A SPECIALIST FACADE ENGINEER CAN HELP RESOLVE DESIGN ISSUES
The specification of a glazed facade is by no means a simple task.
This article examines the complexities of specification, supply chain and technology in the innovative glazed facade of SMC Alsop's Palestra.
As the technology of buildings progresses, new materials, processes and products are continually being developed in the search to provide better performance. Suppliers constantly strive to improve the performance of a particular material or process. Many construction products have a long and fragmented supply chain.
For example, a raw material is processed to a bulk form, semifabricated and perhaps applied with some type of finish, and sent off for another fabrication or treatment process. It is then delivered to another product supplier (as a raw material to be combined with other similar but different raw materials) to be further fabricated or sub-assembled; supplied to another system supplier to be sub-assembled into his product; and, finally, installed on site as a component. The terms may vary with different product areas, but such a long sequence is not unusual.
The sealed double-glazed units used in the unitised curtain-wall system at Palestra exemplify a product with a particularly complicated supply chain, which incorporates toughened and/or laminated glass with a solar control or low-E coating on the glass. This is one of the most common ways of cladding modern glazed buildings and is widely used throughout the industry. For the architect, who is responsible for the design of the form and aesthetic concept of the building, it is becoming increasingly difficult to keep up to date with all the technical developments in specific product and material areas, especially those with long and complex supply chains.
This has led to three problems:
individual suppliers may develop processes which are not always compatible with all other possible processes either upstream or downstream from them in the supply chain, so it can sometimes be impossible to incorporate a particular required performanceenhancing characteristic;
the architect at the top of the design process, as the accepted primary specifier, may not have enough specialist technical knowledge or time to deal with the first problem; and - no single party in the fragmented supply chain has the breadth of design responsibility to control or manage the reconciliation of the conflicting design and performance issues, though one or other may be handed this responsibility. As a result a specification may be issued with a number of performance requirements, each perfectly reasonable in its own right, but impossible to satisfy in a single solution.
When this problem occurs with materials in the building envelope, as with the double-glazing example shown above, a specialist facade engineer can help by filling the gap between architect and supply chain and by bringing specialist technical knowledge to bear, developing the design with the architect so that it can be provided by the supply chain and meet the performance requirements.
Buro Happold Facade Engineers carried out this task for SMC Alsop's Palestra (see Building Study, pages 21-33). The glazing, of which there is a high proportion on all four facades, had to satisfy Part L Building Regulations, which meant high levels of control of energy loss in winter and of solar gain in summer. Parts of the glazing also had to provide good acoustic insulation; and as all the glazing was full-height, it had to provide full restraint against barrier loading to prevent falls through the glazing for uniformly distributed loads, line loads and point loads.
The first problem was to decide how to reconcile the need for a reasonable number of solid panels with high insulation, necessary to meet Part L2 insulation requirements, with complex arrangements of colours.
Three combination solutions were examined:
body-tinted glass in front of vertical bands of coloured aluminium perforated mesh panels for the glazed areas, and similar mesh panels in front of white metal-faced insulated spandrel panels for the non-vision areas. Although this met aesthetic requirements, it was difficult to access the inside face of the glass for cleaning;
full-height body-tinted glass for the vision glazing, with separate glazed lookalike insulated spandrel panels with full ceramic coating on their glass outer panels. This required an external joint between the panels at the junction of the colours, which was not the preferred aesthetic; and glass panels, fritted on part of their surface for the vision glazing, and similar panels in front of solid insulation for the non-vision spandrel panels. This gave the preferred seamless joint detail, but is an expensive approach because the rate for fritting the glass is applied to the whole panel, not just the fritted area.
While these options were being evaluated, attention was focused on the actual fritting process. The architect wanted a dual-colour frit, with different colours visible on each side of the glass. As frit is a fired-on ceramic, this meant that two differentcoloured (liquid) ceramic solutions, one over the other, had to be applied to the same surface and exactly aligned using the silk-screen printing process. Normally a dotted pattern is used, but it was very difficult to align the two colour screens, so the pattern had to be changed to a linear one. Various experiments were made with different colours. Because the frit is not actually opaque, one colour can inuence the appearance of the other. The frit also affects the shading factor of the glass, which has some effect on solar gain and compliance with Part L2. (The amount of framing also has an effect on the overall insulation U-value for Part L2. ) When the relationship of glazing with solid panels, frit colours and patterns had been agreed, the next step was to distribute all the processes in the supply chain among the various panes of glass and their available surfaces. This is often the most complicated part of the process, with numerous constraints on the design. Faces of the glass panes are numbered one to four from the outside inwards. The frit should be on face two, but so should the solar control coating for faces exposed to solar gain. The low-E coating used to prevent heat loss is not required where a solar-control coating is used, but it is required on north-facing elevations, where it has to be placed on face three in order to achieve maximum effect.
The safety containment balustrade loading and glazing safety codes could have been satisfied by using toughened or laminated glass, as both comply with the required standard. But because of the stoving process for the frit on face two, monolithic heat-strengthened glass was used for the outer pane. For cost reasons a solar control film applied in bulk on the production line was used; a limited choice - and hence performance - to hard-coat films. This film would be applied first and frit colours applied over it. Soft-coat films are less durable before they are assembled into the inside of double glazed units, so might be prone to damage and/or discolouration during the processes of fritting and handling.
The image (above left) demonstrates a solution for the glass coatings. The inner pane of glass had to comply with lowlevel glazing safety requirements and accept the low E coating on face three. To provide the high acoustic insulation required in certain situations, a special acoustic laminate film was used - the most economical way to provide acoustic insulation. All the inner panes were laminated, some with special acoustic laminate films, but this was still not enough to achieve the required acoustic performance and so one of the laminate panes was increased in thickness to provide additional mass. This had the additional benefit that with two panes of dissimilar thickness there is a further gain in performance; the resonance that occurs with similar pane thicknesses is avoided.
The image (above right) shows a solution for the acousticcontrol areas. Although these solutions are fairly conventional, the complexity of the design process described above shows that many possibilities need to be addressed. The aforementioned description is very much simplified and reduced; there are a number of other factors, such as thermal shock on the glass, which affect the final choice of materials and processes that have not been covered at all.
Equally critical, the final choice of colours and glazing make-up was also influenced by visual appearance.
The case study demonstrates the technical complexity of a modern construction material, and shows the need to employ technical specialist skills to resolve all the design issues and provide a viable solution that can be delivered by the supply chain from within their standard processes. The provision of facade glazing is clearly not a simple task.