CWCT FACETS

13.01 Design life

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Categories: Durability

Introduction
Design life is generally considered to be the life for which the designer/specifier anticipates that the building will remain economically serviceable.  However, this definition is rather simplistic.  It is now common to talk of life to first maintenance or refurbishment.  Furthermore design life can be taken in the context of the whole building or the design life of a component.

Whatever the definition of design life adopted it should be appropriate for the building and its cladding.  This may range from 5 or 10 years for a temporary structure to 200 years for a prestigous building.  It should be remembered that some so called 'permanent' buildings have a short life span. This is particularly the case for industrial buildings but may also apply to things like transport infrastructure.  For these buildings it will be necessary to consider flexibility of use when considering the life required of the building.

The design life selected for the facade of a building will govern the materials that may be used and in some cases the forms of construction.

For general purpose offices designed to be let, and for many other buildings, architects work to a whole building design life of 60 years.
 


Design life
Design life for the whole building will depend on the design life of individual components and the ability to repair or replace components.  BS 7543 gives guidance on the selection of component design lifes based on consequences of failure.

The economic service life of a component will depend on:

The design life is not the same as warranted life.  Warranted life is generally the lesser of:

  • The period during which the manufacturer anticipates no failure
  • The length of warranty generally available on similar products

Materials and products can generally be expected to remain in a serviceable condition long after any warranties have expired.  Of greater concern is the life expectancy of components.  Furthermore achieving a long average life, measured across all similar components on a building, is of little value if there are sporadic early failures requiring expensive access to repair a few components at a time.
 


Maintenance
Frequency and quality of maintenance play a large part in determining the acieved life of a component or whole facade.  The most important aspect is timely maintenance.

Maintenance may be part of a cleaning programme that occurs at regular intervals.  As a minimum it is good practice to use any cleaning operation as an opportunity to inspect the condition of the facade and its components.  The practice of only undertaking maintenance when faults manifest themselves can be a false economy as remote components may be badly damaged before a fault is observed.
 


Refurbishment
Refurbisment may comprise:

Replacement of components
Components such as window hinges, insulated glazing units and sealant joints may fail to function whilst the other components of the facade still have a long service-life ahead of them.  In the case of masonry walls it may be necessary to replace entire windows after some period. These components may be replaced with similar units to extend the life of the facade.  It should normally be anticipated that some facade components will have to be replaced during the service life of the building.

Components may also be replaced because the appearance of the building has deteriorated although the facade still performs technically.  This will generally be undertaken to replace:

  • Faded panels and frames
  • Stained components
  • Delamination or discoloration of films and interlayers
Enhancement of performance
Refurbishment gives the opportunity to radically improve the performance of the facade.  It may also be undertaken because, although the facade has not lost its original performance, it no longer meets current standards of  comfort, energy use or appearance.  An obvious example is the replacement of single glazing with insulated glazing units.

Facades may be refurbished as part of a general refurbishment of the whole building. This is most likely to be driven by a desire to update the services, heating and lighting within the building.
 


Whole life strategy
At the design and specification stage of any building a strategy should be developed for whole life performance, maintenance, repair, refurbishment and replacement.  An example of good practice in this respect are the requirements of the CWCT Standard for Curtain Walling.  This requires that:

  • The performance criteria of the facade are satisfied for the design life of the facade provided that maintenance is undertaken as specified by the supplier.
  • The design life of the facade need not be equal to that of the building as a whole.  It is acceptable to anticipate replacing the facade to add value to the building.  This typically happens at the end of a lease period.  In the absence of any better guidance the design life of a curtain wall should be assumed to be thirty years. Note that high quality walls have demonstrated design lives well in excess of thirty years.
  • Components should be designated as primary components that require no maintenance, repair or replacement during the design life of the facade or secondary components.
  • Primary components should include:
    • Framing components and fixings
    • Panels and their fixings or fasteners
    • Thermal insulation and vapour barriers
    • Flashings, gutterings, copings and similar metal weathering elements
    • Sealants which are concealed and cannot be inspected without dismantling the wall
  • Secondary components are all components having a service life less than that of the facade as a whole
  • Seondary components should be capable of easy replacement without compromising the integrity of the wall
  • Secondary components may be:
    • Internal linings
    • Exposed finishes to metal components
    • Window and door equipment
    • Glazing
    • Gaskets and compression seals
    • Sealants

Similar strategies can be developed for other forms of facade construction.