- Limit State Design
3.1 Safety and serviceability requirements of structures
• Structures remain fit with adequate reliability and they should be able to sustain all loads and other effects experienced during construction and exploitation.
• The structure should have sufficient restoring moment so that the stability of a structure as a whole against overturning is ensured.
• The structure should have adequate factor of safety against sliding due to the most adverse combination of the applied loads.
• The deflection of the structure or part should not adversely affect the appearance or efficiency of the structure or finishes or partitions.
• The structure should have sufficient durability under normal maintenance.
• The structure does not suffer overall damage or collapse disproportionately under accidental events.
3.2 Different limit states for steel design
Acceptable limit for the safety and serviceability of structure/structural member is called limits state. Structure is considered fail to fulfil the purpose for which it was built beyond this state.
Limit State Design method provides basic frame work within which the performance of steel structures is assessed against various limiting conditions. Objective of limit state design is to achieve a structure that will remain fit for use during its life with acceptable target reliability. In this method the probability of a limit state being reached during its life time is kept very small.
In limit state design method, basically two limit states are considered in design.
- Limit state of strength (limit state of collapse, ultimate limit state)
- Limit state of serviceability
Steel structures are designed for limit state of strength under ultimate loads and checked/verified for limit state of serviceability under service loads.
3.3 Design strength and design load
In limit state design, design strengths and design load are used as strength of steel and as loads experienced during the life time of structure.
Characteristic load (action) – the action to be considered in the design which include direct actions and action induced by imposed deformation. Characteristics actions are the values
of different actions that are not expected to be exceeded with more than 5% probability during the life of the structure.
The value of characteristic actions are taken as
• The permanent load (self weight) in most case calculated on the basis of nominal dimension and unit weight specified in IS875 Pt I
• The variable loads, values of which are specified IS875 and IS1893
• Specified by clients/designers satisfying relevant codes
• Accidental load corresponding the value specified by relevant code.
Design Actions/loads (Refer cl.5.3.3)
Design load is the product of the characteristic load and the relevant partial safety factor for load. It is expressed by
Partial safety factor
Factor by which loads are multiplied to get design loads. Partial safety factor for load accounts for the following uncertainties.
• Possibility of unfavourable deviation of load from characteristic value
• Possibility of inaccurate assessment of load
• Uncertainty in the assessment of limit stats being considered
Value of γf
Value of stress (yield or proof stress) of steel, which have 95%probability of not being decreased during the life of the structure.
Ultimate strength (tensile strength0
Characteristic strength of steel corresponding to its rupture in tension.
Strength obtained by dividing ultimate strength by partial safety factor for material
Sd = Su/ γm
Partial safety factor for material considers
possibility of unfavourable deviation of material strength from characteristic value
possibility of unfavourable variation of member sizes
possibility of unfavourable reduction in member strength due to fabrication and tolerances
uncertainty in calculation of strength of the members
Value of γm
Limit State of strength (ref. cl. 18.104.22.168)
Limit state which is associated with failures under the action of probable and mos unfavouravle combination of design loads on the structure
Limit stat of strength includes
• Loss of equilibrium f structure as whole or ay of its parts
• Loss of stability of structure as whole or ay of its parts
• Failure by rupture of structure as whole or ay of its parts
• Failure due to fatigue
• Brittle fracture
In limit state of strength the reliability of design is ensured by satisfying the requirements
Design action ≤ Design strength
Limit state of serviceability
Limit state , which is associated with the functioning of structure or its components under the action of most unfavourable combination of structure or its components under service or working loads.
Limit state of serviceability includes
• Deformation and deflections , which may adversely affect the appearance or effective use of structure or may cause improper functioning of services, equipments or may cause damages to finishes and non structural members.
• Vibration in the structure or any of its components causing discomfort to people , damages to the structure, its contents or which may limit the functional effectiveness.
• Fire resistance
In limit state of serviceability the following service criterion are ensured
• Deflection Limit
• Vibration Limit
• Fire Resistance