Wood-Glass Composites
Science and engineering of glass and natural stone
- Micro-Project HS15 –
Selma Lehmann, 18.12.2015
Content
1. Introduction ............................................................................................................... 3
1.1. Aims and opportunities ....................................................................................... 3
1.2. A composite material .......................................................................................... 5
The glue ................................................................................................................ 5
Static system ......................................................................................................... 6
Basic principle ....................................................................................................... 7
2. Wood-glass composite element designed by Austrian Cooperative Research .......... 8
2.1. Advantages ........................................................................................................ 9
2.2. Disadvantages.................................................................................................... 9
3. Wood-glass composite element designed by TU Wien ........................................... 10
3.1. Advantages ...................................................................................................... 10
3.2. Disadvantages.................................................................................................. 10
4. Joined research project BUWAL EPFL ................................................................... 11
4.1. Advantages ...................................................................................................... 12
4.2. Disadvantages.................................................................................................. 12
Conclusions ................................................................................................................ 13
References ................................................................................................................. 14
2
1. Introduction
1.1. Aims and opportunities
Wood-Glass composites aim to combine the advantage and properties of glass with
those of a wooden construction. Wooden constructions are often composed by frames,
which especially in behalf of stiffness can be problematic. On the opposite, stiffness in
addition to compressive strength is the main feature of glass. Therefore, it can be
interesting to combine wood and glass to wood-glass composites, especially for
horizontal stiffening in a wooden construction.
Although several researches about wood-glass composites as load bearing structures
have been conducted, there is still a lot of skepticism about the use of these elements
in practice. Glass being a brittle material, it has the reputation that it can fail without
warning and that it is therefore very dangerous.
But there are many advantages to take into account the contribution of the glass to the
horizontal stiffening of the structure [1]:
•
Big transparent surfaces are needed for using passive energy in the form of solar
radiation. If the load bearing capacities of the glass are considered, much bigger
glass surfaces can be designed. By that, more important gains in passive energy
can be achieved and less fossil energy sources are necessary.
•
Also in a concern of sustainable use of resources, the amount of construction
material has to be minimized. If the contribution of glass to a structure is
considered, less wood is needed to guaranty the load bearing capacity and
requirements for the service state.
•
Finally, many architects would like to build more and more transparent and lighttransmissive structures. As mentioned in the first point, this can also be facilitated
if the contribution of glass to the load bearing structure is considered.
3
Figure 1: ECOLAR home (HTWG Konstanz), Solar Decathlon 2012 [4]
If architects, engineers and clients want to use these benefits, both materials must be
connected to work together.
Although glass has already been used a long time ago as a composite material with
steel structures, for example for green houses (Figure 2) or in many train stations [2],
the combination with wood is not yet very common and there is still some lack of
experience. Nevertheless, there are already some examples of practical application
(Figure 1).
Figure 2: Palm House, Bicton Gardens [3]
4
1.2. A composite material
The glue
The glass and the wood must be connected to form a composite material. The most
convenient way to achieve this is to glue them together continuously. In contrast to
most mechanical connections, which are punctual supports, stresses are transmitted
continuously. To obtain a powerful connection, elastic glue (for example silicon) has to
be used. This avoids stress concentrations which would be very dangerous, because
glass has no ability to redistribute stresses. If there are such stress concentrations,
local damage which is already present in the glass is very likely to cause the failure of
the element. Especially the edges are often the starting point of a crack that leads to
failure.
Another advantage of elastic joints is that they allow deformation. Wood and Glass
react very differently to changes in humidity and moisture. If relative displacements
were avoided by a stiff connection, stresses would be induced.
For glass, relaxation and creep are no issues in civil engineering applications, whereas
for wood these are quite important parameter to consider. Also for this reason, relative
displacements must be possible.
The main problem today is the fatigue of the glue joint. But for many types of glue, the
long-term behavior is not well known yet.
Possible elastic glues would be silicon or acrylate glues. In [1] is shown that silicon has
a higher resistance to humidity changes and a better long term behavior. There is also
a lot of experience with this glue from façade and window systems.
Other important parameters for these systems are thermal insulation, water tightness
and wind insulation. In addition to the standards that have to be met, it is important to
consider condensation, because wood is very sensitive to humidity changes and
degrades rapidly at high moisture content due to growth of fungi or decomposition.
5
Static system
To use the full capacity of the different materials, it is important to use them in the way
they behave best. The wood in grain direction has good properties in tension, whereas
the glass has high compressive strength. As a consequence, the wood should be
stressed in tension and the glass in compression. It is important to avoid tensile
stresses in the glass, because the fracture in this case is very brittle and the resistance
very low.
The changes in dimension with humidity and temperature variation of the wood are
more important than of the glass. If they are connected, stresses might be induced by
the differential displacements. Such stresses must be avoided or considered in the
static calculations.
Possible applications are the following products [5]:
•
beams with a glass web:
wood takes normal stresses, glass takes shear stresses
•
T-beam elements:
wood in tension, glass in compression
•
shear walls:
compression diagonals in the glass, tension diagonals in the wood
There are different design methods for Wood-Glass Composites [5]:
•
Composite theory (Verbundtheorie)
•
Framework analogy (Fachwerkanalogie)
•
Analytical model : spring model
•
Numerical model: finite elements
The post-failure behavior of wood-glass composites is highly dependent on the type of
the used glass. Laminated glass for example has a residual strength. In float glass,
there is an early warning system, because the formation of cracks can be observed
before the failure takes place. Single-pane safety glass (ESG) fails without warning and
has no residual strength. [5]
6
Basic principle
The general setup of the wood-glass composite systems is the following:
The glass is glued all along the edge to a connecting frame. The material of this
connecting frame must have a similar thermal expansion coefficient to the one of glass.
Its thermal conductivity must be low and its durability high. It should not be sensitive to
humidity changes, because it should not be source of degradation in case of humidity
ingress or condensation. The glue joint is made in the glass works to guaranty standard
conditions and a constant quality. [6]
The frame is then fixed on the wooden frame on the construction site.
glue joint
glass panel
wooden frame
connecting frame
Figure 3: Wood-glass composite (Holz-Glas-Verbundkonstruktion: HGV) basic principle [7]
7
2. Wood-glass composite element designed by Austrian
Cooperative Research
The element designed by Austrian Cooperative Research is made in accordance with
the basic principle explained in the previous chapter. There are different variations with
changing material for the connecting frame.
The first one which was developed had a connecting frame of glued laminated birch
wood. But these frames had a low durability in case of humidity ingress because of
condensation or water ingress through joints. This is why a second system with a glass
fiber reinforced polymer connecting frame was developed. This material has a similar
heat expansion coefficient, small heat conductivity and a low sensitivity to humidity.
The glass is glued to the connecting frame with one silicon joint as shown in the figure
below (Figure 4, left). The connecting frame is then fixed on the structural wooden
frame by screws. Adjacent elements can be fixed on a relatively narrow frame, thanks
to the “wave-structure” of the frames (Figure 5).
water tight joint
glass panel
connecting frame
screw
wooden frame
Figure 4: fixation of the glass on the frame, shear connection (left) [7]; fixation of two adjacent
elements (right) [8]
8
connecting frame (with
« wave structure »)
glass panel
Figure 5: interlocking of the “wavestructure” of two adjacent elements
[14]
2.1. Advantages
The advantage of this system is a big glass surface on the façade. Only a small part of
the wood is visible. It is possible to fix two adjacent elements very close because the
“wave-structure” of the frames can interlock (Figure 5).
Displacements of the glass are almost not restraint; the transmission of the stresses
from the frame in the glass is very uniform on the whole edge of the glass.
If a wooden frame is used (birch glued laminated timber), the environmental impact is
quite low.
The panel with the small frame can be fixed to the structural wooden frame directly on
the construction site with conventional mechanical connections.
2.2. Disadvantages
Horizontal stresses from the frame are transmitted in the glass by shear only, although
glass is more efficient in compression.
It is important to fill the space between the elements with a water tight filling to assure
that the system is water tight.
If the frame is made of aluminum or another metal, the environmental impact is much
higher. Also the price might go up.
9
3. Wood-glass composite element designed by TU Wien
This wood-glass composite system based on the one developed by the Austrian
Cooperative Research which is presented above (2. Wood-glass composite element
designed by Austrian Cooperative Research) tries to provide improvements. The
difference is that the frame is L-shaped and that there are small block settings
(Verklotzungen) close to the edges (Figure 6). It is important to make sure, that only
compressive stresses and no tensile stresses are transmitted by these elements. If a
small wooden element is used, this is automatically respected. If a liquid block setting
(Flüssigklotz) is used, a separating film can be applied, to avoid adhesion to the glass.
1) glass panels
2)
2) connecting frame
4)
1)
3) glue joint
2)
5)
4) block setting
2)/5)
4)
5) wooden frame
1)
1)
3)
4)
2)
3)
5)
Figure 6: Wood-glass composite element designed by TU Wien with L-frame (left [7], right [9])
3.1. Advantages
The advantage of the L-shaped frame is that a block setting can be added and the
transmission of stresses is thus improved. The element can thus take higher horizontal
forces, because it is stressed in compression only.
If the frame is made of wood, the whole system has a low environmental impact.
3.2. Disadvantages
The wood or joint surface in the façade might be slightly bigger and though the glass
surface might be a little bit smaller. This means that the frame is more visible and a
little bit less light can enter the building. The L-frame must be small enough to but
several elements next to each other and fix them on the same wood structure. The
introduction of stresses is less uniform, because the block setting is not continuous.
10
4. Joined research project BUWAL EPFL
Figure 7: Joined research project BUWAL EPFL, gluing detail [10]
The system used in this project can is illustrated by Figure 7 above. It is composed by
a wooden frame in two parts. The glass panel is first glued to one of them and then, the
second one is added on the other side of the glass panel. A small wood piece is used
to keep the distance between the two parts constant and to guaranty a uniform glue
joint. A space is left between the edge of the glass panel and the frame to avoid stress
because of restrained deformations.
The elements can be integrated in a building by using anchors (Figure 8).
Figure 8: gluing of the first part of the frame (left), fixation of anchors (right) [10]
11
4.1. Advantages
The glue surface is very big, because both sides of the glass are glued. This means
that bigger forces can be transmitted and the distribution of the stress is more uniform.
No additional frame is needed. The frame only can be linked to the rest of the structure
by using the anchors.
4.2. Disadvantages
The surface of the frame is quite large with respect to the glass surface and if two
panels are next to each other, the frame size doubles. Thus less light can enter the
building.
Places for anchors have to be prepared and their placement has to be done in the
glass works during the gluing process. This means that it has to be known in advance
where exactly they have to be placed.
The panels have to be transported to the construction site with the wooden frame and
the anchors. This takes more space than the thin frames from chapter 2 and 3.
12
Conclusions
Wood-glass composites are very powerful elements that help to make a building more
transparent thanks to a very slender visible load bearing structure.
The most challenging part is the link between the two materials, glass and wood. It is
important to transmit only compression stresses in the glass without any stress
concentrations and to allow relative displacements of the elements.
There are different possibilities to achieve this. The most effective way seems to be the
use of a small frame glued to the glass that allows the fixation of the panels to the
wooden frame on the construction site.
The “wave structure” of the element designed by Austrian Cooperative Research is
very interesting to avoid big frames and still assure a continuous load transmission.
The block setting improves the system and increases the capacity of the elements to
support horizontal loads. The difference to a system without block settings is that it
introduces compressive stresses in the glass instead of shear.
A big glue surface is desirable, because it increases the distribution of stress and helps
to prevent stress concentrations.
Gluing the glass directly to the wooden frame is less favorable, because it means that
the panels are more difficult to transport and to integrate in the structure. It probably
also increases the size of the frames and though reduces the glass surface.
13
References
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Weiterentwicklung und Herstellung von Holz-Glas-Verbundkonstruktionen durch
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[Cited: November 16, 2015.]
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