In part one of this multi-part series, Salvatore Ancona looks back at the construction methods used in the World Trade Center.
It is hard to imagine a more tragic and detrimental event to the fire service as a whole than the attacks on September 11, 2001. What occurred that day hurt every fire department, division, battalion, company, and individual on a personal and significant level, some more than others. We can remember and mourn the loss of 343 members of the Fire Department of New York (FDNY) every day until eternity. But one of the most profound aspects of the fire service is that through tragedy we learn from the sacrifice of fellow members and use these lessons to inform our operation knowledge and educate future generations of firefighters. It is obvious to many in our profession that there is really no literature regarding the operation of that day. No recommendations, lessons learned, or even a guy who watches a YouTube video from his mom’s basement and comments how he would have done it differently. The day has become a solemn, holy event which many won’t even touch, and rightfully so. 1mm Aluminium Strip
Never Forgetting at the FDNY Fire Academy: 9/11 Memorials
This article is not about tactical objectives. The focus is the buildings you operate in, on, or around and how the effects of natural, manmade, or fire disaster can affect or compromise the structural integrity of such entities. While analyzing all the parts in this series, we will dive into the World Trade Center (WTC) collapse. We’ll go over its initial construction, engineering concepts, columns, floor systems, and the contributing factors that precipitated its collapse. This event is a once-in-a-lifetime incident; it’s not something you’re going to see frequently, if ever. Many of you don’t have high-rise buildings or even multi-story fire-resistive buildings in your respective cities. That’s okay. As we look into this collapse, try to take little things that you can equate to structures and situations in your response area. All the factors of that day don’t have to exist for us to grasp how structural elements failed and ultimately collapsed.
The Twin Towers of the World Trade Center complex were significant engineering marvels of their time. The tallest buildings in the world, they were the very symbol of American ingenuity and modernization. Buildings of previous generations used a cookbook of construction methods. They used a steel frame method that incorporated many columns and girders. The engineers of the WTC aimed to overcome this historic method.
Architects wanted to create a building that eliminated interior columns that took up space and create an open floor plan using lightweight elements whose strength was consistent with previous methods. Sound familiar? The concept of this construction method was increasing at a substantial rate during the 1960s in smaller commercial and residential applications. Another problem they faced was the use of elevators, which if erected in the traditional method would take up 70% of the available floor space. That was deemed unacceptable.
The engineers overcame these obstacles. The plan was to erect two 110-story buildings, standing 1,368 feet tall (1,730 feet for the North Tower, including the antenna). They would be 210 feet wide and 210 feet long. The buildings weighed over 250,000 tons each. To achieve the open floor plan desired, they employed a tube construction method. This meant they would have a substantial core of massive, braced steel, and load-bearing exterior walls made up of 59 columns per side. The exterior tube wall took most of the weight from the floor systems and all the wind load. The first 10 floors used larger columns, which translated to the foundation. Floors 11-107 used prefabricated column/wall sections that, once completed, incorporated two to three floors for each section. Floors 107-110 included engineering areas, elevator workhouses, and a massive truss hat (We’ll get into that later).
But how did they overcome the elevator issue, right? Well, as previously stated, if builders used traditional methods, they would have used almost 70% of the floor system for just elevator banks. To overcome this, they built three zones of elevators built one on top of the other. Each zone would service a certain number of floors. (Figure 2). Think of it as a train ride. Some zones were local stops, and some were express. If you wanted to go to the 107th floor, you could take an express elevator to the 78th floor sky lobby and transfer to one of the 24 local elevators. Elevators serviced B6–110th floors. B6 was six stories below grade. A total of 99 elevators ran within the core of the WTC, and this method allowed for about an acre of open floor space per floor.
Forty-seven massive columns and beams made up the core, which housed the elevator banks and three stairwells. Think of the WTC as a piece of kernmantle rope. The core and mantle working together to assume the loads. If the exterior column wall of the WTC was damaged, this would transfer loads to the core, and vice versa.
Construction began in 1966 and consisted of two 110-story Type I (fire resistive) structures using steel and concrete. The foundation was dug 70 feet below grade (seven stories). A concrete retaining wall was installed prior to excavation due to the proximity of the Hudson River. They pumped slurry into the trench walls and pumped out the water. They then installed enormous steel cages, submerged them into the slurry, then pumped concrete into these trench walls and pumped out the slurry. (Slurry is denser than water, and concrete denser than slurry.) When the concrete cured, they had a reinforced concrete retaining wall. Following excavation, they drilled steel tendons that were post tensioned through the walls into the bedrock for bracing. They had to be careful during this stage as the two tubes containing the path trains were preexisting, however service was never interrupted during construction (Figure 3).
Following excavation, footings were poured. The foundation was significantly different than many of the high-rise buildings we are used to (pilings, pile caps etc.). The foundation of the WTC used massive steel I-beam grillage placed upon the footings that carried the load-bearing tree columns seven stories above (Figure 4). What’s grillage? Grillage is a system in which beams are placed closely together and then built one atop the other each perpendicular to the other. It’s similar to cross laminated timber, only placed on end vertically, not horizontally.
Construction of the WTC was completed in 1971. The antenna on WTC 1 (North Tower) was installed in 1978. In December of 1971, the designers were awarded the outstanding architect achievement for that year. There were four major structural systems used in the WTC: The exterior walls, the core, the floor system, and the hat truss.
Read part 2, part 3, and part 4.
SALVATORE ANCONA is a deputy chief fire instructor at the Nassau County (NY) Fire Service Academy; a lieutenant with the Seaford (NY) Fire Department; a fire inspector with the North Bellmore (NY) Fire Department; a former captain and training officer for the Bellmore (NY) Fire Department; and a former paramedic supervisor in New York City. He has a bachelor’s degree in fire and emergency services administration from John Jay College of Criminal Justice, an A.S. degree in fire science from Nassau Community College, and is attending Georgetown University for his masters in Emergency & Disaster Management. Ancona is the author of the building construction page “The Sons of Brannigan” on Facebook and was a recipient of the 2019 FDIC International Honeywell DuPont Scholarship.
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