With the recent events in Canterbury and Invercargill it looks like the building standards in NZ will again come under close scrutiny.
The suitability of our standards is a valid question and response, especially when our knowledge has recently been increased by the Earthquake and the collapse of the Invercargill sports stadium roof.
The first design standards for earthquake loadings on buildings were introduced in 1935 following the 1931 Napier earthquake. Since then, significant advances in the required design standards have been made with major changes incorporated in 1965 and 1976.
Even more recently the structural design standard NZS 4203:1992 was replaced with NZS 1170 in 2002, with part 3 and part 5 added in 2003 and 2004. However NZS1170 has only been mandatory in the last couple of years, with engineers able to use either 4203 or 1170 up until then.
Certainly the requirements in 1170 are far greater than 4203 and this has increased building costs by quite a bit as additional structure is required to resist wind, snow and earthquake loads.
As a practical example of the changes over the years, we were involved in refurbishing a reinforced concrete church built 50 years ago. The Structural Engineer doing the assessment was impressed that the building had been well overdesigned for when it was built. Possibly at 1.5 to 2 times the earthquake strength required for its day.
Despite this the building still only came up to 65% of the current requirements in NZS 1170. However this building is still not deemed to be an earthquake risk, which is set 33% or less of the current standards.
On this basis, the structure required to resist earthquake loads has increased about 4 fold in the last 50 years.
Similarly when you look at the requirements for wind and snow loads, some clients have been surprised at what is now being required. We have designed tramping huts to replace some old ones built in the 50s and 60s. Some of these are in high alpine areas with significant loads being imposed. (See the photo below, where there is only 500mm of the roof ridge sticking above the snow, the rest of the hut is completely under snow).
When you look at the existing hut, which has survived storms and the harsh environment for 50 odd years and is still sound, and you see how little timber was used to hold these things together, you wonder how the old buildings survived. Especially compared to the new buildings where there is significant bracing, and timber structure required.
Unfortunately the Stadium Southland failure is a stark reminder of what can happen. I suspect that it will be some time before the full details of what went wrong will be known, and it will be a combination of factors the contributed to the failure.
With snow it is a combination of factors as to how the load gets imposed. Roof shape and slope, the shape or geometry of the structure, the strength of the connections, the types of materials used, the amount of snow, and how long it is there for, all contribute to how well the building structure as a whole performs. It may be that the building was designed to the previous standard, which was deemed suitable 5 years ago, but as we have seen, what was acceptable years ago is now no longer.
Thanks to nature, we now have some real live examples to test the theory and assumptions against. Our building standards will continue to evolve and change as our knowledge improves, and as we have more ’learning experiences’. However it does occur to me that no matter what we do we can never be 100% future proofed against nature.
You can build to resist the shaking induced by the ground acceleration of an earthquake, but you can’t build to resist the ground moving by even 300mm. As we have seen in Canterbury, if the ground doesn’t just shake but permanently shifts up, down, or sideways, it will tear your building apart.