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Recipe for disaster Poor building design and soft ground have been blamed for the devastation caused by August's earthquake in Turkey.

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AN ESTIMATED 30,000 people died as a result of the 45-second earthquake that hit north-west Turkey at 3:02am on 17 August.

In September, the Institution of Structural Engineers' Earthquake Engineering Field Investigation Team (EEFIT) returned to Britain after 10 days in the disaster area. The 12-strong team included structural and geotechnical engineers, as well as seismologists and geophysicists.

It will present its preliminary findings at the Institution of Structural Engineers on 8 November with a formal report to follow.

The earthquake, which affected an area 125km by 50km, occurred just east of Istanbul when the part of Turkey south of the northern Anatolian fault shifted 2m-3m west. The fault forms the edge of the Turkish (Anatolian) crustal block which is being squeezed sideways to the west as the Arabian plate pushes into the Eurasian plate. The area is characterised by relatively frequent and shallow events less than 20km below the surface.

The epicentre of August's earthquake was near the town of Golcuk on the southern shore of the Sea of Marmara. Not surprisingly, this is one of the worst hit towns. Whole districts were destroyed, with nearly all the devastation resulting from the disintegration of poorly built apartment blocks known as beshkats.

Over the past 10 years thousands of these five to six storey concrete blocks were built in the region to house a population boom. Many were thrown up quickly by small local contractors without proper supervision from local authority building inspectors.

Since the earthquake, the Turkish press has been quick to blame cowboy contractors and corrupt building inspectors, but EEFIT found that poor design was as much to blame.

Bath University earthquake engineering expert Dina D'Ayala, who led the EEFIT team, explained: 'Bad construction is clearly a factor but equally significant has been the ground conditions.

'Many of the buildings have toppled or sunk into soft ground. Others have twisted themselves apart because their design did not provide adequate torsion resistance. All of these are design failures.'

'Most of the beshkats were only designed for a static vertical load, with insufficient consideration for the dynamic case,' she added.

Many buildings collapsed because of 'soft storey' failure after walls were removed from the ground floor to accommodate shop fronts. This reduces the stiffness of the lower floors and weakens the structures' resistance to torsion, which leads to shear failure at the beam column connection or at the base of the column.

Adapazari, about 40km east of Golcuk, was also devastated by the earthquake. Much of the destruction here was through liquefaction, with whole floors disappearing into the ground.

University of Colorado professor of geophysics Roger Bilham blames the destruction on the decision to site the town on an alluvial plain.

'The water table is very close to the surface, and the buildings are essentially floating,' he said. 'You can't build anything in these regions. They should relocate all these buildings on the high ground nearby.'

Allott & Lomax technical director Paul Doyle added: 'Jacking-up is required on many buildings and piles should be installed down to the bedrock to stabilise the structures.

'Stone-filled columns could be constructed to lower the water table but this work could be damaging and costly. It would be better to knock them down and start again.'

Soft ground has also been blamed for destruction of an area of west Istanbul more than 100km from the epicentre.

Almost 1,000 people were killed in the Avcilar district of the city, where the soft ground amplified horizontal forces, producing lateral accelerations of 0.25g, a horizontal force equal to 25% of the buildings' mass. In the rest of Istanbul the peak horizontal acceleration was measured at 0.05g.

Turkey's roads survived well, though the large lateral movements damaged many bridge piers and movement joints. Subsidence affected the abutments of many bridges, resulting in a step between bridge decks and their approach roads.

The most serious damage to a road was on a 10km stretch of the Izmit and Adapazari highway where it crosses the faultline several times.

'Because the two plates are pulling in opposite directions tensile forces between the two sides open up tension cracks,' explained Ove Arup geotechnical engineer Matthew Free.

'Compression forces between them push up the road surface into mounds, creating undulations.'

Turkish highways department KGM is carrying out urgent repairs to the road surface where shear forces across the fault have created 100m bands of 0.5m deep undulations.

The only major bridge collapse was also on this section of road, when three spans of a post tensioned concrete box girder overbridge slipped 2m sideways and fell off their piers.

Waterfront structures along much of the south coast of the Sea of Marmara performed badly. Large strips of reclaimed land slumped towards the sea, causing inadequate sea walls to topple.

'What we are seeing generally is that sheet piled structures didn't perform well,' said Babtie technical director Alan Stewart.

Gibb chief geotechnical engineer Robert May added: 'What we have seen perform well are heavy reinforced concrete decks with heavy reinforced piles and properly armoured shoreline protection. I haven't seen any good examples of caisson structure.'

There was virtually no damage to engineered structures. An example of the success of seismically designed buildings can be found at the Toyota car factory at Adapazari.

Huge rupture cracks sliced through one corner of the plant and approach roads were destroyed, but the Ove Arup designed structure survived intact.

British Geological Survey says aftershocks are likely to last for many months, possibly years. They will decrease in frequency and magnitude but, initially, some are likely to have magnitudes of between 5 and 6 which could cause further damage to buildings which suffered in the main shock.

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