Factor of safety in TMT Bars

Factor of safety in TMT bars right steel Fe 415

Engineers build leaving a wide berth of safe loading capacity to construct, let’s get a brief view on how the same is for steel reinforcement bars. We have talked enormously on safety codes listed in IS 1786:2008, in the manufacture of reinforcement bars. Now, let’s discuss how safe loads are calculated in a construction material like reinforcement bars. The min. yield strength a reinforcement bar should have is easily inferred from the grade of the reinforcement purchased. Grade Fe 415 having a min. yield strength of 415 N/mm2, similarly for grade Fe 500 having min. yield strength of 500 N/mm2 and Fe 550 with min. yield strength of 550 N/mm2, so goes for every steel reinforcement grade.

What is the minimum yield strength in TMT reinforcement bars

When we mention min yield strength in TMT reinforcement bars, what it means is a load that do not bring any plastic deformation in the rebar. So for a grade Fe 415, with a min yield strength of 415 N/mm2, no plastic deformation will be observed at 415N/mm2 and thus can be deemed safe to carry such loads. So for testing purposes, when proof stress as high as 450 N/mm2 was applied, it was interesting to find that most TMT bars prominently well known in the market, did not undergo any deformation. So it can be said for a grade of TMT bar, say Fe 415, can withstand a load equal to 415 N/mm2 without any effects and continue so for a finite amount of force applied on it. But the finite amount of force is different for every bar produced, which can be associated with the TMT manufacturing company and the process involved.

Taking a short recap on the TMT manufacturing process, the TMT process can be said to be of three major steps for conversion of the steel billets to TMT steel reinforcement bars.

Manufacture Process of TMT Steel Reinforcement Bars

1. Quenching: The refined steel billets are initially heated to higher temperatures, allowing them to be easily transformed to steel bars by application of force. These hot rolled steel bars are then sent through an automated water spray system with the ability to control the pressure and amount of the water sprayed through its jets instantaneously in response to the instantaneous temperature of the bars. This helps in producing a uniform TMT bars.

2. Self-tempering: After quenching process, in the bars, the core temperature is said to be much higher than the surface temperature of the bar. Thus the core releases temperature, helping in the tempering of the outer martensite layer into a hardened tempered martensite.

3. Annealing: The TMT bars are then let to cool in a specialised cooling beds, where the hotter inner austenite cools down to form a ductile ferrite pearlite. Thus the TMT bars contains a softer flexible ferrite pearlite core and a hardened tempered martensite that provide the bar with the high tensile strength.

As one can see the quenching process is a hard science of controlling of water jets with precision and another interesting factor is that each section of a TMT bar might not have a uniform temperature throughout, so the process is automated in response with the temperature of the bar, thus, the bars so produced cannot be predicted to have the exact same tensile strength, but is within a range of controllable values. This is a reason why a minimum set of requirements are to be met to qualify the IS 1786:2008 standards. The process can be influenced by the environmental temperature factors too. It has been observed that the lower TMT grade like Fe 415 grade can be much more precisely manufactured by TMT process when compared to higher grades as the quenching process becomes even less predictable for the higher grades.

For testing of the Ultimate tensile strength of TMT bars, an increasing test load is added to find out the point of breaking in TMT bars. The ultimate tensile strength of TMT bars on test were found to be much higher than their min. yield strength. For a few TMT bars of grade Fe 415 grade which we tested recorded as high as 550 N/mm2. Any force higher than ultimate tensile strength of the TMT bar causes failure. The ultimate tensile strength of different grades of TMT bars as per standards are as 485 N/mm2 for grade Fe 415, 545 N/mm2 for grade Fe 500 and 585 N/mm2 for grade Fe 550.

So we have discussed about the loads acting on a structure in our previous blog ‘Safe loads and why they are necessity’. So how do the engineers employ TMT bars as reinforcement to concrete. How are they employed to carry the loads generally. Let’s have a short understanding of it.

We do understand TMT bars have an ability to carry a load of 415 N/mm2 for grade Fe 415 and so on for the other grades. So typically in a structure, how much weight are these bars designed to carry by the engineers? On design phase, reinforcement bars are designed to carry only a load keeping on a factor of safety of 1.5 or 1.8, ie, the reinforcement bars are designed on the assumption that the highest loading capacity is at 230 N/mm2 for grade Fe 415. What this leads to is a wide berth for any unforeseeable circumstances that may arise at times, proving a safer construct with a higher life expectancy. This is so, in the case of all reinforcement steel grades.

Stop eating up the safety cushions

Safety of structures is a huge concern of the times with the unexpected man made natural disasters on the rise, so is the need for sustainable structures in construction. The sad fact that have been seen in many construct is the idea to stupidly eat away the safety factors sat, which is even more common in the public works undertaking. Most of the corruption eats away at the quality of the construction material endangering the safety of the construct, but what has kept most of these structures standing can in large be attributed to these factors of safety designed to cushion the blow during unforeseeable times. When it comes to construction, do take a vow to build RIGHT as any shortcomings may be a reason for the reason for unnecessary loss of life, which sadly can never be undone.

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