Limit state method of design was introduced in 1970. As per IS 456 Clause 35.1, The term limit state means an unsatisfactory condition that curtails the specified life of a structure by making it unfit for the use it was intended for. Limit state design aims to achieve an acceptable probability that the structure will not reach a limit state. It ensures proper structural safety and serviceability and covers all forms of failure. The limit state method aims for a comprehensive and rational solution to the design problem, by considering safety at ultimate load and serviceability at working load. A structure may become unfit for use due to the following major categories of limit states:
Limit state of collapse
As per IS 456 clause 35.2 Collapse of structure or its part may take place due to rupture of one or more critical sections, on account of Flexure, Compression, Torsion, Shear of elastic or plastic instability, conversion into a mechanism, and overturning.
Limit State of serviceability
As per IS 456 clause 35.3 beyond a certain limit cracking of the concrete may impair the appearance, efficiency, and durability of the structure, and may allow ingress of moisture which may corrode steel. Excessive deflection or deformation may impair the appearance and efficiency of the structure; adversely affect the non-load-bearing members and finishes, and cause discomfort and anxiety to the users.
As per IS 456 clause 35.3.2, The surface width of the cracks should not, in general, exceed 0.3 mm in members where cracking is not harmful and in members were cracking in the tensile zone is harmful; an upper limit of 0.2mm is suggested for the maximum width of cracks. For particularly aggressive environments, such as the ‘severe’ category, the assessed surface width of cracks should not in general, exceed 0.1 mm
The spacing requirement of reinforcement used by the code is to restrict the cracking. As per IS 456 clause 43.2 If the beam-column carrying compressive load is less than 0.2fckAc (Ac is an area of concrete), it may be treated as a flexural member to control crack and if the load is more than 0.2fckAc the crack due to bending action is not critical.
0ther limit state Method
As per IS 456 clause 35.4 Any relevant additional limit state e.g. fatigue, durability, fire resistance, and lightning, etc. Effects of fatigue should be considered if the imposed load is cyclic in character.
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