# Concrete: Design Philosophies

Structural design is process of determining the configuration (form and proportion) of a structure subject to a load carrying performance requirement.

Concrete: Design Philosophies

Structural design is process of determining the configuration (form and proportion) of a structure subject to a load carrying performance requirement. Form of a structure describes the shape and relative arrangements of its components. The determination of an efficient form is basically a trial and error procedure.

In the beginning of 20th century (1900 to 1960) to late 50's of this century, members were proportioned so that stresses in concrete and steel resulting from service load were within the allowable stresses. Allowable stresses were specified by codes. This method of design is called 'working stress method' (WSM). This method of design resulted in conservative sections and was not economical. This design principle satisfies the relation ___ > _.

Where R is resistance of structural element, RS is factor of safety and L is applied external load.

In 1950's ultimate load method or load factor method was developed. In this method, using non linear stress - strain curve of concrete and steel, the resistance of the element is computed. The safety measure in the design is introduced by an appropriate choice of the load factor (ultimate load/working load). Different load factors are assigned for different loads. Following equations are used for finding ultimate load as per IS456 - 1964

U = 1.5 DL + 2.2 LL

U = 1.5 DL +2.2LL to 5WL or 1.5 DL +0.5LL +2.2 WL

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