By Asitha Jayawardena

There are differences as well as similarities in the disasters at the Central Bank building in Colombo in 1996 and the twin towers of the World Trade Centre in New York in 2001. Both were suicide attacks - the former, using an explosive packed lorry while the latter using a high-jacked aeroplane - that resulted in hundreds and thousands of deaths, respectively, in addition to the destruction of property. In both cases what was mainly responsible for the catastrophic damage was not the explosion or the impact but the fire that followed.

Structural design, according to structural engineers, is a compromise between safety and economy. A structure is designed against substantially larger forces that would occur in practice (i.e. Design Ultimate Load), assuming the strength of material is lower than usual. Thus a structure will properly function upto the Design Ultimate Load. When subject to a force greater than that, it will fail - sometimes by collapse. However, the design ensures that the collapse is gradual, giving warnings of distress such as enlarging deformations and widening cracks, for the users to vacate the structure immediately.

The common structural materials used round the world are reinforced concrete (e.g. Central Bank) and structural steel (e.g. World Trade Centre). Here, the focus is on reinforced concrete, which is more popular in Sri Lanka.

The stresses that arise in structural elements due to external forces can finally be grouped to tension (i.e. pulling apart) and compression (i.e. pushing together). The tensile strength of concrete is only one-tenth of its compressive strength. Therefore it is reinforced with steel, which is equally strong in both tension and compression. Thus reinforced concrete is "manufactured".

When it comes to fire resistance, steel is the weaker of the two. Although concrete could retain its original strength even up to 300C, reinforcing steel starts losing its strength substantially at even lower temperatures. However, since reinforcing steel bars are embedded in concrete, the concrete cover acts as a protection for steel. In fact the thickness of the concrete cover to steel is one factor that determines the duration for which the structure would resist fire without failure. However, due to an impact or explosion, the concrete cover may be damaged, exposing the steel bars within, as was the case with the Central Bank disaster.

There are however lessons learnt from this disaster, especially with respect to the safety of the users of the buildings. As highlighted in a research paper presented by Professor Priyan Dias of Moratuwa University at a seminar of the Society of Structural Engineers Sri Lanka, they are:

1. Governmental and commercial buildings accommodating offices are now popular targets. Therefore, open plan offices are desirable as it reduces the distance to exit points.

2. Exit signs should be marked on the floor as well because, in a fire, smoke could obscure the Exit signs on the walls.

3. Fire escape routes should be made mandatory in new buildings. If unavailable in existing buildings, new routes should be fitted to them.

4. Fire drills should be carried out regularly.

5. In a fire, prevention of spread of fire both horizontally and vertically is highly important.

Some of these measures are almost no-cost ones and the others carry a certain cost element. However, the benefits they bring could be priceless as they could save hundreds - sometimes even thousands - of innocent lives.