Mid-rise Buildings Risky

Medium-rise buildings of two to 10 floors tend to face more risk of damage from earthquakes than taller structures, says a highrise consulting engineer.

The reason is that buildings with two to 10 floors have a natural frequency closer to the frequency of seismic ground motion, said Piyawat Chaisiri, CEO of Arun Chaiseri Consulting Engineer Co.

Natural frequency is a term used to describe the number of times a building will move back and forth between its original position and its displaced position if there is no outside interference.

When a building’s natural frequency matches that of the quake, it will cause resonance that leads to severe shaking.

In the 6.3-magnitude earthquake that shook Christchurch, New Zealand in late February, most of the buildings that collapsed were only five to six floors tall, said Dr Piyawat.

“Meanwhile, higher buildings, let’s say with a height of 20 floors or more, may also experience cracks and other damage to substructures such as the breaking of glass windows or the collapse of surrounding walls,” he said.

Other structures such as pinnacles of pagodas or brick walls also face more risks when an earthquake strikes.Since 2007, all skyscrapers in Thailand, especially in Bangkok, have been subject to tougher seismic building design requirements under an Interior Ministry regulation. The amended regulation covers weight loading, materials, design requirements and construction approaches.

Some people express concern that highrise buildings constructed before 2007 could be at risk of heavier damage or even collapse in an earthquake. However, other regulations are also in place to ensure sound construction.

Dr Piyawat said that municipal regulations required all buildings of 10 stories or more to be able to withstand winds of up to 200 kilometres per hour.

“This municipal regulation is quite conservative. Therefore it covers safety factors that make buildings able to resist conditions beyond the expected or actual wind loads,” he said.

Based on his experience, he said building owners were very aware of earthquake risks and were willing to abide by strict design regulations.

Dr Piyawat suggests two techniques that can be applied to buildings in order to protect them from potentially devastating seismic impact. One is known as base isolation, a collection of structural elements that decouple a building from its foundation.

Another method involves adding strength to joints between beams and pillar to make a building more flexible and resist seismic forces.

“Both approaches may add less than 1% to the construction cost,” he said.

In any case, he said, it was very important for building owners to strictly control every step of the construction process, from blueprint design to onsite work to assure that their buildings are up to standard and safe for residents.

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