An occasional series on science and technology
This week The Ukrainian Weekly introduces a new feature, "Sci-Tech Briefing," an initiative of the Ukrainian Engineers' Society of America. The debut article by Marco Shmerykowsky was prepared for The Weekly in late October.
The collapse of the Twin Towers
It has been two months since the terrorist attack on the World Trade Center, and like most New Yorkers I still can't fully comprehend what happened. "How could two massive high-rise buildings simply vanish in an instant?" was the question a close friend, who happened to be across the street from the Twin Towers at the time of the attack, asked me that day. Although fire caused by burning aviation fuel was a major factor in the collapse, there were also other factors, which draw from the basic elements and theories of high-rise building design.
Buildings and structural loads
The structural elements of any building provide a way for loads from higher-floor levels to travel into the supporting ground. The loads supported by the building can be separated into two general categories: gravity loads and lateral loads. Gravity loads are due to forces caused by the weights of all the materials supported by the building, such as steel, concrete, plumbing pipes, electrical wire, office partitions, furniture and people. Lateral loads are due to the horizontal forces from events such as wind pushing against the walls of a building.
While lateral loads may play a comparatively minor role in the design of a sheltered, low-rise building, they have a significant impact on the design of a high-rise building. This is because a tall building provides a large surface for collecting the wind, which translates into a large horizontal force. The structural engineer also needs to ensure that the building will be stiff enough so that under normal wind conditions it does not sway like a ship caught in a hurricane, but remains comfortable for the people who occupy it.
All high-rise buildings have special portions known as lateral load resisting systems that are specifically designed to address problems due to lateral loads. These systems consist of frames composed of beams, columns and diagonal members that are specially connected to transmit lateral forces. The frames can be located anywhere within the building.
Design of the Twin Towers
The Twin Towers used a lateral load resisting system known as a tube system, in which a series of closely spaced perimeter columns and deep perimeter beams are used to create a shell that has the strength and stiffness of a large square steel "flagpole." In this system, the perimeter columns are designed to carry both lateral loads and gravity loads, and the interior columns are designed to carry only gravity loads. The net result is that the designers obtain a larger area of usable floor space.
The gravity forces on each floor level of the Twin Towers were collected by a floor system that consisted of open-web joists, which spanned 60 feet between the perimeter and core columns. An open web joist is basically a long truss that has top and bottom members (known as a chord), which are connected by diagonal members in a zig-zag pattern. These members are attractive from a design perspective because they can span long distances but weigh less than comparable solid steel beams.
In addition to supporting the floor, the open web joists interacted with the perimeter columns by providing brace points at each floor level. The lateral bracing of a column is an important concept because it has a direct relation to the amount of load a column can carry. As a column becomes longer, the amount of load it can carry decreases. If the length is too long in relation to the magnitude of the load, the column will want to bend sideways or buckle. Since beams connect to the column at each floor level and in each direction, the effective design length is considered to be the distance between floors.
Causes of the collapse
With these basic concepts of structural engineering in mind, it is possible to understand both why the towers withstood the initial impact of the airplanes and why the towers eventually collapsed.
When one of the airplanes collided with the towers, the initial impact destroyed a number of the perimeter columns, floor joists and core columns. Once these structural elements were destroyed, the building essentially re-wired itself and redistributed the loads to the remaining structural elements.
Since the perimeter columns were designed to simultaneously carry both the gravity loads of a fully occupied building and the lateral loads in extreme wind conditions, and to provide the building with enough stiffness for the tower to be comfortable for its occupants, the net result was that the members had extra load carrying capacity under normal conditions. Thus, after the collision of the airplane with the tower, there was sufficient structure remaining to continue supporting the tower.
The fire due to the aviation fuel, however, served to amplify the damage that was done. Typical high-rise construction requires that the builders provide two to three hours worth of fire protection around main structural members. The aviation fuel fire, however, burned much hotter and quicker than a normal office fire. As a result, the fireproofing material most likely disappeared quickly.
Once the steel was unprotected and heated beyond 1,500 degrees Fahrenheit, the steel began to weaken and soften. Since the floor joists are built of thin components, they were most likely damaged by the fire first. As the floor framing failed, the bracing that the joists were providing to the columns was eliminated. Suddenly the remaining already highly loaded columns had their capacity reduced because they effectively became taller. As the fire continued to burn, the combined effect of failing floor members and weakening columns created a condition where the gravity loads from the portion of the tower above the fire could no longer be supported.
Once this point was reached, the top portion of the tower acted like a hammer driving a nail into a piece of wood. The levels at the collision point collapsed, allowing the upper floors to fall and hit the first undamaged floor. This impact was too great for that level to withstand, so it too failed. This sequence of floors stacking up like pancakes kept repeating floor after floor, until the entire structure was destroyed.
The second tower to be attacked was the first one to collapse for the simple reason that the plane hit at a lower point. Thus, there was more gravity load pushing on the tower's damaged section.
The collapse of the towers was a horrific experience for every New Yorker who experienced it first-hand and for millions more who experienced it on television. What should be noted and praised, however, is that the towers were designed so that they stood for nearly an hour after the tragic attacks occurred. This application of engineering principles allowed thousands of people to escape the towers with their lives.
Marco J. Shmerykowsky, P.E., is a principal at Shmerykowsky Consulting Engineers in New York, teaches high-rise design at The Cooper Union School of Engineering, and is president of the New York Chapter of the Ukrainian Engineers' Society of America.
Copyright © The Ukrainian Weekly, November 18, 2001, No. 46, Vol. LXIX