Scott Thompson, Silverstein Properties' project executive for Tower 4
The complexity of erecting a tower at the World Trade Center (WTC) site is unparalleled in New York City. The substructure alone for both 1 WTC and 4 WTC required more than two years’ time to plant the towers’ footings on bedrock, then construct reinforced-steel levels as deep down as 85 feet -- all while coordinating infrastructure for extensive utilities; the rebuilt Cortlandt, Fulton, and Greenwich Streets; and the Vehicular Security Center (VSC).
Then there are things like discovering a million-year-old glacial formation in the heart of the construction site, as was the case at the Tower 4 site. (View a slide show here.)
For Scott Thompson, Silverstein Properties’ project executive for Tower 4, no detail of building the 2.5 million-square-foot skyscraper goes unnoticed. A New York native, Mr. Thompson has been building towers in New York City and the region for more than 20 years. He joined the Silverstein team at the start of 4 WTC construction in 2008, and is now at the center of coordination to get the building to its ultimate height of 64 stories.
The tower reached grade at the end of 2009, and Mr. Thompson is now managing ongoing superstructure erection by contractor Tishman Construction. And while the outcome of WTC arbitration proceedings have yet to be finalized, his team continues to install the steel beams (horizontal members) and columns (vertical members) with the help of two tower cranes and an average of more than 125 daily construction workers and tradesmen.
We asked Mr. Thompson three questions about his work at 4 WTC, and some of the complexities it involves.
Steel superstructure is rising at 4 WTC -- what’s the process for procuring skyscraper steel?
Mr. Thompson: The process begins with the structural engineer for
Tower 4, Leslie E. Robertson Associates (LERA), preparing a set of drawings that show the sizes of standard steel shapes required to hold up the building. Our general contractor, Tishman, takes these drawings and prepares a scope of work which was used to solicit bids from steel contractors. The entire project will have 26,000 tons of structural steel.
|A worker welds
We have five shops across the east coast fabricating the structural steel for Tower 4. The steel used is among the largest pieces rolled in the country -- up to 65 feet long and 110 tons.
The steel contractor fabricates all of the steel on a floor-by-floor basis, typical in groupings of two floors, known as tiers. This is done because a typical steel column will extend across two floors. Once enough steel is fabricated, the steel is shipped to the site by truck, with each truck having a designated crane that will assemble its load of pieces.
Tower 4 has two tower cranes inside the building to erect the steel. We hired DCM Erectors to erect the tower’s steel, which rose above street level in December 2009.
Once the steel arrives to the site, ironworkers attach it to the crane’s hook by steel “choker” cables. Another group of ironworkers grab and guide the steel as it is lowered into place, securing it temporarily with a tapered drift pin shoved through the matching bolt holes of the pieces to be connected. The choker cables are then released and the process starts all over as the ironworkers fasten high-strength steel bolts into the bolt holes.
As there are no floors or walls installed yet to keep the steel from moving, large steel guide cables are secured from the columns to the floor below. These are released over the next week or two as bolts are tightened and welds are made between members. Corrugated metal deck is spread over the steel members and welded down. This deck serves as both formwork and reinforcing steel for the concrete slab to be poured. Once all of the connections are made and all of the deck is installed, the floor is then turned over to our concrete contractor, Roger & Sons, a few floors below the current working erection floor.
(Watch a video about the steel fabrication for Tower 4 here.)
Construction of Tower 4 includes programs for the office building, retail, WTC Transportation Hub and Vehicular Security Center (VSC) -- what are some of the challenges you face?
This job poses very different challenges than I’ve faced before because it is a multi-stakeholder project. We not only have to coordinate base building mechanical systems for a massive project such at this, but also to work with the design teams for the hub, retail, and VSC on the four lower floors as well as up through the fourth floor above street level.
It makes it even more challenging when there are unknown elements to those projects that we have to take into consideration. Once their drawings are submitted, we have to facilitate their needs as best we can and have them comply with our schedule. We also have to coordinate, for example, openings and sleeves within the shear walls and slabs for each of the other stakeholders while working within a schedule.
Once we awarded the trades, we continually make them aware of the design documents for each system from each stakeholder. On this job, we had a room on the 10th floor of 7 WTC where we brought in the awarded contractors, starting with the ventilation system because they set the backgrounds first. Once the ductwork is set, the construction drawings are circulated from trade to trade, with each one putting the overlay of their work on them, distinguished by different colors by trade.
|In late March, work is up to the third level
To manage the data for building Tower 4, we are using Building Information Modeling (BIM), which makes the process more efficient and easier to track. As the information is logged, we get to see a 3D model of what it will look like, allowing us to better coordinate among all the stakeholders.
What was the process for selecting the glass for Tower 4?
Maki & Associates, the architects for Tower 4, designed the tower façade to achieve a mat metallic quality with a luminous sheen that also changes with the light of day. In selecting the glass for the tower’s curtain wall, we looked for reflectivity and flatness to give a uniform exterior appearance. We also wanted the most transparent glass to accomplish Fumihiko Maki’s intention of “bringing the outside in.”
The curtain wall panels for Tower 4 are among the largest anywhere -- 10 feet wide by 12 feet wide at the podium level, and 5 feet wide by 13 feet high in the office tower. Each panel has three layers of glass with a built-in air pocket. In total, there will be 10,000 glass panels for the office tower and about 1,000 at the podium level.
Last spring, a mockup of the storefront for Tower 4’s office lobby façade was completed at the New Jersey headquarters of metal fabricator MetFab Metals. The mockup stood 50 feet tall by 20 feet wide. It was built-to-scale of the tower’s first three floors, and the entire process, including shop drawings, submission and approval process, and erection, took nine months to complete. It is unusual to build such a large mockup, but it was critical to the success of the project. We were able to examine closely the steel, glass, and mullions to ensure the curtain wall would achieve the visual effect we were aiming for.
(View a slide show about the Tower 4 façade mockup here.)