“Steel with higher strength and more ductility is considered better for earthquake resistant design of structures”
Article by Dr. Mohan Kumar Gupta, Professor & Head, Department of Civil Engineering, BIT Durg
Traditionally, steel reinforcement is required to enhance those properties of concrete in which it is otherwise poor, viz. tensile strength and ductility. Steel with higher strength and more ductility is considered better for earthquake resistant design of structures.
Earlier, steels having higher strength were produced by increasing the percentage of carbon, which had an adverse effect on the weldability and ductility of steel, thereby limiting the use of such steels in non-seismic areas. With the passage of time and increasing experience, the structural engineering fraternity world-wide is of the opinion that it is always better to adopt ductile design and detailing practices for most structures, irrespective of the zone in which the structure is situated.
Research is continuously in progress in various parts of the world to improve the properties and behaviour of reinforcing steel further. The outcome of this research is one such product – High strength deformed bars produced through Thermo-Mechanically Treatment (TMT) process. The steel is now produced using a technology such that it has both these desirable properties simultaneously, higher strength and higher ductility, thereby making it most suitable for earthquake resistant structures. Now, higher strength is achieved by the addition of certain alloying elements, keeping the percentage of carbon lower, thereby ensuring that the steel remains sufficiently ductile.
High Strength Deformed bars in strength grades Fe 500, Fe 500D and Fe 500S are specified in the fourth revision of IS 1786:2008 (High strength deformed steel bars and wires for concrete reinforcement – Specification). The figures following the symbol Fe indicate the specified minimum 0.2 percent proof stress or yield stress, in N/mm2. The letters D and S following the strength grade indicates the categories with same specified minimum 0.2 percent proof stress/yield stress but with enhanced and additional requirements.
TMT reinforcement bars in strength grades Fe500D & Fe500S have enhanced physical, chemical and mechanical properties as compared to the bars in strength grade Fe500.
These are produced under controlled manufacturing process and have excellent bendability. External ribs running across the entire length of the TMT bars give superior bonding strength between the bar and the concrete.
They have lower percentage of Carbon, Sulphur and Phosphorous. The low carbon content ensures sufficient ductility and superior weldability for all types of welding without preheating. The impurities like sulphur and phosphorus tend to decrease the strength of steel in extreme weather conditions, hence a lower percent is always desirable.
These steels also have higher yield strength, superior ultimate tensile strength to yield stress ratio, and superior percentage elongation. The higher yield strength of reinforcing bars allows higher loads with the same weight of steel, resulting in optimal utilization and cost saving. The other associated advantages are reduction of reinforcement congestion (few bars are needed; increase in bar spacing possible; lesser bar diameter may be used), relatively faster construction (less time consumed in placing and binding bars; cranes to lift lesser weight) and relative ease in placing concrete. Superior ultimate tensile strength to yield stress ratio and percentage elongation ensure better performance when exposed to unusual events such as earthquakes, collisions and / or fires.
As per IS 13920:1993 (Ductile detailing of reinforced concrete structures subjected to seismic forces – code of practice) High Strength Deformed bars produced through TMT process and having elongation more than 14.5 percent can be used in seismic Zones 3, 4, 5.
It is worth mentioning that the minimum specified elongation for Fe 500D and Fe 500S is 16% and 18% respectively. In Fe 500S, additionally, there is an upper limit specified for yield stress (625 N/mm2) and the tensile strength must be at least 1.25 times the actual yield strength. The properties of FE 500S are designed so as to improve the performance further when greater ductility is required, as in the case of structures in higher seismic zones. Such a provision also helps the structure in experiencing a strong column – weak beam type of behaviour, desired in earthquake resistant design of structures.
The usage of TMT bars facilitates the construction of RCC shear walls, columns, raft foundations and other heavily reinforced structural elements to a great extent, as congestion of reinforcement and placing concrete is always an issue in such structures.
TMT bars may be used with great advantage in a variety of applications such as Bridges, Flyovers, Dams, Retaining walls, High rise buildings with/without shear walls, Industrial structures, Concrete roads etc. The cost benefits, however, may not be fully achieved until there is increased demand for such high-strength reinforcement.
By: Ravi Uppal, Managing Director & Group CEO, Jindal Steel & Power Ltd
Let me, start by thanking all of you—my dear colleagues—for your stellar contributions and spirited performance through 2014-15. In March this year we sold more than 350000 MT of finished steel, the highest monthly sales ever. There could not have been a more befitting toast to your extraordinary efforts through the year.
Be it TMT rebars, plates, SSD, CGP or DRI, through a year of unprecedented challenges we have achieved never-before highs. All the seven gasifiers at our CGP plant were commissioned and the plant achieved full stable operation despite the challenges of fuel supply.
The operations of our Angul DRI plant leaped to nearly full capacity, taking in its resilient stride the poor quality of the input materials. The SMS plant in Angul produced 17 heats with all-time best quality slabs and virtually no rejections. Further, the plate mill achieved first pass yield of greater than 99% and the Raigarh unit added more glory to its glittering record.
The RUBM managed to secure RDSO approval for the manufacture of 260-metre-long rails at the end of a demanding six-month process. The BF-1 touched 2000MT/day production, the sinter plant set new production records and has been operating at full capacity. The cement plant delivered 63000MT of cement—five per cent more than its full capacity during March. Our iconic power plant in Tamnar achieved synchronization of its fourth 600-MW unit in a record time of 22 months from the date activities started at the site. Even more commendable is the fact that it is the only unit in India of its size to be synchronized directly on coal instead of oil. The plant has zoomed effortlessly to a production output of 105% of its rated capacity. Our operations in Tensa and Barbil deserve special mention as they made an extraordinary contribution to supporting JSPL’s efforts to overcome the challenge of material shortages. The list of our achievements is endless and a justifiable cause for all of us Jindalites to feel proud. Our accomplishments are indeed worth celebrating.
During the last year we have faced numerous challenges with regard to the availability of raw materials, most of them because of an adverse external environment. It was a year when our iron ore supplies were suspended due to the non-renewal of SMPL’s EC. Worse, we have been prevented from using our own stock of iron ore fines which is already paid for! These developments left us with no option but to meet our iron ore requirement from external sources.
Through this period of shortages the JSPL team has acted with exemplary prudence and frugality and minimized the adverse impact of external sourcing. Our pellets scaled new highs of quality and we explored myriad ways to measure up to the quality requirements of the DRI plant in Angul. I was overwhelmed to see how our teams across locations worked together seamlessly to overcome the raw material challenges. As a witness to this phenomenon, I have been frequently reminded of the adage “Nothing can overpower Human Will Power”
It is somewhat ironical that societies with minimal natural resources are the ones that have achieved the most success. Israel, Korea, Japan, and Singapore are societies, which have done remarkably well despite having virtually no natural resource. They have done this by being exceptionally innovative and hardworking! We at JSPL should draw inspiration from such examples and move away from depending unduly on any resource.
The recent initiative to reduce dependence on thermal coal by building more blast furnace capacity with minimum additional investment is one such example, which we need to replicate in every domain of our business. The adoption of Tenova technology to reduce power consumption is yet another inspiring example. Our Honourable Chairman has constantly advocated the practice of questioning existing conventions and methods and replacing them with superior practices offering competitive advantage. Since no competitive advantage is permanent, we must constantly innovate to retain our advantages.
The new financial year 2015-16 promises to be yet another challenging but exciting year. It offers us the possibility of fully leveraging our newly built-up capacity and capability in both the steel and power segments. We need to turn around our new as well as underperforming businesses. We need to work speedily towards the completion of Angul Phase-1B, which is critical to our sustainable success. We need to improve the productivity of both our capital and people. In so doing, we need to benchmark ourselves against the global best
Let me welcome you once again to the New Financial Year and wish you all individually and collectively a great success in all your efforts!