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COMBINATIONS – INNOVATIVE BUILDING PRODUCTS WITH ADDED VALUE
of this work is to carry out three research projects and a one aimed
at disseminating the results, to establish an appropriate network, to
employ two PhD students, to hold three theme/seminar meetings, and to
produce at least one new prototype of a combined glass and wood building
can read the first report and about the background
to the project here.
You can read the second report here.
You can read the third report here.
You can read the fourth report here.
You can read the fifth report here.
Prototype 1 -
a glass and timber beam
describing evaluation of the results of testing combination timber/glass
beams has been published, entitled "Timber/Glass
Adhesively Bonded I-beams" (pdf)
by Louise Blyberg and Erik Serrano of LinnaeusUniversity in Växjö.
The results show that glass and timber beams have good residual load-bearing
properties after the first crack has occurred. The load could be increased
by about a further 140 % after the first crack appeared in the glass.
In comparison with what is known as a lightweight beam (an I-beam with
wooden flanges and a web of sheet material, such as the Masonite Beam),
the glass/timber beams that were tested exhibited about the same characteristic
load-carrying capacity (~ 5-10 % higher), but with higher stiffness
(~ 40 %). As far as load-carrying capacity is concerned, it is the measured
maximum load on the beam that is of interest. If the load-carrying capacity
is regarded as being the load on the beam when the first crack occurs,
then the conditions change. It should be noted that, in most cases,
it is the stiffness that determines the load-carrying capacity when
the beam is used in a floor structure.
the beams report: the pattern of cracks during a load test.
- a glass and timber pillar
The glass and timber pillar that had been made was compression-tested.
It was loaded to about 390 kN (39 tonnes), which was the maximum that
the test equipment could deliver, without any failure occurring. A small
crack occurred in one support, but did not affect the pillar's load-carrying
capacity. Buckling failure load is estimated as being about 5-10 times
higher than this load. No further tests of the pillar are planned.
Prototype 3 - a wall element of glass and timber
and last prototype of the project is a wall element of glass and timber,
with a load-bearing glass element. The intention is that it should be
suitable for incorporation in a façade wall in a new building,
or be able to replace existing wall elements in order to admit light.
It should also be able to support a continuous load, such as from a
floor/ceiling structure. It must provide the dual functions of being
load-bearing and helping to save energy.
The element size
is 1200 x 2400 mm: its section can be likened to that of an enlarged
I-beam (c.f. the glass and timber beam), with a 10 mm load-bearing sheet
of glass in the middle.
In order to help
to save energy, the element should incorporate at least three layers
of glass, i.e. the "I-beam" design, complemented with additional
sheets of glass on each side. This will enable us to include functions,
such as sunshading, on and between the sheets of glass. The distance
between the panes is limited to a maximum of 10 cm in order to avoid
The flanges and the sides are made from timber, which simplifies joining
the elements to other parts of the structure. Detailed design of the
flanges depends on a number of technical problems, which must be solved.
The aim to avoid thermal bridges affects the design of the flanges and
the joint between the glass and the timber.
timber surfaces of the support on which the element rests can still
cause spot loads due to the structure of the timber. There should be
a ventilating layer on the outside in order to avoid condensation on
Both we, and production
companies whose opinion we sought, feel that the prototype provides
potential for creating good living conditions in terms of safety, indoor
environment and energy use. A prototype element will be manufactured
in the spring.
drawings have been prepared for use in procurement.
The following parameters, which are important in connection with
energy calculations have been identified: the distance between the sheets
of glass, the number of panes, optical coatings on the glass, the ability
to incorporate a sealed glazing unit, and the need for ventilation in
order to avoid condensation.
Parameters that can be varied.
For the purpose of calculating heat flows through thermal bridges in
glass constructions, we have noted that two-dimensional calculations
are adequate. Coefficients of thermal conductivity (U), and the effect
of thermal bridges, have been calculated for the prototype using the
U-Norm program and the Heat 2 program. Optical coatings and distances
between panes have been varied, and both programs have given similar
results: Heat 2, however, also permits calculation of the effects of
such items as screws etc.: see the diagram below. Screws conduct heat
and act as thermal bridges.
Heat flow through the
prototype, with screws and aluminium mounts around the edge. The blue
side is the exterior, at a temperature of 0 °C, and the red side
the interior, at a temperature of +20 °C.
When calculating the design prior to construction of the prototype (see
the table below), we found that the distance between the panes is important,
and that at least one pane of low-energy-transmittance glass (i.e. optically
coated glass) must be used. The use of more coatings does not produce
the same marked improvement. Two panes are not sufficient: three are
needed if the U-value is to be below 1 W/m2K.
U-value calculations for a prototype with two or three panes and different
between panes, mm
||1 energy saving
||2 energy saving
||1 energy saving
The U-Norm program
was used for calculating the U-value of the prototype in the format
of a sealed glazing unit, a load-bearing 10 mm sheet of glass and an
exterior glass pane. With a sealed glazing unit with a U-value of 1,0
W/m2K and two 20 mm airgaps, the application of a soft energy
conservation coating on the inside of the load-bearing glass sheet reduced
the U-value by more than half. Without the coating, the U-value was
1,41 W/m2K, but fell to 0,57 W/m2K with the coating.
The width of the
airgaps does not have as great an effect as does the coating. Application
of a sunshading coating on the inside of the outermost pane and the
use of an optimised sealed glazing unit with a U-value of 0,8 W/m2K
can reduce the overall U-value somewhat to 0,52 W/m2K for
a 60 mm airgap width.
in a single-storey house
Various performance results for the prototype have been used as inputs
in the U-Norm program, and the results used as input data for Isover
Energi. The calculations have been made using energy data from the SVEBY
project (Standardise and Verify Energy performance for Buildings).
energy use have been made for a typical passive house, giving it between
five and ten prototype wall elements and no windows. Five prototype
panels are equivalent to 10 % of the building's floor area, and ten
panels to 21 % . The general recommendations for passive houses are
that the window area should be equal to about 15 % of the floor area.
The lowest value
of specific energy use is obtained when using eight or nine prototype
elements, depending on the element's performance. However, the variations
are not particularly large, and the highest value occurs with the use
of five elements. This indicates that the glazed area can be increased
somewhat and give an improved energy balance, with solar input meaning
that less energy is required for heating.
An important question is that of how to prevent excessive indoor temperatures
when using the prototype. 'Parasol' is a program that specifically allows
for the effects of different types of sunshading, and we shall start
to use it in the autumn.
The effect of
temperature gradients on stress
Finite element analysis (FE) has been used to investigate how stresses
in the surface of the glass are affected by various temperature gradients
resulting from exposure to direct sunlight. This has been investigated
for various insolation intensities and shadow effects on three different
glass sizes. The results show that the size of the glass has some, but
not particularly large, effect on thermal stresses. The extent and distribution
of the stresses induced in the glass is determined mainly by how the
glass is secured.
An important part
of the modelling work has been to optimise the distribution and type
of the analysis elements in order to obtain a relevant and acceptable
result, while at the same time keeping calculation times reasonable.
Present work is concentrated on modelling a glass and timber structure.
prototype wall element
Test arrangements have been planned for horizontal and vertical loading,
with simultaneous shear loading.
As the wall element is a completely new product, and considerably larger
than an ordinary window, there are not many organisations in Sweden
with sufficiently large test facilities to perform U-value tests. We
plan to perform the tests at Lund Institute of Technology, as SP's equipment
is not large enough.
For the glass part, it is interesting to investigate how stresses in
the glass are affected by the loads in a load-bearing structure. Emmaboda
Glass, a company in the reference group, demonstrated its Grazing Angle
Surface Polarimeter (GASP), which is used for measuring surface stresses.
The disadvantage of this equipment is that it is uncertain as to whether
it can be used for measurements in the vertical direction, which could
be a problem for the planned tests. Glass Stress in Estonia manufactures
an alternative stress measurement device that could be of interest.
Small test pieces
of glass and timber
Testing the adhesive joint
Adhesive Bonds - Experimental testing and evaluation methods"
by Louise Blyberg, Erik Serrano, Bertil Enquist and Magdalena Sterley,
of the Linnaeus University in Växjö, presents the results
of an investigation of three very different adhesives: silicone, acrylate
and polyurethane. Of them, the acrylate adhesive, SikaFast 5215, was
the best in terms of both tensile strength and shear strength: 3,0 MPa
in tension and 4,5 MPa in shear. Although the ability to distribute
stresses evenly across the joint is an important property when adhesively
bonding glass, the flexible silicone adhesive had insufficient rigidity
and strength for use in load-bearing components where the component
must carry more than its own weight.
A total of 35 tests of small test pieces have been carried out: 15 at
60 % relative humidity (RH), 10 at 85 % RH and 10 at 98 % RH). The results
will be evaluated after all the tests have been completed.
- Conclude the humidity tests on the small test pieces.
- Manufacture the wall element prototype.
- Run the Parasol sunshading program.
- Decide on a suitable stress measurement device.
- Perform energy, load and stress tests.