19.1 Limit State Design
The structural design shall be based on limit state concepts. In this method of design, the structure shall be designed to withstand safely all loads liable to act
on it throughout its life; it shall also satisfy the serviceability requirements, such as limitations on deflection, cracking and maximum compression. The acceptable limit for the safety and serviceability requirements before failure occurs is called a ‘Limit State’. The aim of design is to achieve acceptable probabilities that the structure will not become unfit for the use for which it is intended, that is, it will not reach a limit state.
19.1.1 All relevant limit states shall be considered in design to ensure an adequate degree of safety and serviceability. In general, the structure shall be designed on the basis of the most critical limit state and shall be checked for other limit states.
19.1.2 For ensuring the specified objective, the design should be based on characteristic values for material strengths and applied loads, which take into account the variations in the material strengths and in the loads to be supported. The characteristic values should be based on statistical data if available; where such data are not available, they should be based on experience. The ‘design values’ are derived from the characteristic value through the use of partial safety factors, one for material strengths and the other for loads. In the absence of special considerations, these factors should have the values given in 20.4 according to the material, the type of loading and the limit state being considered.
19.2 Limit State of Collapse
The limit state of collapse of the structure or part of the structure could be assessed from rupture of one or more critical sections and from buckling due to elastic or plastic instability (including the effects of sway where appropriate) or overturning. The resistance to bending, shear, torsion and axial loads at every section shall not be less than appropriate value at that section produced by the probable most unfavourable combination of loads on the structure using the appropriate partial safety factors.
19.3 Limit States of Serviceability
19.3.1 Limit State of Serviceability : Deflection
The deflection of a structure or part thereof shall not adversely affect the appearance or functional requirements of the structure or finishes or partitions. The deflection shall generally be limited to the following:
a) The final deflection, due to all loads including the effects of temperature, creep and shrinkage and measured from the as-cast level of the supports of floors, roofs and all other horizontal members, should not normally exceed span/250.
b) The deflection including the effects of temperature, creep and shrinkage occurring after erection of partitions and the application of finishes should not normally exceed span/350 or 20 mm, whichever is less.
c) If finishes are to be applied to prestressed concrete members, the total upward deflection should not exceed span/300, unless uniformity of camber between adjacent units can be ensured.
19.3.2 Limit State of Serviceability : Cracking
Cracking of concrete shall not affect the appearance or durability of the structure. The criteria of limit state of cracking for the three types of prestressed concrete members shall be as follows:
a) For Type 1, no tensile stress.
b) For Type 2, tensile stresses are allowed but no visible cracking.
c) For Type 3, cracking is allowed, but should not affect the appearance or durability of the structure; the acceptable limits of cracking would vary with the type of structure and environment and will vary between wide limits and the prediction of absolute maximum width is not possible.
NOTE — For design of Type 3 members, as a guide, the following may be regarded as reasonable limits. The surface width of cracks should not, in general, exceed 0.1 mm for members exposed to a particularly aggressive environment such as the ‘severe category’ and not exceeding 0.2 mm for all other members.
19.3.3 The flexural tensile stress at any section of the structure, both at transfer and under the most unfavourable combination of design loads, shall satisfy the criteria for the corresponding type of structure.
19.3.4 Limit State of Serviceability : Maximum Compression
The compressive stresses both at transfer and under design loads shall be limited to the values given in 23.3 for all types of structures.
19.3.5 Other Limit States
Additional limit states such as limiting vibrations for human comforts shall be complied with to meet the functional requirements of such structures as may be applicable.