Limit State Design Philosophy

LIMIT STATE DESIGN PHILOSOPHY

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; and b) Limit state of serviceability.

1. The limit states of strength

The limit states of strength, as detailed in Cl. 5.2.2.1 of IS 800:2007, are those associated with failures (or imminent failure), under the action of probable and most unfavourable 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.

2. The limit state of serviceability

The limit state of serviceability, as detailed in Cl. 5.2.2.1 of IS 800:2007 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 (see Annex C of IS 800:2007). c) Repairable damage or crack due to fatigue. d) Corrosion, durability and e) Fire.

3. Actions

The actions (loads), as detailed in Cl. 5.3 of IS 800:2007, to be considered in design include direct actions (loads) experienced by the structure due to self weight, external actions etc., and imposed deformations such as that due to temperature and settlements.

3.1. Classification of Actions

Actions are classified by Cl. 5.3.1 of IS 800:2007, by their variation with time as given below:

• Permanent actions (Q_p): Actions due to self weight of structural and non-structural components, fittings, ancillaries, and fixed equipment, etc.

•         Variable actions (Q_v): Actions due to construction and service stage loads such as imposed (live) loads (crane loads, snow loads, etc.), wind loads, and earthquake loads, etc.

•     Accidental actions (Q_a): Actions expected due to explosions, and impact of vehicles, etc.

 Characteristic Actions (Loads)

The Characteristic Actions, Q_C, as defined by the code in Cl.5.3.2, are the values of the different actions that are not expected to be exceeded with more than 5 percent probability, during the life of the structure and they are taken as: a) the self-weight, in most cases calculated on the basis of nominal dimensions and unit weights [see IS 875 (Part 1)], b) the variable loads, values of which are specified in relevant standard [see IS 875 (all Parts) and IS 1893 (Part l)], c) the upper limit with a specified probability (usually 5 percent) not exceeding during some reference period (design life) and

d) specified by client, or by designer in consultation with client, provided they satisfy the minimum provisions of the relevant loading standard.

3.3. Design Actions

The Design Actions, Q_d, is expressed as ??_fkQ_ck, where ?_fk = partial safety factor for different loads k, given in Table 4 of IS 800:2007 to account for: a) Possibility of unfavourable deviation of the load from the characteristic value, b) Possibility of inaccurate assessment of the load, c) Uncertainty in the assessment of effects of the load, and d) Uncertainty in the assessment of the limit states being considered. This is detailed in Cl. 5.3.3 of IS 800:2007.

4. Strength

The ultimate strength calculation as detailed in Cl. 5.4 of IS 800: 2000 require consideration of the following: a) Loss of equilibrium of the structure or any part of it, considered as a rigid body; and

b) Failure by excessive deformation, rupture or loss of stability of the structure or any part of it including support and foundation.

4.1. Design Strength

The Design Strength given in 5.4.1 of IS 800:2007, S_d, is obtained from ultimate strength, S_u and partial safety factors for materials, ?_m given in Table 5 of IS 800:2007 by the relation S_d ? S_u/?_m, where partial safety factor for materials, ?_m account for: a) Possibility of unfavourable deviation of material

strength from the characteristic value, b) Possibility of unfavourable variation of member sizes, c) Possibility of unfavourable reduction in member strength due to fabrication and tolerances, and d) Uncertainty in the calculation

5. Factors Governing the Ultimate Strength

The following factors are considered by IS 800:2007 as those governing the ultimate strength.

5.1. Stability

Stability shall be ensured for the structure as a whole and for each of its elements. This should include overall frame stability against overturning and sway, as given in Clause 5.5.1.1 and 5.5.1.2 of IS 800:2007.

5.2. Stability against overturning

The structure as a whole or any part of it shall be designed to prevent instability due to overturning, uplift or sliding under factored load as given below: a) The Actions shall be divided into components aiding instability and components resisting instability. b) The permanent and variable actions and their effects causing instability shall be combined using appropriate load factors as per the Limit State requirements, to obtain maximum destabilizing effect.

5.3. Sway stability

The whole structure, including portions between expansion joints, shall be adequately stiff against sway. To ensure this, in addition to designing for applied horizontal loads, a separate check should be carried out for notional horizontal loads such as given in Cl. 4.3.6 of IS 800:2007 to evaluate the sway under gravity loads.

5.4. Fatigue

Generally fatigue need not be considered unless a structure or element is subjected to numerous significant fluctuations of stress. Stress changes due to fluctuations in wind loading normally need not be

considered. Fatigue design shall be in accordance with Section 13 of IS 800:2007. When designing for fatigue, the partial safety factor for load, ?_f, equal to unity shall be used for the load causing stress

fluctuation and stress range.

5.5. Plastic Collapse

Plastic analysis and design may be used, if the requirement specified under the plastic method of analysis (Cl. 4.5 of IS 800:2007) are satisfied.

6. Limit State of Serviceability

Serviceability limit state is related to the criteria governing normal use. Serviceability limit state is limit state beyond which the service criteria specified below, are no longer met: a) Deflection limit, b) Vibration limit, c) Durability consideration, and d) Fire resistance.

6.1. Deflection

The deflection under serviceability loads of a building or a building component should not impair the strength of the structure or components or cause damage to finishings. Deflections are to be checked for the most adverse but realistic combination of service loads and their arrangement, by elastic analysis, using a load factor of 1.0. Table 6 of IS 800:2007 gives recommended limits of deflections for certain structural members and systems. Circumstances may arise where greater or lesser values would be more appropriate depending upon the nature of material in element to be supported (vulnerable to cracking or not) and intended use of the structure, as required by client.

6.2. Vibration

Suitable provisions in the design shall be made for the dynamic effects of live loads, impact loads and vibration due to machinery operating loads. In severe cases, possibility of resonance, fatigue or unacceptable vibrations shall be investigated. Unusually flexible structures (generally the height to effective width of lateral load resistance system exceeding 5:1) shall be investigated for lateral vibration under dynamic wind loads. Structures subjected to large number of cycles of loading shall be designed against fatigue failure, as specified in Section 13 of the code. Annex C of the code can be used for accommodating the floor vibration.

6.3. Durability

Factors that affect the durability of the buildings, under conditions relevant to their intended life like a) Environment, b) Degree of exposure, c) Shape of the member and the structural detail, d) Protective measure, and e) Ease of maintenance. The durability of steel structures shall be ensured by recommendations in Section 15 of the code.

6.4. Fire Resistance

Fire resistance of a steel member is a function of its mass, its geometry, actions to which it is subjected, its structural support condition, fire protection measures adopted and the fire to which it is exposed. Design provision is to resist fire are discussed in Section 16 of the code.