Earthquakes and Building Design

Earthquakes, Ground stability and Building Structural Strength

Earthquake Risk and Warnings

In the article The risk of earthquakes in Bali and Indonesia we looked at how earthquakes happen and the level of risk in Bali. It was not a pretty picture but if we take care we can be safe.

The scale of earthquakes and the earth movements that cause them are rather large, in fact I might be so bold as to describe them as simply gihumungously big.

Earthquakes occur without warning. Sometimes we might get some rumblings now and again to warn us of that ground movement is to be xpected but we won’t know when it's going to happen. We won’t know how big it will be, there is nothing we can do to stop it and our location in relation to the epicentre of the quake will have an important bearing on how and to what degree we will be affected.

Earth movements in an earthquake

A starting point is “how will my bit of ground and my house be affected should an earthquake occur?”.

Unfortunately this is totally unpredictable. The earth has been around for a while now and it’s ancient old structure is both complex and variable. It has been uplifted, folded over, split, crushed, melted, squeezed out and thrusted so many times that the geology of the earth’s crust can be very complex.

Earthquake Distance, Depth and Size

More than anything else the amount of movement, distance and depth of the epicentre will decide how serious a tremor you will get. Looking at historical information we see that when earthquakes are deep the damage on the surface is considerably reduced.

We also have to consider how the vibrations from an earthquake will travel through the earth’s crust from the source of the quake to where we are and how the earth will behave in our vicinity once it arrives.

We may find that strong shudders can travel long distances along lines of rigidity while in other places more elastic ground may absorb the shocks. There are a million permutations of course and if you visit an earthquake hit area it is not uncommon to find one house totally destroyed while the one next door is untouched.

The Nature of The Ground

The nature of the ground has a major impact on how buildings should be designed and constructed.

When asking people about their experiences you will get many different responses. Some will talk of hearing gates rattling, they get closer and closer until the earthquake strikes, they could hear it coming. Others may report sounds like an approaching train.

In the Yogyakarta earthquake of 2006 people described seeing waves a metre high, like ripples on a pond, coming towards them across the rice fields and we are not talking about waves of water these were waves in the earth.

Liquefaction

There is also an effect called liquifaction in which the vibrations cause water to rise to the surface from the water table below where the ground is shaken into a liquid mud. Liquifaction can make even the strongest buildings simply fall over.

If we are in a fairly stable rocky area we can expect the ground to be resilient and fairly predictable however we can also expect that the rigidity of the ground will transmit hard shocks into buildings.

Peak Ground Acceleration and Building Design

All this is taken into consideration when the expected PGA (Peak Ground Acceleration - see article here) is estimated.

In earthquake prones areas there is usually a government standard which states the building design standards that are required to withstand earthquakes.

These standards work to a particular purpose - to save lives. Designs standards have been determined such that even if a building is seriously damaged people will stand a good chance of survival. The idea is that while walls or roof may crack or fall out the structure itself will not collapse.

So what happened in the earthquake of 2009 in Padang where a hotel collapsed and so many people died in collapsing buildings? Many government buildings collapsed - surely they were built to the government standards.

Well sadly it appears that they weren’t.

An evaluation of collapsed buildings in Padang makes disturbing reading. The experts tell us that the earthquake in Padang was only a PGA of 0.113g which is considerably less than the rating in that area which states that buildings should be able to withstand a PGA of 0.28g to 0.36g. This suggests that the many buildings that collapsed were not built to anywhere near the correct standards.

The Ambacang Hotel that collapsed was originally an old 2 storey building, it had four extra floors added without having its structure appropriately strengthened. This looks like a serious case of incompetence however many other buildings that should have survived didn’t.

Many buildings are not safe.

There are serious implications for building owners all over the world. Many of us live in countries where in many cases structural engineers are not engaged to ensure structures are designed and built properly and of course there is that inevitable old poverty avoidance gene that means that the terribly expensive steel is left out or reduced in size and cement is used as a colouring pigment rather than a bonding agent.

Probably the most common cause of fatalities during earthquakes is from falling buildings and the key factor in protecting you is the integrity of the structure of your building.

Interlocking lightweight wooden or bamboo structures fare best (as long as they don't have heavy tiled roofs). They tend to sway and flex dissipating the energy but most importantly they tend to stay upright and are light if they do come down.

Reinforced concrete buildings must be properly designed and built

Most modern buildings are made from reinforced concrete, they are rigid and heavy. Concrete cracks and breaks easily but, when combined with steel, concrete structures can be made to withstand pretty heavy shocks.
Three factors are essential:

1. The structure must be correctly designed by an expert (and make no mistake there are some excellent Indonesian structural engineers here).
2. The steel must be to the engineer's design. When the builder saves money by leaving out some bars or reducing their size the building is considerably weakened.
3. There must be sufficient cement in the concrete and the concrete must be cast in single pieces to have sufficient strength.

It is vital that these factors are checked while the building is being built. It is very difficult to know if a building was built properly after it is finished.

Once a building is built it is generally a difficult and expensive task to make it more earthquake resistant.

An important point to note is that architects are NOT structural engineers.

Only last week I saw some beautiful architectural drawings, very detailed and very professional looking. It was immediately obvious that there was no structural engineer involved and there were some areas of serious concern. The developer has been advised to modify the design to make it safe.

In another recent case a very expensive villa almost completed was found to have insufficient structural strength and, believe it or not, no foundations. The villa has now been rectified but it has been a major undertaking involving some very specialist engineering techniques and all because the builder saved a little bit of money by not engaging a structural engineer and skimping on the steel.

As a final comment it is the conventional wisdom to advise people to stay inside a building when an earthquake hits rather than risk falling debris outside. This advice assumes that buildings are properly designed and constructed. In a country where correctly constructed buildings are the exception rather than the rule I think I would get out as fast as my little legs can carry me.

See also:

• The risk of earthquakes in Bali and Indonesia
• Why houses fall down in earthquakes
• Dangerous developments in architectural design
• Checking your building for earthquake damage