Project manager: Elisabeth Flygt
Project status: on-going

GLASS-WOOD COMBINATIONS – INNOVATIVE BUILDING PRODUCTS WITH ADDED VALUE

The target 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 construction component.


You 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
A report 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.

From the beams report: the pattern of cracks during a load test.

Prototype 2 - 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
The third 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 excessive convection.

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.

Apparently flat 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 the glass.

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.

Manufacturing drawings have been prepared for use in procurement.

Energy calculations
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.

Thermal bridges
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.

Calculation of U-values
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 optical coatings.

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.

Use 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).

Calculations of 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.

Indoor climate
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.

Testing the prototype wall element
Load tests
Test arrangements have been planned for horizontal and vertical loading, with simultaneous shear loading.

Energy tests
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.

Stress testing
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
The "Timber/Glass Adhesive Bonds - Experimental testing and evaluation methods" report (pdf) 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.

Moisture testing
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.

Continued work
- 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.