The new 7 World Trade Center (WTC) building in New York City is a notable structure in many ways. The 741-foot high structure, completed in mid-2006, is the first major building to be built in lower Manhattan’s financial district since the attacks of Sept 11, 2001. It replaces the original 47-story 7 WTC built in 1985, which was the last building to fall in the 2001 attacks.

The $700 million center’s innovative design reflects the city’s new stricter safety standards and incorporates sustainable design features that included the use of slag cement.  In recognition of this, the project has received the Slag Cement Association’s 2006 Best Use – Strength award.

Construction began in May 2002 on the site of the original center and amidst the challenges of clearing away rubble and working around closed roads in the aftermath of the attacks. The new 7 WTC features a 52-story office tower above a 10-story Con Ed substation, which is a restored and enhanced version of the Con Ed substation in the first building.

The new steel structure rises above a concrete core that required a strength of 10,000 psi. The concrete mixture containing 40 percent slag cement provided the strength the designers were seeking without inordinately increasing the total amount of cementitious materials required in the mixture.  Concrete mixtures with slag cement mixtures typically achieve higher long-term strength than ordinary portland cement mixtures;  optimum strength gain is normally achieved around the 40 percent replacement level.   Besides strength, slag cement helped achieve optimal resiliency through increased structural stiffness, increasing tenant safety,  Slag also helped the building achieve a Gold Certification under the U.S. Green Building Council’s LEED (Leadership in Energy and Environmental Design) Rating System. Contributing to this green building status—one of the first in New York City—was the large amount of recycled products used in the building, including slag cement. 

The structure’s design also meets New York City’s stricter codes established in March 2005. Prior to that, concrete buildings in the city were built according to a prescription design, which meant that the strength of the concrete was dictated by specified components. The new code requires concrete strength to be specified in the design and allows suppliers to create the optimal mix to meet that requirement.

The new 7 WTC is now considered one of the city’s safest buildings against fire and terrorist attacks because it goes beyond minimum code requirements with core concrete walls as thick as 27 inches, floor slabs ranging from 8 inches to 14 inches thick and walls from 12 inches to 27 inches thick.

Project Team