LIMIT STATE METHOD OF DESIGN- IS 800-2007

LIMIT STATE METHOD OF DESIGN-IS 800-2007:

Basis for Design:

In the limit state method of design IS 800-2007,the structure shall be designed to withstand safely all loads likely to act on it throughout its life. It shall not suffer total collapse under accidental loads such as from explosions or impact or due to consequences of human error to an extent beyond the local damages.

The objective of the design is,

To achieve a structure that will remain fit for use during its life with acceptable target reliability.
In other words, the probability of a limit state being reached during its lifetime should be very low.
The acceptable limit for the safety and serviceability requirements before failure occurs is called a limit state.
In general, the structure shall be designed on the basis of the most critical limit state and shall be checked for other limit states.

2.Steel structures are to be designed and constructed to satisfy the design requirements with regard to stability, strength, serviceability, brittle fracture, fatigue, fire, and durability such that they meet the following:
a) Remain fit with adequate reliability and be able to sustain all actions (loads) and other influences experienced during construction and use;
b) Have adequate durability under normal maintenance;
c) Do not suffer overall damage or collapse disproportionately under accidental events like explosions, vehicle impact or due to consequences of human error to an extent beyond local damage.

The potential for catastrophic damage shall be limited or avoided by appropriate choice of one or more of the following:

1) Avoiding, eliminating or reducing exposure to hazards, which the structure is likely to sustain.
2) Choosing structural forms, layouts and details and designing such that:
i) the structure has low sensitivity to hazardous conditions; and
ii) the structure survives with only local damage even after serious damage to any one individual element by the hazard.
3) Choosing suitable material, design and detailing procedure, construction specifications, and control procedures for shop fabrication and field construction as relevant to the particular structure.

Limit State Design:

For achieving the design objectives, the design shall be based on characteristic values for material strengths and applied loads (actions), which take into account the probability of variations in the material strengths and in the loads to be supported.
The characteristic values shall be based on statistical data, if available.
Where such data is not available, these shall be based on experience.
The design values are derived from the characteristic values through the use of partial safety factors, both for material strengths and for loads.
In the absence of special considerations, these factors shall have the values given in this section according to the material, the type of load and the limit state being considered.
The reliability of design is ensured by satisfying the requirement:
Design action ≤ Design strength
Limit states are the states beyond which the structure no longer satisfies the performance requirements specified. The limit states are classified as:
a) Limit state of strength; b) Limit state of serviceability.

Limit state of strength

Limit state of serviceability.

It is associated with failures (or imminent failure), under the action of probable and most unfavorable combination of loads on the structure using the appropriate partial safety factors, which may endanger the safety of life and property.
The limit state of strength includes:
a) Loss of equilibrium of the structure as a whole or any of its parts or components.
b) Loss of stability of the structure (including the effect of sway where appropriate and overturning) or any of its parts including supports and foundations.
c) Failure by excessive deformation, rupture of the structure or any of its parts or components,
d) Fracture due to fatigue,
e) Brittle fracture.

The limit state of serviceability include:
a)Deformation and deflections, which may adversely affect the appearance or effective use of the structure or may cause improper functioning of equipment or services or may cause damages to finishes and non-structural members.
b)Vibrations in the structure
or any of its components causing discomfort to people, damages to the structure, its contents or which may limit its functional effectiveness. Special consideration shall be given to systems susceptible to vibration, such as large open floor areas free of partitions to ensure that such vibrations are acceptable for the intended use and occupancy.
c)Repairable damage or crack due to fatigue.
d)Corrosion, durability.
e)Fire.

LIMIT STATE METHOD OF DESIGN- IS 800-2007