Glass – we love it – it lets light into buildings enhancing well-being. It allows views out so occupants can relate to their surroundings and it helps to form part of the building envelope to provide a safe and comfortable environment. It all sounds rather perfect! However, so often the visual outcome is not what we are expecting as the glass we specify does not provide the effect we want – so we hate it! I have learned through experience that glass has certain limitations and that what is specified does not always give us the result we seek.
The following are a few examples of projects that have enabled me to have a better understanding of these limitations and through this understanding has informed me how to specify glass more appropriately for particular applications.
The visual myth!
As an architect and façade consultant, I understand the aspiration to develop the best solution for the project brief and that this needs to be translated into a design concept. Commonly this is visually presented using computer generated images. It is not often, however, that the concept image is so representative of the finished article. A presentation I made at the CWCT Members Meeting in March 2013, titled ‘Architectural Glass, Challenges Specifying Visual Quality of Glass’ illustrated this using the following example of a project which whitbybird façade engineers were appointed on when I joined them. This very transparent façade was represented in the design images. At this stage it is unlikely the requirements for the solar control of the glass or the reflectivity of the glazing were known and how this could impact the transparency of the façade.
What designers and the client need to appreciate is that although they may wish the glass to be completely transparent as shown by the computer rendering, the façade must still meet certain performances that will influence its appearance. For example, coatings applied to the glass to reduce solar gain can cause the glass to appear dark and reflective. If no other means of solar shading is used, then solar control performance coatings are needed to ensure the building is comfortable for the occupiers and that the façade design does not contribute to an excessive cooling load.
Renderings are a great tool though and this example is over 15 years old. Since then modelling techniques have advanced and now glass suppliers can also use these models and apply the visual attributes for different glass build ups and coatings to better inform the client and design team of the potential final visual. It is also important to get initial samples of the proposed glass and view these in natural daylight conditions.
It is sensible to source from different suppliers as the products differ in ‘colour’ depending on the base material and types of coatings they offer.
When the design progresses and a preferred sample(s) are chosen, then it is important to order project specific samples with the glass type – annealed / heat treated, correct pane thicknesses including any laminated panes and the correct coating on the face required. This project specific glass sample should also be incorporated into the visual mock-up. The
mock up needs to be viewed in natural lighting conditions and the sample must be kept until completion of the project.
Safety glass – we love what it is does but sometimes hate the way it looks!
In the last 10 years I have seen a rise in the specification of laminated safety glass. With this build-up particularly when the inner and outer pane of a double-glazed unit (DGU) specification has been heat treated and laminated, there have been some quite disturbing visual attributes.
The next example needed safety glass for the inner and outer pane of the DGU. Internally, the glass needed to be safety glass as the DGU was on an incline and so the internal pane was treated as overhead. Additionally, due to the possibility of thermal stress breakage this inner glass was also heat strengthened. Externally the glass was also proposed as safety glass as it was subject to possible BMU impact and again laminated heat strengthened glass was proposed. It was after the glass was installed that the distortions in the glass became apparent. The images in figures 3 to 8 are excerpts from my dissertation which I completed for the MSc in Façade Engineering at the University of Bath in 2015.
Following investigation of the installation, it was suggested that the glass was experiencing multi-layer lensing effects. That is to say that the panes of glass when laminated together were creating lenses as the roller waves in each pane did not correspond.
This phenomenon can be really disturbing to the occupants and for this project some of the glass was replaced. For the replacement glass, the specification was agreed and updated to reduce the allowable roller wave and each glass pane was reviewed at the factory in an inclined view so that it could be seen in its proposed final position.
A key lesson learned from this is if a specification requires laminated heat-treated glass, then it is essential to review the roller wave specification. The value of 0.3mm in the British Standards for Heat Strengthened and Fully Tempered glass can be tightened. As a base for our specification we require 0.15mm and this can be tightened further to 0.08mm. This does come with some limitations however as not all glass manufacturers and processors will agree to this and it comes at a cost premium. In addition, it is also subject to the glass thicknesses being proposed as panes less than 5mm are more difficult to heat treat and more susceptible to roller wave.
There are several steps that can be taken to mitigate the risk of disagreements for installed glass. A key is to agree the visual acceptance criteria for the glass at tender stage. Distances for internal and external viewing and angles at which the glass are to be inspected must be agreed. If this is not done until the glass installation, then any misunderstandings of the specification are unlikely to be rectified for the installed glass, as this will inevitably affect the programme. Ensure that full size project specific samples are included in the visual mock-up and ensure these samples are kept as a record. Again, technology is advancing and there are certain processes being developed to improve the quality for tempering glass that eradicate roller waves such as tempering glass on an air bed.
Certain projects during our careers enthuse us. A key project I worked on whilst at Ramboll did exactly that and inspired my dissertation for the Façade Engineering MSc, ‘The consequences of panelisation on visual inconsistency of curved glazed façades’.
The project was never built due to the economic crisis in 2008. However, this project gave me insight into the pitfalls of specifying curved glass. The project was designed by Zaha Hadid Architects and the colour of the glass was to be like ‘sand dunes at sunset’ as requested by the client at a meeting I attended.
The first challenge was to define the panelisation. There was little repetition and the project would require both hot and cold bent glass. The hot bent would need to be both slump formed and roller formed. The cold bent could use both forced bent and laminated. All these different bending methods have different visual attributes. The façade was proposed as a double skin. The outer skin was to be single glazed to create the complex form and the inner skin double glazed forming the weather and thermal line with an accessible ventilated cavity between.
Having finally tracked down 2 glass samples that in certain conditions resembled ‘a sand dune at sunset’, the samples were subjected to curving using the slump formed hot bending method. The glass had a hard coating and the samples were tested with the coating side down and coating side up on the mould. The tests gave some quite dramatic results and showed how the colour was affected by the hot slumping process as seen in figures 11 to 14.
For my dissertation I looked at other examples of curved glass and the effects of the curving on the coatings. The example in figure 15 was sent by one of my colleagues at Meinhardt Façade Technology (MFT). They had been requested to review colour inconsistency between curved and flat glass. In this case, the façade contractor had suggested different low e coatings for the curved glass and flat glass, stating that post bending both would match. Unfortunately, that was not the case.
The lessons learned from both these projects is that sampling and testing are vital for every project. So often samples and testing are omitted at tender stage due to cost savings. This inevitably leads to subsequent disputes on site.
My next project is an example of how precise the glass industry is when a challenging specification is adopted. The project is well known and seen on the news daily. It was the BBC Broadcasting House extension and was the first major project I was involved in as a façade consultant at whitbybird engineers.
The glass screen to the cyclorama of the building was composed of laminated glass with 2 different frit patterns. This was to give a chequered effect to the glass. The glass panels, many of which exceeded 2m x 2m were all laminated by hand. The pattern was set to very tight tolerances of +/- 1mm and if these were exceeded then the overall effect was distorted.
The team at whitbybird had to inspect every panel very carefully, although it was very easy to see when the tolerances had been exceeded and some panels had to be replaced.
What was remarkable and credit to the manufacturer was that very few panels did not meet the tight criteria. However, the tight tolerance in the specification limited the number of suppliers able to tender the project.
These are some of the experiences that have made me think more carefully about glass specification. If there were particular aspects to be considered regarding the specification I would recommend the following:
Be pragmatic and clear in your specification. Asking for the impossible will add cost and limit the suppliers you can go to. However, this may be appropriate for very high-end and unique projects.
Always request samples and for unusual specification specify a mock up. Do not discard the approved samples before construction is completed.
Agree acceptance criteria for the glass at the tender stage. In particular viewing distances internally and externally and the viewing angles allowed. Carry out factory inspections and ensure benchmarking is established. Only inspect the glass when it is clean.
Glass is only glass – so specify it in a way that will not give you surprises when it is finally installed!