Investigators in Atlantic City are probing connection failure as a possible cause of the progressive collapse of a multistorey car park, which killed four people.
Contractors from Fabi Construction were pouring the deck of the tenth floor of the final bay at the Tropicana Casino car park when the floor collapsed.
Local structural engineers are claiming that the collapse was triggered when contractors removed temporary supports on lower floors before insitu concrete had gained sufficient strength.
This, they said, coupled with a lack of any kind of substantial tie between the deck and columns led to the progressive collapse of the top five floors of the ten-storey structure.
The connections with the outer columns failed during the collapse but the columns and a shear wall were left standing after the accident.
Investigators will focus on whether a proprietary deck system had been adapted outside of the manufacturer's instructions.
The deck construction was a combination of precast and insitu concrete using the filigree beam and slab system.
Mid State Filigree vice president of engineering Ted Betz stressed that topping concrete is poured insitu and must be cast into the columns.
Engineers in the UK said that this type of failure would not happen there.
'It begs the question on designing for progressive collapse.
'The UK code dictates that sufficient ties are in place between columns and the decks but the American codes don't have the same precautions to protect against accidental damage, ' said Ian Laing, director of the property management team at Halcrow.
Another theory is that a central column gave way.
'Buckling failure of the middle column would cause excessive sagging to occur simultaneously on all floors.
'The collapse would then progress along the length of the structure, ' said Gordon Masterton, managing director at Babtie and vice president of the ICE.
Engineers on site are currently working to secure the external shear wall, which remained in place when the floor decks collapsed.
The Federal Occupational Safety and Health Administration (OSHA) is carrying out the investigation and has six months to compile a report.
It is the second time that the OSHA has been called in to investigate this particular site.
In October last year Fabi was fined US$8,300 after a prefabricated concrete section gave way injuring three workers.
The car park is being built for Aztar Corporation. Main Contractor is Keating Building Corporation and consultant is DeSimmone