CONSTRUCTION WORK has started on the first structure to be rebuilt at New York's World Trade Center site since the 11 September terror attacks.
The new World Trade Center 7 building is being constructed to replace the structure that collapsed around eight hours after the Twin Towers were struck.
The sudden collapse of WTC7 raised many questions with structural engineers as it is believed to be the first modern steel-framed building to have collapsed due to fire alone. A series of blazes are thought to have been fed by generator fuel at different locations in the building.
The original 47-storey structure was effectively built on a huge steel girder bridge spanning a large electricity substation in the lower storeys.
Massive girders and trusses up to the seventh floor, transferred the vertical loads from steel frame to caisson foundations which were irregularly arranged around the substation.
Speaking at the Institution of Civil Engineers last week, president of the Structural Engineers Association of New York (SEAoNY) Ed DePaola said that the new building would now have a reinforced concrete core.
The building is designed by New York consultant Cantor Seinuk which is owned by UK consultant WSP. The structure differs significantly as, apart from some composite flooring, the original WTC7 was almost all steel-framed.
No details are publicly available about the new building, but it is thought to be primarily concrete in the lower floors which again incorporate an electricity substation.
This substation will help power other areas of the World Trade Center complex as they are reconstructed.
Consultant Mueser Rutledge, led by geotechnical guru George Tamaro, has the contract to design the new building's foundation, believed to consist of caissons with massive load carrying capacity.
But the choice of a different designs and materials suggests that clients are now demanding more robust and fire-resistant features in future structures.