Over
the years, various design philosophies have evolved in different parts all over
the world, regarding to the structural concrete design. The earliest design
philosophy is the working stress method of design, which evolved around 1900
when the theory was proposed by Coignet and Tedesco was accepted. The elastic
theory has been the basis of Reinforced Concrete design for many years. In the
recent (2000) revision of IS 456 code, the provisions relating to this method
of design procedure have been relegated from the main text of the code.
The working stress method was followed by the ultimate
strength design, which was developed in the 1950s. This design became accepted
as an alternative to working stress design in the ACI code in 1956 and in the
British code in 1957. It was based on the ultimate load carrying capacity of
the Reinforced concrete at collapse. This method was introduced as an
alternative to working stress method in Appendix B of IS 456 code in 1964.
In the mid-1960s, the probabilistic concepts of design
receive a major impetus (Madsen, et al. 1986; Subramanian 1974). The philosophy
was based on the theory that the various uncertainties in design could be
handled more rationally in the mathematical framework of probability theory.
The risk involved in the design was quantified in terms of the probability of
failure. Such probabilistic methods are known as reliability – based methods.
However, this theory was not accepted in professional practice, mainly because
the theory appeared to be complicated (mathematically and numerically).
For codification, the probabilistic reliability method
approach was simplified and reduced to a deterministic format involving
multiple (partial) safety factors (rather than the probability of failure).
Based on the CEB-FIP recommendations, the philosophy of limit state method was
introduced in the British code CP 110 in 1972 (now BS 8110), and the Indian
Concrete Code IS 456 in 1978. In the USA, the ACI introduced the limit states
method in the form of load and resistant factor design (LRFD) in 1963. In the
1971 version of the code (ACI 318-71), the LRFD method was fully adopted and
the working stress was moved to an Appendix and later deleted in 2002.
In general:
1) Safety
2) Economy
3) Serviceability
a) In
working stress method only safety and serviceability is attained.
b) In
ultimate load method only safety and economy is attained.
c) In
limit state method safety, economy and serviceability is attained.
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