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Civil Engineer blog

How can our buildings be more resilient against terrorist threats?

Date
16 January 2025

Mina Maxi, Emerging Engineers Award 2024 finalist, explains how retrofitting could help protect our existing buildings from the pancake effect.

How can our buildings be more resilient against terrorist threats?
The World Trade Center Memorial Plaza in New York City. Image credit: Shutterstock

In 2001, nearly 3,000 casualties were recorded in the infamous 9/11 attacks on the Twin Towers located in Manhattan, New York, United States.

In the last 20 years, there have been many terrorist attacks worldwide on similar structures.

Counter-terrorism organisations were created and are working to eliminate these attacks.

But they aren’t the only ones with the power to make a difference – engineers can help by ensuring that our structures are resilient.

As part of my Emerging Engineers research, I set out to prove that retrofitting is a viable option to protect existing buildings against terrorism.

Understanding the pancake effect

In the 9/11 attacks, many people expected the failure of the Twin Towers to be horizontal, but they collapsed vertically. The pancake effect was the main reason for this.

This effect created a vertical, progressive building collapse due to the impact of upper storeys onto lower ones.

Twin Tower progressive collapse phases. Image credit: ASCE Library
Twin Tower progressive collapse phases. Image credit: ASCE Library

Mitigating the effect of the collapse

While completely stopping a collapse may not be achievable, slowing it down could make a big difference.

In the case of 9/11, the northern tower collapsed 102 minutes after the collision and the southern tower collapsed after 56 minutes.

The northern tower had 8,700 personnel in the building approximately and 1,344 (15.4%) people were killed.

The southern tower had approximately 6,154 personnel and around 1000 (16.2%) were killed.

Now imagine how many could’ve died if the building had collapsed instantly after the collision from the aeroplanes.

As such, prolonging the time it takes for a building to collapse could be an answer.

Reducing the threat of terrorist attacks

Much research has been conducted to reduce the threats of terrorist attacks on strategically important buildings.

The American Department of Defense (DOD) issued the Unified Facilities Criteria (UFC-2001) to make buildings with strategic safety measures.

They took it from the strategic point of view (for example, where should the checkpoints be, etc.) or the engineering point of view (which material should be used to protect the building from a blast wave for example, etc).

However, not on protecting existing buildings.

What about existing buildings and structures?

Reusing structures is now a very common sustainable choice in cities.

This means there’s a need to make existing buildings more resilient against terrorist threats.

The right use of new materials is essential for retrofitting. Traditional materials like steel and concrete may no longer be enough.

The most promising materials –according to research – are fibre-reinforced polymer (FRP)-based materials, particularly carbon fibre ones (CFRP).

These materials are considered in such projects because:

  • They have a high strength to weight ratio
  • They show resistance to corrosion
  • Ease of application and flexibility
  • They have a better load carrying capacity and redundancy (how much degradation a structure can suffer without losing certain elements of functionality)

How to address the pancake effect

One of the main reasons behind the pancake effect is columns breaking down.

As such, for my Emerging Engineers Award research, I set out to find ways to strengthen the columns using CFRP materials.

To simulate the effect, I used software to create a prototype model – a five-storey building with four columns.

The columns of the third storey were eliminated in the simulation’s loading stage so that the upper three floors would impact the lower two floors.

When these types of loadings occur, the load tries to go through the shortest path. So, it was either through the building’ slabs or columns.

What the simulations revealed

I simulated four different scenarios.

First scenario

In the first case, I simulated the impact without any enhancement to test the theory (i.e. if the building could sustain the impact without changes). It resulted in total failure.

Second scenario

The second case was to enhance the slab with strips of CFRP diagonally at the four corners. This resulted in the force of the impact being slightly spread out, or dissipated.

Third scenario

The third case targetted the column enhancement. CFRP was used as column jackets on different zones of the column.

Having the jackets at varying heights and thickness led to a worse force dissipation than the first two cases.

However, it took almost twice as long for the building to collapse.

Fourth scenario

The findings from the third simulation led to the fourth, with a full concrete column jacket added to make the building withstand this impact.

Even the slab was enhanced with CFRP layers placed diagonally at the corners, similar to case two.

The results of the fourth simulation were excellent.

The enhancement not only stopped the progressive collapse of the pancake effect, but completely eliminated it, as the building didn’t fail like in the previous cases.

The downside is that this would reduce the usable floor area.

Summary of the research path and best findings. Image credit: Mina Maxi
Summary of the research path and best findings. Image credit: Mina Maxi

What could this research achieve?

This column jacket retrofitted enhancement would serve two of the UN Sustainable Design Goals (SDGs):

  • Goal 3: good health and well-being of residents; and
  • Goal 9: reuse existing structures for different (even more strategically important) purposes.

By implementing this approach in real-life structures, occupants of the building will have much more time to evacuate in these circumstances.

Thus, saving as many lives as possible through engineering.

  • Mina Maxi, PhD candidate at the Norwegian University of Science and Technology (NTNU)