By Sean O’Sullivan
The New Zealand Building Code requires structural elements of a building, with only normal maintenance, to satisfy the performance requirement of the Code for the lesser of the specified intended life of the building or the life of the building be not less than 50 years.
Given that most buildings do not have a specified life, it always appears to me to a somewhat curious and undefined statement. However, we can take some solace in that most buildings are required to last at least 50 years with normal maintenance.
Mild steel is a major structural element in many buildings and to be durable must resist corrosion, which depends very much on environmental factors and protective systems used. In a dry internal environment very little, if any protection is required to steel structures for them to last at least 50 years. However, in New Zealand external environments the adequacy of the protective coatings is paramount.
Protective coatings are not going to last 50 years and therefore the steel structure elements must be maintainable and for that maintenance to be carried out access to the structures is required. However, in many instances the design of structures makes reasonable and complete access to all faces near impossible and well beyond what could be expected as part of normal maintenance. The key to most coatings used in external situations is surface preparation prior to recoating. I believe that when designing building structures, Engineers need to take into account how the structure is to be maintained.
One of the worst examples is the use of galvanised metal tray decking as the structural component to concrete floor slabs in the external environment. While I am yet to see corrosion of decking due to unwashed surfaces, which is a common enough problem with galvanised roofing and cladding, I have on a number of occasions seen failure of metal decking where it is used to suspend slabs of swimming pool surrounds. I do realise that metal decking may also be used as permanent formwork rather than a structural component. However, given the spans I have seen, I suspect, that metal tray has been used as a structural component. The Engineer may have expected a waterproofing membrane to prevent water entering through onto the metal decking, but to rely on the integrity of waterproofing membrane as sole protection to a structural component is folly. Unfortunately, the corrosion failures under a swimming pool surround can go largely unnoticed and collapse of a suspended floor is a possibility.
We have not seen the extent of the problems because they will only become apparent with time. The 10 year longstop provision within the Building Act makes it unlikely that legal action will occur. However, this does not take away from the requirement to meet the Code.
To remedy poor durability in structural design is going to prove difficult and expensive in the future.
In the world there is often the catch cry ‘What are we leaving for future generations?’ The answer for a number of our buildings is – a very serious problem.