Earthquake Proof Building Materials

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.

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Why can’t we fly through volcanic ash?

Lots of people were getting very frustrated about not being able to fly through the volcanic ash cloud last year.
I read many angry comments online from people trying to go on holiday saying that airlines should just fly through the cloud and that it’s not going to cause any problems.
Let me just say that with the billions of dollars that it is costing the airline industry, the no-fly rule is not something that had been decided lightly. This is to protect lives, and for the safety of passengers and crew alike, I’m sure it’s frustrating if you are stuck somewhere, or missing your holiday, trust me I have a flight to England scheduled for next week, but if the airlines say I can’t go – I’m OK with that.
A lot of the comments have been from people who don’t really understand the consequences of volcanic ash in a jet turbine, so I thought I would try to explain it in a simple a form as possible. Some academics may rip this apart, but I’m not writing this for them, I’m writing it for you in the least technical format I can.
Firstly let’s start with how a jet turbine engine works:
When you look at an airplane, you will notice under the wings there are big cylindrical devices with fans inside – this is what gives the plane the power it needs to fly and are called turbine engines.
If we were to cut one in half and look through the cross section it would look like this:

The air is sucked into the engine by the rotating fan blades and then squeezed by the compressor; it is then injected into a combustion chamber and sprayed with fuel then ignited by a flame. This combustion is blown out of the exhaust via a turbine which accelerates the air out pushing the engine, and thus the airplane forward. The exhaustion gases also turn the turbine which keeps the compression cycle going.

So now we know how it works, what are the dangers of volcanic ash?
Let’s start at the front – those nice shiny metal fan blades.
Volcanic ash is very abrasive; it can scratch and wear away the fan blades causing damage.
Think of sand on the beach, sand is also abrasive; you wouldn’t scratch sand across anything valuable because you know that sand can damage surfaces – the same goes for the particles in the ash. Volcanic ash is like sand, but much, much finer, like flour, so it can go higher in the atmosphere and get into all the nooks and crannies of a plane.
Next let’s move into the combustion part of the engine – I have labelled this region hot, but what I really mean is 1200ºC to over 2000ºC! (put this into perspective, aluminium melts at 660ºC and titanium at 1670ºC). The little ash particles are made up of rock and glass and they melt into a liquid inside the combustion chamber where they pass through and hit the cooler surfaces of the turbine. This causes the liquid glass to solidify and create a glassy film on the turbine, which can prevent them from rotating.
There are vents on engines designed to flow air towards the turbines for cooling and these can become blocked. This results in the engines burning too hot causing them to stall and scarily shooting flames from the back!
In addition to the engine problems, another dangerous consequence of the ash is if it blocks the Pitot tube. This is a little tube mounted on the wing which has two holes and some sensors. The side hole measures the still air, and the front hole measures the airstream pressure, by measuring the difference between these two you can calculate airspeed.
Unfortunately if this tube gets blocked (for example by being clogged with volcanic ash) you will get an incorrect airspeed reading. Not knowing your airspeed makes piloting a plane very difficult (imagine trying to park or drive your car without a speedometer) and can affect the autopilot computer models.

So would I fly through a no-fly airspace?
Nope.

Glass – a peek into the future

Glass is one of those materials that is actually designed to go unnoticed. When was the last time that you noticed the glass windows in your office, or the glass windshield on your car? The odds are you only noticed them when there was something wrong, a crack from a stone, or residual rain marks.

Corning have just come out with a new video showing how technical glass can be and what we can look forward to in the near future:

Spray on “Liquid glass” is also hitting the news with its ability to provide a thin (100 nanometer) coating that protects surfaces by preventing bacteria from attaching to them.

Pretty amazing for something that is made from sand!!!

We are used to glass bowls and maybe even have a Pyrex bowl or two at home. I’ve always loved the story about how they were actually discovered, so here is a brief history of Pyrex Glass.

On a rainy winter night in 1901, a speeding train hurtled through the dark.
A signalman stepped from the warmth of his station to warn of a slow freight train ahead on the same set of tracks.
He started to swing his kerosene lantern at the train driver to warn him, but when cold rain hit the heated glass globe, it shattered, extinguishing the signal light.
(This is because if you suddenly cool warm glass it will crack or shatter -you may have experienced this by trying to put a hot drink in a cold glass).
Without a light the signalman shouted at the train to stop, but his shouts were lost in the roar of the train as it sped past the station.
Seconds later came the sound of an enormous collision as the two train’s crashed causing death and destruction 😦
This was a common problem with lantern glass and so scientists frantically tried to make a glass that could withstand sudden temperature changes.
Early experiments proved that if the raw materials contained borax, the glass would be far more resistant to heat and temperature change.
However, the first glasses they made with Borax were so weak chemically that they deteriorated in water (not good on a rainy day).
In their search for a balanced recipe for heat-resistant glass, scientists tried one formula after another, seeking a stable mixture of silica sand and boric oxide.
With little to guide them but trial and error, two scientists found a formula that would combine heat resistance with chemical stability in 1912 (it took 11 years to find a good combination).
Soon after, lantern globes and battery acid jars made of the new glass were in production and lives were saved.
About a year later a young physicist had joined the research team. He had a theory that the lantern glass would make a good cooking dish because it absorbs radiant heat, while most metals reflect it so he took home a jar made from the lantern glass.
That night, the physicist’s wife baked in glass and the next day her husband brought it to the laboratory and everybody ate cake that was baked in the lantern.
The wife used the glass utensil for other recipes, and she reported that food didn’t stick to the glass, cooking time was shorter, the glass didn’t transfer a flavour to its contents, and that she could watch the food brown and know when to take it from the oven.

And that is the history of Pyrex bowls 🙂