Recently I’ve had a few conversations with people about the so-called “New Building Standard”, which is part of the suite of ideas intended to make buildings in NZ “safe”. By now it’s pretty well percolated through the world at large that 33%NBS is “Earthquake Prone” and that the nominal minimum target being touted by engineers is 67%NBS. Probably what’s less widely mentioned is the “star rating” system which some engineers are proposing as a replacement. However, %NBS or Stars out of 5, both essentially amalgamate all aspects of a building’s performance into a single convenient number.
As with all simplifications, that has its merits and its problems. The concept is basically that a low number represents an unsuitable building and a high number represents a suitable building. But there is a lot of confusion about both ends of the spectrum, and some surprising results when you start to pull apart the terms and see what they mean. I worry that in the slavish adherence to producing this number in the way specified in the building act, that some buildings are miss-classified both as being dangerous and as being safe.
The basis of the 33%NBS as spelt out in the act is that it is an earthquake with the same duration, but one third the magnitude of a design-level earthquake. In terms of engineering design however, we don’t actually directly calculate the effects of shaking – that’s all aggregated into a set of streamlined “response spectra” for different soil types and different building fundamental periods.
The trouble with these as a basis for design is that the earthquake has no idea what it’s aggregated response is supposed to be, it just shakes. In the recent Canterbury earthquakes for example, the effective ground acceleration for long-period (i.e. tall) buildings was something like 2.5 times that predicted by the code. Suddenly your 100%NBS building is now responding like a 40%NBS building because the loading is higher than expected.
In order to cope with this variability, engineers use a concept called “capacity design” which you could think of in simplistic terms like an electrical fuse. If you put more electricity than the fuse likes into the system, the fuse blows and protects all of the rest of the electrical components. Well, when a larger-than-anticipated earthquake arrives, capacity design ensures that those weakest elements yield and prevent load transferring through the rest of the building. This means that, broadly, the performance of the building is similar whatever the level of the earthquake is once it’s larger than the fuse elements allow.
This design approach is relatively new. It first began to be applied in the 1960s, but in detailed terms, engineers are still figuring out how to apply this concept to irregular buildings, and it is often difficult to achieve this paradigm in seismic retrofits because by definition all of your existing structure is already weaker than you would ideally want your fuse to be in a new building. Unfortunately, the way that the Act defines the earthquake prone terminology does not address this fundamental design paradigm at all, because the definition of the level of shaking is the determinant, rather than the performance of the building.
I have posted at length in the past about brick and how it is problematic in seismic zones. The main problem with brick is that it has poor post-elastic performance. That means that it goes from being essentially intact to being essentially a pile of bricks. Whereas a steel frame has a long section in between these two modes where it is deformed but still has some ability to carry load.
Let’s take a particularly well-built brick building as an example. At shaking of one-half of the nominal design level, our building remains completely intact, showing no damage. Therefore the building is not “earthquake prone”. However, at a 100% (or, indeed 51%) design earthquake it will suffer a total collapse. That means that while it is not “earthquake prone” it is far from “safe” because the earthquake can be of any magnitude, including many times higher than a “design quake”. Here the criterion as it stands over-estimates the building performance.
If we look at a Steel framed building with a similar level of strength, we will find that at 50% earthquake, it will show signs of damage, probably mostly aesthetic. At 100% it will show significant damage, but is very unlikely to have collapsed and so while it has the same nominal %NBS, it is a far more desirable building. Here the criterion as it stands under-estimates the building performance.
There are a number of other potential problems with the system, depending on how you interpret “failure”. For example, the codes specify nominal drift limits in order to prevent second-order events, but a strong-but-flexible structure can often exceed these limits relatively safely.
The system we have is far better than nothing. It’s basically good at identifying risky structures – those with less than 33%NBS. However, the situation for partially-compliant buildings is far more complex and subject to all kinds of limitations. In effect, a screening tool is now being used as a classification tool, not what it was intended for at all. This is something that the non-engineering community at large is going to need to come to terms with: that simply slapping a number on something is not adequate. And I think that as this post demonstrates, the concepts involved are not all that complex if someone takes the time to break them down at a non-technical level.