|Friday, 21 September 2001|
WTC disaster: Lessons of safety
by Asitha Jayawardena
The dust has settled at the World Trade Canter (WTC) in New York, but questions haven't. One is, Can a building collapse like that?
When completed in 1973, the WTC twin towers were the tallest in the world - rising to 110 storeys and beyond 0.4 kilometres in height. Since then, they dominated the New York skyline until that disastrous day. After witnessing the horror on the television screen, the question that rings in minds is: Is it really safe to visit, let alone occupy, a highrise building?
To shed some light on this issue, I turned to Dr. Thishan Jayasinghe, Department of Civil Engineering, University of Moratuwa. He not only talked about the concepts behind structural design but also highlighted priceless lessons we should learn from the WTC disaster.
A structure serves a purpose: bridges give access, dams retain water, and buildings provide shelter. In service a structure is subject to various forces: wind, earthquake, snow, weight of the people and furniture etc, and weight of the structure itself. The combinations of forces that cause the worst effect on the structure are identified through calculations. This maximum load is further increased by multiplying it by a safety factor (generally greater than one), resulting in the Design Ultimate Load. This could exceed the actual loads by a substantial margin.
Structural Steel are the most commonly used materials for structures. In reinforced concrete, concrete is reinforced by embedding round steel bars in it. Concrete is weak in tension while steel is strong in both tension and compression. Therefore, Structural Steel, usually found in I, L, U or box sections, need no reinforcement as such. In structural design, the strength of materials is further reduced by dividing it with a safety factor (generally greater than one). This means that a lesser strength than the actual value is used for the design.
This is how safety is enhanced. The model of the structure, which is weaker than the actual structure is now resisting a set of forces larger than those the structure would encounter in practice. Therefore, properly designed structures can easily withstand the forces that can be predicted with reasonable accuracy.
Structural design can be described as striking a balance between safety and economy. Therefore, it is not economical to design a structure against a force so large that it is unlikely to occur during the design life of the structure. However, the design ensures that, when the ultimate load is reached, the structure collapse gradually - showing signs of distress such as enlarging deformations and widening cracks. This is escape warning for the users of the structure. If they do not take such warnings seriously, the results could be catastrophic as seen in the WTC nightmare.
The structure of the WTC towers consisted of structural steel, which was the preferred material for very tall buildings in the early seventies as concrete technology had not been well developed by then. Today, due to rapid advancement of concrete technology paving the way for high strength concrete etc, reinforced concrete is also a strong candidate for tall buildings. However, there are merits and demerits in both. Relatively, structural steel is strong against impact but weak against fire, and the reverse could be said of concrete as a material. However, due to relatively low strength-to-density ratio of concrete, concrete beams and columns are likely to be bulky, increasing their resistance to impact but contributing lavishly to the weight of the structure unnecessarily. Fire is not a serious problem to the reinforcing steel embedded in concrete as the latter acts as a fire protection.
Reports citing experts have revealed that what made the WTC towers crash down was really not the impact of the jets but the fire that followed due to the burning of jet fuel resulting in a staggering temperature of 815 C.
It is also reported that the collision had scraped off the fire protection layer, therefore the fire had no trouble in melting the key structural elements around the crashed area into a rubber-like consistency, eventually engineering a pancake-like collapse, taking thousands of lives in the process.
Even though the structure was steel, the fire protection layer of some columns was damaged and a fire was raging, the towers gave some time for the people to escape. When the first plane crashed into a tower, not only the people in that tower but also those in the other tower should have evacuated the structures. However, some if not most of those in the second tower opted to watch the fire in the first tower instead of fleeing. They could have used the shuttle cars (i.e. high speed lifts) before the second plane crashed into their tower. Even without the second plane's crashing, their lives were in absolute danger because of the disaster in the neighbouring first tower. The structures of the WTC gave time for escape; unfortunately, their users Were too late to realize the gravity of the situation.
The lesson to learn from this unfortunate nightmare is this. Structures are safe but
learn to use them properly. When something is wrong in the building you're in or even in a
nearby building, vacate the building quickly. If you must have a look, do so at a
considerable distance. Let the professionals such as fire-fighters handle the rescue
operations. Help but at a distance because you don't have the necessary knowledge or
training. Never sacrifice your life by becoming an observer of the disaster. Remember
this: To tell the others what you've seen, you should be alive in the first place,
concluded Dr. Jayasinghe.
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