EXPLODING FUEL tanks are unlikely to have caused structural damage to Milan's Pirelli building, a leading aircraft accident investigator told NCEI.
Four people died and 34 were injured when the Rockwell Commander 112 plane hit the 42 year old, 30 storey Grattacielo Pirelli at 5.47pm on Thursday 18 April, bringing rush hour Milan to a standstill.
Several fires broke out after the plane ripped through the building at a diagonal angle from left to right and its wing fuel tanks exploded. But impact with the two structural shear walls which run the full height of the concrete structure (see diagram), apparently did little damage to either.
One of the doors to the central group of lift shafts was damaged, probably by the port wing, but the main fuselage of the plane passed through the gap between the shear wall and the lift and smashed through the facade on the other side of the building.
Preliminary investigations had concluded that blast pressure from exploding fuel tanks cracked the joint between the very shallow long span beams and the massive shear walls that take most of the weight of the 31 storey building.
These long span concrete beams on the 1950s structure's 26th floor were found to have deflected by 250mm after the crash.
But Air Accidents Investigation Board senior inspector of air accidents (engineering) Robert Carter said that fuel tanks rarely explode when light aircraft crash.
'It only happens when the tank is almost empty and full of vapour and there is some internal source of ignition, ' he explained. 'What usually happens is that the wings fold back, the fuel tanks rupture and spill their fuel, and there's a fireball.
This gives the illusion of an explosion, but the energy release is very small.'
The Rockwell Commander 112 aircraft which struck the Pirelli tower had a maximum fuel capacity of 250 litres held in two tanks in the wings.
It is still not clear how much fuel would have remained when it ploughed into the Pirelli tower, although it is unlikely to have been so little as to have created necessary conditions for an explosion. The aircraft which has a range of 1,500km, flew to Milan from Locarno in southern Switzerland.
On the 26th floor, the carpet is virtually unmarked, Carter said: 'I find it hard to believe that a blast powerful enough to cause large beams to deflect wouldn't also have damaged the floor the beams were supporting.'
The 80mm thick insitu floors of the tower are supported by 750mm deep beams at 1,500mm centres. These have always been hailed as pioneering examples of post-tensioned concrete design, but now doubts have emerged over the 24m span beams in the central section of the tower.
A report in Concrete Quarterly magazine from 1961 suggests that these beams may have been modified during construction and relied only on normal reinforcement plus load sharing with the insitu lift shaft. But former Jubilee Line Extension architect Roland Paoletti, who was a colleague of the Pirelli building's structural engineer Pier Luigi Nervi in the 1960s, said he found it inconceivable that the design would have been modified without Nervi's approval.
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