The recent events in Christchurch have got me thinking about earthquake proof materials.
Since the mid 70’s New Zealand buildings have been designed for earthquake resistance with stringent building rules and some great technology advances over the last 20 years.
Unfortunately there is no such thing as an “earthquake proof material” to use for building. However, it is known that glass, bricks and concrete, the materials that we use the most, are actually very bad materials in earthquakes. This is because these types of materials are very weak when forces such as pulling (tensile forces) and sideways pushing (shear forces) are applied to them. When an earthquake hits a building it causes it to be pushed and pulled in lots of different directions, which is very different than the usual compressive force that a building experiences when things are normal.
We still build out of bricks and concrete but help to reinforce the building by adding steel rods which add strength to the building when it is exposed to forces other than compression.
So if we can’t make earthquake proof materials, can we make earthquake proof buildings?
It’s very common to build an earthquake resistant building by building it on an isolating base which puts the whole building on top of springs. A great example of this in New Zealand is the Te Papa building in Wellington which would probably be one of the safest places to be in an earthquake.
When it was being built the ground site was stabilised by dropping 30 tonne weights on the ground 50,000 times to create a hard, strong and well packed base. The building was also fitted with shock absorbers made of rubber which let the building move up to half a metre in any direction during earthquakes.
These shock absorbers are knows as “base isolation” and they transfer very little force to the building from the ground if its shaking. This means that the building will experience a lot less force from the earthquake and hopefully prevent it from collapsing.
Another type of building technology is to use energy dissipation devices which are usually diagonal braces that absorb the energy from the shake. They work like shock absorbers on your car or your mountain bike and are commonly either viscous dampers or friction dampers.
With viscous dampers, the energy is absorbed by the viscous (thick and honey like) fluid which passes through the piston/cylinder.
With friction dampers, the energy is absorbed from the friction of the two surfaces rubbing against each other.
The picture below shows a cartoon of how viscous dampers can be placed in a building. If a building is exposed to sudden jerks from an earthquake, a lot of the energy is absorbed by the viscous fluid and not transferred to the building itself. This “damps’ the motion of the building, meaning it will move less in an earthquake and be less likely to collapse.
Cartoon image: Left – schematic of how viscous dampers can be placed diagonally through a building to absorb some of the energy of an earthquake. Right – schematic of a close up of a damper consisting of a piston with a viscous fluid which damps the energy and minimises the motion within the building.
Why don’t we do this will all buildings in New Zealand?
The problem with putting in these extra earthquake prevention schemes is that they add about 5% to the total building cost. When you consider that Te Papa cost $300 million New Zealand Dollars to build, it’s not a decision that is made lightly.
Why did the buildings in Christchurch collapse?
Its too early to make conclusions, and there will be several months of investigations into why the buildings collapsed, however many of those buildings that failed were older buildings. New laws were brought in around the mid 1970’s which enforced buildings to be able withstand specific earthquake forces.
The two biggest building collapses in Christchurch were the Pyne Gould building and the Canterbury TV building, both of which were built in the 1960’s before the tough earthquake-resistant standards were brought in.
The other factor is that the ground motion during the earthquake in Christchurch was way over what was predicted for a 1000 year period. Most buildings in New Zealand are designed for a 475 year return period (I learned this from Professor Bruce Melville while in the lift this morning).
So even though many of the older buildings had been reinforced to withstand earthquakes, they were not strengthened enough for the earthquake that hit. This earthquake not only exceeded the force of any predictions that would hit Christchurch in the next 1000 years, but was also on a fault line that wasn’t even identified as being at risk.
Its a sad time in New Zealand, and many engineering lessons have been learned. From a materials engineer point of view, we need to keep making materials that are stronger in all force directions (tension, compression and shear) to help improve the safety of new buildings and prevent more tragedies from occurring.