UNITED STATES fire codes face a major overhaul as a result of investigations into the terrorist attacks on New York's World Trade Center (WTC) in 2001, following publication of a key report into the disaster.
There is also new evidence that tower designs were done on the basis of wind tunnel tests which under-estimated wind loadings.
Last week the US National Institute of Standards & Technology published its second progress report on investigations into the September 11 attacks.
It highlights the variable and untested levels of fire protection for the twin towers' steelwork.
The report claims that this may have been a factor in the unprecedented structural collapses which came after two Boeing 767 aircraft crashed into the WTC towers.
It also claims wind tunnel tests were flawed.
Recent tests suggest that wind load factors would have been between 20% and 60% higher than originally expected.
This means the buildings' structural strength redundancy in a fire was much reduced.
But the report adds that two recent tests, carried out in 2002, produced 40% variations in predicted wind forces on the structures.
The towers' original structural engineer Leslie Robertson said this week that the wind loadings used in the design exceeded New York City codes of the time and the current New York State code. 'And they also exceed the current American Society of Civil Engineers code 7(02), ' he told NCEI.
On fire protection, the report points in particular to variations in the sprayed concrete protection of steel beams and columns.
This may explain why one of the towers, WTC2, collapsed more quickly than the other.
It fell 56 minutes after aircraft impact while WTC1 fell after 103 minutes.
The report says fireproofing for WTC1's truss supported floor system had been upgraded to 37mm from an initial 12mm while protection in WTC2 remained at 12mm.