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  • Writer's pictureSi Shen

Progressive Collapse

There was a famous event for tower blocks in London. In 1968 there was a gas explosion on the 18th floor of Ronan Point, but the consequence you can see a whole corner of the building collapsed. Many people died. This is a typical example of progressive collapse. The consequence is out of proportion compared to the original impact.

Let’s now look at a simplistic design for a cable stayed bridge. The bridge deck is supported by a series of cables. There is a uniform loading on the deck, W. the spacing between two adjacent cables is L. so how much force the cables must be designed to take? So, the calculation is very simple. The design force should be F = W x L. That is absolutely correct, but only on paper. In real life, cables are not as effective as they are on paper. A cable could loosen up or snap, or relax over time. If the cables are designed to take W times L exactly, what happens is that as soon as a cable loosens up, the forces in the cables next to it will get bigger than W times L, and they are very likely to snap. And as soon as they break, the ones next to them will take on more load and break. And this bridge will collapse like a domino. This is called progressive collapse. This is a very simplistic example of progressive collapse, but the principle holds true for all situations. The prevention of progressive collapse again is a risk management principle. It basically means the consequence or damage should be proportionate to the scale of the failure. A snapped cable can lead to malfunction of a local stretch of the bridge, but it is not acceptable if it leads to a total collapse of the bridge.

Typical approaches of preventing progressive collapse:

  • Alternative load path. A pack-up structural system is set up ready to step up in case the primary structural system is taken out. This is the most commonly used approach. It typically requires the structure to tolerate the loss of any one column without collapse.

  • Local strengthening. The governing factor to the capacity of the structural system is its shortest plank; therefore this approach aims at finding the weak points in the structural system and making it stronger, whereby making the system stronger. This requires knowledge in both how the structural system functions and how threats are imposed onto the structures. This could take the form of improving ductility of the critical parts.

  • Interconnection. Structural elements are tied together one way or another, so that in case one fails, others could come to help. This improves ductility of the structural system as a whole.

Looking at the approaches above, summarising them, we can actually see that preventing progressive collapse is all about redundancy:

  • Redundancy in load paths

  • Redundancy in vulnerability protection

  • Redundancy in connectivity

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