Undoubtedly construction quality is a factor in the devastation of last month's Turkish earthquake. But structural engineers on the Institution of Structural Engineers' Earthquake Engineering Field Investigation Team (EEFIT) believe that inadequate design is just as much at fault.
Of the thousands of residential blocks that collapsed, a number showed evidence of common design failings. The EEFIT team found that most of the collapsed multi storey buildings could carry static loads but could not resist the loads imposed by an earthquake.
'Each member has been designed satisfactorily for a static vertical load but not for dynamic horizontal loads,' said University of Westminster concrete structures expert Abbas Al-Hussaini. 'The overall structural integrity is missing. There is no strength or stiffness to resist the lateral loads.'
Allott & Lomax technical director Paul Doyle explained that asymmetry in many of the damaged buildings' column layouts meant that centres of stiffness did not coincide with the centre of mass. This meant that when a horizontal force was applied, the building twisted, inducing torsion forces on the beam column connections.
Non-structural masonry infill walls added on only two sides added to the asymmetry. 'They have added masonry infill walls without recognising they become part of the load path in an earthquake,' he said.
Most of the collapsed buildings have failed because columns sheared at their top and bottom due to torsion loading. Damaged columns reveal inadequate shear reinforcement at the joints at each floor level. Shear links were at 200mm-300mm spacings, when engineers could expect them to be around 50mm apart. Main column bars often do not continue through beams.
Walls and sometimes even columns had been removed from the ground floor of many buildings to accommodate shop fronts. This further reduced torsion resistance, creating a 'soft storey'. In the earthquake zone it is common to see buildings still standing but with their ground and first floors missing.