13.04 Design and specification
Open full view...Categories: Durability
Introduction
Buildings have traditionally been built with the initial cost of construction as a major influence over the design but with little thought to the durability and maintenance costs. The increasing costs of maintenance of buildings resulting from poor durability has lead to greater interest in durability and life cycle costing. Appropriate design and specification can reduce the whole life maintenance costs. This Section describes some of the issues to be considered.
Specification
Specification is an important aspect of design, and poor specification can lead to future problems. There are two aspects to specification - the selection of a particular aspect of a component (for example the grade of material) and the way the designer describes his intention so that the contractor or manufacturer provides what the designer intended. Both aspects can clearly affect the durability of the resulting structure.
Specification can either be too specific, or not specific enough. For example, specifying the general use of 304 grade stainless steel for the fixings in a cladding system might lead to premature corrosion of the fixings if the cladding is then installed in a marine environment, where a Molybdenum-containing stainless steel should have been used; this is a case of over-specification. However, saying that the fixings should be ‘stainless steel’ would be under-specification.
Ease/frequency of component replacement
The ease with which components can be replaced is a design issue - components which are inaccessible may lead to damage to adjacent components during attempts at repair; the problem is accentuated if replacement is frequent.
Exposure
The performance of materials and components is dependent on the environmental conditions to which they are exposed. These depend on the prevailing external and internal conditions but can be modified by design. For example, external exposure may be varied by overhangs and recesses whilst levels of insulation may affect temperatures experienced and risk of condensation. Decisions affecting exposure may be made with the aim of improving durability but where the decision is made for other reasons, for example appearance of the structure, the effects on durability need to be considered. Whilst providing shelter will normally be beneficial there may be situations where the resulting lack of rain washing is detrimental.
Workmanship
Workmanship is an obvious cause of premature failure - a good example is the failure to properly clean a joint before applying sealant, leading to poor adhesion and subsequent failure. The exposed nature of a typical sealant joint will increase the risk of failure but will not necessarily aid removal and replacement.
Supervision
Supervision plays an important role in improving durability, and can limit the effects of poor workmanship. However, the supervisor can also reduce durability by failing to understand the demands of particular materials and components - a little knowledge can be very dangerous.
Protection
Protecting components during installation is essential, particularly where finished surfaces are exposed, but the protection can also cause damage if left in place too long. It is important that trades understand the effect that they can have on already installed components, for example the damage that can be caused to glass by weld spatter.