The Constructor


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A major defect of the modular ratio or working stress method of design is that it does not give a true factor of safety against failure. To overcome this, the ultimate load method of design was introduced in reinforced concrete design. This method, later was modified as Load Factor Method, was introduced in USA in 1956, in UK in 1957, and later on in India. In this method, the strength of the reinforced concrete section at working load is estimated from the ultimate strength of the section. The concept of load factor, which is defined as the ratio of the ultimate load the section can carry to the working load it has to carry, was also introduced in UK. Usually, reinforced concrete structures are designed for suitable separate load factors for dead loads and for live loads with additional safety factors for strength of concrete.

After the introduction of the load factor method, in order to make calculations comparable with the modular ratio (working stress) method, some codes like British and the Indian codes adopted the Modified Load factor method. This method used the ultimate load principles for design, but retained the allowable service stresses concept in the calculations. Thus, CP 114 used a load factor (ratio of ultimate load to working load) of 2 with additional safety factor applied to material strength, to arrive at the permissible service stresses. As the variation of strength of concrete is much more than that in steel, an additional factor of safety 1.5 for design mixes and 1.67 for nominal mixes were used when calculating the permissible concrete stresses. This additional factor of safety for concrete also ensured that failure always took place due to tension failure of steel, and not by sudden compression failure of concrete. It should be noted that historically the load factor method was the first method which did not use the imaginary modular ration for design of reinforced concrete structures. This has been superseded by the limit state method today in codes of practice.

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