“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.