The mega-cities, mushrooming around the world, are putting millions of lives at risk because the buildings cannot withstand natural disasters, the Royal Geographical Society will hear next week.

But cheap alterations to building methods to improve resilience are being ignored, according to Scott Steedman, a former fellow of St Catherine's College, Cambridge.

There will be about 30 more mega-cities of eight to ten million people in a decade's time. "I'm really quite concerned about the risk that we're posing ourselves," said Dr Steedman, "because the vulnerability of our cities is very serious. The risk is really huge."

"Many mega-cities have grown up within one or two generations so human memory of natural disasters is poor," said Dr Steedman, who will talk at a meeting to mark the United Nations International Decade for Natural Disaster Reduction.

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"You ask them if their area suffers from typhoons and they think they don't have them." Yet, he said, the records show that they do.

One vital step in construction that is often ignored is to join floors and walls together. Attaching them is not necessary if the only force on the building is the downwards force of gravity, but as soon as the building experiences a sideways force, from a typhoon or earthquake, for example, it has the strength of a pack of cards.

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The majority of the 5,000 buildings that fell down during the Kobe earthquake in Japan this year had no such connections, he said.

Yet it would be cheap to put pins into the walls to attach them to the floors, he will tell the meeting, arranged by the Royal Academy of Engineering.

Dr Steedman, who was a lecturer in engineering at Cambridge and is now director of engineering at the consulting engineering firm Sir Alexander Gibb, also said: "One of the tragedies of rapid urbanisation in developing countries is that they are using reinforced concrete because that's what they see everyone else using."

While it is an "exciting" material, he said, it can crack, and, if it does, it has to be completely replaced.

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