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Earthquake resistant

As temblors shook Dubai and people fled high rises, Ben Millington found out how safe we truly are from a seismic disaster as buildings reach even further into the sky.

High-rise buildings along Sheikh Zayed Road were some of those to be evacuated after earthquake temblors hit Dubai last week.
High-rise buildings along Sheikh Zayed Road were some of those to be evacuated after earthquake temblors hit Dubai last week.

As temblors shook Dubai and people fled high rises, Ben Millington found out how safe we truly are from a seismic disaster as buildings reach even further into the sky.

Hundreds of people evacuated tall buildings in Dubai last week when long range temblors from an earthquake in Iran caused structures here to shake.

Rachel Cox was on the 29th floor of the World Trade Centre at the time of the temblors and describes her fear.

 

If this was in Tokyo you'd have far stronger earthquakes than we did last week and people will just ignore them.

"The alarms didn't go off, everyone in the office just felt it and headed straight for the stairs," she said.

"The whole building was shaking; I saw all of the stuff on my desk moving and felt a little giddy when I stood up.

"It was quite frightening because we didn't know if it was an earthquake or if the building was crumbling."

Rachel joined hundreds of other evacuated workers and residents waiting outside buildings across Dubai. But was this measure an overreaction? Are buildings in Dubai safe from seismic disaster?

The Dubai Municipality (DM) says "yes" - earthquakes pose no risk to the city's towers and they are in fact planning to relax earthquake building regulations next year.

They currently adhere to the internationally-used Uniform Building Code 97 (UBC) and certify all buildings over four stories to a zone 2A standard which assumes buildings can withstand local earthquakes up to 5.5 on the Richter scale.

But the DM is likely to scrap the UBC next year and replace it with a new code specific to Dubai.

"We have an outside company analysing all geological factors specific to this region to get the right parameters and formulas for our own unique code," said Moawya Safarini, head of the DM's structural engineering unit.

"When it's introduced I expect earthquake building requirements will be more relaxed because Dubai is not a seismic zone and there is no chance of having an earthquake over 5.5.

"We are actually classified as zone 0 under UBC, but we use zone 2A which makes us much more earthquake-proof than required under international standards."

While most people seem to agree that the UAE is a low-risk zone for earthquakes, the Council of Tall Buildings and Urban Habitat (CTBUH) says caution still needs to be shown.
 

They say the codes being used by the DM are outdated and do not apply to the types of buildings being constructed in Dubai.

"Their 'tall' is not the kind of 'tall' that the world is used to so the codes have to be re-examined with this in mind," said CTBUH co-chair Michael Willford.

"The current codes were developed to address the seismic design of low to moderate-rise buildings."

"As such, they fall short in conveying specific modeling, analysis and acceptance criteria for tall buildings and lead to unsafe construction."

The CTBUH recommends a 'performance based' approach to seismic design be introduced in the UAE, similar to systems used in Japan and China.

This would require new building designs to be exposed to a series of computer simulated earthquake events consistent with any potential seismic hazard at the site.

Technical director at Hyder Consulting Angus MacFarlane said they carry out such tests for all of their building projects which include the Burj Dubai, Emirates Towers and Burj Al Arab.

"I think most competent consultants do it, but not all clients are willing to pay for it," he said.

"It's difficult to say whether it should be made compulsory because it tends to be the wind loading that makes more of a difference to tall buildings in Dubai.

"I'd be more concerned about being in low-rise building during an earthquake because they don't have to comply with any earthquake regulations."

MacFarlane supports the DM's plans to relax the earthquake code, but said the new code should apply to all buildings, not just those above four stories.

In the latest incident it was people in high rises that seemed to feel the temblors, but MacFarlane said that is to be expected.

"What actually happens is those distant earthquakes send out waves at all different frequencies and the short wave lengths die off very quickly and the long waves carry on quite a considerable distance.

"Although they are not very strong they're at a frequency that can excite tall buildings but there is not risk or structural damage. If you vibrate any structure at its natural frequency it will resonate."

He said last week's temblors caused the spire of the Burj Dubai to move less than 100mm which is well below the 2.8 metres it moves under a simulated design earthquake.

Construction workers were evacuated from the site as a safety precaution, but Macfarlane said Dubai's tall building residents and office workers shouldn't have bothered.

"If this was in Tokyo you'd have far stronger earthquakes than we did last week and people will just ignore them."

"I think what we need to do is raise public awareness that all tall buildings in Dubai are safe from earthquakes."

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