A shallow 6.8 magnitude quake in the morning of Nov 10 hit a rural area 116km north of Mandalay which felt by people on high-rise buildings in Bangkok may take people aback to rethink about construction safety in Thailand.
This issue has earlier received an attention earlier this year when a series of 25 earthquakes over five days from April 16-20 occurred in Phuket.
The quakes measuring between 1.9 and 4.3, with an epicentre in Thalang District, caused cracks and damage in more than 30 houses and some buildings on the island. There were no reports of casualties.
The incidents underline the importance of renovating properties to improve earthquake resistance as a precaution against future occurrences, says Associate Professor Dr Amorn Pimanmas, a lecturer at the Sirindhorn International Institute of Technology's School of Civil Engineering and Technology at Thammasat University.
There are simple ways to strengthen and protect existing buildings in areas prone to earthquakes so they will not collapse or hurt people inside, he says.
"[Inadequate] columns are the most significant cause of building collapses from earthquakes. General buildings in Thailand were not designed for earthquake resistance so the amount of steel inside columns is insufficient."
General buildings, he says, are those constructed before the issuance of Ministerial Regulation B.E. 2550 on Earthquake Resistance Design for Structures, which came into force in 2007.
They also include other buildings and structures that are not covered by the 2007 regulation.
When a building shakes during an earthquake, concrete at the columns, particularly at the ground floor or near the foundation, will be easily crushed and flake off from the outer surfaces.
When shakes continue or repeat, reinforcing vertical steel bars inside the column will bend as concrete increasingly flakes off, and the spaces become too wide between each stirrup that ties reinforcing vertical steel bars together. The structure can then collapse easily.
For a new building, Dr Amorn suggests double the usual number of stirrups be used in columns to increase the building's compressive strength and to help hold concrete more tightly.
This may cost 3-4% more than usual but can save the building and people inside.
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More stirrups to join reinforcing rods should be used near column bases.
Even more stirrups to tie reinforcing vertical steel bars together should be used in spaces at the column's base or near the ground as the column's base is the most prone to damage during earthquake.
Stirrups may be applied at the beam-column joint to increase strength and column sections should be bigger, the earthquake civil engineering expert adds.
For buildings lower than five storeys or 15 metres, additional stirrups should be used at the first two levels from the ground while those higher than five storeys need to have an engineer to design the structure.
Stirrups may not work at existing buildings as they need to be installed at the beginning of construction. Dr Amorn suggests a carbon-fibre sheet to wrap all column bases at the ground and second levels to lock concrete and prevent it from flaking off.
This material is easy to install: just coat it and the column with epoxy cement before wrapping it around the column. The price of around 2,000 baht per square metre is too high for home use but builders of high-rises and other structures such as expressway or elevated train track pilings need to consider it.
"Carbon fibre sheets were invented by the Japanese after they faced Kobe earthquake in 1995, Japan's worst earthquake in the 20th century with a magnitude of 7.2," Dr Amorn said.
For a house, he suggests chicken mesh as an alternative to carbon-fibre sheets as it costs 10 times less. Chicken mesh should be used together with stirrups by wrapping them around the existing column before applying cement over it. This can be done in one day.
In single-storey rural houses, to increase wall strength and prevent falling bricks, chicken mesh should be applied along the whole wall.
Dr Amorn says buildings that are constructed with flat slabs or precast concrete panels are also risky as precast walls are used as a load-bearing element.
One solution is to add steel brackets and steel collars under the floors of each level of the building to brace them. Floors are precast concrete slabs that are not adequately tied together.
Related search: earthquake, Myanmar, design, Singgu
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About the author
Writer: Kanana KatharangsipornPosition: Business Reporter
Source: http://www.news.thethailandlinks.com/2012/11/17/design-gets-a-shake-up/
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