ENGINEERS IN the United States believe movement in a slender concrete shear wall triggered for the fatal collapse of a 12 storey car park in Atlantic City in last October (NCEI last month).
Immediately after the collapse it was apparent that the 30m high by 350mm thick wall had tilted outwards, although it is not yet clear whether this was a result of the failure or the cause.
The wall was later demolished as a safety precaution.
Four people died in the collapse, which occurred at 10.40am on Thursday 30 October.
'The first thing I noticed was that the shear wall had moved outwards, ' said a contractor called in to examine the wreckage by Alter Corporation, owner of the Tropicana Casino site where the collapse occurred.
The contractor said he had a close knowledge of the car park project and the construction techniques used at the Tropicana site.
Movement of the wall was exacerbated by its slenderness and the density of reinforcement used, he said.
'It's only 14 inches (350mm) wide, which is real thin for a shear wall. The rebar is only T6 (20mm) when I would always use a T8 (25mm) in a shear wall like this, ' he said.
The contractor who spoke to NCEI has been involved in the construction of 12 such structures in Atlantic City over the past 18 years. He is involved in other construction projects at the Tropicana site.
Construction of the car park floor slabs was based on the use of a proprietary permanent formwork system known as the Filigree system (see diagram).
Thin precast, prestressed concrete planks and tubs are supported mainly by falsework and then topped with insitu concrete.
At the perimeter wall where the failure occurred, the shear wall under investigation replaced four columns.
During construction of the outer end of the floor, precast permanent formwork planks were apparently supported directly by the columns and a section of shear wall. The planks rested on a lip cast into the shear wall and run 50mm into the 350mm thick wall itself.
According to the contractor who spoke to NCEI, loads from the precast planks gradually forced the wall outwards during the pour to create the 12th floor slab.
He claimed that as the floor slab concrete was pumped into place, the outer end of the precast plank slipped off the shear wall triggering a progressive collapse, first of the falsework and then of the five floors below.
Wet concrete and the Filigree deck plunged down approximately 2m to the 10th floor, starting a progressive failure.
The day after the collapse the Federal Occupational Safety & Health administration (OSHA) took control of the $225M development, which included a 31 storey hotel tower and 2,400 space car park.
OSHA is investigating whether there had been any violation of safety procedures.
Reports that falsework had been removed too early and of lower floors being loaded before the concrete had gained enough strength were rubbished by the engineer who spoke to NCEI.
'It's a stiff concrete mix with a 6 inch (150mm) slump and an accelerator called Positech is added to increase the rate at which it gains strength, ' he said.
'There's no way these guys would have removed shoring early. These are family companies and they just wouldn't take stupid risks like that, ' he said. Consultant for the project is DeSimmone.
No one involved in the project would comment on the collapse.
How the storeys went up
Contractors from Fabi Construction were half way through pouring the 11th storey deck when the collapse occurred.
Pouring the concrete is the final stage in constructing each level. It all starts with the columns.
These are cast from east to west at 5m centres and are reinforced with eight 48mm diameter steel bars.
A flat-bottomed 'U' shaped prefabricated section called a filigree tub is then run along the top of the columns.
Hollow sections in the base of each tub allow the column reinforcement to be tied in to horizontal reinforcement that runs through the tub.
Polystyrene blocks are then laid end to end either side of the columns along the length of the tub.
'These allow you to keep a wide span without creating a lot of mass, ' said Ted Betz, vice president of Mid State Filigree in New Jersey.
When two rows of columns have been prepared this way and the tubs are supported with falsework, the precast planks can then be laid from between the edges of the tubs to form a bay.
These in turn are supported with falsework until after the insitu concrete is poured.
Reinforcement is then placed along the deck on top of the planks and tied in to the columns.
Finally, contractors can pour the deck slab and fill in the tubs before a galvanised steel mesh is laid over the whole system to prevent shrinkage cracking as the slab hardens.
When complete the deck is a total of 220mm thick and tubs are filled in, ready to act as a base for the next column.
The standard connection method for the outer edge of this type of structure is to use half tubs to support the slab at the outer edge of the structure.
But this was not used in the Tropicana car park.
Here a combination of shear wall and columns support the slab without the use of a tub. The deck simply sits on a lip cast onto the shear wall to receive and support the deck.
The floor slab is then cast onto the top of the shear wall, which is formed a floor at a time, just ahead of floor slab construction.
Pouring of the ill-fated bay was divided into three longitudinal strips running from the eastern edge of the structure, by the shear wall.
Contractors had just finished work on the first, southernmost strip when the failure occurred.