Construction crews on the massive One World Trade Center site Wednesday reached a milestone by placing the largest steel column to date for the $3.1 billion tower in Lower Manhattan.

The 60-ft, 70 ton beam will serve as one of 24 perimeter columns that surround the building’s core. Once placed, the columns will allow the initial floors of the tower – including the lobby – to be built out. Each of the 24 columns was manufactured at the ArcelorMittal plant in Luxembourg. The steel plates were shipped to North America and fabricated in shops in South Plainfield, N.J. and Terrebonne, Canada. The columns are being delivered to the site from New Jersey over the George Washington Bridge.

Tishman Construction is managing construction on the 2.6 million-sq-ft, 102-story structure for the Port Authority of New York and New Jersey, which will own and manage the building. Construction began in April 2006 on the steel-frame tower designed by Skidmore, Owings & Merrill of New York. As of last month, the superstructure had risen 105 ft above grade. More than 2,300 tons of steel had been erected and Collavino Construction Corp. of New York had placed more than 33,000 cu yd of concrete.

The team is using 14,000-psi iCrete, a precisely blended mix of aggregates and cement, for the up to 6-ft, 6-in thick core shear walls. Thirteen shear walls had been poured as of last month, with the psi reaching 17,000 on several occasions. Maximum concrete temperatures must stay below 160 degrees F, with maximum differential temperatures between the wall center and face maintained within 35 degrees F. The mix employs fly ash and slag, using only 300 pounds of cement per cu yd instead of more than 1,000 pounds of a comparable mix. Collavino is pumping at a rate of 120 cu yds per hour, at a working pressure of 240 bars, over a distance of 400 ft with a 10-in clump and a 28-in spread. The team is utilizing “tree-saving” steel forms and externally mounted vibrators.    

“Workers are erecting steel and pouring concrete on a daily basis, placing some of the largest steel members ever fabricated and completing some of the largest continuous pours on record,” according to a written statement from Tishman.

In early July, construction workers spent 14 consecutive hours pouring 1,250 cubic yards of concrete on the eastern side of the tower, bringing it to street level. The pour began in the rain and was the largest performed to date for One World Trade Center. It formed the base of a large fountain on the plaza level, as well as concrete boxes where Swamp White Oak trees will be planted. More than 125 trucks delivered the concrete to the site.

The Port Authority has secured the first private tenant for One World Trade Center –Vantone Industrial Co. It joins prospective state and federal government tenants and brings the building to 50% occupancy.

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Port Authority spokesman Steve Coleman stated in an email that One World Trade Center, the National September 11 Memorial and Museum, and the WTC Transportation Hub are on schedule. Current target dates, according to the Lower Manhattan Construction Command Center are 2013 for Tower One; September 11, 2011, for the memorial plaza and mid-2013 for the rest of the memorial; and mid-2014 for the transportation hub.