How important is Buckling analysis? - Collapse of the WTC: Case study

Credit image: public image
Credit image: public image

Article by: Abhinav Tanksale, on 30 May 2023 at 07:21 am PDT

Let's have a closer look at a well-known incident in history and the real reason behind the catastrophe from an Engineering point of view. Why did the World Trade Centre (WTC) collapse after being struck by airplanes?

Although, the official report on the 9/11 event was not released. It's fair to say there are still many questions about why the buildings collapsed. However, one thing is clear: to ensure safety and stability, buildings should be designed to withstand the impact of airplanes. Other than that, all else is speculation.

Before we look at the case of World Trade Center, let's answer a basic question - How important is Buckling analysis?

By knowing how buckling causes a building to collapse, we can learn how to design buildings that are more likely to survive.

Buckling is a structural failure in a frame that results from the combined effect of lateral loads and axial loads. In the event of a seismic event, buckling can cause a building to collapse. 

Some probable causes for the catastrophic failure of WTC towers are:

  • The structural materials of the WTC were not suited for the hurricane-force winds that came with the initial impact of an airplane. The impact caused the building to quickly sag, and the columns of the buildings were no longer vertical, therefore they buckled and the building began to collapse.

  • The WTC was designed and built before airplane attacks were a consideration in skyscraper design, so the towers were not built to handle the impact of airplanes. The vertical columns, which were the main supporting elements, were built with a very light cross-section. Materials for the building were selected for their low weight, not for their ability to withstand impact.

  • To be more specific, the towers were built using a conventional frame design called a truss-web system. This type of frame is not typically designed to withstand the combined effects of lateral and axial loads. When the towers collapsed, their frames buckled and caused their cores to fail.

Now that we got some idea about the building's design, material, and structure, the next big question to ask is, what factors could have triggered the consequent events which led to a sudden collapse of the towers?

To be precise, the WTC collapse sequence went something like this:

First, the airplane collision and ignition of the airplane's fuel led to a fire with a temperature of more than 800°C. This high temperature caused the columns to soften and weaken under the weight of all the floors higher in the building.

Next, the higher part of the building collapsed onto the lower floor. Consequently, this sudden and huge impact caused its supporting columns to buckle.

About 3000 people died and more than 6000 were injured in this terror attack on WTC.

So, in order to avoid such accidents, it is crucial to design and analyze any structure with respect to buckling.

In conclusion, all buildings will collapse when the load on the structure exceeds the strength of the building. Of course, building regulations are now in place to prevent this from happening. Some ways to prevent this from happening again are to strengthen the lower floors and to build a structure that is less likely to buckle if it is hit by an airplane.