LONG SPAN post-tensioned floor beams on Milan's historic 127m high Pirelli building have sagged up to 250mm after last week's tragedy in which a single engined plane smashed into the 26th floor, NCE has learned.
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.
Firefighters from a station at the rear of the building reached the 26th floor 'in minutes' and are reported to have extinguished the flames in three minutes after the sprinklers stopped it spreading.
Subsequent investigations by building owner and regional government Regione Lombardia (RL) discovered that five, 750mm deep beams spanning 24m between the shear walls at the affected floor had undergone a permanent deflection of up to 250mm. RL senior engineer Franco Caputo blamed dynamic loading from the fuel tank explosion for cracking the construction joint between the floor beams and the shear walls and allowing the beams to behave as simply supported units rather than encastre as the designer had envisaged.
'It is too early to say how the floor structure will be repaired but it will probably not require total reconstruction of floor 26.
Replacing the floor beams at that height will be challenging though, ' Caputo said.
Assessment of other parts of the building has shown that damage to aluminium mullions and glass on the side of the building where the plane impacted could lead to the whole facade being replaced because of the need for consistent use of materials.
'We may have to replace the facades completely because of the difficulty of getting the same materials. It could be very expensive, ' said Caputo. Damage to the interior of the 700m 226th floor is extensive. The interiors of the 24th and 25th were also badly damaged by the explosion.
Three days of structural inspections were completed on Sunday, allowing the bottom 11 floors to reopen for essential workers on Monday.
A commission made up of structural engineering experts from Milan Politecnico, fire chiefs and RL engineers has been set up to investigate the accident and recommend repairs.
Damian Arnold in Milan and Dave Parker