So, what is the best way to specify and procure glass? Where should responsibility lie? How can failures be minimised? BS 952 and the Glass and Glazing Federation recommendations deal with tried-and-tested uses of glass in domestic and standard window unit technology. However, when glass is used structurally, or as external or internal walls, roofs, floors or horizontal canopies, more care is required.
Chris Jofeh’s book Structural Use of Glass in Buildings deals with these technical issues, and section 3 is an invaluable aid to specification writers. But beware: it assumes that the designer/specifier always makes the design decisions specific to the contract. This is taken by many to mean that the designer/specifier should select glass types and configurations after considering all the relevant issues. Specialist consultants are only too willing to assist in this, but it can lead to mistakes.
Who specifies the glass product for any particular situation is a key issue. It begs the question of who is legally responsible for selecting the glass that is to be incorporated into the works. The glass specification has to reflect the form of the contract and the responsibilities agreed in the designer’s appointment. It is essential, therefore, that these key documents correctly stipulate design responsibility. This often does not happen, or, to be more specific, they put responsibility on the wrong shoulders.
Only two parties matter in determining the final glass specification: the designer and the glass manufacturer. Unfortunately, they rarely have direct contact with each other during the design or procurement phases, as they are separated by managers, contractors, subcontractors and forms of contract. This can lead to a gap between what is expected and what can be achieved, with factors such as cost, programme, profit margins and favoured suppliers becoming dominant influences. They are legitimate factors, but the overriding requirement should be the correct selection of glass, its fitness for purpose, safety, performance and appearance.
How does a specifier overcome these problems? Answer: by persuading the construction team and client to adopt a glass selection process that allows the designer and its advisers access to the glass industry while maintaining a competitive platform. Selection should be a progressive process that includes testing through the procurement process and the post-contract detailed design. The best way to do this is to use performance specifications leading to a selection process. This includes tenderers declaring their proposals followed by a full technical evaluation pre-contract and the provision of samples that are extensively viewed and tested with the manufacturer.
On projects incorporating large areas of glass, prototypes should be built as part of a small design contract with potential contractors. It can save enormous amounts of time and expense. This also allows the designer to ensure that visual requirements are achievable using the contractor’s proposed product. The wrong visual appearance of a major glass wall can ruin a building and decrease its value.
A good glass performance specification should include precise details relating to:
- Position: horizontal, vertical or sloping
- Construction: framed, structural silicone, pressure plate and so on
Only two parties really matter in determining the glass specification: the designer and the glass manufacturer
- Finish: appearance, coatings, reflectivity, colour, flatness, visual intent
- Performance: acoustic, environmental, fire, safety, structural, design life
- Responsibility: who provides the guarantees/warranties? The manufacturer should respond with the following before going into construction:
- Product: supplier’s name and glass reference
- Type: float, toughened, tempered and so on
- Construction: laminated, thickness, double glazed and so on
- Finish: type of coating
- Data: compliance with performance criteria
- Samples for visual checking
Nick Schumann is a partner in specification-writer and quantity surveyor Schumann-Smith. The company has a web site at www.schumannsmith.com.