Natural disasters are usually one of the troublesome challenges for building structures to overcome particularly, the reinforced steel. Natural disasters are of many types, but the most common disaster occurrence in our country are earthquakes. There are many scenarios of structural collapses due to earthquakes. Although the causes can be attributed to several factors such as the age of building structure, lack of maintenance etc, the most important factor to be attributed is the ductile nature.
How does Ductility withstand earthquakes?
In India, the earthquake zoning map is divided into 4 seismic zones namely Zone 2, 3, 4 and 5, with Zone 5 being the highest level and Zone 2 being the lowest level for seismic activity. In order to resist the seismic vibrations, the building structure must have sufficient ductility. The major source of ductility is from the TMT reinforced bars. Even though earthquake resistant TMT bars are produced, it is the ductility factor that must be given the most priority.
When ductile components are used in a building structure, in the event of an earthquake, the seismic vibrations will be absorbed by the reinforced steel, hence the damages caused will be considerably reduced. The TMT steel has several grades based on their strength namely Fe 415, Fe 500, Fe 550, Fe 600. The higher grades for these reinforced bars are achieved by compromising the ductility.
How is ductility compromised?
The TMT steel reinforced bar manufacturing involves an additional process which consists of rapid quenching and cooling. This process consists of three steps which are Quenching, Self Tempering and Annealing. The quenching process involves rapid cooling by a series of water jets which results in an outer hardened martensite layer and a hot inner core layer. The self-tempering process involves the transfer of heat from the core to the outer surface which further makes the rebar harder, thus forming the tempered martensite layer. In the annealing stage, the soft ferrite-pearlite core is left to cool at atmospheric temperature.
When the core cools down, this becomes the ductility component in the TMT steel reinforced bar. The important point is that to get the correct ductility all these three processes must be done at an optimum ratio. When there is a change in that ratio, the ductility and strength factors vary, i.e when the reinforced bar is quenched for longer periods, it makes the rebar harder, resulting in higher tensile strength but the ductility content reduces. This is how the ductility is reduced and the higher grades of TMT steel bars are produced.
What makes Fe 415 the right choice
Even though the higher grade bars have more tensile strength, the major flaw in them is that they are brittle. Using these brittle rebars in earthquake-prone areas makes them vulnerable in handling the seismic loads which in turn results in structural integrity failure. With sufficient tensile strength and right ductility ratio, the Fe 415 is definitely the right choice for building homes. Like the higher grades, ductility is not compromised in favour for more strength.
This is for you people, when choosing steel bars for reinforcement, choose the right grade., which is the Fe 415. When compared to higher grade steels, Fe 415 grade steel rebars are able to withstand the seismic forces of earthquakes much better.