TIMBER-CONCRETE COMPOSITE SYSTEMS WITH CROSSLAMINATED TIMBER
Luís Jorge1, Johannes Habenbacher2, Bruno Dujic3
ABSTRACT: Cross-laminated timber (Xlam) bending elements are known to have design governed by serviceability
issues. Therefore, the addition of concrete layer on the top could be one solution to consider and which has already
known features for glulam or sawn timber elements. The work presented here shows results from few finite element
modelling simulations and allows understanding the relevance of different kind of timber-concrete connection systems.
Despite the demonstrated advantages in using the concrete topping as structural component – increase load capacity,
bending stiffness and natural frequency, the choice for connections with higher slip modulus will lose relevance for
longer spans.
KEYWORDS: Xlam, composite structures, bending, deflection
1 INTRODUCTION 123
The development of cross-laminated timber (Xlam)
technology has opened up new perspectives for
numerous applications in architecture and structural
engineering, which used to be restricted to other
construction materials. Multi-storey buildings for
residential or office purposes can be nowadays erected
using timber as the main structural component.
Several characteristics of the system, namely, the
thermal performance, seismic behaviour and speed of
erection, have increased its interest among designers.
Meanwhile, some problems persist and request improved
solutions to overcome. In this framework, serviceability
performance on big spans, related to deflection and
vibration, are addressed in the present text.
Serviceability problems in timber floors are well known
since they generally control design on medium to longspans. To overcome those problems, the increase of mass
and stiffness are the most evident possibilities and
consequently the use of a composite system adding a
concrete layer above the timber floor is already a well
known possibility [5].
The composite section with concrete, increases also the
thermal mass, has better acoustic separation and is less
susceptible to vibration, taking the composite solution
with concrete something to consider.
The additional mass of the structural concrete layer will
reduce the natural frequency of the original timber floor.
Meanwhile, the connection stiffness has to be studied,
particularly if it is not stiff, which may lead to
unsatisfactorily dynamic performance.
2 MODELLING
2.1 THE STATIC MODEL AND CROSSSECTIONS
The analysed timber-concrete composite panels,
intended to perform in bending on floors and roofs, are
simply supported in two opposite sides and have been
used in simulations with spans from 4.0 to 12.0 meters
(at 1.0m step). Applied loads are vertical and uniform,
acting in the analysed width of 1.0m.
Timber-concrete composite cross-sections are presented
below in further chapters with detail. Three different
cross-laminated timber panels are used, combined with
also three distinct timber-concrete connections
(totalizing 9 configurations). In all configurations was
used the same 7.0cm concrete layer thickness. The
elastic properties assigned for components were 12.0GPa
for Xlam panels and 33.0GPa for concrete.
1
Luís Jorge, School of Technology, Polytechnic Institute of
Castelo Branco, Avenida do Empresário, 6000-767 Castelo
Branco, Portugal. Email: [email protected]
2
Johannes Habenbacher, KLH Massivholz GmbH, 8842
Katsch / Mur 202, Austria. Email: [email protected]
3
Bruno Dujic, CBD d.o.o. - Contemporary Building Design,
Lopata 19 g, SI-3000 Celje, Slovenia. E-mail:
[email protected]
2.2 THE COMPOSITE BEHAVIOUR
The composite theory will be used to model the
behaviour of the cross-laminated timber elements, based
on the shear deformation of transversal timber layers
[6][7][10]. Adding the concrete layer, slip between
timber and concrete has also to be considered, with
directly dependence on the connection used. A linear
elastic finite element model implemented in the SAP
2000® software package [3] enables the appropriate
analysis to parameterize the influence of the cross
section components.
If the acoustic impact sound insulation is needed, it can
be also considered a gap between concrete and timber.
These insulation materials have no structural strength or
even stiffness, but have to be accounted in modelling
through the gap between timber and concrete layers.
This kind of connection has not been used in the work
presented here.
For long time it is known the feasibility of the simplified
model proposed on the Annex B from the Eurocode 5 –
Part 1.1, meanwhile the FEM presented here will enable
to further developments, which are not in the scope of
the simplified model (e.g. point loads, continuous beams,
non constant spacing and non-linear behaviour for the
timber-concrete connection).
As it is well known, Xlam panels exhibit composite
behaviour due to rolling shear from intermediate layers
(those with transverse boards) . The cross-laminated
timber panels used in simulations are presented in
Figures 1 to 3.
same concrete thickness in independence of the Xlam
panel thickness. By the same reason, using different
concrete strength class or thickness will establish
different quantitative results. The same care should be
taken regarding Xlam panels. The panels’ characteristics
used here were taken from technical specification of the
Austrian producer KLH® Massivholz GmbH [8]. Other
producers will have different values for geometrical
characteristics and mechanical properties, leading to
small differences in result values. Nevertheless,
qualitative trends and conclusions given here from
simulations keep their accuracy.
2.3 TIMBER-CONCRETE CONNECTIONS
For simulations, three connections systems were chosen,
based on their different characteristics – screws, notches
and grooves. All connections are drawn in Figures 4 to 6.
The main features of the screws (e.g. SFS Intec selfdrilling screw VB-48-7,5x100) are related to their ductile
behaviour, simple application and the capacity of
coexistence with sound insulation between the timber
plate and concrete layer. Figure 4 presents the pairs of
screws placed at ±45º, which means that, according to
the model presented in Eurocode 5 – Part 2, one is
carrying tension and the other compression forces. Other
layouts are possible (vertical position or both in same
direction at 45º), but differences in load-slip diagrams
should be expected (load capacity, slip modulus and
ductility).
Figure 1: Cross-section of the 146mm Xlam plate [8]
Figure 2: Cross-section of the 230mm Xlam plate [8]
Figure 3: Cross-section of the 320mm Xlam plate [8]
Comparison analysis between different timber-concrete
panels can not be done since simulations are using the
Figure 4: Screw type timber-concrete connection (type I)
On the other hand, the notch and groove connections
configurations are stiffer but generally don’t exhibit
much ductile behaviour. Their fabrication could be done
with advantage in production line and delivering at
construction site ready for use.
The notch connection (Figure 5) should be fabricated
with dense sawn wood or wood based product (above
900kg/m3) in order to avoid, as much as possible,
deformation in contact area with concrete due to shear.
Square pieces of 10.0x10.0x1.5cm3 are enough to
achieve the value of slip modulus presented in Table 1.
The use of continuous notch will disregard composite
behaviour in two directions, unlike the square piece
which could perform identically (especially, if it has
equal properties in both directions, e.g. densified veener
wood).
Special attention must be given to the gluing process if it
is done at construction site.
Table 1: Slip modulus for timber-concrete connection
Connection
Screw [12]
Notch [4]
Groove [9]
ks (kN/mm)
29,2
304,8
500,0
ks (kN/mm/m)
146,0
1524,0
500,0
2.4 FINITE ELEMENT MODEL
Figure 5: Notch type timber-concrete connection (type II)
For the groove connection (Figure 6), indentation could
be done all along the panel width. Discrete grooves will
perform distinctly in each direction, related to the
direction to the grain (perpendicular to the grain vs.
parallel to the grain). Even in the direction parallel to the
grain, mechanical properties of groove connection would
be worse than notch connection due to lower wood
density of the boards used in Xlam panels.
For both notch and groove configurations, a few
mechanical fasteners should be placed in order to carry
uplift force components. Eurocode 5 – Part 2, establish
for this tension strength a minimum of 10% of the shear
force [1].
Figure 6: Groove type timber-concrete connection (type
III)
Timber-concrete connections used in these simulations
have their slip modulus presented in Table 1. The values
for the screw and notch connections are regarding to
each pair of screws or one notch. For simulations, were
considered 5 in each row, as presented in the right
column of Table 1.
The numerical model is intended to analyse the system
in bending, thereby, special care should be attended to
slipping between layers (one timber layer per each
longitudinal board or group of consecutive boards and
one layer representing the concrete layer). The concrete
is the upper element, which is mainly conducting
compression stresses. Timber panel is placed downward
concrete in order to carry tension stresses and its linked
to concrete across special intend connections accounting
for shear. Due to the connection effectiveness, in
practice, these composite structures in bending cannot
avoid slip between components. This means that the real
behaviour of timber-concrete composite structures are
characterized by partial interaction.
The timber and concrete components were modelled as
simple beam-type finite elements and placed parallel to
each other’, equals to the distance between its centroids.
The beam element is modelled as a straight line
connecting two points (nodes), with 3 degrees of
freedom available by node in order to guarantee stability
in solution (one out-of-plane rotation and two in-plane
translations) and is only able to carry linear-elastic
behaviour.
The vertical connection between layers is modelled with
‘link’ elements [3], connecting two existing nodes and
allowing just one deformation between them, while the
others were restrained (imposing the same curvature in
both layers and avoiding vertical discontinuity). The
available deformational degree of freedom enables the
uniaxial elastic pattern of load-slip behaviour,
representing the timber-concrete connection or the
rolling shear deformation in transversal boards.
3 RESULTS OF DEFLECTION
Results from simulations are presented in Figures 7 to 9.
Labels “TC type I”, “TC type II”, “TC type III” means
the use of, respectively, screws, notch and groove
connection. On the vertical axis of the diagrams it is
presented the relation between effective bending
stiffness of the cross-laminated panel, EIXlam, and the
effective bending stiffness of the timber-concrete
composite panel, EITC. This relation means the gain of
stiffness obtained by adding the concrete layer over the
Xlam panel.
Both diagrams tend asymptotically to a value that
represents the perfect interaction behaviour (no slip
between layers), achieved by those heights of Xlam
panel (boards thickness and their arrangements) and
concrete.
3.0
2.5
EITC / EIXLAM
Few conclusions arise from the diagrams:
i) As expected, the structural timber-concrete
connection strongly increases the stiffness of the
cross-section, with values between around 1.75 and
4.7 times;
ii) The bending stiffness of the panel enhance with
increasing the span as well as the slip modulus of
the connection used;
iii) The advantage in bending stiffness by using each
connection system decreases with the increasing of
the span.
2.0
1.5
1.0
5.0
4
6
8
4.5
EITC / EIXLAM
10
12
Span [m]
TC type I
4.0
TC type II
TC type III
Figure 9: Deflection analysis results for 320mm Xlam
panel
3.5
3.0
4 RESULTS FOR VIBRATION
2.5
4
6
8
10
12
Span [m]
TC type I
TC type II
TC type III
Figure 7: Deflection analysis results with 146mm Xlam
panel
3.5
EITC / EIXLAM
3.0
The natural frequency, damping ratio and mode shapes
characterise the dynamic performance of the floor and
should be evaluated regarding to their use. Induced
resonant vibrations caused by human walking, machine
or traffic should be designed and assessed by different
methods, which has also to consider the perception of
vibrations by persons and their feeling of annoyance
[2][5][11].
However, the evaluation reported here is only based on
the natural frequency assessment for each configuration
presented above. The natural frequency was determined
according to the Expression 1 [1][2].
2.5
f =
2.0
1.5
4
6
8
10
12
Span [m]
TC type I
TC type II
TC type III
Figure 8: Deflection analysis results for 230mm Xlam
panel
π 1
2 L2
EI
m
(1)
where L=span, EI= bending stiffness and m=mass.
The type of support is also known to affect mostly the
dynamic performance of the composite section but it is
not in evaluation, since only the simply supported static
model is considered here.
The mass was calculated from frequent combination
through permanent loading actualized in each
configuration – panel and concrete self weight.
On the vertical axis of the diagrams it is presented the
relation between natural frequency of the crosslaminated panel, fXLAM, and the natural frequency of the
timber-concrete composite panel, fTC. This relation
represents the increase of natural frequency obtained by
adding the concrete layer over the Xlam panel.
1.75
f TC/fXlam
1.50
1.25
1.00
4
6
8
10
12
Span [m]
TC type I
TC type II
TC type III
Figure 10: Modal analysis results for 146mm Xlam panel
1.75
f TC/fXlam
1.50
As for the vibrations, the composite system enables to
increase natural frequency for spans over 6 meters,
between 20 and 70%. Further, special attention should be
paid to damping evaluation, which has relevant influence
on perception of vibrations made by persons and their
feeling of annoyance [5].
The work presented here has simulated unidirectional
behaviour of the timber-concrete composite plates.
Nevertheless, bidirectional behaviour could also be
performed by the system, having in mind different
characteristics for the connection, namely, due to
direction of load to the grain or the load to the screw
axis.
The use of lightweight aggregate concrete could be the
opportunity to optimize the system performance, having
in mind the reducing of density in concrete as well as
better creep and shrinkage behaviour.
Future developments should evaluate the compatibility
and influence of the timber-concrete connection system
with the acoustic insulation needs.
REFERENCES
1.25
1.00
4
6
8
10
12
Span [m]
TC type I
TC type II
TC type III
Figure 11: Modal analysis results for 230mm Xlam panel
1.75
f TC/fXlam
1.50
1.25
1.00
4
6
8
10
12
Span [m]
TC type I
TC type II
TC type III
Figure 12: Modal analysis for results 320mm Xlam panel
Besides the obvious mass increasing (between 45% to
60%), the presence of concrete with structural
connection enables to move natural frequencies up to
70%. Comparing the performance of the system using
each connection scheme, it could be concluded a
maximum growth around 10% above the 6.0m spans.
5 CONCLUSIONS
The conclusions of this study indicate the reliability of
this system reducing the traditional serviceability
problems in design when increasing the spans.
[1] CEN: Eurocode 5: Design of timber structures. Part
2: Bridges. 2004.
[2] Clough, R., Penzien, J.: Dynamics of Structures (3rd
Ed.). Computers & Structures, Inc. 2003.
[3] CSI: SAP2000 – Integrated Software for Structural
Analysis &Design, Technical Reference Manual.
Computer & Structures, Inc., 2007.
[4] Dias, A., Kuilen, J., Cruz, H., Lopes, S.: Densified
Veneer Wood for Notched Joints in Timber
Concrete Composite Structures. Proceedings of the
9th World Conference in Timber Engineering.
Portland, USA. 2006.
[5] European Commission: Generalisation of criteria for
floor vibrations for industrial, office, residential and
public building and gymnastic halls. RFCS Report
EUR 21972 EN. ISBN 92-79-01705-5. 2006.
[6] Gsell, D., Feltrin, G., Schubert, S., Steiger, R.:
Cross-Laminated Timber Plates: Evaluation and
Verification of Homogenized Elastic Properties.
Journal of Structural Engineering, Vol. 133, No. 1,
January 2007, pp. 132-138.
[7] Guggenberger, W., Moosbrugger, T.: Mechanics of
Cross-Laminated Timber Plates under Uniaxial
Bending. Proceedings of the 9th World Conference
in Timber Engineering. Portland, USA. 2006
[8] KLH: Engineering. KLH® Massivholz GmbH.
Austria. 2008.
[9] Kuhlmann, U., Michelfelder, B.: Grooves as shearconnectors in timber-concrete composite structures.
8th World Conference in Timber Engineering. Lahti,
Finland. 2004.
[10] Mestek, P., Kreuzinger, H., Winter, S.: Design of
Cross Laminated Timber (CLT). Proceedings of the
10th World Conference in Timber Engineering.
Miyazaki, Japan. 2008.
[11] Toratti, T., Talja, A.: Classification of human
induced floor vibrations. Proceedings of the 9th
World Conference in Timber Engineering. Portland,
USA. 2006
[12] Van der Linden, M.: Timber-concrete composite
structures. Ph.D. Thesis. Delft University of
Technology, Delft, 1999.