Earthquakes_and

Earthquakes 2

What can be done to stop buildings falling over in an earthquake?
The simple answer is - nothing!

Earthquake-proof Buildings
After massive earthquakes, such as near Japan one wonders if it’s possible to build an earthquake-proof building. The answer is ‘yes’ and ‘no’.

There are engineering techniques that can be used to create a very solid structure that will endure a modest or even a strong quake.

However, during a very strong earthquake, even the best engineered building may suffer severe damage. Engineers design buildings to withstand as much sideways motion as possible in order to minimise damage to the structure and give the occupants time to get out safely.

Buildings are designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standing.

Earthquakes present a lateral, or sideways, load to the building structure which is more complicated to account for.

One way to make a simple structure more resistant to these lateral forces is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake. This picture shows a stainless steel wall tie, which literally binds together the outer and inner wall. This keeps both the cavity (gap) regular and increases the strength of the wall.

Steel cross-bolts which give flexibility to building whilst still allowing tremendous strength.

The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block. Generally, this type of construction has walls that are made of bricks stacked on top of each other and held together with mortar. The roof is laid across the top. The weight of the roof is carried straight down through the wall to the foundation. When this type of construction is subject to a lateral force from an earthquake the walls tip over or crumble and the roof falls in like a house of cards. Just like a normal house.

Buildings made traditionally, with rigid structures, which allow no movement either horizontally or vertically, always suffer badly in the event of an earthquake. Lateral movement ability allows a building to move a little with the earthquake and suffer less damage.

Construction techniques can have a huge impact on the death toll from earthquakes. An 8.8-magnitude earthquake in Chile in 2010 killed more than 700 people. On January 12, 2010, a less powerful earthquake, measuring 7.0, killed more than 200,000 in Haiti.

The difference in those death tolls comes from building construction and technology. In Haiti, the buildings were constructed quickly and cheaply. Chile, a more rich and industrialised nation, adheres to more stringent building codes.

Skyscrapers
As the buildings get bigger and taller other techniques are employed such as “base isolation.” During the past 30 years, engineers have constructed skyscrapers that float on systems of ball bearings, springs and padded cylinders. Acting like shock absorbers in a car, these systems allow the building to be decoupled from the shaking of the ground.

Base isolation - where the building is not even ‘on the ground’ - it’s on massive rubber balls.

Another technique to dampen the swaying of a tall building is to build in a large (several tons) mass that can sway at the top of the building in opposition to the building sway. Known as “tuned mass dampers”, these devices can reduce the sway of a building up to 30 to 40 percent. The Taipei 101, formerly known as the Taipei World Financial Center, has just such a giant pendulum mounted between the 88th and 92nd floors. Weighing 730 tons and capable of moving 1 1⁄2 m in any direction, it takes the prize as the world’s largest and heaviest building damper. In fact, it is so heavy that it had to be constructed on site because it is too heavy to be lifted by a crane.

As the building’s structure moves to the left or right, the vast sphere is moved in the opposite direction to counterbalance the movement, and keep the building in a stable position.

There are also smaller ‘tuned mass dampers’ located in the tip of the spire which daily help against movement cause by winds.

The counterweight inside the tower weighs 660 tonnes and is able to steady the structure in bad weather conditions. The most the sphere has had to move is 100 cm, in 2015.

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