and
the application of a nail plate as reinforcement of the connection zone. In total, seventeen different types (each type comprising three identical samples) of STC joints
whose geometry, detail and setup are shown in Fig. 2 and in Table 1 were tested.
The primary variables within the different push-out specimens were the loading
direction with respect to the orientation of the LVL grain (i.e. parallel or perpendicular
to the grain), the type of the mechanical connectors (i.e. bolts or screws), the diameter
of the coach screws and the use of glue in conjunction with screws. Furthermore, the
influence of using nail plates to reinforce the timber around the location of coach
screws that are both parallel and perpendicular to the grain direction are considered.
2.2. Material properties
2.2.1. LVL timber
In this study, hySPAN cross-banded LVL panels were used to fabricate the pushout specimens. The LVL panels had been manufactured from Radiata Pinewood
structural laminated veneer lumber and tested in accordance with the relevant Australian AS/NZS 4357:2005 standard [32]. Phenolic adhesive, producing a type (A)
bond as per AS/NZS 2098 [33] had been used for manufacturing the LVLs. The average density and moisture content of the LVL panels were 600 kg/m3 and 9% respectively. The mechanical properties of the LVL panels in both parallel and
perpendicular to the grain directions are given in Table 2.
2.2.2. Steel profile
The steel profile used for fabrication of the push-out specimens was an Australian 310UB32 hot rolled section of Grade-300PLUS steel with no coating or painting. The dimensions and mechanical properties of this cross-section given in Table 3
complies with the specifications of AS/NZS 3679.1 [34].
A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
65
Fig. 2. (a) Configuration of the six different STC joints and (b) cross-sections of STC joints for push-out tests.
Table 1
Size and configuration of connectors and loading direction in push-out specimens
(3 identical specimens of each type).
With Nail Plate
Without Nail Pate
Glued
Parallel
Perpendicular
Parallel
Perpendicular
Parallel
Screw 8, 12,16, & Bolt 12
Screw 8, 12,16, & Bolt 12
Screw 8, 12,16, 20
Screw 8, 12,16, 20
Glue and Screw 16
2.2.3. Coach screws and bolts
Hexagonal coach screws having 8, 12 and 16 mm diameter and 12 mm diameter
high-strength 8.8 hexagonal bolts were used in the fabrication of the specimens
(Fig. 3a). Uniaxial tension test was carried out on coach screws and high-strength
bolts and the stress-strain plots are shown in Fig. 3b. The screws were made of
Grade 4.6 steel with a characteristic (95 percentile) yield strength of 240 MPa
and an ultimate tensile strength of 400 MPa, and they comply with the minimum
requirements of AS/NZS 1393 [35]. The average yield and ultimate strength of coach
screws (see Fig. 3b) were 320 MPa and 440 MPa, respectively. The 12 mm diameter
high-strength 8.8 hexagonal bolts had an average yield strength of 660 MPa and an
ultimate tensile strength of 830 MPa as per the specifications of AS1110.1 [36] and
AS1112.1 [37]. The 8 and 12 mm screws were 65 mm long, and the lengths of the
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A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
Table 2
Mechanical properties of hySPAN LVLs (in MPa).
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)
50
25
4.6
41
12
13,200
660
Table 3
Mechanical characteristics of steel profile.
Section depth d
(mm)
Flange width bf
(mm)
Flange thickness tf
(mm)
Web thickness tw
(mm)
Yield strength fy
(MPa)
Ultimate strength fu
(MPa)
Elastic modulus Es
(GPa)
298
149
8
5.5
320
450
205
2.2.4. Nail plate
A 100 mm 50 mm 1 mm nail plate (Fig. 3c) was used in this study to reinforce the timber around the connector hole and to improve the stiffness and
strength of the dowel joints. The nail plates were made of Grade G300 steel with
characteristic yield strength of 300 MPa and a tensile strength of 340 MPa.
2.2.5. Glue
To develop full composite action between the LVL timber panels and the steel
profile, a two part non-sag gel type structural epoxy adhesive was used in some
of the specimens in conjunction with 16 mm screws. The mechanical properties
and curing requirements of the epoxy adhesive are provided in Table 4.
2.3. Test setup, instrumentation and fabrication of specimens
It is well-known that the configuration of the push-out test can have a significant effect on the test results by introducing some undesirable phenomena such as
unwanted friction between different parts of the specimen and an unsymmetrical
load distribution on the sample [3]. Therefore, in this study a symmetric timbersteel-timber arrangement has been adopted for fabrication of the specimens
(Fig. 2). This symmetric test set up can be assembled more easily than unsymmetrical specimens, and also it provides a close to uniform distribution of load with
predictable friction effects on the load-slip results.
Four linear variable differential transformers (LVDTs) were used to measure the
relative displacement between the steel profile and timber panels, being FR, FL, RR
and RL (the first character denoting the Front or Rear side and the second denoting
the Right or Left side of the specimen). These had a maximum stroke of 100 mm and
were mounted on each specimen (Fig. 4a). The LVDTs were positioned on the LVL
panels and their bases were attached to the steel profile. In addition to the four
LVDTs on the specimen, a LVDT for the testing machine recorded the vertical movement of the specimens.
The specimens were fabricated following a procedure similar to that representative of construction practice. After cutting the LVL panels to a specific size, both
the LVL panels and the flange of the steel profile were predrilled. The diameter of
the predrilled holes in the LVL panels was 1–2 mm smaller than the diameter of
the screws, and 1–2 mm larger than the diameter of the bolts. The dimeter of the
predrilled holes in the flanges of the steel profile was 0.2 mm larger than the dimeter of the dowels (the screws or bolts). The bolts were tightened using a manual torque wrench and a post-tensioning force of 10 kN (equivalent to 0.14fy) was induced
in the 12 mm bolts, where fy is the yield stress of the bolt, and a 50 mm 50 mm
washer was used in conjunction with the bolted dowels to prevent crushing of
the timber. The post-tensioning force f (in kN) in the bolts was related to torque
T (in Nm) and dimeter d (in mm) of bolt using equation T ¼ 0:2 d f . The calibration accuracy of manual torque wrench was checked by installing strain gauges on
the bolts and measuring the post-tensioning strain/stress induced in the bolts. For
the glued joints, the epoxy adhesive was spread on the LVL panel surface and, after
mounting the LVL panels, the specimen was left for one hour and the 16 mm coach
screws were installed after the glue hardened. During hardening of the glue, no
extra pressure was applied to press the LVL panels against flange of steel profile
and the glue line hardened under self-weight of steel profile while STC joint lying
on the ground. In some of the specimens, a nail plate was used to reinforce the timber around each screw connector as shown in Fig. 4b.
Fig. 3. (a) Screws and high-strength 8.8 bolts used in the push-out specimens (b)
uniaxial stress-strain obtained from direct tension tests on high-strength 8.8 bolts
and coach screws and (c) nail plate.
16 mm and 20 mm screws were 75 mm and 90 mm respectively (Fig. 3a). The coach
screws were zinc coated, but the high-strength bolts were not treated (had no
coating).
Table 4
Mechanical properties and curing requirements of the epoxy adhesive.
Tensile
Strength
Compressive
Strength
Shear
Strength
Coefficient of
Linear Expansion
Minimum Cure
Time at 25 °C
30 MPa
70 MPa
15 MPa
60 106 mm 1/°C
24 h
A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
67
if necessary. The rate of loading under displacement control was 1–3 mm per second. The load-slip response and the failure modes of the specimens were recorded
in the push-out tests carried out in this way.
3. Discussion of test results
3.1. Modes of failure
(a)
Loading direction
50
m
m
(b)
Fig. 4. Arrangement of (a) push-out test setup and LVDTs (b) installed nail plates
with respect to loading direction.
2.4. Testing procedure
The loading procedure adopted for the push-out tests is shown in Fig. 5. For the
first stage, the load was increased from 0 to 40% of the estimated ultimate load (Fu)
of the specimen over 120 s and it remained at this level for 30 s. Following this, an
unloading phase commenced and the load was decreased from 40% to 10% of Fu. The
load was maintained at this level for another 30 s, and subsequently the second
loading phase commenced. The testing was then conducted under a load control
regime up to 70% of Fu, and it then reverted to displacement control and continued
until failure of the specimen. The 0.4Fu limit in the first unloading/reloading cycle
and the 0.7Fu limit to switch from the load control to displacement control procedure are respectively corresponding to serviceability limit state (SLS) and ultimate
limit state (ULS) loading conditions specified in European standards (EN) for timber
structures [38]. The value of Fu for the subsequent test was modified accordingly
with respect to the previous test result, and the loading procedure was redefined
The push-out tests conducted on the doweled timber-timber,
timber-concrete and timber-steel joints show three distinctive
modes of failure (see Fig. 6), identified as Modes I, II and III
[9,39,40]. The occurrence of each failure mode depends on D/L
(with D being the diameter and L the length of the dowel) and
the ratio of the fastener material strength fs to the bearing strength
bs of the primary element (in this study, bs is the embedding
strength of the LVL) [40]. Failure Mode I occurs when the timber
in the vicinity of the dowel connector is crushed, without the formation of any plastic deformation in the dowel. Mode II is associated with crushing of the timber and the formation of one plastic
hinge within the dowel connector near the steel flange, whilst for
Mode III, crushing of the timber is associated with the formation
of two plastic hinges in the dowel connector, with one plastic hinge
appearing at the middle and the other in the vicinity of the steel
flange (Fig. 6). In this study, similar modes of failure were observed
in STC joints with screw connectors. More specifically, for the
8 mm screws the dominant failure mode was Mode III while for
the 12 mm screws, the failure mode was either Mode II or III
(Fig. 7a and b). In STC joints with 16 mm and 20 mm screws, Mode
II was the dominant failure mode (Fig. 7c). The STC joints with
screw connectors exhibited relatively ductile behaviour, in which
the load-slip response revealed a large post-peak branch with a
gradual reduction in the strength accompanied by mild softening.
However, the STC joints with bolted connectors produced a somewhat brittle mode of failure that was associated with fracture of
the bolts (Fig. 8).
The mode of failure in STC joints with screw connectors and a
nail plate was relatively brittle; the brittleness of the failure mode
being evident from the sudden drop of the load after the peak load
was attained. The failure mode in specimens with nail plates was
associated with localised crushing of the timber and rupture of
the nail plate (Fig. 9). Furthermore, for the STC joints with 8 mm
and 12 mm screws as well as with nail plate reinforcement, the
screw head detached at the final stage of loading. It is noteworthy
that the STC joints with screw connectors and nail plates exhibited
very similar behaviour when loaded in both parallel and orthogonal to the grain direction.
For the glued specimens, the thickness of the epoxy adhesive
layer after drying was between 2 and 3 mm. In order to prevent
Fig. 5. Adopted loading procedure for the push-out tests [38].
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A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
grain (Fig. 11a) and 3d–4d when the loading is perpendicular to the
grain direction (Fig. 12a).
3.2. Load-slip response
Fig. 6. Different modes of failure in steel-timber composite joints.
sudden failure and damage in the testing equipment, two screws
were used in the glued STC joints. However, the screws provided
no contribution to the ultimate loading capacity of the specimen.
The failure mode of the glued connection was brittle, and it was
associated with fracture of the glue and separation of the LVL panel
from the steel flange, as shown in Fig. 10, with no cracking or damage being observed in the LVL panels. In addition, separation
between LVL the panel and the steel profile not was observed in
the glued connection, but in the dowel connections with no epoxy
adhesive, the LVL panel separated from the flange of the steel girder after the peak load of the connection had been reached.
Crushing of the timber and the extent of the damage in the timber panel loaded in parallel and perpendicular to the direction of
the grain for STC joints with dowel connectors (without glue) are
shown in Figs. 11 and 12 respectively. In the joints with LVL panels
loaded parallel to the grain direction, crushing of the LVL timer is
localised mainly in the vicinity of the screw and bolt and within
a small zone equal approximately to the diameter of the connector
(as in Fig. 11). However, in the LVL panels loaded in the direction
perpendicular to the grain, crushing and damage of the timber
propagated through a relatively large area in the vicinity of the
connectors, as shown in Fig. 12. The width of crushing zone (measured perpendicular to the direction of loading) is around 1d–1.5d
(d being the diameter of dowel) when the loading is parallel to the
It is known that the behaviour of dowel connectors in timber
panels depends extensively on the angle between the direction of
loading and the grain [41,42]. Accordingly, the load-slip results
for the STC joints were obtained from push-out tests with the loading directions both parallel and perpendicular to the grain.
The variability of the materials (in particular of the timber) and
the fabrication techniques can affect the load-slip and peak load
capacity of timber, timber-concrete and timber-steel connections
significantly. So as to ensure the accuracy and repeatability of
the experimental results, three identical specimens were fabricated for each type of STC joint (Table 1 and Fig. 2). The parallel
and perpendicular to the grain load-slip responses of the STC joints
with 16 mm screws and 12 mm bolted connectors obtained from
the three identical specimens, as well as the mean load-slip curve
of the specimens, are shown in Figs. 13 and 14, which demonstrate
clearly the small variation in the peak load capacity and of the
load-slip response of the STC joints. The influence of friction
(between the steel beam and the LVL timber) mobilised by the
small post-tensioning force in the bolted connectors is evident at
early stages of the load-slip behaviour, where no slip occurs up
to a shear force of 20 kN (as in Fig. 14a). The maximum coefficient
of variation (CoV) of the peak load capacity of the specimens tested
was limited to CoVpeak = 5.7% and 6.7% for the screw and bolted
connections respectively. With regard to the small variation
observed in the results for identical specimens, only the mean of
the experimental load-slip response in both the parallel and perpendicular to the grain directions and with or without nail plates
for the STC joints with screws are provided in Figs. 15–18, and
the mean and CoV of the peak load capacity (viz. the strength)
and the mean of the serviceability stiffness for the STC joints with
screws and bolts are given in Table 5. In this study, the serviceability stiffness of a STC joint is represented by the slope of the
load-slip curve between 10% and 60% of the peak load capacity
(i.e. the ks,0.6 slip moduli) in the second cycle of the loading.
The experimental load-slip responses of the STC joints parallel
to the grain direction (Figs. 15–18) exhibited close to elasticperfectly plastic behaviour, whilst the STC joints loaded in the
direction orthogonal to the grain showed some hardening characteristics that could be attributed to densification of the timber
grain for that regime. Furthermore, it is seen that after reinforcing
the LVL panels by nail plates, the STC joints exhibited very similar
load-slip responses, peak load capacities and serviceability
stiffnesses (ks,0.6 slip moduli) for loading both parallel and
perpendicular to the direction of the grain.
Fig. 7. Failure (a) mode III for 8 mm, (b) mode II and III for 12 mm and (c) mode II for 16 mm screws, (d) mode II for 20 mm screws.
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A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
With regard to Figs. 13–18 as well as Table 5, the following conclusions can be established.
Fig. 8. Failure mode of 12 mm bolt.
Fig. 9. Crushing of timber and nail plate rupture in the joints with screw connectors
and nail plate reinforcement.
Fig. 10. Failure mode of glued connection with screws.
For the STC joints with screw connectors, the peak load capacity
(strength) in the direction perpendicular to the grain is higher
than the peak load capacity in the direction parallel to the grain.
However, the serviceability stiffness (i.e. ks,0.6 slip moduli) of the
STC joints with screw connectors is lower (55% and less) in the
perpendicular to the grain direction than that in the parallel to
the grain direction. This observation is consistent with previous
studies, and with the fact that the elastic modulus of timber in
the perpendicular to the grain direction is significantly lower
than in the parallel to the grain direction [11,43].
For the STC joints with bolted connectors, the peak load capacities in both the parallel and perpendicular to the grain directions are similar. However, the serviceability stiffness (ks,0.6) of
the bolted joints in the perpendicular to the grain direction is
much smaller than in the parallel to the grain direction
(Table 5).
Comparing the peak load capacities and serviceability stiffnesses of STC joints with and without nail plates shows that
reinforcing the LVL panel by nail plates can significantly
increase the serviceability stiffness of STC joints with screws,
particularly in the direction perpendicular to the grain. A minimum stiffness increase of 24% (for S8) and 220% (for S16) is
observed in the directions parallel and perpendicular to the
grain respectively.
Reinforcing the LVL timber by nail plates can enhance the load
capacity of STC joints with screws when direction of the shear
loading is parallel to the grain. A minimum enhancement of
19% was achieved in STC joints with S16 screws. However, using
nail plate reinforcement had minor influence on the peak load
capacity of STC joints loaded in the direction perpendicular to
the grain, particularly when the diameter d of screws was relatively large (i.e. d P 12 mm).
The load carrying capacity and service stiffness of STC joints
with bolted connectors is higher than for joints with coach
screws, because of the higher strength of the steel used for manufacturing the Grade 8.8 bolts compared to the mild steel used
for manufacturing the coach screws.
In addition to STC joints with dowel (i.e. screw and bolt) connectors, six identical STC joints with combinations of glue and
screws were fabricated and tested. The load-slip and shear
stress-slip results obtained from these joints and the mean loadslip and mean shear stress-slip relationships for the specimens
are shown in Fig. 19. The glued STC joints exhibited significantly
d: diameter of
dowel
Width of crushed
zone= 1d-1.5d
(a)
(b)
Fig. 11. Timber crushing in the parallel to grain direction for the STC connection with (a) screw and (b) bolt.
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A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
d: diameter of
dowel
Width of crushed
zone= 3d-4d
(a)
(b)
Fig. 12. Timber crushing in the perpendicular to grain direction for the STC connection with (a) screw and (b) bolt.
Fig. 13. Load-slip response of STC joint with 2 S16 (16 mm screw) connectors for (a) parallel and (b) perpendicular to the grain loading obtained from three identical
specimens.
Fig. 14. Load-slip response of the STC joint with 2 B12 (12 mm bolt) connectors for (a) parallel and (b) perpendicular to the grain loading obtained from three identical
specimens.
higher strengths and stiffnesses compared to the STC joints with
only mechanical connectors. The short horizontal plateau in
Fig. 19 is indicative of transition from composite action provided
by glue + coach screw to composite action provided by only coach
screw (following glue fracture).
4. Analytical model for load-slip of STC joints with screw and
bolt connectors
In this section, an empirical load-slip behavioural model is proposed and calibrated for each group of the tested STC joints with
screw and bolt connectors, and with and without reinforcing nail
plates. The load-slip function that is proposed can be incorporated
into component-based numerical and analytical models and used
for non-linear short-term analysis of hybrid steel-timber composite beams and parallel continuous timber beam-to-steel column
joints (e.g. Fig. 1b).
For both the sake of simplicity and convenience for numerical
implementation, a continuous function with seven input parameters is proposed, being given by
f ¼n
h
ðk0 kp Þs
1 þ ðk0 kp Þ fs0
in1 on1 þ n
1
h
ðkp þ ks Þs
s
1 þ ðkp þ ks Þ f 1 f
0
in2 on1 ks s;
2
ð1Þ
A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
Fig. 15. Load-slip response of STC joint with 2 S8 (8 mm screw) and with/without nail plate for (a) parallel and (b) perpendicular to the grain loading.
Fig. 16. Load-slip response of STC joint with 2 S12 (12 mm screw) and with/without nail plate for both parallel and perpendicular to the grain loading.
Fig. 17. Load-slip response of STC joint with 2 S16 (16 mm screw) and with/without nail plate for both parallel and perpendicular to grain loading.
Fig. 18. Load-slip response of STC joint with 2 S20 (20 mm screw) and without nail plate for (a) parallel and (b) perpendicular to grain loading.
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Table 5
Mean and CoV (in%) of the peak load capacity and mean of the slip moduli ks,0.6a for STC joints.
Strength kN
Loading direction
Reinforcement
||
Without nail plate
With nail plate
Without nail plate
With nail plate
Without nail plate
With nail plate
Without nail plate
With nail plate
∟
Slip moduli ks,0.6 kN/mm
||
∟
Screwb
Boltb
8 mm
12 mm
16 mm
20 mm
12 mm
25.9
32.0
26.7
33.7
9.9
12.3
2.4
12.5
40.5
52.4
48.1
48.8
8.2
15.7
4.3
12.8
58.9
69.9
69.9
65.6
11.6
18.8
6.4
14.2
99.1 [0.43]
–
100.6 []
–
15.3
–
6.1
–
92.6 [0.6]
–
92.6 [6.7]
–
28.6
–
11.1
–
[0.2]
[2.9]
[3.3]
[3.0]
[5.2]
[2.1]
[2.7]
[1.4]
[2.8]
[1.3]
[1.7]
[5.7]
a
The slip moduli ks,0.6 is used as a representative of the connection stiffness within the service range of loading. The ks,0.6 slip moduli is the slope of the load-slip curve
between 10% and 60% of the peak load capacity.
b
The values in [] are CoV (in%) obtained from test results on three identical specimens.
Fig. 19. (a) Load-slip response and (b) shear stress-slip results for 6 identical glued STC joints.
where f is the shear force, s the slip, k0 the initial stiffness, kp the
pre-peak stiffness, ks the post-peak stiffness, f0 the first reference
shear force corresponding to the pre-peak branch and f1 the second
reference shear force corresponding to the post-peak branch, and
where n1 and n2 are two parameters that control the curvature of
the first and second curves respectively (Fig. 20). The function proposed can capture the pre-peak and post-peak behaviour of STC
joints with screws and bolts and with and without nail plates and
it is inspired by the Richard-Abbott four-parameter model that
has been used for modelling non-linear behaviour of semi-rigid
steel beam-to-column connections [44].
Fig. 20. Proposed analytical model for load-slip response of STC joints with dowel
connectors.
The analytical load-slip function is built on three asymptotic
lines, with the slope of the first line k0 representing the initial stiffness, the slope of the second line kp representing the pre-peak stiffness and the slope of third line ks being the pre-peak or post-peak
stiffness, depending on the experimental load-slip curve (Fig. 20).
The stiffness values k0, kp and ks can be positive, negative or zero
and they correspond to the three stages of connections load-slip
behaviour.
MATLAB software was used for non-linear regression and calculating the seven input parameters of the proposed analytical loadslip model. The non-linear regression procedure is based on the
least squares method with trust-region algorithm and bisquare
robustness. At the first stage, to implement the regression, the
average of the experimental data for each group tested was calculated. In the next step, by defining the initial values and upper and
lower bounds of each of the seven unknowns (the input parameters), the software calculates the value of each unknown/parameter within a 95% confidence bound. The value of the seven input
parameters (i.e. k0, kp, ks, f0, f1, n1 and n2) for the analytical loadslip model of the STC joints with and without nail plates are given
in Tables 6 and 7 respectively. In addition, the R-square values for
the calibrated load-slip curves are provided in Tables 6 and 7. It is
seen that the R-square values for all models are greater than 0.99,
demonstrating excellent correlation between the analytical and
experimental load-slip curves. The close correlation between the
calibrated analytical load-slip model and the experimental results
is also evident from Figs. 21 and 22.
The peak load capacities and slip moduli ks,0.6 of the STC lap
joints with coach screws versus the diameter of the screws are
shown in Fig. 23a and b respectively. Furthermore, the results of
the linear regression analysis of peak load capacity and slip moduli
ks,0.6 of the STC joints with coach screws are provided in
73
A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
Table 6
Input parameters for analytical load-slip model of doweled STC joints with nail plate.
Parallel to grain
Perpendicular to grain
Connector
S8
S12
S16
S8
S12
S16
f0
f1
k0
kp
ks
n1
n2
R2
22.16
45.77
16.8
1.24
1.421
3.072
10
0.9977
57.18
147.5
20.15
0.3213
3.964
2.058
7.53
0.9989
80.91
376.8
39.08
0.0
2.056
1.258
0.6765
0.9888
19.91
37.92
14.81
1.634
0.3942
4.539
10
0.9937
24.02
55.38
15.53
3.748
0.4229
3.383
3.271
0.9992
65.01
162.0
44.98
11.95
1.452
0.7974
0.5186
0.9947
Table 7
Input parameters for analytical load-slip model of doweled STC joints without nail plate.
Parallel to grain
Dowel type
f0
f1
k0
kp
ks
n1
n2
R2
Perpendicular to grain
Screw
Bolt
Screw
Bolt
S8
S12
S16
S20
B12
S8
S12
S16
S20
B12
15.65
28.51
79.61
1.103
0.2473
1.254
24.19
0.9963
39.99
53.93
18.11
0.2308
0.4143
1.228
3
0.9982
66.97
188.5
53.22
0
1.607
0.8712
1.62
0.9928
91.3
117.1
199
2.1
0.3535
0.6418
4.265
0.9971
24.32
67.15
262.2
13.51
1.71
45.76
3.185
0.9961
10.41
108.8
53.06
1.542
2.511
1.047
2.295
0.9973
43.9
160.3
10.66
0.5092
2.67
1.049
4.039
0.9973
99.19
193.9
62.21
0.1989
2.938
0.5078
16.41
0.9978
126.4
144.4
190.1
2.02
0.468
0.3543
5
0.9985
64.44
156.8
1002
2.76
1.01
0.3443
3.308
0.9958
Fig. 21. Correlation between analytical model and mean of experimental load-slip
response for STC joints without nail plate and loaded in (a) parallel and (b)
perpendicular to grain direction.
Fig. 22. Correlation between analytical model and mean of experimental load-slip
response for STC joints with nail plate and loaded in (a) parallel and (b)
perpendicular to grain direction.
74
A. Hassanieh et al. / Construction and Building Materials 118 (2016) 63–75
load-slip data by using a least-squares non-linear regression technique. In addition, a linear regression analysis was conducted to
derive formulae that can characterise the peak load capacities
and slip moduli ks,0.6 of the STC joints with respect to the screw
diameter d. The analytical models proposed can be incorporated
into component-based finite element models and used for the
non-linear analysis and design of hybrid steel-timber composite
lap joints.
Acknowledgement
This project was funded by ARC Discovery Grant DP160104092.
The support of the ARC to the project is acknowledged with thanks.
(a)
References
(b)
Fig. 23. Correlation between coach screw diameter and mean of experimental (a)
peak load capacity and (b) slip moduli ks,0.6 in parallel and perpendicular to grain
discretion.
Fig. 23a and b. Accordingly, formulae are proposed for calculating
the peak load capacity P as
P Perpendicular ¼ 6:0875d 23:9 ðR2 ¼ 0:99Þ
ð2Þ
ðR2 ¼ 0:94Þ
P Parallel ¼ 5:95d 27:2
and the slip moduli ks,0.6 of the STC lap joint with coach screws as
2
ks;0:6;Parallel ¼ 0:0844d 1:8725d þ 19:24
2
ðR2 ¼ 0:96Þ
ks;0:6;Perpendicular ¼ 0:0344d þ 1:2925d 5:87 ðR2 ¼ 0:97Þ;
ð3Þ
both parallel and perpendicular to the direction of the grain. In Eqs.
(2) and (3), P is the peak load capacity (in kN), d the screw dimeter
(in mm) and ks,0.6 the slip modulus (in kN/mm).
5. Conclusions
In this paper, the load-slip response, failure modes, peak load
capacities and the ductility of STC joints with having screws and
bolted connectors, and with and without nail plate reinforcing
for the timber, were investigated by conducting push-out tests
with a symmetric configuration. Furthermore, push-out tests were
conducted on STC joints with a combination of glue and coach
screws to characterise the failure mode and load carrying capacity
of the STC joints with near to full composite action.
A seven-parameter load-slip model was proposed for the dowelled (screw or bolted) STC joints with and without nail plate reinforcement, and the model was calibrated against the experimental
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