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BEHAVIOUR OF MASS TIMBER PANEL-CONCRETE CONNECTIONS WITH INCLINED SELF-TAPPING SCREWS AND INSULATION LAYER

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The Mass Timber Panel-Concrete (MTPC) composite floor system considered in this paper consists of a Mass Timber Panel (MTP) connected to reinforced concrete slab with Self-Tapping Screw (STS) connector and a sound insulation layer in between. This type of composite floor system is intended for mid- to high-rise building applications. Two types of MTPs with normal weight concrete, two insulation thicknesses, two screw embedment lengths and two screw angles were investigated through connection tests to characterize connection stiffness and strength. The main goal of this connection test program was to provide preliminary test data to assist in the development of a model to predict connections lateral stiffness and strength under consideration of insulation thickness, screw angle, withdrawal and embedment properties of screws in MTP. Connection test results show that screws at an insertion angle of 30 have a higher stiffness and strength along with a larger embedment length compared to the screws at a 45 angle and smaller embedment length. Stiffness seemed to be more susceptible to the influence of presence of insulation compared to strength with 40-65% reduction of stiffness and 10-20% reduction of strength were noticed for an insulation thickness of 5 mm. Screws in CLT showed higher strength while screws in CLP showed higher stiffness but the difference is insignificant.
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BEHAVIOUR OF MASS TIMBER PANEL-CONCRETE CONNECTIONS
WITH INCLINED SELF-TAPPING SCREWS AND INSULATION LAYER
Md Abdul Hamid Mirdad1, Ying Hei Chui2
ABSTRACT: The Mass Timber Panel-Concrete (MTPC) composite floor system considered in this paper consists of a
Mass Timber Panel (MTP) connected to reinforced concrete slab with Self-Tapping Screw (STS) connector and a sound
insulation layer in between. This type of composite floor system is intended for mid- to high-rise building applications.
Two types of MTPs with normal weight concrete, two insulation thicknesses, two screw embedment lengths and two
screw angles were investigated through connection tests to characterize connection stiffness and strength. The main
goal of this connection test program was to provide preliminary test data to assist in the development of a model to
predict connections lateral stiffness and strength under consideration of insulation thickness, screw angle, withdrawal
and embedment properties of screws in MTP. Connection test results show that screws at an insertion angle of 30 have
a higher stiffness and strength along with a larger embedment length compared to the screws at a 45 angle and smaller
embedment length. Stiffness seemed to be more susceptible to the influence of presence of insulation compared to
strength with 40-65% reduction of stiffness and 10-20% reduction of strength were noticed for an insulation thickness
of 5 mm. Screws in CLT showed higher strength while screws in CLP showed higher stiffness but the difference is
insignificant.
KEYWORDS: Composite floor systems, Mass timber panel, Insulation layer, Self-tapping screw
1 INTRODUCTION 123
The Mass Timber Panel-Concrete (MTPC) composite
system considered in this paper consists of a Mass
Timber Panel (MTP) connected to a reinforced concrete
slab by mechanical fasteners. In floors under out-of-
plane bending situations the MTP primarily resist tensile
stress while the concrete slab resists compressive stress.
An insulation layer is sandwiched between the timber
panel and concrete slab which provides better acoustic
and thermal performance. The mechanical fasteners,
which penetrate all three components, allow for partial
shear transfer and therefore partial composite behaviour
between the components. Figure 1 illustrates the type of
MTPC composite system considered in this project. The
advantages of this MTPC composite system are high
strength and stiffness to weight ratio, large span to total
depth ratio, high in-plane rigidity and better acoustics,
damping, thermal and fire performances [1, 2]. Since
full-scale floor tests are expensive to conduct, initial
small scale connection tests are desirable to study how
the floor system stiffness and strength may be affected
by component characteristics. In this paper, a connection
test program is presented and the goal is to characterize
the stiffness and strength of the connections, which will
aid the future development of a design approach for this
type of floor system.
1 Md Abdul Hamid Mirdad, University of Alberta, Canada,
mdabdul@ualberta.ca
2 Ying Hei Chui, University of Alberta, Canada,
yhc@ualberta.ca
Figure 1: Typical MTPC Composite System
2 CONNECTION
The structural efficiency of MTPC composite floor
systems highly depends on the stiffness of the interlayer
connection. The stiffer the connection, the higher the
level of composite action and therefore the lower the
deflection of the floor structure under transverse loading.
It is most advantageous to use a connection that provides
1) a stiff connection between the wood and the
concrete while undergoing stresses in the elastic range
and 2) a ductile response while undergoing stresses in
the plastic range as both concrete and timber are
naturally brittle [3].
In order to characterize Timber-Concrete Composite
(TCC) systems, tests are often performed on connection
specimens. EN 26891:1991 [4] provides a standardized
procedure for such a test where load-displacement curve
is produced to determine the strength and stiffness of the
connection. Load-carrying capacity (strength), slip
modulus (stiffness) and ultimate deformation capacity
(ductility) are the most important mechanical properties
of the connection. The slip modulus of the connector
represents the relative displacement between the timber
and concrete under an applied shear force. The ultimate
deformation capacity of the connection would be that
where the ultimate slip of the connection is not reached
before the failure of the connection [5, 6 and 7]. By
avoiding failures in the connections in the timber-
concrete composite system, it is possible to maximize
the load-carrying capacity and increase the ultimate
deformation capacity.
A comparison of load-displacement relationships for
different categories of connection obtained from the
literature is shown in Figure 2. From Figure 2, it can be
seen that glued and notched connections are stronger and
stiffer than the other presented connections. The glued
and notched connections show almost linear behaviour
up to failure but their failure mode is brittle. Slender
dowel-type fasteners, such as screws, are usually ductile
with nonlinear behaviour under lateral load, with
relatively low strenght and stiffness. Axially loaded
screws can be ductile and their load-carrying capacity as
well as stiffness is in-between dowel-type and notch type
connections.
Figure 2: Load-displacement relationship for different types of
connections [8]
The use of aggregate has very limited influence on the
behaviour of the timber-concrete connection. Use of
lightweight concrete instead of normal weight concrete
reduces the permanent load of the floor by up to 15%
and high strength concrete gives the option to reduce the
thickness of the concrete slab [5]. Load-carrying
capacity of the connection slightly increases with the
increase of compressive strength of concrete. Normal
weight concrete shows higher stiffness followed by
lightweight concrete and high strength concrete [6].
Overall, the best choice is to use normal weight concrete
considering the cost, stiffness and strength.
3 TEST MATERIALS
3.1 SELF-TAPPING SCREW (STS)
Self-tapping screws were developed as an improved
threaded fastener over the traditional lag screws or wood
screws for the applications in mass timber structures. In
contrast to traditional lag screws which have their
threaded part machined from the original rod diameter,
the thread of self-tapping screws is produced by rolling
or forging a wire rod around the shank, which
consequently features a smaller diameter when compared
to the outer cross-sectional thread diameter, or smooth
shank for partially threaded screws. Self-tapping screws
often feature a continuous thread over the whole length
(fully-thread) which leads to a more uniform load
transfer between the screw and the wood material as well
as a considerably enhanced withdrawal resistance, the
type of loading for which they are optimized [9]. If these
screws are loaded axially when inserted at an angle,
especially at 30° and 45°, higher withdrawal strength and
stiffness can be achieved when compared to an insertion
angle of 90° angle which gives lower stiffness. These
screws can be up to 16 mm in diameter and 1200 mm in
length. Because of their self-drilling tip, no pre-drilling
is required, except for long screws or dense wood. In
Figure 3, different types of screw heads, threads and drill
tips are shown in comparison to lag screws.
Figure 3: Different types of drill tips, threads, and heads in
comparison with a lag screw (left) [9]
3.2 CROSS LAMINATED TIMBER (CLT)
CLT consists of layers of dimension lumber (typically
three, five, seven or nine layers) oriented at right angles
to one another. The layers are face-glued to form
structural panels with high strength, dimensional
stability, and rigidity. Panel sizes vary by manufacturers;
typically, widths are up to 3 m while length can be 20 m,
and the thickness can be up to 400 mm. For floor and
roof panels, the outer layers run parallel to the span
direction [10].
3.3 COMPOSITE LAMINATED PANEL (CLP)
CLP is a new type of mass timber made with lumber and
Structural Composite Lumber (SCL) panels. CLP is
under development. It can be produced with various lay-
ups with alternating layer orientation or all layers being
parallel to each other. In the present study, 5-layer CLP
panels with lumber in the outer layers and SCL in the
inner layer is used. Both materials run in the same
direction along the length of the member. CLP has
improved rolling shear and stiffness over CLT.
Dimensions of CLP can be the same as CLT.
3.4 SILENT FLOOR
The silent floor has a sound-proofing layer made of
polyester felt and elastoplastomer bitumen, designed as
an acoustic insulating material for absorbing noise and
vibrations resulting from foot traffic. Due to the
bituminous base composition, it is also impermeable to
air, water and humidity. The bituminous layer is able to
adhere and close around connections, compensating for
any tears. Silent floor creates an elastic separation
between stiff elements, slabs and walls, dampening
vibrations due to foot traffic and to the various sound
sources in the rooms [11].
4 METHODOLOGY
4.1 TEST SETUP
Two types of Mass Timber Panel (MTP), namely Cross
Laminated Timber (CLT) and Composite Laminated
Panel (CLP) were used in the tests. The CLT elements
had a length of 400 mm, a width of 200 mm and a
thickness of 175 mm, while the CLP elements had
dimensions of 400 mm length, 200 mm width and 185
mm thickness. Normal weight concrete (35 MPa) of 75
mm thickness with 19 mm aggregate size was used.
Insulation thicknesses were selected as 0 and 5 mm. This
selection was according to Eurocode 5 [12], where the
minimum thickness of concrete is mentioned as 50 mm,
the maximum aggregate size mentioned is 20 mm and
with maximum intermediate layer thickness of 50 mm.
Stucco steel wire mesh was used to limit crack
propagation in concrete in order to maintain the integrity
of the connection close to timber according to Eurocode
5 [12]. Plastic separation sheets were used in between
the concrete and timber surface to remove any bond at
the interface which increases the load-slip modulus at
low load levels [13]. A symmetrical test setup was
selected since asymmetrical shear test setup leads to a
slight overestimation of the shear strength and load-slip
modulus [14]. Among the different setups, it seems that
the strength and load-slip modulus obtained from
concrete-timber-concrete specimens are a better
representative compared with the results obtained from
other setups [15], therefore the concrete-timber-concrete
configuration was selected instead of timber-concrete-
timber.
Self-tapping screws were selected for the connections;
only one screw diameter (11 mm) was included. Two
different angles of insertion relative to timber grain (30
and 45) were tested. The screws were installed in pairs
cross-wise (per pair, one screw each in tension and
compression). Two cross-wise pairs (4 screws total) per
shear plane with two in tension and two in compression
were installed. The spacing of the Self-Tapping Screw
was based on Canadian Construction Materials Centre
(CCMC) [16] for STS to avoid group effect and is shown
in Figure 4. Screws were located at 3d distance from the
parallel edge, 11d in the unloaded edge and 13d in the
loaded edge. Here, d refers to the diameter of the screw.
A waterproof and sound insulating membrane made of
polyester felt and elastoplastomer bitumen was used as
insulation for better acoustic, damping, and thermal
performance. The connection test setup is shown in
Figure 5. Top and bottom plate along with L shape
angles were used to avoid the lateral movement of the
specimen. Linear Variable Differential Transformers
(LVDT’s) were used on both sides of two shear planes to
measure the relative slip between timber and concrete.
Figure 4: Screw orientation according to CCMC 13677-R [16]
Figure 5: A typical connection test setup with L80-I5-45°
configuration (top) and an actual connection test setup of a
CLP specimen (bottom)
A total of 16 configurations (2 MTP x 2 angles x 2
embedment length x 2 insulation thicknesses) with three
replicates each leading to a total of 48 connection
specimens. The investigation parameters are listed in
Table 1. There were 8 major screw configurations tested
for each MTP. The specifications of the screws for each
composite material are shown in Table 2, where L#
refers to the embedment length of screw into MTP, I#
refers to the acoustic insulation thickness and # refers to
the insertion angle of screw to the timber grain.
Table 1: Investigation parameter
Material
Parameter
MTP
Cross Laminated Timber (CLT)
Composite Laminated Panel (CLP)
Insulation
0 mm
5 mm
Screw Angle
30 (screw pair)
45 (screw pair)
Screw
Embedment
80 mm
100 mm
Table 2: Screw configuration for composite material
Screw
length
(mm)
Concrete
(mm)
Insulation
(mm)
MTP
(mm)
150
70
0
80
150
62.9
7.1
80
150
70
0
80
150
60
10
80
200
100
0
100
200
92.9
7.1
100
200
100
0
100
200
90
10
100
The specimens were prepared by inserting the screws
into the MTP block with proper orientation, therefore
casting a normal weight concrete on site. The specimens
were cured for 28 days before testing. In Figure 6,
pictures of the specimen preparation are shown.
Figure 6: Specimen preparation; a) insertion of screw at 30
angle to the grain, b) screws on the specimen, c) specimen on
forms, d) casting concrete on forms, e) specimens ready for
casting and f) full specimen before test.
4.2 TEST PROCEDURE
The loading protocol according to EN 26891:1991 [4]
was followed, where the specimen was loaded initially
up to 40% of the estimated failure load, then the loading
was stopped for 30 seconds and then unloaded to 10%
and stopped 30 seconds. Finally, the specimen was
loaded to failure. The load was applied at a loading rate
of 5 mm/min. The maximum allowable slip level was
chosen to be 15 mm in accordance to EN 26891:1991
[4]. The reason for the unloading step was to allow the
specimen to settle and to eliminate the internal friction
between timber and concrete and to ensure that the
connection does not fail due to initial slip or slack [17].
The connection strength was defined as the maximum
load before failure of the specimen. The stiffness was
quantified by the load-slip modulus at three different
load levels (40, 80 and 100% of the estimated failure
load) corresponding to Serviceability Limit State (SLS),
Ultimate Limit State (ULS), and collapse load level
respectively [18]. Figure 7 shows the applied loading
procedure and an idealized load-slip curve.
Figure 7: Loading procedure (a) and idealized load-slip
curves (b) base on EN 26891:1991 [4]
a
b
c
d
e
f
f
The slip modulus for SLS, K0.4, slip modulus for ULS,
K0.8 and slip modulus at collapse load level, Kult were
calculated using the formulas shown in Equation 1.
ult
ult
est
est
s
F
K
ssss
F
K
ss
F
K
max
01042428
8.0
0104
4.0
)(
3
4
)(
8.0
)(
3
44.0
(1)
Here, Fest is the estimated maximum load adjusted from
previous test, Fmax is the maximum load at failure and s
refers to the relative slip at specified points shown on
Figure 7.
4.3 MATERIAL PROPERTIES
For all batches, concrete cylinders were tested 28 days
after the casting and the average compressive strength
was found to be 38.82 MPa with a modulus of elasticity
of 29.28 GPa and a density of 2345 kg/m3. Fully
threaded Self-Tapping Screw from Rothoblaas with a
diameter of 11mm was used. The screw had yield
strength of 1000 MPa, a modulus of elasticity of 210
GPa and a tensile strength of 38 kN [19]. Nordic X-Lam
5 ply E1 stress grade CLT was used which has Machine
Stress Rated Spruce-Pine-Fir (S-P-F) lumber in
longitudinal and No. 3/Stud (S-P-F) in transverse layers
[20]. The CLP elements all the layers with wood grain or
strands aligned in the same direction. The top and
bottom layers were visually graded Spruce-Pine-Fir (S-
P-F) lumber and the middle three layers were Laminated
Strand Lumber (LSL). Silent Floor from Rothoblaas was
used as insulation layer which has the dynamic stiffness
of 7 MN/m3 and up to 27 dB reduction level in foot
traffic noise [11]. The material mechanical properties are
shown in Table 3.
Table 3: Material Properties
Material
Fc (MPa)
E (MPa)
Fb (MPa)
Concrete(1)
38.82
29,284
-
CLT(2)
-
7,020
21.93
CLP(2)
-
9,149
33.56
Where,
Fc = Compressive strength,
E = Modulus of elasticity and
Fb = Bending strength.
(1) Tested average cylinder value
(2) Tested mean value
5 RESULTS
The load-displacement curves were drawn by taking the
average displacement of four LVDT and applied load on
the specimen recorded by calibrated load cell and
presented in Figure 8 and Figure 9 for CLP and CLT
specimen respectively. The solid lines represent the
specimens without insulation and dotted lines represent
the specimens with 5 mm insulation. It can be seen
clearly that stiffness is reduced significantly with the use
of insulation layers in between concrete and MTP.
Figure 8: Load-displacement curve of CLP specimen
Figure 9: Load-displacement curve of CLT specimen
Overall connection test results for all configurations are
shown on Table 4 where, ultimate strength, stiffness at
SLS, ULS, and collapse load level and reduction
percentage in strength and stiffness due to the use of
insulation are included based on a pair of screws. Also,
to investigate the influence of the screw embedment
length, screw angle and insulation thickness, a
comparison chart was drawn for strength and stiffness at
SLS values separately which are the most important
factors in TCC and shown in Figure 10 and Figure 11
respectively. From the connection test results, it can be
seen that the ultimate strength of the connection
increases with the increase of embedment length of the
screws into MTP. Also the screws inserted at 30° grain
angle have higher strength compared to the screws
inserted at 45° grain angle for both MTP. The strength
decreases by 10-20% in all configurations when an
insulation layer of 5 mm was used. In Figure 10, strength
per pairs of screw for all configurations can be seen.
Also, a significant difference in stiffness for all
parameters can be noticed. The trends of the stiffness
result are almost the same as for the strength where 30°
angled screws showed a higher stiffness compared to 45°
angled screws along with a higher stiffness at a larger
embedment length of screws into the MTP. The stiffness
decreases by 40-65% when 5 mm insulation layer was
used. Here, 50-65% stiffness reduction was noticed in
case of screws in CLP while 40-50% stiffness reduction
in CLT due to the presence of insulation. Also, the
ultimate stiffness and collapse stiffness follows almost
the same ratio of reduction as serviceability stiffness.
Overall, it seems that the stiffness is more sensitive to
the influence of the insulation than the strength.
Comparison of the stiffness at SLS per pairs of screw
can be seen in Figure 11. Although CLP is a stiffer
structural material than CLT, strength and stiffness of
the connections in CLP and CLT seemed not that
significant.
Table 4: Connection test results
Specimen
Fult/Pair
(kN)
% Reduction
K0.4/Pair
(kN/mm)
K0.8/Pair
(kN/mm)
Kult/Pair
(kN/mm)
CLP-L80-I0-45°
30.3
-
34.7
23.5
14.0
CLP-L80-I5-45°
27.4
9.6
13.1
8.5
5.1
CLP-L80-I0-30°
35.0
-
56.3
40.3
21.3
CLP-L80-I5-30°
29.0
16.9
20.0
14.7
6.9
CLP-L100-I0-45°
41.9
-
45.8
36.4
22.3
CLP-L100-I5-45°
37.0
11.7
16.6
10.2
7.0
CLP-L100-I0-30°
42.1
-
59.7
44.3
26.0
CLP-L100-I5-30°
36.5
13.2
29.0
17.5
9.8
CLT-L80-I0-45°
33.6
-
27.7
22.4
12.4
CLT-L80-I5-45°
28.7
14.6
15.6
9.4
4.6
CLT-L80-I0-30°
41.9
-
46.3
46.5
28.2
CLT-L80-I5-30°
31.5
24.7
27.2
18.4
9.6
CLT-L100-I0-45°
44.2
-
32.4
34.4
15.9
CLT-L100-I5-45°
35.9
18.9
16.5
10.0
6.5
CLT-L100-I0-30°
39.7
-
47.9
28.3
17.6
CLT-L100-I5-30°
36.1
9.0
28.7
18.4
8.5
Figure 10: Comparison of strength per pair of screws
Figure 11: Comparison of stiffness at SLS per pair of screws
The failure modes in the cases without insulation were a
combination of a plastic hinge formed in the screws
within the MTP and wood embedment and concrete
crushing. In the cases with insulation, the failure
occurred due to wood embedment and plastic hinges
formed in the MTP. This is because; the insulation does
not contribute to the system stiffness and strength at all
and behave like a gap. When there is a gap, the stress on
timber is more compared to the stress in concrete due to
additional lever arm in bending. In the specimens
without insulation, cracking of concrete was dominant.
Also because of the stiffer behaviour of CLP compared
to CLT, concrete crushing was found to be more
dominant in CLP.
Overall, initially the screw connections were fully rigid
followed by bond failure where load was transferred to
the less stiff screw and therefore, the screws started to
pick up additional load before failure in either tension or
withdrawal. Some of the failure modes in the tested
specimens are shown in Figure 12 and Figure 13.After
cutting the specimen, plastic hinge of screw in MTP and
wood embedment was observed which are shown in
these figures with red circle.
Figure 12: Failure mode concrete crushing without
insulation (top left), plastic hinge with insulation (top right),
embedment failure of timber (bottom left and right) in CLP
Figure 13: Failure mode plastic hinge with insulation (top
left), wood embedment without insulation (top right), concrete
crushing (bottom left) and plastic hinge on single screw taken
from the specimen in CLT
6 CONCLUSIONS
The stiffness of the connections appears to be strongly
influenced by the insulation layer. The use of an
insulation layer resulted in a significant reduction in
stiffness even for very small insulation thickness. The
strength seems to be less affected. Further, 30° angled
screws relative to the timber grain showed higher
strength and stiffness compared to 45° angled ones. A
larger embedment length of screws into MTP showed a
higher stiffness and strength. Test results for CLP and
CLT were nearly the same despite the slightly higher
stiffness of connection in CLP. Therefore, the
connection properties are heavily influenced by the
insertion angle along with the embedment length.
Special consideration has to be taken while designing
MTPC composite floors using insulation layers between
timber and concrete. These test results provide a general
idea about the influence of insulation and inclined self-
tapping screws in MTPC composite systems and will
help in the development and validation of an analytical
model for connection to predict stiffness and strength.
ACKNOWLEDGEMENTS
The authors would like to gratefully thank Nordic
Structures, Rothoblaas, Western Archrib and InnoTech
Alberta for providing the materials for the test. This
research is supported by Natural Sciences and
Engineering Research Council of Canada’s (NSERC)
Industrial Research Chair grant program.
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... They also illustrated that using a larger screw diameter with a longer embedment length into the glulam joist improved the bearing capacity of the connection. Similar results were also observed by [33][34][35]. Recently, Kyprianou et al. [36,37] have conducted a comprehensive experimental and analytical study on the connection mechanical behaviour of sheathed cold-formed steel studs. ...
Article
This study carries out numerical and experimental investigation on shear connection performance of flooring systems comprising of cold-formed steel joists and engineered plywood panels. A 3-D finite element model has been developed using ANSYS software which considers the nonlinear behaviour and damage of plywood panels using a maximum stress-based criterion incorporated in the FE model. The yielding behaviour of steel joists and fasteners (screws and bolts) is defined as a multilinear plastic hardening material model defined in ANSYS software. The FE model was validated against the experimental results obtained from push-out tests conducted on specimens utilising different connection arrangements. The validated FE models showed a good agreement with the experimental results of the push-out tests in terms of load-slip behaviour, ultimate peak capacity, and failure modes. Finally, the validated FE models are utilised to perform a parametric study that investigates the effect of varying various parameters, including diameter and yield strength of self-drilling screws, the thickness of plywood panels, value of friction coefficient, and the ratio of fastener diameter to the joist thickness, on the initial stiffness, failure mode and ultimate load capacity of the cold-formed steel and plywood composite connections.
... In fact, TCC structures have attracted much attentions for its good structural performance, high economic value, and environmental sustainability, which have become a hot research topic during the past few decades [26][27][28][29][30][31]. Most investigations of TCC structures focused on the bending performance of TCC beams [32][33][34][35][36][37], the shear behavior of timber-concrete connections [38][39][40][41], the fire or vibration performance of TCC floors [42][43][44][45], and the long-term creep behavior of TCC structures [46][47][48], while no studies have considered the influence of the slab width on the structural performances. The existed studies have proved that the concept of combining the good tension strength of timber and the high compressive properties of concrete is feasible in practice [49]. ...
Article
To support the design and application of timber-concrete composite (TCC) floor systems, the effective width of the TCC beams with crossed inclined coach screw connections at the serviceability state was evaluated in this study. The bending tests on the TCC beams with different slab widths were conducted to assess the shear lag effect in concrete slab and discuss the influence of slab width on the bending performance of TCC beams. A nonlinear finite element (FE) model for TCC beams was established in ABAQUS 6.14 and validated by the experimental results. Based on the experimental and FE studies, an extensive parametrical analysis was performed to evaluate the effect of various factors on the effective width of TCC beams. The presence of shear lag effect was proved by the experimental strain distributions in the concrete slab, and the shear lag effect was found more apparent for the tension strain at the bottom of concrete slab than the compression strain at the upper surface of slab. The FE numerical results showed high agreement with the test results in terms of the macro and micro bending performances of the TCC beams. In addition, the parametrical studies indicated that the effective width of TCC beams mainly depends on the load type, slab width-to-span ratio, slab thickness, and connection stiffness. Finally, for practical use, a general formula was proposed to calculate the effective width of the TCC beams.
... e two components are connected by mechanical connectors, as shown in Figure 1, such as dowels (e.g., self-tapping screw (STS)), notches, or proprietary connectors (e.g., glued in plate/HBV plate). e added concrete increases the floor mass and stiffness which subsequently mitigates the vibrations and excessive deflection issues [6][7][8]. ...
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Over the last few decades, there has been growing interest in the use of low-carbon materials to reduce the environmental impacts of the construction industry. The advent of mass timber panels (MTP), such as cross laminated timber (CLT), has allowed structural engineers to specify a low-carbon material for a variety of floor design considerations. However, serviceability issues such as vibration and deflection are limiting the construction of longer span timber-only floor systems and have encouraged the development of timber-concrete composite (TCC) systems. The use of concrete would negatively impact on the carbon footprint of the TCC floor system and should be minimized. The purpose of this study was to study the impact on embodied carbon in the TCC system, when the ratio of timber and concrete was varied for specific floor spans. Two MTP products were considered, CLT and glued laminated timber (GLT). The floors were designed to satisfy structural, acoustic, and vibration criteria, and the results were presented in the form of span tables. It was found that using thicker MTP instead of adding concrete thickness to meet a specific span requirement can lead to lower embodied carbon values. Increasing concrete thickness for long-span floor systems led to a reduction in allowable floor span due to the vibration criterion being the controlling design parameter. Increasing timber thickness also resulted in higher strength and stiffness to weight ratios, which would contribute toward reducing the size of lateral load resisting systems and foundations, resulting in further reductions in the embodied carbon of the entire structure.
Chapter
An experimental campaign was carried out for dowel type joints made of two wood species, Maritime Pine (Pinus pinaster) and Scot’s Pine (Pinus sylvestris). Monotonic (EN 26891) and cyclic (EN 12512) tests were performed on the joints, with and without reinforcement, and the effect of the reinforcement in the overall capacity of the joint is presented and analyzed. The effectiveness of the reinforcement was found to be linked directly to the slenderness of the dowel, which has proven to have an essential role in determining the failure mode of the joints. Reinforcement in general have projected different effects by means of the joints’ load-carrying capacity, ductility and strength degradation, depending on many factors, such as the different densities and mechanical properties of each of the wood species, as well as the different loading procedures, as the effect was more evident with cyclic tests.
Article
Mass timber panel-concrete (MTPC) composite floor systems are currently the preferred choice of designers for multi-storey modern mass timber construction. An analytical model for one-way acting composite panels has been developed to predict the effective bending stiffness and load-deflection response of MTPC composite floor systems by considering the elastic-plastic behaviour of interlayer connectors and the presence of a soft acoustic layer between concrete and timber. One-way acting composite floor panels were tested under four-point bending and vibration with various configurations to validate the developed stiffness prediction model and investigate the influence of different parameters. The experiments showed that the model can predict the effective bending stiffness of MTPC composite systems within 10% of the bending test values in the elastic range and 14% of the modal test results. The so-called Gamma method, commonly used for designing composite systems, provides predictions within 12% of the test bending stiffness, on average. Although the Gamma and proposed method yield similar results, the proposed method is more comprehensive and less restrictive in terms of the underlying assumptions that underpin the method. The proposed model is also capable of predicting the post-yielding load-deflection response of the composite system. A comparison of the predicted and tested load-deflection responses indicates a reasonable agreement between the two, although the predicted effective bending stiffness tends to be higher. The predictive capability of the model for the post-yielding response can be further improved by considering the influence of cracking in concrete slabs.
Article
In many practical applications of timber-concrete composite (TCC) structures, especially in rehabilitation of timber buildings, the floorboard interlayer is utilized as formwork for pouring concrete slab. So far, few researches have been conducted to study the shear performance of inclined crossing screws in TCC beam with an interlayer. In this paper, seven groups of shear tests were conducted to study the shear performance of inclined crossing screws in glulam-concrete composite beam with an interlayer. The analyses focused on the effects of embedded length of screw into glulam, thickness of the concrete slab and screw diameter on the failure mechanism, the load-slip behavior and the load carrying capacity. Two typical failure mechanisms were observed: cone failure of the concrete layer around shear-compression screw and pull-out failure of shear-tension loaded screw. The test results demonstrated that with the increase of screw diameter and embedded length of screw into glulam, the load carrying capacity of the inclined crossing screws was enhanced. According to theoretical studies, in timber-concrete composite beams with interlayer, the calculation method for predicting load carrying capacity of inclined crossing screws was proposed, and significantly the predicted results agreed well with the experimental results.
Article
Full-text available
This paper analyzes the load-carrying capacity of timber-concrete joints made with dowel-type fasteners. The scope of the work was to obtain information to be used in the design of timber-concrete composite structures. In order to achieve that, two different approaches were used: experiments with timber-concrete joints loaded in shear, and the use of nonlinear analytical models. Shear tests were performed using various timber species, concrete mixtures, and fasteners. The results from these tests are presented and compared with the results determined with three different analytical models. These models differ in the approach used to simulate concrete: linear-elastic, linear-elastic with crushing, and elastic-perfectly plastic. The analysis of the test results shows that the load-carrying capacity of this type of joint is significantly influenced not only by the strength of the materials but also by the shape of the fastener. From the three models proposed, the one assuming elastic-perfectly plastic behavior for concrete leads to the results closer from the test results, however, the best statistical correlations between model results and test results were obtained with the model assuming linear-elastic behavior with crushing.
Article
Full-text available
The mechanical behaviour of timber-concrete structures is significantly influenced by the stiffness of the joints, both with regard to deformations and to strength. This paper analyses the experimental and numerical assessment of the stiffness of timber-concrete joints, made with dowel-type fasteners. The objective of this analysis is to provide information to be used in the design of timber-concrete composite structures. In order to achieve this, two approaches were used: experimental tests with timber-concrete joints and numerical predictions using models available in the literature. As part of the experimental assessment, shear tests were performed with various joint configurations aiming to be representative of a wide range of practical conditions. The results obtained are presented and discussed. In terms of numerical assessment, two types of models were considered: analytical models, based on the beam on elastic foundation and empirical models. The results obtained in the experimental tests are compared with those obtained from the design models. Based on that analysis the accuracy of the numerical predictions is discussed.
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Timber-Concrete Composite (TCC) systems have been employed as an efficient solution in medium span structural applications; their use remains largely confined to European countries. TCC systems are generally comprised of a timber and concrete element with a shear connection between. A large number of precedents for T-beam configurations exist; however, the growing availability of flat plate engineered wood products (EWPs) in North America has offered designers greater versatility in terms of floor plans and architectural expression in modern timber and hybrid structures. The opportunity exists to enhance the strength, stiffness, fire, and vibration performance of floors using these products by introducing a concrete topping, connected to the timber to form a composite. A research program at the University of British Columbia Vancouver investigates the performance of five different connector types (a post-installed screw system, cast-in screws, glued-in steel mesh, adhesive bonded, and mechanical interlocking) in three different EWPs (Cross-Laminated-Timber, Laminated-Veneer-Lumber, and Laminated-Strand-Lumber). Over 200 mid-scale push-out tests were performed in the first stage of experimental work to evaluate the connector performance and to optimize the design of subsequent vibration and bending testing of full-scale specimens, including specimens subjected to long-term loading.
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In timber engineering, self-tapping screws, optimized primarily for axial loading, represent the state-of-the-art in fastener and reinforcement technology. Their economic advantages and comparatively easy handling make them one of the first choices for application in both domains. This paper focuses on self-tapping screws and threaded rods applied as reinforcement, illustrating the state-of-the-art in application and design approaches in Europe, in conjunction with numerous references for background information. With regard to medium to large span timber structures which are predominately erected by using linear timber members, from e.g. glued laminated timber, the focus of this paper is on their reinforcement against stresses perpendicular to grain as well as shear. However, latest findings with respect to cross laminated timber are included as well.
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This paper presents a survey on the state-of-the-art of timber-concrete composite research in the past and recent years. The most important literature references were carefully selected and reviewed to provide an overview and some depth in the development of this construction technique. After highlighting the advantages of the composite system, the standards and design methods currently available are presented. An extensive description of the connection systems developed around the world is also provided. The experimental and numerical investigations performed on connections and beams in both the short- and long-term (at collapse and under sustained load, respectively) are discussed at length in the paper. Other aspects covered are prefabrication, the influence of concrete properties, fatigue tests, fire resistance, vibrations, and acoustics. DOI: 10.1061/(ASCE)ST.1943-541X.0000353. (C) 2011 American Society of Civil Engineers.
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Composite concrete–timber building and bridge structures are gaining in importance throughout the world. The approach is to pour fresh concrete over the timber. Mechanical linkage between hardened concrete and timber is provided through keyed indentations in the timber or various sorts of mechanical fasteners inserted in the timber before the concrete is poured. The technique is commented on from research and design points of view.
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Composite concrete slab and timber flooring systems are commonly used in many parts of the world to exploit the high strength-to-weight ratio of timber and the good acoustic separation provided by concrete floor slabs. This paper describes the results of an experimental programme that investigated the suitability of a range of connectors to transfer shear between a concrete slab and a laminated veneer lumber (LVL) beam. Shear tests on reduced scale specimens were performed with the aim of comparing the strength, stiffness, and post-peak performance of different connectors such as round and rectangular concrete plugs with and without screw and steel pipe reinforcement, proprietary (SFS) screws, coach screws with different diameters, sheet brace anchors, and framing brackets. The rectangular concrete plug reinforced with a coach screw was found to provide the greatest stiffness and strength, as well as favourable post-peak behaviour. Such a system can be used for cost-effective composite floor systems due to its efficient cost-to-capacity ratio, which reduces the number of connectors needed along the beam axis to achieve the composite action.