In November 2013, 4 World Trade Center became the first tower to open on the original 16-acre World Trade Center site, 12 years after the September 11 attacks. The goal of the design of the 72-story, 977-ft, $1.2 billion tower was two-fold: to construct a quietly powerful and efficient structure that would emanate a dignified presence, and to enrich and enliven the surrounding urban environment.

Bounded by Greenwich, Liberty, Church and Cortlandt Streets, Tower 4 contains 2.3 million sf of above-grade area, with 1.9 million sf of office space. Floors 15 to 54 have a 44,000-sf typical floor plate in the shape of a parallelogram, while floors 57 to 72 are trapezoidal in shape and open toward the southern tip of Manhattan, triangulating from the lower floors to gently bring focus down to the National September 11 Memorial. The lease span distance from core wall to perimeter wall is 45 ft on the west, north and south sides, and 35 ft on the east side.

As is the case with all buildings, column placement is critical to structural and programmatic efficiency. On Tower 4, LERA worked closely with the design architect, Maki & Associates, to balance these efficiencies in an effort to create the desired open space. As a result, there are only four perimeter columns on each side of the tower (compared to the typical seven to 10 used in buildings of similar size and shape). The limited number of perimeter columns allowed for six column-free building corners and four 80-ft-long clear spans, creating expansive views of the city. The corner cantilevers range in length from 20 ft to 45 ft. The column locations were also optimized to pass continuously through the rigidly-programmed sub-grade levels -- a precarious arrangement of subway lines, commuter circulation, mechanical spaces and more -- in order to avoid costly column transfers.

Outside of New York, the core wall is the first major component to be erected in the standard sequence of construction for shear wall buildings. For Tower 4, however, the construction manager, Tishman Construction -- working with local labor officials -- deemed that a structural steel frame would rise first before being encapsulated by reinforced concrete. LERA embraced this approach and recommended that the perimeter columns also be encapsulated. The use of composite columns along the perimeter allowed the steel spandrel girder to be offset from the interior steel erection column when installed, thereby maximizing piece length, increasing the speed of erection by decreasing the number of crane picks per floor and reducing the number of moment connections by 75%. The necessary stiffness required for the long spandrel was accomplished by using a nominal W36 rolled steel section with cover plates located at the points of maximum positive and negative bending. The cover plates were shop-welded, and stop short of spandrel field splice locations. Overall, the reduction and simplification from more traditional design and construction techniques enabled the building to efficiently achieve the cantilevers and long spans of the four-column layout, and helped to offset their costs.

The building's core structure, surrounding the elevators, stairs, bathrooms and mechanical shafts, is composed of two linked concrete tubes. These tubes are connected using built-up steel link beams that span over a central corridor and attach to the concrete walls. Typically, link beams attach to an erection column set near the face of the concrete wall, but this condition can require a massive moment connection. Instead, LERA proposed a shift of the erection column away from the face of the wall, such that the engagement of the steel link beam to the concrete wall was provided by embedment. The resulting increase in material was offset by the elimination of steel-to-steel moment connections.

Throughout the building, concrete strengths vary from 2,000 psi lean concrete fill at the foundation level to 12,000 psi for the tower columns and walls. Concrete mixes were required to be well-graded low-shrinkage, and incorporate significant quantities of supplemental cementitious material. Tower 4 was one of the first buildings in New York City to use high-strength thread bars in concrete columns and core walls. These thread bars decreased erection time and reduced both bar-placing complexity and the size and complexity of the rebar couplers. Thermal concerns were addressed through the use of comprehensive pretesting and the approval of several mix designs, tuned for the appropriate ambient temperature range, element volume and concrete delivery temperatures. Thermal control was made possible through the use of Engius thermal monitoring technology, which LERA employed in collaboration with Roger & Sons, the concrete contractor.

Tower 4 was designed in accordance with the highest energy efficiency standards and was given a LEED-certified Gold rating by the United States Green Building Council.