By Cymon Allfrey
As an architectural practice, we often have people talking to us about changes in structural requirements to buildings.
People want to be building something ‘earthquake proof’. However, while we can design for better performance, nothing is certain.
What we do need to remember is that damage to the point in which buildings and homes are lost, is about economics, not about total catastrophic loss of building. Despite some recent evidence to the contrary, this is rare.
The Building Act in essence is focussed around ensuring people are safe. Should the worst occur, people could be safely evacuated from the building and that in the event of an emergency, the structure of the building is safe enough for emergency services to do their job.
But that level of strength is only a fraction of what is required for a building to meet full structural code. With its focus on people safety, the Act itself doesn’t consider the condition of the building, just that it will remain structurally sound.
Post-earthquakes we have seen some changes to the building design, but in many cases they are more pragmatic than embedded in law. The minimum Building Act standards deal with lives, while as a design team it falls within our scope of works, and that of the engineer, to find a solution which would see the building repaired, rather than demolished.
As a consequence of this, we have seen practical alternatives to problematic solutions emerge – such as the use of lightweight cladding materials over heavier alternatives.
The pragmatic approach to structural safety is best seen in the commercial arena where we are seeing fusible links included in buildings, which allow elements of the structure to attract and absorb load, and then be easily repaired or replaced if damaged.
Essentially, the building is being designed to fail in a controlled way. However, this is not a new concept and has been used in both large scale timber buildings’ and commercial steel structures well before the earthquakes, such as in the HSBC Club Tower building in central Christchurch (which was found to have only used a small percentage of its strength, ensuring it was easily repaired and back into operation).
Residentially, the majority of the strength of the home is found in the plasterboard lining which is anchored into the floor system to form sheer walls. The problem is differential ductility between the flexible timber frame and the rigid plasterboard, when given a shunt, the lining or the finish to the lining is scarified.
It is important to note that seismic hazard factors calculated within structural design have been increased in Canterbury since the earthquakes. Which means that a building which was 100 percent to code in 2010, is now 72 percent of current code.
However, this doesn’t mean these buildings are now unsafe. The increase in code is a turn of events based around knowledge of fault lines and seismic regularity. It is a guidance measure for new construction and repairs; it doesn’t mean the old buildings’ are going to fail.
There has been some discussion around repair work to commercial buildings’ only being undertaken to two thirds of the code. Essentially, two thirds of code is pretty much a hundred percent of code in 2010, and it has been widely accepted that this is an acceptable, yet informal, level to repair to.
When it comes to our existing homes, there is no requirement to strengthen them to a certain standard as this only applies to publicly occupied buildings. But we do need to start considering design technology in the residential market that allows our homes to perform, and then be easily repaired.
While there are new standards for new light timber frame construction that were cited after the quakes, its engineering and drafting precedes the quakes so would have come into play regardless. In essence, we are not required to do anything we weren’t previously. Homes built a year before the quakes are very similar to those being built today.
It is also interesting to note that across the city we have seen an eradication of bricks, due to its perception of weight. However the reason so many brick chimneys, walls and buildings failed in our recent seismic activity was because the structural system to restrain them either wasn’t sufficient or didn’t exist.
And while fashion maybe part of the reason this masonry material has fallen from grace, there is still a place for it in both residential and commercial design and we can accommodate them structurally.
As we rebuild the city, we need to ensure we ar rebuilding and designing with a focus on knowledge, technology and effective structural systems.