STRUCTURAL DESIGNER AND QUALITY IN STEEL
The answer to the statement “why high percentage sulfur and phosphorous steel was accepted earlier but is unacceptable now” is not hidden in the shape of the product or temperature condition but the answer is hidden the design methods for building / structures and the material property of steel.
A complete understanding and knowledge of the real behavior of construction materials is of prime importance for the proper behavior of engineered structures.
NORMAL STREES – STRAIN CURVE
In order to understand the statement it is important to understand the stress – strain graph for steel. A typical stress strain curve for mild steel looks like
Stress is a function of load while strain is a function of deformation. Now referring to the graph the BLUE line known as the ELASTIC ZONE – in this zone the steel behaves just like elastic i.e. under a load it will deform but will come back to its original shape as soon as the load is removed.
As we move further from BLUE towards RED line known as YIELD PLATEAU - in this zone the material starts behaving somewhat like loosened elastic i.e. once the load is removed steel will try to come back to its original shape but some amount of deformity will still remain.
As we move further to GREEN line known as the STRAIN HARDENING zone – in this zone the material remains deformed even after the load is removed but the material still has not failed. After which the material rapidly fails.
DESIGN CODES BEFORE AND AFTER 2000
The design codes before the year 2000 worked on the principle of WORKING STRESS method where the elastic limit was considered in all calculations. In other words steel was never designed to be loaded beyond the BLUE line. During 80's and 90's a lot of research was done on steel as a material thereby giving better understanding about the material to designers. In INDIA in the year 2000 BIS issued the revision of RCC Building design code IS456:2000 in which the LIMIT STATE design method was adopted. In a limit state design method the calculations are done in the plastic range also up to the yield point i.e. now the steel is designed up to the end of RED line instead of stopping at the BLUE line as done earlier thereby providing material economy.
CHANGES IN STREES – STRAIN CURVE DUE TO HIGH SULFUR AND PHOSPHOROUS
Typical stress–strain curves for standard steel bars used in reinforced concrete construction when loaded in tension are shown below. The curves exhibit an initial elastic portion(marked in BLUE), a yield plateau (marked in RED), a strain hardening range (marked in GREEN) in which stress again increases with strain and finally, a range in which the stress drops off (marked in BLACK) before fracture occurs.
The length of the yield plateau is generally a function of the strength of the steel.
Since it is essential for the safety of the structure that the steel be ductile enough to undergo large deformations before fracture, sufficient length of the yield plateau is required.
In the practical experiments done in Ghana it is found that excess sulfur and phosphorus contents though increase the strength and hardness of the steels, but at the same time decrease their ductility, making them brittle this can be seen by a very small length of the yield plateau.
The major effect of this shortened yield plateau is seen under the seismic condition. Since in seismic design, even though both structural and non-structural damage could occur, collapse of the whole structure should exhibit sufficient ductility during an earthquake. It is important to ensure that in the extreme event of a structure being loaded to failure, it will behave in a ductile manner with large deformations at near maximum load-carrying capacity. The large deflections at near maximum load give ample warning of failure, and by maintaining load carrying capacity, total collapse may be prevented and lives saved. Also, ductile behavior of members enables the use in design of distributions of bending moments that take into account the redistribution possible from the elastic bending moment pattern. In areas requiring design for seismic loading, ductility becomes an extremely important consideration. This is because the present seismic design philosophy relies on energy absorption and dissipation of post elastic deformation for survival in major earthquakes. Thus structures incapable of behaving in a ductile fashion must be designed for much higher seismic forces if collapse is to be avoided. Most building codes for seismic loading, however, recommend design of structures to resist only relatively moderate earthquakes elastically; in the case of a severe earthquake, reliance is placed on the availability of sufficient ductility after yielding to enable a structure to survive without collapse. Hence, the recommendations for seismic loading can be justified only if the structure has sufficient ductility to absorb and dissipate energy by post-elastic deformations when subjected to several cycles of loading far within the yield range.
As can be seen from the two graphs till the material is designed within the ELASTIC limit the high percentage of Sulfur and Phosphorus does not have major impact in the design, but with the plastic limit design having a defined yield plateau is a must.
Thus instead of simply refusing the industry to manufacture high sulfur and phosphorus content material, the decision should be left to the structural designer on what Design methodology he wishes to use considering the client requirements and accordingly what material can be used based on that Design.