Building Construction, Renovation, Maintenance & Advice

How do Houses Withstand Earthquakes

Earthquakes

Here we look at building design, why houses fall down and how we design buildings so that they can withstand earthquakes.


See the full Fixed Abode article "When The Earth Moves" here


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"When The Earth Moves"

Many parts of the world are susceptible to earthquakes. “The ring of fire” refers to the high level of geological activity that results in volcanoes and earthquakes. In Indonesia a joint between the Indo Australia and Eurasia tectonic plates runs right along the western and southern edges of the Indonesian archipelago only a few kilometres offshore. Moving an average of 6 cms a year this boundary has been very active in recent years and caused the major earthquake and Tsunami that devastated Aceh and, more recently, the major earthquake in Yogyakarta.

Could such an earthquake occur in Bali? Well unfortunately yes it could. In fact damaging earthquakes have occurred in the East Java Bali region 15 times in the last century the last one being in 1979.

There are a few things we need to know about earthquakes to get things into perspective.

Firstly there are some places in the world such as Tokyo that have earthquakes every day. The people live with it on a daily basis.

Secondly large earthquakes are very rare and you are more likely to get injured on the way to the airport than by a major earthquake. Finally most people are killed not by the earthquake but by falling masonry and if buildings are properly designed and well built most will survive an earthquake.

Why did houses collapse during the Yogyakarta earthquake of 2006

In the aftermath of the recent earthquake in Southern Java I went to Yogyakarta to asses the damage and examined many buildings affected by the earthquake. In some areas buildings were left in relatively good condition while in others almost every house was flattened. All in all 400,000 houses were either seriously damaged or totally destroyed in the province. It is rather sobering to sit and talk to hard working people who, because they didn’t understand or couldn’t afford the basics of housing construction, are still living under tarpaulins amongst piles of shattered bricks. It is even more heartbreaking to hear the stories of the many people who escaped from their houses only to be killed or seriously injured by walls falling on them in the narrow streets.

5 construction faults

Five basic factors can be blamed for most of the house failures in Yogya:

1. The absence of reinforced concrete columns and beams in the building structures.

2. The practice of reducing cement content in building mortar to save money (mortar should be 2 parts of sand to 1 of cement, in some cases 10 parts of sand to 1 of cement had been used!)

3. The use of weak, low temperature red bricks to build structural walls.

4. Heavy tiled roofs on weak wall structures.

5. Poor foundations.

Well designed and built houses survive earthquakes

It was very noticeable that houses that were well built with integral reinforced concrete columns and beams survived with surprisingly little damage. The Yogya earthquake was not only strong (6.5 on the Richter scale) but it was also long, 50 seconds, which is a very long time for a building to withstand. 50 seconds is also a terrifyingly long time for victims surrounded by falling buildings, just try timing it and you’ll see what I mean.

In an earthquake ground movement varies enormously including up and down fractures, sideways fractures, the ground moving apart leaving open voids or moving together so the land rises up. Some people in Yogya reported seeing waves rippling through the paddy fields like waves on the sea.

Earthquake resistance of houses was a subject discussed by building experts at Gajah Mada University in the aftermath of the earthquake. Traditional buildings, particularly made from bamboo or wood, are better able to withstand earthquakes than modern “brittle” buildings made from concrete.

Wood and bamboo houses can resist earthquakes

Houses built with wood or bamboo frames tend to withstand earthquakes with relative ease because the main structure of such houses is locked together and not rigid. These buildings absorb the ground movement by swaying and flexing and so are far less likely to fall.

The point was proven in Yogya where I found a house fully intact in the centre of an area where barely a wall was left standing. It was a very old timber and bamboo house with a tiled roof. Hardly a tile was out of place.

Concrete buildings should have a well designed structure of reinforced concrete beams and columns to provide stability. Raft construction where the building is built on a single reinforced concrete slab is another technique which allows the building to “float” as a solid single unit on moving ground.

The Japanese deal with so many earthquakes they have developed some interesting techniques.

Many multistory office buildings in Japan are built with steel girder framed structures mounted on coil springs that absorb earthquake vibrations. I suppose this is why the Japanese always have that inscrutable look. They are either wondering when the next tremour will be or “how on earth do those coil springs work?”

But what about Bali?

Well Bali is a place that has more personal wealth than other parts of Indonesia. As a result most houses and hotels here tend to be well built with properly designed and constructed reinforced concrete structures.

Many villas have Alang Alang roofs which are light and of a construction that can sway without disintegrating.

So what do you do if an earthquake strikes? The Yogya experience says:
Get out of the house and away from walls or other structures. You are likely to have around 10 seconds to get out. If you can’t get out of the house get under something as solid as possible such as a heavy table leaning against a structural wall. Find something that will deflect falling masonry rather than try to withstand it.

With a bit of thought at the building design stage and good supervision during construction living in an earthquake zone need not be a cause for serious concern.

Copyright © Phil Wilson 2009
This article or any part of it cannot be copied or reproduced without permission from the copyright owner.


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17 July 2017 Copyright © Mr Fixit,
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