06-10-2016, 09:55 AM
Structural and thermal behaviour of a timber-concrete prefabricated composite wall system
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Abstract
Wood is the oldest building materials and still now it plays an important role in the construction sector. There are
many general advantages in using timber for building purposes. First of all, it is an environmentally friendly, easily
recyclable material; it has a low weight in relation to strength, which is advantageous for transport, handling and
production; moreover wood has aesthetic qualities, which give great possibilities in architectural design. Lastly
wooden structures have an excellent performance in case of earthquake if compared to traditional structures. In Europe
the development of the timber-concrete composite structures (TCC) began during a shortage of steel for reinforcement
in concrete in the beginning of XX century. TCC application was primarily a refurbishment technique for old historical
buildings, during the last 50 years interest in TCC systems has increased, resulting in the construction also of new
buildings. This paper presents the analysis of the structural and thermal behaviour of an timber-concrete prefabricated
composite wall system, the Concrete Glulam Framed Panel (CGFP) which is a panel made of a concrete slab and a
structural glulam frame. The research analyses the structural performance with quasi-static in-plane tests, focused on
the in-plane strength and stiffness of individual panels, and the thermal behaviour of the system with steady state tests
using an hot box apparatus. The results validate the efficacy of proposed system ensuring the resistance and the
dissipative structural behaviour through the hierarchy response characterized by the wood frame, the braced reinforced
concrete panel of the singular module and by the rocking effects of global system. On the other side hot-box measures
demonstrated a high level of thermal resistance of the system reaching U-values around 0,20 W m-2 K-1. Moreover
experimental data permitted to calibrate a FEM model with which will be possible to study and analyse the panels in
different conditions and configuration in both mechanical and thermal field.
Introduction
In Europe wood has been used as construction material mainly coupled with other traditional materials such as
brickwork or stone. The usage of wooden structural elements in order to improve the seismic resistance of masonry
buildings has been a practice widespread as consequence of disastrous earthquakes that destroyed buildings made with
traditional constructive systems [1]. Timber-concrete composite structures (TCC) were developed in the first decades
of 1900 and a system of nails and steel braces aimed to connect concrete slab and timber beams was patented by Muller
(1922). TCC application was primarily a refurbishment technique for old historical buildings, during the last 50 years
interest in TCC systems has increased, resulting in the construction of bridges, upgrading of existing timber floors,
and the construction of new buildings. [2]. Recently the development of this kind of systems takes always more spaces
in the scientific literature with the introduction of new materials and new inter-layers connections. In this terms, [3]
and [4] presented studies and analysis of properties of timber-concrete composite systems, with different types of shear
connectors.
Pozza et al. [5] presented a wide investigation of constructive system which mixes a typical platform frame with
three thin external reinforced concrete boards acting together as a diaphragm against the horizontal forces. The
construction system presented here in addition considers the sustainability throughout all their life cycle: from
conception and construction to decommissioning. Keeping in mind that the management of building will greatly affect
its impact on the environment, the paper approach is aimed at defining, at the same time, the structural and the thermal
behavior in the conviction that these two aspects have to be considered together in the development of a sustainable
building system.
2. Description of the CGFP specimens
In order to accurately characterize the thermal and structural properties of concrete-wood composite systems a
framed panel (Figure.1a) was analyzed. It was composed essentially of two parts, a slab of reinforced concrete (RC)
with a thickness of 50 mm connected with special connectors, integrated to the armature, at the timber frame of spruce
homogeneous glulam with resistance class GL24h. This frame is formed by two posts of 80mm x 320mm section and
by a crosspiece of 300mm depth, same width of the panel and variable height according to the slab resulted. In order
to allow a wide range of architectural solutions the system provides certain types of standard panels. All the panels
have basic characteristics like depth, type of concrete slab and section of the timber frame which are the same for all.
The differences are on the geometric characteristics as widely explained in Boscato et al [6]. From inside to outside
there are: two plasterboard sheet, an air gap, the insulation layer (the standard is made of polystyrene foam with
graphite), a ventilated air gap, the reinforced concrete slab and the external smoothing with the colored cement
finishing. A building made of CGF (Concrete Glulam Framed) panels for load-bearing walls and floors is a modular
system where, according to architectural and structural requirements, all the panels are prefabricated. For each panel
an innovative type of connection enables the manufacture of the reinforced concrete slab, with a specially designed
mesh, separately from the laminated wood frame. The individual panels are then assembled providing insulation inside
the frames and then are easily transported to the site thanks to its small size. In the ground, after having set up a
foundation curb, the panels are hooked to it and to each other with nails and screws. Once all the modules are
assembled the construction ends with the plant, doors, windows and interior and exterior finishes.
3. Experimental characterization of CGF panels
For the reasons above, the research program proceeded keeping in parallel the structural and the thermal point of
view. It can be summarized in three main phases: the first phase was the laboratory experimentation, the second one
was the analysis of the results and the related FEM modeling and calibration, and the last one was the numerical
analysis of aspects that have not been tested experimentally. The experimental step, concerning the structural aspects,
started with tests in order to measure the strength and stiffness in the plane of wall panels in various combinations
according to the UNI EN 594:2011 [7]. In the thermal field, it was tested the behavior of the system, determining the
steady state thermal transmittance with an hot box apparatus.
3.1. Mechanical Experimental analysis.
Mechanical characterization of the panels started with a Quasi-Static Ramp Tests test of in-plane stiffness according
to UNI EN 594:2011 [6]. They were performed on panels with geometric characteristics as it is shown in Figure 1b.
An horizontal load was applied in the plane at the top of the panel by an hydraulic actuator fixed, in turn, to a contrast
structure wall [7]. At the same time, a vertical load was also applied at the top of the panel by an hydraulic jack, fixed
to another contrast frame, in order to simulate the load resulting from an upper floor (Figure 3). Three tests were
performed on a single panel “Type A” (test 1, 2, 3). Figure 3 presents the relationship between the force applied by
the actuator and the displacement of the top of the panel. This shift was measured by a wire and displacement
transducers, [7]. The results show a good agreement between the structural response of the three tests. The curves
confirm the linear behavior until the ultimate load (circa 22kN) of CGFP panels; a dissipative capacity was recorded
by Test 2 with bi-linear response. In detail, after the first failure load achieved together Test 1 (around 17kN), the
Test 2 guarantees a strength capacity with deformable behavior (between 60 to 95mm) up to the collapse load. The
failure mechanisms involve the link at the base, with local damage, and the bracing role of RC panel with flexural
behavior followed by in plane shear collapse.
3.2. Thermal Experimental analysis.
Concerning to the control and limitation of energy consumption, the building system has been subjected to
experimental tests carried out using an hot-box with reference to the standard UNI EN ISO 8990 [8]. In the tests
performed, it has been considered the standard solution that involves the use of polystyrene foam as insulation. In
particular, according to this solution two different samples were tested. In the first configuration was analyzed the
system without the concrete slab and the ventilated air gap outside the package, while in the second configuration was
measured the transmittance of the complete package, considering the air gap outside as a non-ventilated air gap (see
Figure 2). For each samples we performed two tests: the results are presented below. In the first configuration the two
tests performed the air temperature difference of the two wall sides was of 29,6°C and the transmittance value resultant
were respectively 0,20 W m-2 K-1 and 0,19 W m-2 K-1, therefore the results can be considered converged to the average
value of 0.195 W m-2 K-1. In the second configuration the air temperature difference of the two sides of the wall was
of 29,4°C and the transmittance value resultant was 0,19 W m-2 K-1. As we expected the concrete slab has not a great
relevance in term of thermal resistance, therefore the values measured were approximately the same of the first
configuration.
4. Numerical simulation
After the experimental characterization of the panel a FEM modeling phase was carried obtaining a numerical
model of the system. The model was tuned on the basis of the collected data so that its static and thermal behavior
reflects the values measured experimentally. In relation to the structural outlook, the calibration was performed taking
into account the load applied and the displacement at the top of the panel; otherwise, concerning the thermal prospect,
the correct material properties were set up and finally the experimental result was verified. The numerical analysis
was carried out applying the Strand 7 code [9]. In the structural standpoint, the FEM models obtained will be useful
in other researches in order to compare, for example, this system with others or to verify the performance of the system
resisting earthquake action. In the thermal point of view, we could model the main building interface between
technological units and we could investigate, verify and quantify the presence of thermal bridges. The phases of
numerical modeling carried out are the following: creation of the numeric model of the individual panels and of the
experimental mechanical configurations; set up of special connectors between the parties and their calibration
according to the experimental results.
5. Concluding remarks
This paper presents the outcomes of an experimental and numerical research project on the Concrete Glulam
Framed Panel system (CGFP). The constructive system combines, with innovative dry connections, a slab of
reinforced concrete to a frame of laminated wood. Panels that are mainly designed for the construction of residential
buildings aimed at improving the quality construction, maintaining the link between economic sustainability and
environmental sustainability. Through this research the first considerations can be drawn: 1) the structural
performance results show that the assembling phase not implies imperfections and different response of CGFP panels;
2) in seismic field the dissipative capacity, that could be improved by complex configuration through the rocking
response between different panels assembled together, offers encouraging results, 3) the results show the agreement
between experimental and numerical approach confirming the simplicity of the proposed system thus reducing the
variables at play. The U-value appears very low around 0,20 W m-2 K-1 in both analyzed configurations, and also the
thermal bridges characterizing the use of this construction systems show to have limited dispersion effects compared
to similar situation in traditional construction systems.
The completely dry prefabrication in small panels of this new construction system goes to meet different needs in
terms of environmental, economic and logistics sustainability. The possibility of building the concrete slab separately
from the frame of glulam allows great flexibility and speed in terms of both production and transport.