Rust Never Sleeps – Undue Risk in Multi-Storey Buildings

December 16, 2013

Sean By Sean O’Sullivan MNZIBSRegistered Building Surveyor, BRANZ Accredited Adviser, Corporate Representative for Prendos on the Australasian Corrosion Association
While surveying a multi-storey building recently, I was surprised to find bare steel brackets used to support large exterior pre-caste concrete panels. On checking with the design engineer I was told this is common practice. The rationale was that in a dry environment – even without a protective coating – the brackets will not rust to a degree that will compromise their integrity. This, I was told, meets the 50 year durability requirement for structure contained in Clause B2 ? Durability of the New Zealand Building Code.Unfortunately I believe the engineer is wrong. I am not confident those external brackets can be guaranteed to remain in a dry state. If water or dampness reaches the bracket corrosion will occur at a rapid rate. Given our marine environment this corrosion could be even more pronounced than average.

This particular building – and others like it – rely on sealant between panels for water tightness. Sealant has a life much less than 50 years and should be replaced between three and five times during that period. It is most likely that sealant replacement will only occur after significant leaks and on going attempts at repair, not in the course of normal preventative maintenance.

Furthermore, this type of construction creates a cavity between the back of the pre-caste panel and the apartment walls. Consequently, leaks are likely to go unnoticed – they can drain away without causing obvious damage, but will still be affecting the brackets.

Most importantly, because the brackets are hidden from view in the wall cavity, they cannot be inspected or maintained. On close inspection, these brackets were already starting to corrode.

Unfortunately the failure of these brackets is likely to occur suddenly and without warning during an event such as severe storm or earthquake. The images of catastrophic failures of buildings that are seen on television could occur in New Zealand. Maybe, our construction standards are not as high as we would like to think.

If we are to avoid the possibility of large pre-caste slabs crashing down onto the streets below, action is needed. As a minimum, I believe that brackets in these types of buildings need to have planned early replacements. Their integrity and durability cannot be guaranteed over the expected life of the building.

Given that prevention is better than cure, this mistake should not be repeated in other new buildings. In Britain, brackets for a similar purpose are manufactured from silicone bronze not mild steel. I believe we should be following this practice.

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